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AS Chemistry Unit 2 AQA AS-LEVEL Student Guide to Unit 2 Chemistry in Action See me in glorious All programs Shared Areas at: Chemistry Read Mr Lund’s Classes LUND Apr-17 1 AS Chemistry Unit 2 AS Chemistry Unit 2 Chemical Energetics Chemical bonding a chemical bond is an electrostatic force of attraction between oppositely charged particles Ionic Metallic Covalent ‘an electrostatic force of attraction between a lattice of oppositely charged ions – cations (which are ‘pussytive’) and anions (which are negative) ‘an electrostatic force of attraction between delocalised valence electrons and a lattice of positive ions’ (the positive centres are what remain behind as a result of the valence electrons being delocalised) ‘a shared pair of electrons’ obviously the true nature of a covalent bond in terms of electrostatic interactions is not fully explained in the description above but it is acceptable at this level (a simple interpretation of the potential well concept can better explain the concepts of bond length, bond energy and why helium does not form a diatomic molecule). http://mysite.verizon.net/kdrews47/bonding/bonding3.html#Well there is a general association between bond dissociation enthalpy and bond length in that longer bonds (involving larger atoms) are typically weaker – see Coulomb's Law the more able could possibly explain E(F-F) in the context of the trend in group 7 bond dissociation enthalpy of triple bonds > double bonds > single bonds although the individual component bonds of a multiple bond are not of equal strength i.e. a double bond is not twice the value of a single bond between the same atoms (the nature/efficiency of orbital overlap plays a part – look up ‘hybridisation’ on chemguide if you want to understand more) in the case of C=C the double bond is less than twice the value of the single (σ) bond as the orbital overlap of the second bond (known as the π ‘pi’ bond) is less efficient chemical reactions involve breaking bonds and then forming new bonds as all that has taken place is a rearrangement of the participating particles (hence the underlying basis of a balanced chemical equation) LUND Apr-17 2 AS Chemistry Unit 2 breaking bonds (bond dissociation) requires energy forming chemical bonds releases energy the energy required to break a given bond is the opposite of that released when it is formed under identical conditions (the dehydration and re-hydration of hydrated copper(II) sulphate is a good example of this – and fun to do) the overall outcome (exothermic or endothermic) of a chemical reaction is a consequence of the energy input to break bonds and the energy output when new bonds are formed the amount of energy exchanged with the environment varies pro-rata with the quantities of chemicals reacted (somewhat obvious if you think about it … hotter fire = larger gas bill) if a reaction is exothermic overall then energy will have been released to the environment as the energy supplied to break the old bonds is less than the energy released when the new bonds are formed in effect the new bonds are stronger than the old bonds exothermic examples: combustion, respiration ENDOTHERMIC EXOTHERMIC +H -H if a reaction is endothermic overall then energy will have been taken from the environment (the degree of this will determine whether an additional energy source such as a Bunsen burner is necessary) as the energy supplied to break the old bonds is more than the energy released when the new bonds are formed in effect the new bonds are weaker than the old bonds (at first sight this may seem implausible as it suggest a less stable situation arises, however, there is another energetic concept called Entropy which we’ll explain that to you next year) endothermic examples: thermal decomposition, electrolysis, photosynthesis Summary Questions How Science Works Page 114 page 113 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2, 3 and 4 ‘The energy value of fuels’ 112 -- 114 Energetics s-cool: Chemical Energetics LUND Apr-17 3 AS Chemistry Unit 2 Mean Bond Enthalpy values for bond dissociation enthalpies represent average values in the gaseous state (this will result in slightly different values to those obtained for specific reagents) using average bond energies to do calculations is pretty much the same level as what you did in GCSE Unit 3 so first of all lets re-visit those calculations (i) (ii) How Science Works: E combustion of methane and other hydrocarbons combustion of alcohols in the homologous series (where each alcohol differs by CH2 and so there will be extra bonds to break and an extra CO2 and H2O produced per increment – perhaps you can work out the expected values starting from methanol) representing the above as an energy cycle – see page 133 although thermochemical cycles will be explained in the next section in more detail Summary Questions Exam Style Questions Page 133 Page 134 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2, 3, 4 and 5 1 131 - 133 Bond enthalpy (the first bit) s-cool: Chemical Energetics LUND Apr-17 4 AS Chemistry Unit 2 Enthalpy Change enthalpy (H) is the total heat content of a system and cannot be determined experimentally ALL chemical reactions are accompanied by energy changes (principally in the form of heat energy but also as light, sound etc) which are transferred to/from the system/surroundings (exothermic or endothermic) note: heat and temperature are related they are not the same quantity e.g. consider the skin damage caused by a kettle of boiling water at 100oC and sparks from a sparkler at ~2000 oC reaction pathway diagrams can be used to show exothermic (-H) and endothermic (+H) reactions. even in an exothermic reaction some (but not all) bond breaking must occur first so some energy must be initially supplied (e.g. a match, or being rather clumsy with nitro-glycerine!) - this is the activation energy once the reaction has started, energy released as new bonds form is used to break more bonds hence an exothermic reaction continues without a continuous supply of energy. this is not the case for an endothermic reaction where energy must be continuously supplied for the reaction to take place e.g. in photosynthesis enthalpy changeH = heat exchange at constant pressure (open container) enthalpy change cannot be measured directly but can be determined experimentally (usually done by measuring temperature change at constant pressure in a calorimeter) H varies with temperature and pressure so Standard Conditions (Ho298) are required: temperature pressure physical state 298 K (25ºC i.e. nominal room temperature) 100kPa don’t put ‘1 atmosphere’ in the exam!!! at room temperature and most stable allotrope (e.g. graphite) LUND Apr-17 5 AS Chemistry Unit 2 you MUST learn the definition of the enthalpy of formation and the enthalpy of combustion (one of them is a certainty – well almost – in the exam!) the use of these values is considered in the next section the enthalpy of formation of an element is by definition zero there is a lot of commonality in the wording – hence the method used below to help you learn: the enthalpy of is the enthalpy change that occurs when one mole of in their standard states at 298K and 100 kPa (i.e. standard conditions) formation (Hof,298) a compound is formed from its elements combustion (oc,298) an element or compound is completely combusted in xs oxygen Summary Question Page 116 AS Chemistry (Nelson Thornes) AQA Chemguide 1 115 -- 117 Enthalpy change s-cool: Chemical Energetics LUND Apr-17 6 AS Chemistry Unit 2 Hess’s LAW Hess’s Law: the enthalpy change for a reaction is dependent only on the initial and final states of the system and is independent of the route taken. this is a consequence of the first law of thermodynamics - energy can neither be created or destroyed (Hess’s law is analogous to saying that the height of the mountain you climb (hence the gain in potential energy) is independent of your route up it – even if you have a rather cunning short cut – consider the consequences for an external observer watching several mountaineers using different routes if this was not true!) enthalpy of reaction (Hor) = enthalpy change for the specific reaction equation given under the stated conditions i.e. this is specific to the equation as it is written (again another obvious idea … you get twice as much energy released from two moles of fuel than is released from one) – this will become better understood after a few calculations in the next section we use Hess’s Law to construct simple energy cycles (NOTE: inverse sign for reverse path – again lets use our mountain analogy – if you climb a 1000m cliff half way up you gain +500m, but if you fall of when you get there it’s -500m height change that will occur as you are now travelling in the opposite direction to the data specified) enthalpy changes that cannot be determined by direct measurement can be determined indirectly using Hess’s Law e.g. Hof,298 (CO) or thermal decomposition reactions (the reasons for that one should be obvious) this is analogous to it being possible to determine the height of an impossible cliff by using a GPS on a viable route – its still the same height irrespective of the route taken Enthalpy Level Diagrams and Thermochemical Cycles – what’s the difference? USING ENTHALPY LEVEL DIAGRAMS TO DO CALCULATIONS IS MENTIONED HERE SO THAT YOU ARE AWARE OF AN ALTERNATIVE APPROACH – EXAMPLES ARE SHOWN IN YOUR TEXTBOOK you will most likely find that thermochemical cycles are easier to use to solve simple problems There will be situations in which energy level diagrams are required at A2 e.g. when you meet the Born-Haber Cycle thermochemical cycle diagrams have no energy scale y-axis so can be written in any orientation (however writing the reaction you are investigating at the top of an inverted triangle is best practise) however, enthalpy level diagrams set the energy value of elements (in their most stable allotropic form at room temperature) as a zero reference (just like sea level). exothermic changes go down relative to the vertical axis and endothermic changes go up LUND Apr-17 7 AS Chemistry Unit 2 Using Enthalpy of Formation Reactants Products 1 Hess’s Law states that 1 = 2 A MARK IN THE EXAM !! Hor,298 = Hof,298 (Products) - Hof,298 (Reactants) 2 Elements Tips: Remember that the enthalpy of formation of an element is 0 Hof,298 (H2O) = Hoc,298 (H2) and Hof,298 (CO2) = Hoc,298 (C) Summary Questions Page 124 Page 130 Page 134 Page 232 Exam Style Questions 1 1 2 1 Using Enthalpy of Combustion Reactants s Products 1 Hess’s Law states that 1 = 2 Hor,298 = Hoc,298 (Reactants) - Hoc,298 (Products) 2 Combustion Products Summary Question Exam Style Questions Page 126 Page 134 AS Chemistry (Nelson Thornes) AQA Chemguide 1 4 127 - 130 Hess, Bond enthalpy (the rest of this section) s-cool: Chemical Energetics LUND Apr-17 8 AS Chemistry Unit 2 Calorimetry it is not possible to measure heat exchange directly but it can be determined from temperature changes the usual way to determine an enthalpy change is to measure the temperature change of a given mass of water (or solution) q = m c m= mass of water/solution being heated/cooled c is the specific heat capacity (the heat required to raise 1g of a substance by 1K – UNITS ?) the next step is scaling to the enthalpy change that would have occurred had one mole of reactant been used (avoid the common error of mixing up mass of water/solution heated with mass of chemical used to do the heating) SCALE up to one mole using: Enthalpy change n= q n use How Science Works: C, D moles of limiting reagent i.e. the one not in xs solids n = m/Mr solutions n = cV Use common sense to make sure that the sign of the FINAL answer is correct by considering the change in temperature. e.g. –ve if it RISES AND DON’T FORGET THE UNITS!!! A polystyrene cup is used for the determination of reactions in solution (e.g. neutralisation and displacement) ensure that you consider the total mass being heated if two solutions are mixed together and don’t include the mass of any solid added in the total mass being heated it is assumed that the density of dilute solutions approximates to that of water for converting volume into mass in a displacement reaction you must determine which reagent was in excess before carrying out the scaling calculation LUND Apr-17 9 AS Chemistry Unit 2 the energy released by fuels involves heating a water tank and measuring its temperature gain investigating a homogenous series of fuels (incrementing by -CH2-) can provide an experimental basis for mean bond enthalpy values How Science Works: E experimentally determined values obtained in the school laboratory will inevitably differ from those quoted in data books due to the limitations in the accuracy of the equipment and the nature of method used the thermometer will probably contribute the most significant apparatus error (particularly if the temperature change is small) however, even if the result lies within the range (from the data book value) associated with instrumental (measurement) error there will always be experimental error due to heat loss and this must be commented upon (errors can cancel one another out to give the impression that the experiment is better than it was) this is minimised if improved insulation is used How science works Page 118 - 119 Questions 1 - 3 The Flame Calorimeter – missing diagram in textbook How Science Works: D extrapolating a cooling curve back to the time where the reaction was started improves accuracy also allowing time for the two solutions to stand while recording their temperature prior to mixing (such measurements will be used to derive a line of best fit prior to the reaction being initiated) it must be remembered that the experiment you carried out was not done under standard conditions (this is a secondary consideration to the far more significant heat loss) Summary Questions Exam Style Questions Page 121 Page 134 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2, 3 and 4 3 117 - 121 s-cool: Chemical Energetics LUND Apr-17 10 AS Chemistry Unit 2 Reaction Kinetics Reaction Rate = change of the concentration of products (or reactants) wrt time. Measuring Reaction Rates Continuous measurements by remote sensing (i) GAS PRODUCED e.g. Mg + HCl(aq) or marble chips and acid the volume of gas produced or mass loss as gas leaves the reaction vessel (ii) PRECIPITATION – a solid is formed in a reaction between two solutions e.g. the reaction between hydrochloric acid and sodium thiosulphate (the ‘thiosulphate cross experiment’) or analysing the relative rates of hydrolysis of alkyl halides using silver halide precipitation How Science Works: F (iii) A COLOUR CHANGE IN SOLUTION e.g. zinc and copper(II) sulphate where the blue colour fades as blue Cu2+ ions are displaced from solution by the relatively more reactive zinc metal and precipitated as metallic copper e.g. the ‘iodine clock’ experiment a colorimeter can be used for accurate measurements (i.e. less human error) (iv) CHANGES IN ELECTRICAL CONDUCTION i.e. the number of ions in a reaction mixture changes as the reaction proceeds e.g. hydrolysis of bromobutane by water Direct Chemical Analysis samples are taken at given time intervals, and the reaction rate in the sample slowed by rapid cooling, dilution, removing the catalyst or one of the reactants – quenching before subsequent analysis by, for example, titration LUND Apr-17 11 AS Chemistry Unit 2 Collision Theory this is basically the same as for GCSE (which means any GCSE book will be good for learning the basics) i.e. REACTION RATE VARIES WITH COLLISION FREQUENCY NOT ALL COLLISIONS LEAD TO A REACTION (MOST DON’T) THEY MUST HAVE ADEQUATE ENERGY UPON COLLISION TO OVERCOME ACTIVATION ENERGY (KINETIC BARRIER) COLLISIONS THAT DO RESULT IN A REACTION ARE DEEMED ‘SUCCESSFUL COLLISIONS’ the effect of concentration, pressure and surface area can be adequately explained with this theory on a pro-rata basis e.g. DOUBLING A REAGENTS CONCENTRATION → DOUBLES THE NUMBER OF PARTICLES → DOUBLES THE NUMBER OF (SUCCESSFUL) COLLISIONS → DOUBLES THE RATE OF REACTION Activation Energy however temperature changes and the effect of catalysts require greater consideration collision theory does not explain the significant effect of raising the temperature activation energy (kinetic stability) must be considered: the minimum collision energy between reactant particles per mole of collisions for a reaction to occur to give products the peak in an energy level diagram represents a ‘transition state’ or ‘activated complex’ – some bonds have been broken and the product is now going to be formed temperature increases mean more effective (with energy greater than the activation energy) collisions (hence increased rate (e.g. food sell by dates and refrigeration) same concept can explain the effect of light (e.g. in photosynthesis) Summary Questions Page 137 1, 2, 3 LUND Apr-17 12 AS Chemistry Unit 2 Maxwell-Boltzmann Distribution Curve There is a distribution of energies amongst particles (i.e. they don’t all have the same energy) no molecules have zero energy Few molecules have very high energy The Effect of Temperature Notice that the peaks shift to the right AND get lower as temperature rises BUT EA Those molecules that can engage in successful collisions are represented by the area under the curve where E > EA You MUST know the effect of temperature change on the shape of the curve and take great care to use precision when drawing it implications for reaction rate NOTE EA DOES NOT CHANGE Summary Questions Exam Style Questions Page 137 Page 144 AS Chemistry (Nelson Thornes) AQA Chemguide 1 2 136 - 139 Collision, Maxwell LUND Apr-17 13 AS Chemistry Unit 2 Catalysis you should know the correct definition of a catalyst catalysts enable a different mechanism with a lower activation energy NOTE CATALYSTS CHANGE THE VALUE EA NOT THE SHAPE OF THE CURVE. you must ensure that you are able to clearly differentiate between the use of the MBDC to explain the effect of increased temperature and how a catalyst works in the context of why more particles find themselves with enough energy to react. Homogeneous Catalysis same phase as reactants (e.g. all in solution) acid catalysed esterification enzymes in biological systems chlorine free radicals (formed by the action of UV light on CFC’s) and ozone (O3) depletion Heterogeneous Catalysis different phase to the reactants typically transition metals or their compounds are used e.g. manufacture of ammonia Haber Process Fe catalytic converters Pt and Rh hardening fats (making margarine) Hydrogenation Ni (adsorption onto the surface of the solid nickel catalyst weakens π bonds) manufacture of nitric acid Ostwald Process Pt and Rh manufacture of sulphuric acid Contact Process V2O5 other (non transition metal) examples include: hydration of ethene to produce ethanol cracking of hydrocarbons silica/H3PO4 aluminium oxide/silicon dioxide LUND Apr-17 14 AS Chemistry Unit 2 How Science Works: L Catalytic Converters platinum and rhodium are coated onto a honeycomb ceramic material (large surface area = increased rate) since adsorption only occurs at the surface (expensive metal underneath would be wasted) the reactant gases form weak bonds with the surface of the catalyst (adsorption) this weakens their bonds thus lowering the activation energy (additionally the catalyst also helps promote more favourable molecular orientation) this is followed by desorption in which the products depart the catalyst selected provides bonding strong enough to hold the reactant gases on the surface whilst not preventing the products from leaving thus blocking an active site Write equations for these CO and NO react to form CO2 and N2 NO also reacts with uncombusted hydrocarbons to produce CO2, H2O and N2 Hardening Fats – catalytic hydrogenation the raw materials for the manufacture of margarine are vegetable oils e.g. sunflower, olive) which contain triglyceride esters derived from propane-1,2,3-triol (glycerol), and polyunsaturated fatty acids such as linoleic acid. adsorption onto the surface of a nickel catalyst at about 200oC weakens π bonds allowing hydrogenation the relative strength of the VdW increases as the removal of semi-rigid double bonds allows more efficient overlapping thus increasing the relative (i.e. the oil becomes a fat) Here’s a few links for those of you interested in the history and health aspects of margarine and spreads (you ALL will eat this stuff at some point even though you might think that you don’t) http://www.margarine.org.uk/whatisspread-history.html How Science http://www.margarine.org/historyofmargarine.html Works: B How science works Summary Questions Exam Style Questions Make notes on Margarine and Zeolites (142) Page 143 1 Page 144 1, 3, 4, 5 Page 234 8 AS Chemistry (Nelson Thornes) AQA Chemguide 140 - 143 Catalyst, margarine LUND Apr-17 15 I can’t believe it’s not Chemistry AS Chemistry Unit 2 Chemical Equilibrium reversible reactions can occur in a closed system e.g. in solution (where no gas is given off) eventually a position of DYNAMIC equilibrium will be achieved (as opposed to static) where the relative concentrations of the reactants and products will remain constant providing that the reaction conditions remain unchanged at chemical equilibrium the rate of the forward and reverse reaction are identical this does not mean that the relative amount of reactants and products are identical examples of a homogeneous equilibrium in which all the reactants and products are in the same phase include the Haber Process and the Contact Process note that the catalyst used can still be in a different phase (heterogeneous catalysis) CATALYSTS DO NOT AFFECT THE POSITION OF EQUILIBRIUM (SINCE THE FORWARD AND BACKWARD REACTIONS ARE BOTH SPEEDED UP) BUT DO ALLOW IT TO BE ACHIEVED FASTER equilibria can be monitored using similar techniques to those used to investigate rates of reaction Summary Questions Page 147 1, 2 Le Chatelier’s Principle. LCP is a predictive tool NOT an explanation of the reason why the position of equilibria shifts when reaction conditions (concentration, temperature, pressure) are changed Le Chatelier’s Principle states that if you change the reaction conditions then the position of equilibrium changes in the direction that seems to oppose that change LCP is NOT an explanation of WHY it happens so avoid statements such as ‘because of LCP’, ‘LCP causes …’ and learn to state ‘LCP predicts that …..’ LCP is not suggesting that the system completely reverses the change imposed, e.g. reversing a temperature increase when establishing a new equilibrium it implies that the shift in the position (in terms of reactants and products) of equilibria is in the direction that seems to minimize the effect of that change a new position of equilibria in which the relative rates of the forward and backward reaction are once again in balance under the new set of conditions is eventually arrived at by custom the chemicals on the RHS of a chemical equation are deemed the products a shift in equilibria to the right (i.e. the imposed constraint favoured the forward reaction) increases the yield of the reaction LUND Apr-17 16 AS Chemistry Unit 2 LCP and Concentration the effect of concentration changes can be predicted by LCP and explained by relative changes in the rate of the forward and backward reaction when a change in concentration is made and therefore a change in collision frequency LCP and Temperature an increase in temperature favours the endothermic process (which uses energy and so ‘opposes’ that increase) e.g. the equilibrium between dinitrogen tetroxide and nitrogen dioxide e.g. the complex looking reactions below [Cu(H2O)6]2+(aq) + 4Cl-(aq) Blue CuCl42-(aq) + 6H2O(l) Yellow [Co(H2O)6]2+(aq) + 4Cl-(aq) CoCl42-(aq) + 6H2O(l) Pink Mauve the reason this is so reflects the fact that EA will be larger for the endothermic process so it will be relatively more favoured by a rise in temperature LCP and Pressure this is only applicable where there is an imbalance between the number of moles of gaseous particles on either side of the equation an increase in pressure favours the direction that reduces the total number of gaseous particles in effect ‘opposing’ the pressure increase by reducing the number of gaseous particles in the reaction vessel e.g. the equilibrium between dinitrogen tetroxide and nitrogen dioxide and also the Haber process Summary Questions Exam Style Question Page 150 Page 154 1, 2 3 LUND Apr-17 17 AS Chemistry Unit 2 How Science Works: A Haber Process know how we obtain nitrogen and hydrogen nitrogen is un-reactive (consider the car engine conditions) have a look at this animation: http://www.absorblearning.com/media/attachment.action?quic k=128&att=2741 consider the opposing affect of temperature on rate and equilibrium and the compromise made recycling the un-reacted hydrogen and nitrogen (ammonia is liquefied and removed) helps to compensate for the poorer yield thus saving costs whilst increased pressure provides both an increased yield AND an increased rate this is limited by cost (energy for pumps, high pressure vessels) and safety considerations a catalyst (Fe in small lumps – large surface area) is used to increase rate without requiring an even higher temperature (energy cost) and of course it can be re-used (cost saving) compounds derived from ammonia are important – e.g. nitric acid, fertilizers, nylon, dyes, explosives etc How science works Summary Questions Exam Style Questions Page 151 - 152 Page 153 Page 154 1-3 1 4, 5 and 6 Hydration of Ethene to form Ethanol ethanol can be manufactured as a batch process using fermentation it can also be made from the direct hydration of ethene obtained from the fractional distillation of crude oil in a continuous process the reaction involves the use of a catalyst – phosphoric acid on silica the position of equilibrium for this process will be affected by concentration, temperature and pressure as with the Haber process there are trade offs of which you should be aware and practicalities to be overcome. Summary Question Page 153 2 LUND Apr-17 18 AS Chemistry Unit 2 Carbon Monoxide and Hydrogen to form Methanol methanol is used as a chemical feedstock and as an additive to petrol it can be manufactured by the reversible reaction between carbon monoxide and hydrogen in the presence of a copper catalyst the reactants (‘synthesis gas’) are manufactured from the reaction of methane or propane with steam as with ethanol the temperature and pressure that are used represent a compromise Summary Question Exam Style Questions Page 153 Page 154 AS Chemistry (Nelson Thornes) AQA Chemguide 3 1, 2 146 - 153 Haber, Reversible, Le Chatelier’s Principle LUND Apr-17 19 AS Chemistry Unit 2 Acid-Base Equilibria This section is not officially part of Module 2, it is a revision of GCSE Chemistry but its contents are vital to your understanding of the rest of the course so work through this yourselves. Arrhenius definition of an acid – releases hydrogen ions into solution (alkali releases hydroxide ions) Bronsted-Lowry Theory of Acids and Bases - acids are proton donors, bases are proton acceptors (e.g. hydroxide ions accept protons to form water). Chemistry of Acids and Bases when a substance dissolves in water it forms an aqueous solution which may be (i) acidic (ii) alkaline (iii) neutral. e.g. soluble oxides of non-metals (for example, carbon dioxide, sulphur dioxide and nitrogen dioxide) e.g. soluble metal oxides and hydroxides (for example, the oxides and hydroxides of sodium, potassium, and to some extent calcium) e.g. salt water pure water is neutral (i) (ii) distilled water can be slightly acidic due to dissolved CO2 tap water is often slightly alkaline due to dissolved minerals pH scale is used to show how acidic or alkaline a solution is indicators (which are pH sensitive dyes) are different colours at different pH’s acids (or alkalis) of the same concentration are not necessarily of the same strength a strong acid or strong alkali is one that is 100% ionised in water. HCl(aq) H+(aq) + Cl-(aq) NaOH (aq) Na+(aq) + OH-(aq) A weak acid or weak alkali is only partially ionised in water. e.g. CH3COOH(aq) ethanoic acid e.g. NH3 (aq) ammonia + H+(aq) + CH3COO-(aq) ethanoate ions NH4+(aq) + ammonium ion H2O(aq) LUND Apr-17 20 OH-(aq) AS Chemistry Unit 2 Neutralisation Reactions examples include: antacids; spreading lime on fields to reduce soil acidity, or in lakes to reduce acidity caused by acid rain. acid + base salt + water salt produced when an acid is neutralised depends on the metal in the substance neutralising the acid, and the acid used ammonia can also neutralise an acid (ammonium sulphate (a fertilizer) is produced with sulphuric acid) when neutralisation occurs between acids and alkalis, hydrogen ions react with hydroxide ions forming water molecules H+(aq) + OH-(aq) H2O(1) other ions are ‘spectator ions’ ‘they do nowt’ sulphuric acid will require twice as much alkali for neutralisation as two hydrogen ions are released per molecule. This means it will be twice as strong as hydrochloric acid of the same concentration. H2SO4(aq) + SO42-(aq) metal carbonates and hydrogencarbonates react with acids to produce a salt, carbon dioxide and water. acid + metal (hydrogen)carbonate 2H+(aq) salt + water + carbon dioxide when a reactive metal reacts with an acid: metal + acid LUND Apr-17 21 salt + hydrogen AS Chemistry Unit 2 REDOX REACTIONS How Science Works: A simplest model involves the gain (oxidation) or loss (reduction) of oxygen similarly the gain or loss of hydrogen can be used these are still used to explain oxidation and reduction in organic chemistry they do not however include all instances of redox reactions this idea can be improved by considering redox reactions as an electron transfer process oxidation is the LOSS of electrons, reduction is the gain of electrons OILRIG oxidising agents (OXIDANTS) accept electrons and are themselves reduced reducing agents (REDUCTANTS) donate electrons and are themselves oxidised Half (ionic) equations since oxidation and reduction MUST both occur in the same reaction in order for electrons to be transferred then it is possible to write separate equations to show each in turn you will already have encountered this in electrolysis separate equations were written describing what takes place at the anode and cathode cations arrived at the cathode an accepted electrons hence reduction anions arrived at the anode and surrendered electrons hence oxidation remember that charges on ions and electrons must be shown in addition to state symbols(note electrons don’t have a state symbol) e.g. electrolysis of brine ANODE OXIDATION 2Cl CATHODE REDUCTION 2H Summary Question - (aq) + (aq) Page 157 AS Chemistry (Nelson Thornes) AQA Chemguide Cl2(g) + + 2e 1 156 - 157 Oxidation and reduction LUND Apr-17 22 - - 2e H2(g) AS Chemistry Unit 2 Oxidation Numbers oxidation is the LOSS of electrons, reduction is the gain of electrons OILRIG oxidising agents (oxidants) accept electrons and are themselves reduced reducing agents (reductants) donate electrons and are themselves oxidised Oxidation State this is a ‘book keeping’ method of the effective control of electrons used in bonding elements = 0 oxidation state of elements in simple ions = charge on ion oxidation state of elements in polyatomic ions = charge on ion oxidation state of elements of a compound = 0 the relatively more electronegative element is assigned the negative oxidation state hydrogen = +1 (except in metal hydrides where it is -1) oxygen = -2 (except in peroxide O22-) where it = -1) group 1 metals = +1 group 2 metals = +2 fluorine = -1 (even with oxygen, which is +2 in OF2) Aluminium = +3 metals are always positive in a compound or polyatomic ion maximum possible oxidation state oxidation numbers and nomenclature e.g. cobalt(II) nitrate(V), phosphorus(V) oxide = group number (note not always possible for various reasons – see later) take care not to mix up charge and oxidation numbers in polyatomic species e.g. a sulphate(IV) ion does NOT have a 4- charge (the IV refers to the oxidation state of the sulphur LUND Apr-17 23 AS Chemistry Unit 2 changes in oxidation numbers can be used to identify redox reactions in inorganic and organic reactions e.g. metal or halogen displacement reactions we specifically refer to an element in a species (e.g. ‘the iron in Fe2O3 is reduduced’) assume that multiple instances of an element in a species have the same value (e.g. both carbons in ethene are -2) OXIDATION REDUCTION oxidation number becomes relatively more positive oxidation number becomes relatively more negative in a disproportionation reaction atoms of the same element are oxidised and others are reduced in the same reaction e.g. chlorine with water to form chlorate(I) and chloride (note: chlorine with alkali goes to completion) decomposition of hydrogen peroxide Summary Questions Page 159 AS Chemistry (Nelson Thornes) AQA Chemguide 1-5 158 - 159 Oxidation number LUND Apr-17 24 AS Chemistry Unit 2 Balancing Redox Equations protocol for constructing half equations 1 2 3 4 5 get the formula correct (and stoichiometry – see example (iii) below) balance the oxygen adding H2O(l)’s to the side with least O’s balance the hydrogens using H+(aq)’s to the side with least H’s balance the charge on each side by adding e- to relatively more positive side add state symbols (i) Fe3+(aq)/ Fe2+(aq) (ii) MnO4-(aq)/Mn2+(aq) (iii) Cr2O72-(aq)/Cr3+(aq) (iv) S4O62- (aq)/ S2O32- (aq) (tetrathionate/thiosulphate) combining two half equations: as with any equation cancelling out/down should be undertaken (i) iodine and thiosulphate ions (ii) iron(II) ions and manganate(VII) ions (iii) dichromate(VI) ions and ethanol (to ethanal and to ethanoic acid) (note that the use of [O] is acceptable as a simplification where the specific details of the oxidising agent used are superfluous) Summary Questions Exam Style Questions Page 163 Page 164 – 5 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2 (2a is a little tricky at this stage) 1-8 160 - 163 redox equations LUND Apr-17 25 AS Chemistry Unit 2 Group 7 Elements And Compounds Physical Properties colour of chlorine, bromine and iodine deepens down the group. trends in the volatility of the elements reflect increase in Mr and increased Van der Waals’ forces relative electronegativity decreases down the group as atomic radii increases (proton increase is cancelled by a similar increase in the number of screening electrons) Summary Questions Page 167 Group 7 1-3 Halogens as Oxidizing Agents halogens are oxidising agents with decreasing oxidising ability (and hence relative reactivity) down the group. the increased number of protons are cancelled by more screening electrons, however as the atoms are larger the halogen less readily gains an electron the above idea is actually a simplification of the overall process but is adequate for chlorine, bromine and iodine as this trend reflects the outcome although in reality there are other contributory factors to consider a more reactive halogen can displace a less reactive halide from a solution of its salt, in effect oxidising it (the halogen is used in solution in practicals – less dangerous) be able to write half equations for displacement reactions and associated observations examples are the extraction of bromine from sea water using chlorine, and iodine from kelp How science works Summary Questions Page 169 Page 169 1-2 1 test for iodine – turns starch black (NOTE: IODIDE IONS DO NOT TURN STARCH BLACK !!!!!) AS Chemistry (Nelson Thornes) AQA Chemguide 166 - 169 Halogens LUND Apr-17 26 AS Chemistry Unit 2 Halide Ions as Reducing Agents halide ions become relatively stronger reducing agents down the group, themselves being oxidised to the respective halogen as halide ion size increases, the effective nuclear charge (i.e. actual nuclear charge diminished by screening electrons and distance) decreases and the ease with which the outer electron is lost increases. remember starch is NOT a test for iodide ions !!!!!! Reactions with conc. H2SO4 in all cases the hydrogen halide is produced when concentrated sulphuric acid is added to the solid halide – this is a displacement reaction NOT REDOX (check the oxidation numbers) e.g. S= +6 +6 NaCl(s) + H2SO4(aq) NaHSO4(aq) + HCl(g) bromides and iodides (but NOT chlorides) then undergo a redox reaction +4 2HBr(aq) + H2SO4(aq) Br2(l) + SO2(g) + 8HI(aq) + H2SO4(aq) 4I2(s) + H2S(g) + H2O(l) -2 4H2O(l) iodides are a better reducing agent than bromide ions hence they reduce the sulphur initially to SO2 (+4) as with bromide but subsequently further reduce it to S (O) and finally H2S (-2) you should be able to write equations separately for each of these redox reactions you must be aware of the associated observations for each of the above – covering colour changes, gases evolved and precipitates (sulphur in this case) Note: whilst in reality it may not be possible to see each one as it may be masked by other observations you should still be able to describe what might possibly be observed as a consequence of each step, particularly for iodide ions Summary Questions Page 172 AS Chemistry (Nelson Thornes) AQA Chemguide 1 170 - 171 Halogens LUND Apr-17 27 AS Chemistry Unit 2 Test for Halide Ions halide ion solutions are all colourless remember starch is NOT a test for iodide ions !!!!!! add dilute nitric acid* followed by silver nitrate solution – which will produce a silver halide precipitate Silver Fluoride Silver Chloride Silver Bromide Silver Iodide No precipitate – it’s soluble White soluble in dilute ammonia solution Cream soluble in conc. ammonia Yellow insoluble in conc. Ammonia * Nitric acid is used as the nitrate anion will not produce a precipitate with any metal ions present. It also ensures that no silver oxide precipitate is formed. silver salts decompose in strong light (hence they are stored in dark bottles) two of the precipitates are quickly affected by sunlight - silver chloride turns purple/grey and silver bromide turns green/yellow silver is deposited as an opaque layer on black and white film 2AgCl(s) 2AgBr(s) 2Ag(s) + 2Ag(s) + Cl2(g) Br2(l) photochromic sunglasses contain a mixture of silver chloride and copper(I) chloride – find out how they work (and don’t work too well for driving) http://www.explainthatstuff.com/photochromiclenses.html http://www.district87.org/staff/sutterm/Chem%20Matters/Organized%20according%20to% 20topics/Acid%20Bases/Automatic%2520Sunglasses.pdf Summary Questions Exam Style Questions Page 172 Page 174 - 175 AS Chemistry (Nelson Thornes) AQA Chemguide 2 3, 5, 8, 9 171 - 172 Halogens LUND Apr-17 28 AS Chemistry Unit 2 How Science Works: I Reaction of Chlorine with Water chlorine gas is poisonous but its solution is used in water purification (drinking water and swimming pools) to kill bacteria chlorine reacts with water (‘chlorine water’) to produce a mixture of two acids hydrochloric and chloric(I) - by disproportionation (chlorine atoms are simultaneously oxidised and reduced) 0 Cl2(g) + H2O(l) + +1 HOCl(aq) a similar reaction occurs but to a relatively lesser extent with other halogens down the group moist blue litmus paper is first turned red, then bleached by the chloric(I) acid in drinking water it prevents typhoid and cholera in swimming pools the chloric(I) acid, HOCl, (an oxidising agent) kills bacteria by oxidation and it is also a bleach (swimming costumes might fade over time as result) in sunlight chlorine oxidises water to oxygen hence the need to add more chlorine, particularly in shallow pools, but the concentration of the chlorine solution must be monitored to ensure that toxic levels are avoided alternatively sodium or calcium chlorate(I) can be added to water which reacts with it to yield choric(I) acid ClO-(g) -1 HCl(aq) + H2O(l) HOCl(aq) + OH-(aq) the pH kept slightly acidic – use LCP to suggest WHY Reaction of Chlorine with Sodium Hydroxide Solution disproportionation reaction occurs to yield sodium chlorate(I) which is used to make bleach 0 Cl2(g) + 2OH-(aq) -1 +1 Cl (aq) + ClO-(aq) + H2O(l) adding acid to bleach is dangerous Cl-(aq) + ClO-(aq) + 2H+ (aq) Cl2(g) + H2O(l) solutions of sodium chlorate(I) (i.e. bleaches) react with KI to liberate iodine How could I2 can be estimated by titration against Na2S2O3(aq) using starch as an indicator this be used? Summary Questions Exam Style Questions Page 173 Page 174 - 175 AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 1, 2, 7 173 Disproportionation, halogens LUND Apr-17 29 AS Chemistry Unit 2 Group 2 Alkaline Earth Elements Physical Properties atomic radius increases down the group as there are more electron shells both first AND second ionisation energy decrease down a group as the size of the atom and number of shielding electrons increases (which offsets increasing nuclear charge). increasing ease of ionisation contributes to the general increase in reactivity down the group melting point decreases down the group as increasing size means that the distances between the positive nuclei and delocalised electrons are greater hence the metallic bonding is weaker (this also make them softer down the group) Reactions of Group 2 Elements with Water when metals react they form positive ion and therefore are oxidised hence the chemical with which they react is reduced metals are therefore potential reducing agents with the more reactive metals being better reducing agents magnesium reacts with steam to form magnesium oxide and hydrogen Mg(s) + H2O(aq) MgO(s) + H2(g) others react with water to form metal hydroxides Ca(s) + 2H2O(aq) Ca(OH)2(aq) + H2(g) Ba(s) + 2H2O(aq) Ba(OH)2(aq) + H2(g) increase in reactivity down the group reflects ease of removal of the two outer s sub-shell electrons Solubility of Group 2 Hydroxides and Sulphates solubility of hydroxides increases down group while sulphates decrease the alkalinity of solutions of the hydroxides thus increases down the group LUND Apr-17 30 AS Chemistry Unit 2 Uses of Group 2 Metals compounds Magnesium hydroxide very insoluble and used medicinally as a slurry (a solid suspension known as milk of magnesia) and in toothpaste in both cases it acts as an antacid and is better than a carbonates since no gassiness results from the production of CO2 as when the latter reacts with acid in the stomach Magnesium Sulphate very soluble, Epsom salts – a mild laxative Calcium sulphate Plaster of Paris – poorly soluble Calcium hydroxide (slaked lime) sparingly soluble, used in lime water Ca(OH)2(aq + CO2(g) CaCO3(s) + H2O(l) note that the cloudy precipitate re-dissolves in xs CO2 CaCO3(s) + CO2(g) + H2O(l) Ca(HCO3)2(aq) this is in effect the same equation as that associated with the formation of hard water by the action of rain water on limestone the reverse process takes place when the product is evaporated, which is the cause of lime scale deposits, stalagmites and stalactites Ca(HCO3)2(aq) CaCO3(s) + CO2(g) + H2O(l) Barium sulphate barium meal used in medicine to outline the gut in X-rays fairly safe - relatively low solubility – particularly with addition of sodium sulphate (see ‘solubility product’ to know more) which ensures that toxic barium ions do not dissolve into the body Acidified Barium chloride = test for sulphate ions SO42- - white ppt (acidified with HCl(aq) (or HNO3(aq) since all metal nitrates are soluble) since it’s not going to introduce any additional anions whilst ensuring no carbonate ions are present since barium carbonate will also give a white precipitate) Summary Questions Exam Style Questions Page 178 Page 179 Page 233 AS Chemistry (Nelson Thornes) AQA Chemguide 1-6 1-8 4 176 - 178 Group 2 LUND Apr-17 31 AS Chemistry Unit 2 METAL EXTRACTION Some environmental considerations There will probably be a lot of ‘How science works’ type questions on this topic. Try to be specific in your answer e.g. avoid statements such as ‘it causes pollution’ try to clearly say what form of pollution and why e.g. ‘smelting the ore causes the release of carbon dioxide which is a greenhouse gas and contributes to global warming. Mining, transporting and extracting the ore loss of landscape due to mining air and noise pollution due to dust from mining air pollution from transporting the ore – CO2 emissions – greenhouse effect disposal of slag, some of which is sometimes just dumped solid waste is called gangue, typically silica rocks and clays for which other uses would be found to reduce ££££ air pollution from the extraction process, in particular carbon dioxide (greenhouse gas) and sulphur dioxide (acid rain) in the roasting process but note the latter can be used in the manufacture of sulphuric acid Recycling saving of raw materials and energy by not having to first extract the ore avoiding the pollution problems in the extraction of the metal from its ore (see above) not having to find space to dump the unwanted metal if it wasn't recycled (offsetting these to a minor extent - energy and pollution ££££ in collecting and transporting the recycled materials) Rocks, Ores and Minerals an ore is a rock containing an economically viable quantity of the metal to be extracted a mineral is the particular compound in an ore that contains the metal these minerals are typically metal oxides or metal sulphides the latter are converted to an oxide by heating them in air prior to extraction – this is called roasting this produces an acidic gas SO2 which can cause acid rain and so it is used to make sulphuric acid thus minimising waste and reducing pollution → SO2(g) + H2O(l) + ½ O2(l) LUND Apr-17 32 H2SO4(aq) AS Chemistry Unit 2 Extraction Method metals are removed from their ore by reduction (either chemical or electrolytic depending upon the relative position of the metal in the reactivity series) Continuous saves energy in high temperature processes (heating and cooling) raw materials are constantly added and the products produced are constantly removed which is a very efficient and ££££ saving way to produce materials in large quantities Batch useful when small amounts are required better when purity is important involves extra ££££ due to equipment cleaning and non use between batches Method used depends on: purity required energy requirements position of metal in reactivity series ££££ of reducing agent Iron metal oxide with carbon Titanium Metal chloride with a more reactive metal egg sodium or magnesium Aluminium electrolysis of metal oxide Tungsten metal oxide with hydrogen Carbon coke (mainly carbon) is a low ££££ choice it may require excessively high (££££) temperatures with some ores the purity of the metal obtained must be considered as metal carbides can be formed that weaken the metal structurally – it is thus unsuitable for extracting titanium , tungsten and aluminium. it is used for the extraction of iron, copper and manganese in preference to Pepsi waste, particularly polluting gases such as CO2 when using coke, is an issue Hydrogen made from methane and water – see the Haber process water is the co-product so pollution is relatively low it has potential dangers (explosions) and ££££ more than coke Electrolysis used to extract metals high in the reactivity series lots of energy is required to melt/dissolve the ore and also for the electrolysis process itself plants are often associated with hydroelectric sites to minimise energy ££££ LUND Apr-17 33 AS Chemistry Unit 2 Continuous process - cheap the most commonly used iron ores are haematite, Fe2O3, and magnetite, Fe3O4. these can be reduced to iron by heating them with carbon in the form of coke which is cheap and produced by heating coal in the absence of air (effectively distilling off the organics) coke provides both the reducing agent for the reaction and also the heat source - as you will see below iron ore is added to the blast furnace with coke and limestone coke is a solid fuel composed largely of carbon (formed after the distillation of coal, which removes volatile elements) limestone is a sedimentary rock consisting largely of calcium carbonate (CaCO3). the mixture is heated by blasts of air, preheated to about 750°C, blown in at the bottom of the furnace (note this is partially heated by hot waste gases from the top to reduce energy ££££) the coke is oxidised under the strong heat, the carbon reacts with oxygen to produce carbon dioxide and this process is highly exothermic, raising the temperature to about 2000°C C(s) + O2(g) CO2(g) the carbon dioxide formed then reacts (endothermically) with the coke to form carbon monoxide CO2(g) + C(s) 2CO(g) the carbon monoxide produced then reacts with the iron ore acting as a reducing agent CO is better for reduction than C as it is a gas hence more collisions hence faster rate Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g) the dense molten iron sinks to the bottom of the furnace and is tapped off regularly as cast iron cast iron is very runny when it is molten and doesn't shrink much when it solidifies so is ideal for making castings - hence its name. cast iron is very impure, containing about 4% of carbon which makes it very hard, but also very brittle (if you hit it hard, it tends to shatter rather than bend or dent). cast iron is used for things like manhole covers, guttering and drainpipes, cylinder blocks in car engines, Aga-type cookers, and very expensive and very heavy cookware. silica (silicon(IV) oxide SiO2) impurities that exist in the iron ore react with the limestone added to form slag. LUND Apr-17 34 AS Chemistry Unit 2 calcium carbonate undergoes thermal decomposition to form calcium oxide and carbon dioxide. CaCO3(s) CaO(s) + CO2(g) the calcium oxide (a basic oxide) then reacts with the silica (an acidic oxide) to form calcium silicate (this is slag and is removed from the surface – relatively low density) CaO(s) + SiO2(s) CaSiO3(s) CaO(s) + Al2O3(s) CaAl2O4(s) slag is used in road making and as "slag cement" - a finely ground slag which can be used in cement, often mixed with Portland cement. Batch process resists corrosion hence useful for - cutlery, cooking utensils, kitchen sinks, industrial equipment for food and drink processing made from purified pig iron and scrap iron chromium and nickel also added to make an alloy (this makes it more expensive than mild steel) as well as small amounts of carbon scrap iron used in the process because: less mining – so less eyesore, traffic, noise, dust less energy required than extracting the iron from its ore recycles scrap (obviously) hence less landfill requirements less pollutant gases (e.g. CO2, SO2) economically sound (££££ of extraction and transportation of the ore as well as energy saved) scrap iron is easy to recycle as it can be extracted with a magnet scrap iron can also be used to extract copper from its solution since it is higher in the reactivity series, much cheaper than copper in the past copper was extracted from malachite which contains copper(II) carbonate the carbonate was thermally decomposed to the oxide then the copper extracted with carbon, both processes produced CO2 and require a lot of energy nowadays it is possible to extract copper from low grade ores and mining waste by firstly extracting Cu2+(aq) using dilute acid and bacteria then reducing it using the scrap iron How Science Works Summary Questions Exam Style Questions Page 182 Page 182 Page 185 AS Chemistry (Nelson Thornes) AQA Chemguide 1-5 4 180 - 182 Extraction iron LUND Apr-17 35 AS Chemistry Unit 2 Continuous process the main ore is bauxite that contains economically viable quantities of the mineral alumina (Al2O3) it cannot be extracted using carbon due to the relatively high position of aluminium in the reactivity series. alumina (an amphoteric oxide – reacts with acid and alkali) is extracted from its ore using NaOH (details not required) alumina has a high melting point >2000oC (strong electrostatic forces of attraction between 3+ cations and 2- anions) hence the direct electrolysis of molten alumina is not economically viable. alumina is dissolved in molten cryolite at 1000oC (compare this with the notion of melting salt or dissolving it in water at room temperature to separate the ions) electrolysis requires energy locating the smelting plant near a hydroelectric dam is advantageous given the relatively low ££££ energy recycling aluminium also saves energy Al3+ + 3e- → Al Al is formed at the cathode (pussytive cations → -ve electrode i.e. the cathode) O2 is formed at the anode and reacts with the graphite electrodes at high temperatures producing CO2 hence they must be replaced periodically - ££££ you MUST be able to write the associated redox equations 2O2- → O2 + 4e- note that 3 moles of electrons are required per mole of aluminium produced – energy ££££ Recycling: melting of the metal is used requires a fraction of the energy to extract the same quantity of aluminium from its ore – huge ££££ saving significantly reduces CO2 emissions there are ££££ in sorting and transportation although specialised recycling points (aluminium can banks) help to reduce this How science works Summary Questions Exam Style Questions Page 184 Page 184 Page 185 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2 5 182 - 184 Extraction LUND Apr-17 36 AS Chemistry Unit 2 Batch process expensive carbon could be used to extract Ti at very high temperatures (££££) but is NOT used because titanium carbide is produced making the metal brittle. titanium is abundant in the crust so this is NOT the reason for its ££££. titanium ore (Rutile) contains an economically viable quantity of titanium(IV) oxide (TiO2) this is converted to the chloride: TiO2 + 2Cl2 + 2C - → TiCl4 + 2CO (or TiO2 + 2Cl2 + C → TiCl4 + CO2) Titanium chloride is covalent so can be distilled off (strong bonds within TiCl4 but weak IMF in between molecules) but it’s covalent nature rules out electrolysis to extract the metal. titanium is extracted using a more reactive metal e.g. sodium at high temperature in an argon atmosphere (to prevent oxidation of the hot metal) once the reaction is complete, and everything has cooled (several days in total - an obvious inefficiency of the batch process), the mixture is crushed and washed with dilute hydrochloric acid to remove the sodium chloride. TiCl4 + 4Na → 4NaCl + Ti Titanium is expensive because of: raw materials – chlorine, argon, and in particular sodium used in extraction since it’s a batch process rather than continuous (like iron) Batch process expensive a relatively rare metal – ££££ implications used in incandescent light bulb filaments due to its high melting point although this will be less common as energy saving light bulbs are becoming more widespread. carbon could be used to extract W at very high temperatures (cost) but is NOT used because tungsten carbide is produced making the metal brittle. it is extracted from its oxide WO3 (oxidation state of tungsten) by reduction with hydrogen (the relatively high ££££ of tungsten makes this method viable). WO3 + 3H2 - → W + 3H2O this must be carefully controlled because of the dangerous nature of hydrogen at high temperatures. Summary Questions Exam Style Questions Page 184 Page 185 Page 233 AS Chemistry (Nelson Thornes) AQA Chemguide 3, 4 1, 2, 3, 6, 7 5 183 - 184 Extraction LUND Apr-17 37 AS Chemistry Unit 2 Haloalkanes haloalkanes are rarely found naturally but are important in many synthetic products they are used extensively as solvents 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) is a modern anaesthetic chloroethene and tetrafluoroethene are used to make PVC and PTFE Nomenclature general formula CnH2n+1X prefixes chloro, bromo and iodo used e.g. 1-bromobutane structure of primary, secondary and tertiary haloalkanes as with alkyl branches substituents are named in alphabetical order NOT numerical order (ignore di, tri etc. for order determination) numbers are used to individually locate each and every substituent e.g. 2-bromo-1,1-dichloroethane Physical Properties boiling points generally determined by VdW intermolecular forces and largely depend on Ar of the halogen rather than dipole 1-iodobutane > 1-bromobutane > 1-chlorobutane >> butane (in the latter case there is the double advantage of higher VdW and the presence of a dipole) immiscible with water as they cannot form hydrogen bonds with it (unlike don’t mix) hence a universal solvent such as ethanol is used when carrying out practical’s they do mix with hydrocarbons (grease and oil) so can be used as dry cleaning fluids (being fairly volatile also assists drying) Summary Questions Exam Style Questions Page 187 AS Chemistry (Nelson Thornes) AQA Chemguide 1 186 - 187 Haloalkanes LUND Apr-17 38 AS Chemistry Unit 2 Hydrolysis of Bromoethane hydrolysis is the breaking of bonds in a substance by reaction with water or its ions (e.g. OH-). bromoethane + OH- ethanol + Br- NS dissolve in ethanol (universal solvent) then add dilute aqueous sodium hydroxide solution (alkaline hydrolysis) a nucleophile is an electron pair donor e.g. :OH-, :CN-, H2O:, :NH3 mechanism of nucleophilic substitution and the use of curly arrows the halide ion is called the leaving group haloalkanes are also hydrolysed by water, but more slowly since water is a weaker nucleophile than hydroxide ions Factors Affecting the Rate of Hydrolysis relative polarity of C-X bond suggest reactivity of chloro > bromo > iodo haloalkanes as greater polarity should more strongly attract the nucleophile BUT actual relative rate is iodo > bromo > chloro relative strengths of the C-Hal bond (idea of ‘good leaving group’) determine the rate practically demonstrated by carrying out hydrolysis in the presence of aqueous silver nitrate solution (and ethanol as a universal solvent) rate of formation of silver halide precipitate is proportional to rate of displacement of the halide ion which is proportional to the rate of substitution. colour of silver halide precipitate can also be used to determine the halogen present in organic analysis chloroalkanes bromoalkanes white ppt of AgCl pale yellow ppt of AgBr iodoalkanes yellow ppt of AgI LUND Apr-17 39 soluble in dilute ammonia soluble in conc. (or xs) ammonia insoluble in ammonia AS Chemistry Unit 2 Nucleophilic Substitution with CN- bromoethane + CN- propanenitrile boil under reflux with NaCN or KCN (not HCN) + Br- NS dissolved in ethanol the product is called a nitrile AN IMPORTANT REACTION SINCE IT INCREASES THE CARBON CHAIN LENGTH BY ONE CARBON ATOM the nitrile group is converted into a carboxylic acid by acid hydrolysis propanenitrile + 2H2O + H+ propanoic acid + NH4+ boil under reflux with dilute sulphuric acid Nucleophilic Substitution with NH3 bromoethane + 2NH3 ethylamine + NH4Br NS heat with xs ethanolic ammonia under pressure NOTE: the HBr produced will then form the salt NH4Br in the presence of excess NH3 a primary amine is formed xs ammonia ensures the primary amine is the main product the ethylamine produced is also a nucleophile (stronger than ammonia due to the +ve inductive effect of the alkyl group) so if excess bromoethane is used instead of xs ammonia further substitution is possible to give diethylamine, triethylamine and finally tetraethylammonium bromide (a quaternary ammonium salt) Summary Questions Exam Style Questions Page 190 AS Chemistry (Nelson Thornes) AQA Chemguide 1, 2 187 - 190 Nucleophilic, hydrolysis, silver nitrate LUND Apr-17 40 AS Chemistry Unit 2 Elimination Reactions bromoethane + OH- ethene + H2 O + Br- E heat with a solution of sodium hydroxide dissolved in ethanol NOT water notice that the conditions are different even though the same reagent that was used for nucleophilic substitution is used in this case the hydroxide ion is behaving as a base and abstracting a proton in effect HBr is eliminated resulting in the formation of a double bond (evidence for this is the fact that the gaseous product decolourises bromine water) in reality elimination and substitution reactions compete with the major product determined by the conditions dissolving the hydroxide ions in water and at room temperature favours substitution dissolving the hydroxide ions in hot ethanol favours elimination the outcome is affected by the type of haloalkane favours elimination 1o 2o 3o favours substitution 2-bromobutane yields two products – but-2-ene (which can be present as two isomers – see Alkenes) and but-1-ene Summary Questions Exam Style Questions Page 193 Page 196 AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 2, 4 191 - 193 Elimination LUND Apr-17 41 AS Chemistry Unit 2 The formation of Haloalkanes from Alkanes alkanes are non-polar molecules with strong bonds between C-H and C-C hence are relatively unreactive (so far we have only seen combustion) they do however react with halogens in UV (or strong sunlight) (remember that ALKENES react with bromine water even in the dark) to form a haloalkane by free radical substitution Cl2 2Cl∙ INITIATION (UV breaks C-Cl bonds homolytically) homolytic fission (rather than heterolytic fission) takes place as the UV light provides adequate energy (which visible light would not) for the dissociation of chlorine molecules the halogen bonds are broken first as they are the weakest Cl∙ + CH4 ∙CH3 + Cl2 ∙CH3 + HCl CH3Cl + Cl∙ PROPAGATION Radical is regenerated hence a chain reaction takes place ∙CH3 ∙CH3 + ∙CH3 CH3CH3 + Cl∙ CH3Cl overall equation – determined by the propagation cycle rather than termination since this happens on numerous occasions compared to the termination Cl2 + CH4 TERMINATION HCl + CH3Cl OVERALL further substitutions are possible so these reactions typically produce a mixture of haloalkanes (which will have yield and cost implications) other halogenation reactions work the same way e.g. ethane + bromine, bromoethane + bromine etc Summary Questions Exam Style Questions Page 195 Page 196 AS Chemistry (Nelson Thornes) AQA Chemguide 1 3, 5, 6 194 - 195 Free radical Look at s-cool also LUND Apr-17 42 AS Chemistry Unit 2 How Science Works: B, L Haloalkanes – Environmental Issues after the 30’s CFC’s replaced toxic and smelly ammonia in fridges CFCs are unreactive so were also used extensively for – aerosol propellants this relative lack of reactivity initially seemed like a good thing however, their lack of reactivity meant that they did not decompose over a short timescale hence were able to make their way into the upper atmosphere concern was raise about CFC when it became apparent that they were acting as catalysts in the destruction of the ozone layer CFC’s interact with UV solar radiation in the upper atmosphere to release separate chlorine atoms (chlorine free radicals) CFC’s Cl∙ INITIATION (UV breaks C-Cl bonds homolytically) Cl∙ + O3 ClO∙ + O2 PROPAGATION ClO∙ + O3 2O2 + Cl∙ Radical is regenerated hence a chain reaction takes place 2O3 3O2 OVERALL this means that an awful lot of ozone can be destroyed by the presence of a relatively small proportion of CFC’s – an ‘ozone hole’ came to be the ozone layer is an important shield in our planets defence against harmful levels of UV solar radiation overexposure to which can cause cancers and more worryingly a reduction in the amount of Plankton in the oceans – removing a major component of the food chain following the Montreal Protocol of 1987 it was agreed that they would be phased out. Ozone does form naturally in the upper atmosphere, however, it will take time for the ozone hole to be ‘refilled’ as there was a reservoir of CFC’s still present after 1987 Chemists have since developed safer alternatives to replace CFC’s (e.g. hydrochlorofluorocarbons, HCFC’s and hydrofluorocarbons, HFC’s) http://www.atm.ch.cam.ac.uk/tour/ http://www.theozonehole.com/ Summary Questions Exam Style Questions Page 195 Page 196 AS Chemistry (Nelson Thornes) AQA Chemguide 2 1 143, 193, 195 Ozone hole LUND Apr-17 43 AS Chemistry Unit 2 Alkenes nomenclature of alkenes (unsaturated hydrocarbons), general formula CnH2n locant for the double bond formula confusion with cycloalkanes homologous series similar to alkanes Isomerism in Alkenes positional isomerism (a form of structural isomerism) also occurs in higher alkenes where the name will be identical apart from different numbers being used e.g. but-1-ene, but-2-ene alkenes can also exhibit a type of stereoisomerism called geometrical isomerism this is consequential of the non-rotation of a double bond (unlike in alkanes) – which is what you will state as the fundamental requirement in the exam How Science this lack of free rotation is consequential of the π bond present in the alkene Works: A additionally for geometrical isomerism to be possible both carbon atoms on the double bond must have different atoms/groups attached to themselves, however, the carbon atoms can still both be identical in that respect Geometrical isomers have the same molecular formula, same structural formula but a different spatial arrangement of the atoms due to the non rotation of the carbon-carbon double bond E and Z are used to distinguish between the two isomers (its quite EZy to do will a little practice) E isomers have the main grouping diagonally across the double bond Z isomers have the main grouping on the same side of the double bond cis and trans were previously used and in most cases E corresponds to trans and Z to cis BUT NOT ALWAYS How Science Works: H the trick is to identify what the main groupings are: where the two atoms directly bonded to the carbons of the double bond with the largest atomic numbers (highest priority) are diagonally opposite then it is deemed an E isomer where the two atoms directly bonded to the carbons of the double bond with the largest atomic numbers (highest priority) on the same side then it is deemed an Z isomer if on one of the carbons the atoms directly bonded are identical then to establish the highest priority grouping a tie break situation arises in which you look at the next highest priority atom attached to each of them e.g -CH2Br beats CH2Cl and so on LUND Apr-17 44 AS Chemistry Unit 2 Example: but-2-ene Step 1: split the alkene Step 2: assign the relative priorities. The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Step 3: look at the relative positions of the higher priority groups : same side = Z, hence (Z)-but-2-ene. The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Therefore the two high priority groups are on the opposite side, then this is (E)-but-2-ene. Combustion of Alkenes combustion reactions – same general idea as alkanes – practice a few just in case since alkenes are an important chemical feedstock (used to make other stuff) they are not economically used as fuels Summary Questions Page 201 AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 198 - 201 Alkenes, geometrical isomerism Electrophilic Addition alkenes are more reactive than alkanes, despite the fact that a double bond is stronger overall, due to relatively weak bond that constitutes the second bond being weaker than a single bond it requires less energy to break double bonds present a high electron density so are attractive to electron deficient species called electrophiles such species can add onto unsaturated compounds such a alkenes in an electrophilic addition electrophiles are electron pair acceptors reaction with HBr and Br2 (in the dark – unlike alkanes) ethene + HBr bromoethane EA ethene + Br2 1,2-dibromoethane EA LUND Apr-17 45 AS Chemistry Unit 2 bromine water is the standard test for alkenes (carbon-carbon double bond) heterolytic fission takes place in these reactions again, when outlining the mechanism of electrophilic addition take the trouble to learn the correct use curly arrows in these reactions a positive carbocation intermediate is generated evidence for the intermediate carbocation can be obtained by carrying out the addition of Br2 in the presence of chloride ions which then also produces 1-bromo-2-chloroethane Other Addition Reactions ethene + H2 O ethanol EA 1 2 cold cH2SO4 to produce ethyl hydrogensulphate THEN warm with water to produce ethanol (overall sulphuric acid is a catalyst) industrially alcohols are produced by reacting alkenes with steam and phosphoric acid – see ALCOHOLS Electrophilic Addition to Unsymmetrical Alkenes reactions of HBr and H2SO4 with unsymmetrical alkenes (i.e. where the double bond is not in the middle) e.g. propene yield two isomeric major and minor products Markovnikov’s rule determines the outcome relative stability 3o>2o>1o carbocation intermediate due to positive inductive effect (+I) of the alkyl group this arises because the alkyl group is a relatively better e- source than H which thus stabilises the positive charge this in turns means EA is relatively lower thus favouring that reaction path Summary Questions Exam Style Questions Page 205 Page 210 AS Chemistry (Nelson Thornes) AQA Chemguide 1-5 1, 2, 3, 5, 7 202 - 205 Electrophilic addition, Markovnikov LUND Apr-17 46 AS Chemistry Unit 2 Addition Polymerisation monomers can join together to form polymers with a large Mr the conditions are typically heat, pressure, catalyst variations of the reaction conditions can yield different types of polymer – with variations in length and the degree of branching low density poly(ethene) utilises high temperature and pressure via a free radical mechanism which increases the extent to which branching occurs (due to the random nature of the radical mechanism) and reduces density high density poly(ethene) uses lower temperature and pressure and a Ziegler catalyst resulting in significantly reduced branching hence closer packing you should be able to show (or identify) the repeating monomer units in a polymer poly(ethene) low density high density plastic bags, plastic bottles stronger – milk crates poly(phenylethene) normal pipes, chairs (Polystyrene) poly(propene) toys, mobile phones poly(chloroethene) ‘imitation leather’, drain pipes poly(methyl 2-methylpropenoate) ‘imitation glass’, lenses (cheap) poly(propenenitrile) acrylic fibre - clothing poly(tetrafluoroethene) lubrication, non-stick pans (PVC) (Perspex) (PTFE aka Teflon) problems non biodegradability (since most these polymers are typically an alkane hence unreactive) so take centuries to decompose in landfill combustion yields toxic gases solutions mechanical recycling – after sorting out the different types of plastic the thermosoftening types can be remoulded (but only a few times due to degradation of the plastic as chains break down ) feedstock recycling – cracking is used to break down the polymers prior to making new plastics replace poly(alkenes) with biodegradable and photodegradable polymers Summary Questions Exam Style Questions Page 209 Page 210 Page 234 AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 4, 6 7 206 - 209 Polymerisation, Ziegler LUND Apr-17 47 AS Chemistry Unit 2 Alcohols Nomenclature -OH hydroxyl group present with general formula CnH2n+1OH (or ROH is its short form) form a homologous series of chemicals with similar properties names end in ‘ol’ but will be used as a ‘hydroxy’ prefix if a higher rated group e.g. one with a C=O is involved e.g. 2-hydroxypropanoic acid CH3CH2(OH)COOH primary, secondary and tertiary alcohols – same idea as haloalkanes di-ols e.g. ethane-1,2-diol (used in antifreeze) (the ‘e’ is kept as the next letter is a consonant) Physical Properties alcohols are structurally similar to water i.e. R-O-H 105o bond angles exist on the oxygen as was the case for water capable of hydrogen bonding in addition to VdW – strong intermolecular force hence boiling points higher than alkanes with comparable Mr tri-ols e.g. propane-1,2,3-triol (glycerol) have a particularly high viscosity due to extensive hydrogen bonding can form hydrogen bonds with water, hence lower members miscible with it miscibility decreases with increasing chain length as contribution of –OH becomes progressively less significant and stronger VdW between larger alcohol molecules make them harder to separate Summary Questions Exam Style Questions Page 213 Page AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 212 - 213 alcohol Uses of Ethanol ethanol burns with light blue flame with little smoke (this reflects the relatively low percentage by mass of carbon) and is a constituent (along with methanol) of methylated spirits (picnic stoves) – to which an unpalatable purple dye is added thus avoiding taxation useful as a petrol substitute in countries like Brazil (Gasohol) with limited oil reserves where up to 20% can be added to petrol without engine modification DON’T FORGET THAT IT ALREADY HAS AN OXYGEN ATOM WHEN WRITING A COMBUSTION EQUATION presence of -OH group and hydrocarbon part C2H5 makes ethanol a useful universal solvent – where ease of evaporation is also a benefit e.g. in perfumes it is also used as a feedstock to manufacture other chemicals LUND Apr-17 48 AS Chemistry Unit 2 Production of Ethanol Fermentation carbohydrates (from sugar can or sugar beet – a renewable resource as they can be grown annually) are broken down into simple sugars and then to alcohols by enzymes (present in yeast) the reaction is exothermic which provides energy for yeast metabolism and these warm conditions provide a reasonable rate of reaction (NOTE: higher temperatures will not give a higher rate as the enzymes become denatured) an anaerobic respiration process takes place (air excluded to prevent oxidation of ethanol to ethanoic acid) the yield of ethanol is poor as the increase in alcohol inhibits the fermentation process Ethanol produced this way can be used as a BIOFUEL nominally carbon input (photosynthesis) = carbon output (fermentation + combustion) but other factors such as distribution mean that it is not 100% CARBON NEUTRAL C6H12O6 2CH3CH2OH catalysed by yeast, warm conditions BAD BAD GOOD GOOD + 2CO2 fermentation is an example of a batch process – high production costs slow reaction, yielding impure product (distillation of the aqueous solution necessary) uses renewable resources - sugar cane mostly requires little energy Industrial production ethene + H2 O o H3PO4 catalyst, ~300 C, 70atm ethanol covered in detail in the section on equilibria GOOD GOOD BAD BAD continuous process – lower production costs fast reaction, good yield of relatively pure product uses non-renewable resources (oil) energy required (high temperature and pressure) Summary Questions Exam Style Questions 1–3 2 (if not already done) 1, 11, 12 Page 215 Page 153 Page 220 -3 AS Chemistry (Nelson Thornes) AQA Chemguide 152 – 153, 214 - 215 Ethanol, fermentation LUND Apr-17 49 EA AS Chemistry Unit 2 Oxidation of Alcohols primary, secondary can be easily oxidised using the reagents K2Cr2O7(aq)/dilute H2SO4(aq) remember ‘H+(aq)’ is REACTANT, sulphuric acid is the REAGENT that provides it tertiary alcohols cannot be easily oxidised (no C-H bond to break) primary alcohol + [O] aldehyde + H2 O mild i.e. NO REFLUX conditions - K2Cr2O7(aq)/H2SO4(aq) distil off aldehyde as formed to prevent further oxidation to a carboxylic acid the aldehyde initially produced is relatively less volatile than the alcohol due to reduced opportunity for hydrogen bonding hence the distillate will be rich in the aldehyde with most of the alcohol remaining in the reaction vessel primary alcohol + 2[O[ REFLUX with K2Cr2O7(aq)/H2SO4(aq) secondary alcohol + [O] reflux with K2Cr2O7(aq)/H2SO4(aq) carboxylic acid + ketone H2 O + H2 O orange dichromate(VI) ions (Cr2O72-(aq)) are reduced to green chromium(III) ions (Cr3+(aq)) – PLEASE avoid putting statements such as ‘it changes from orange to green’ important test to differentiate between primary/secondary and tertiary alcohols cannot differentiate between primary and secondary as both give a positive result however the products can be isolated and then tested with Tollens or Fehlings (see Module 4) carbonyl compounds such as aldehydes and ketones have CnH2nO as their common general formula (unsaturated due to carbon-oxygen double bond) aldehydes and ketones can be functional group isomers (a form of structural isomerism) aldehydes are RCHO ketones are RCOR’ higher Ketones can also exhibit positional isomerism (another form of structural isomerism) LUND Apr-17 50 AS Chemistry Unit 2 Distinguishing Between Aldehydes and Ketones tests are based on the fact that aldehydes can be easily oxidised to a carboxylic acid while ketones cannot be – i.e. a redox reaction can occur Aldehyde (i) + [O] Carboxylic Acid orange dichromate(VI) ions (Cr2O72-(aq)) are reduced to green chromium(III) ions (Cr3+(aq)) reflux with K2Cr2O7(aq)/H2SO4(aq) NOTE THAT THE ABOVE REAGENT WILL OXIDISE PRIMARY AND SECONDARY ALCOHOLS. HOWEVER, THE TWO REAGENTS BELOW ONLY OXIDISE ALDEHYDES (ii) warm with Fehling’s solution (an alkaline solution containing complexed copper(II) ions) blue Cu2+(aq) is reduced to brick red Cu2O(s) (this is the basis of the test for reducing sugars) (iii) warm with Tollen’s reagent (aqueous silver nitrate in xs ammonia) Ag+(aq) is reduced to Ag(s), hence the silver mirror effect Dehydration of Alcohols alcohol alkene heat with excess cH2SO4 at 170oC + H2 O OR passing the vapour over HOT Al2O3 elimination can yield more than one product positional isomers and/or geometrical (E-Z) isomers can be produced alcohols produced from renewable sources can be used to produce alkenes which can then be used to produce polymers Summary Questions Exam Style Questions 1–6 2 – 11 9, 10 Page 219 Page 220 - 3 Page 235 AS Chemistry (Nelson Thornes) AQA Chemguide 216 - 219 Oxidation alcohol, Fehling, dehydration LUND Apr-17 51 AS Chemistry Unit 2 Mass Spectrometry this provides us with: Mr, Relative abundance and (fragmentation Patterns – see A2 Chemistry) do you know how a mass spectrometer works: vaporisation ionisation acceleration deflection detection you must be able to interpret simple mass spectra to determine the elements present from their relative isotopic mass and also their relative abundance the value of the relative isotopic mass can be read directly from the m/z value of a given peak on the spectra from a mass spectrometer relative abundance is reflected in the heights of the peaks when a molecule is subject to ionisation a molecular ion is formed M(g) M(g)+ + e- This molecular ion can also undergo fragmentation which results in lots of peaks corresponding to many smaller positive ions which will help to identify the molecule chloro-alkanes will produce two molecular ion peaks when mono substituted in a ratio commensurate with halogen abundance. e.g. chloromethane gives M(g)+ peaks at 50 and 52 in a 3:1 ratio Di-substituted chloro-alkanes will yield three molecular ion peaks of relative intensity for dichloro 9:6:1 – those who do Maths Statistics will be able to explain that: e.g. dichloromethane gives M(g)+ peaks at 84, 86 and 88 in a 9:6:1 ratio high resolution mass spectra can distinguish between compounds with similar Mr e.g. ~123 for C6H5NO2 and C7H7O2 however, they will not be 123.000 exactly and will differ from one another as a consequence allowing for individual identification Mr data is particularly useful with IR data to establish the maximum carbons in a molecule known to contain one or two oxygen atoms Summary Questions Exam Style Questions Page 225 Page 230 AS Chemistry (Nelson Thornes) AQA Chemguide 1-3 1, 3, 4 224 - 225 Mass spectrometry LUND Apr-17 52 AS Chemistry Unit 2 IR Spectroscopy molecules absorb IR energy at values corresponding to a natural vibration frequency associated with asymmetric stretching and bending of the covalent bonds present which is dependent on the bond energy and mass involved (the ability of CO2 to do this is why there is a relationship between atmospheric CO2 and global warming) a beam of variable IR frequencies is shone through a sample (after calibration) and the transmittance is plotted against frequency (wavenumber is used - more user friendly) above 1500 cm-1 two important peak values are: 1620 – 1680 cm-1 C=C can distinguish between alkenes and isomeric cycloalkanes 1680 – 1750 cm-1 C=O strong and sharp with slight variations in position depending on the type of compound (found in acids, aldehydes and ketones) 3230 – 3550 cm-1 O-H this is broad (due to hydrogen bonding) cf the narrow C– H of 2850 – 3300 cm-1 note that the O-H for alcohols lies further to the left than for acids revealing the narrower C-H absorption which might be partially or totally obscured with an acid A data sheet will be provided in the exam – similar to table 1 on page 227 used with mass spec they will help establish a maximum number of carbon atoms in the compound by indicating the presence of at least one or two oxygen atoms. the fingerprint region 400 – 1500 cm-1 is unique to each organic molecule and allows identification by matching against a computer database the C-O peak lies in this region so may be hard to see clearly impurities will yield additional peaks on the IR spectra – in particular water in a wet sample Summary Questions Exam Style Questions Page 229 Page 230 - 1 Page 233 AS Chemistry (Nelson Thornes) AQA Chemguide 1-6 2, 5 – 8 6 226 - 229 Infra red spectroscopy LUND Apr-17 53 AS Chemistry Unit 2 Amine AS ORGANIC PATHWAYS Haloalkane Alkane Alkene Nitrile Carbonyl Carboxylic Acid Alcohol Reagents/conditions Mechanism required LUND Apr-17 54 AS Unit 2 Chemistry in Action AS ORGANIC CONVERSIONS You should be able to write full chemical equations, and identify the type for all the reactions listed: Oxidation, Reduction, Addition, Elimination, Hydrogenation, Dehydrogenation, Hydrolysis Substitution, Hydration, Dehydration, Mechanisms should be known for all those shaded: Nucleophilic Substitution NS, Electrophilic Addition EA or Elimination E. Alkenes to: haloalkane, alkane, alcohol, alkoxyalkane Alkenes are obtained from alkanes via cracking ethene + HBr bromoethane EA ethene + Br2 1,2-dibromoethane EA ethene + H2 ethane Ni catalyst, ~200oC (catalytic hydrogenation) + Method 1 Method 2 cH3PO4 on a silica support with high temperature and pressure, first react with cold cH2SO4, then warm with water Alkyl Halides H2 O ethene to: ethanol EA amine, alcohol, nitrile, alkene Alkyl halides can be made from alkanes by free radical substitution bromoethane + 2NH3 ethylamine + NH4Br heat with xs ammonia under pressure to minimise further substitution NS bromoethane + OH-(aq) ethanol + BrNS dissolve in ethanol (universal solvent) then boil under reflux with dilute aqueous NaOH bromoethane + CN- propanenitrile boil under reflux with alcoholic NaCN or KCN (NOT HCN) + Br- bromoethane + OH- ethene reflux with an alcoholic solution of NaOH H2 O + Mr Lund January Apr-17 55 + NS Br- E AS Unit 2 Chemistry in Action Alcohols to: carbonyl (aldehyde and ketone), carboxylic acid, alkene ethanol + [O] ethanal + H2 O mild conditions - K2Cr2O7(aq)/H2SO4(aq) distil off aldehyde as formed to prevent further oxidation to a carboxylic acid ethanol + 2[O] reflux with K2Cr2O7(aq)/H2SO4(aq) ethanoic acid + H2 O propan-2-ol + [O] reflux with K2Cr2O7(aq)/H2SO4(aq) propanone + H2 O ethanol ethene heat with excess cH2SO4 at 170oC + H2 O Mr Lund January Apr-17 56 AS Unit 2 Chemistry in Action Testing for functional groups Bromine Water (Br2(aq)) Test for alkenes (unsaturated hydrocarbons) Add bromine water dropwise to ~1 cm3 of the unknown substance. OBSERVATION INFERENCE EXPLANATION orange colourless alkene electrophilic addition. product is colourless Acidified Potassium Dichromate Solution Test for 10 and 20 alcohols (CARE it is also positive with an aliphatic aldehyde) To ~ 1 cm3 of the substance under test add ~ 1 cm3 of acidified (1 mol dm-3 sulphuric acid) potassium dichromate solution then heat gently in a water bath. Acidified purple potassium 23+ manganate(VII) will be Redox reaction in which orange Cr2O7 is reduced to green Cr decolourised in a similar test 2+ 3+ Cr2O7 (aq) + 14H (aq) + 6e 2Cr (aq) + 7H2O(l) OBSERVATION INFERENCE orange green 1o or 2o alcohol EXPLANATION 1o alcohol aldehyde acid 2o alcohol ketone aldehyde aldehyde carboxylic acid Methanoic acid can also be oxidized to carbon dioxide NOT 3o alcohol or ketone Tollens’ Reagent (Silver Mirror Test) Test for aldehydes NOT ketone Don’t confuse with the test for alkyl halides To ~1 cm3 of your sample add ~ 1 cm3 of Tollen’s reagent* and warm in a hot water bath. (*to ~2 cm3 of ~ 0.1 mol dm-3 silver nitrate solution add ~2.0 mol dm-3 sodium hydroxide solution 1 drop at a time until a brown precipitate just forms. Add ~2.0 mol dm-3 ammonia solution to it dropwise until the precipitate just dissolves - (ammoniacal silver nitrate)). OBSERVATION INFERENCE silver mirror formed on test tube aldehyde EXPLANATION + Ag(NH3)2 (aq) + e- Ag(s) + 2NH3(aq) aldehyde acid NOTE THAT ALCOHOLS DO NOT GIVE A POSITIVE RESULT WITH THIS TEST Mr Lund January Apr-17 57 AS Unit 2 Chemistry in Action Fehlings’ Test Test for aldehydes NOT ketone NOT benzaldehyde To ~1 cm3 of your sample add ~ 1 cm3 of Fehling’s solution* (BLUE) and warm in a water bath. (*made by mixing equal volumes of: Fehling’s A (dissolve 17g of CuSO4.5H2O in 250 cm3 of water) and; Fehling’s B (dissolve 86 g of Rochelle salt (potassium sodium 2,3-dihydroxybutanedioate (tartrate) and 30 g of NaOH in 250 cm3 of water with gentle warming). OBSERVATION INFERENCE EXPLANATION orange-red precipitate aldehyde reduction of copper(II) copper (I) Cu2O is precipitated aldehyde acid NOTE THAT ALCOHOLS DO NOT GIVE A POSITIVE RESULT WITH THIS TEST Test for carboxylic acids Sodium Carbonate Solution To a small quantity of the unknown substance in a boiling tube add ~1 cm3 of sodium carbonate solution. Test any gas evolved using a drop of lime water at the end of a glass rod. OBSERVATION INFERENCE EXPLANATION effervescence gas evolved turns lime water cloudy carboxylic acid acid + metal carbonate salt + water + CO2 Acidified Silver Nitrate Solution Test for alkyl halides Don’t confuse with Tollen’s! To test for an alkyl halide, the halogen atom must first be released as a halide ion by hydrolysis. Dissolve ~ 1 cm3 of the alkyl halide in ~1 cm3 of ethanol, then add ~ 1 cm3 of sodium hydroxide solution and warm in a water bath. Add ~ 1 cm3 of silver nitrate solution that has been acidified with dilute nitric acid (this removes excess OH-(aq) which would otherwise precipitate out Ag2O(s) thus masking the test results). OBSERVATION white precipitate soluble in ammonia pale yellow precipitate soluble in xs ammonia yellow precipitate insoluble in ammonia INFERENCE EXPLANATION alkyl chloride AgCl(s) alkyl bromide AgCl(s) alkyl iodide AgI(s) Relative rates of hydrolysis of alkyl halides are: iodo > bromo > chloro Mr Lund January Apr-17 58 AS Unit 2 Chemistry in Action Mr Lund January Apr-17 59 AS Unit 2 Chemistry in Action Mr Lund January Apr-17 60 AS Unit 2 Chemistry in Action Mr Lund January Apr-17 61 AS Unit 2 Chemistry in Action Mr Lund January Apr-17 62 AS Chemistry Unit 2 63 AS Chemistry Unit 2 64