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2/17/2009 Lecture 11: Stoichiometry between reactants; electrolytes; precipitation reactions • Readings for next class – 4.4 Acid-Base Reactions Electrolytes • Ionic compounds and acids and bases form free ions (DISSOCIATE) when they dissolve NaCl(s) → Na+(aq) + Cl-(aq) Ionic solid Electrolyte solution Conducts electricity HCl(g) → H+(aq) + Cl-(aq) Molecular Electrolyte solution electrolyte • Example: Write the chemical equation for the dissociation of aluminum sulfate in water Ions in solution.MOV 1 2/17/2009 Electrolytes • Strong electrolytes dissociate completely (or almost) – Most soluble ionic compounds are strong electrolytes – HCl(g), H2SO4(l) and HNO3(aq) are strong electrolytes • Weak electrolytes form ions to only a small extent – E.g. NH3(g), acetic acid (l) (CH3CO2H) • Solutions of non-electrolytes have very low conductivities, since they produce no ions – Most molecular substances are non-electrolytes – Exceptions are acidic and basic molecules such as HCl(g) and NH3(g) Precipitation Reactions • Occur when TWO ELECTROLYTE SOLUTIONS are mixed AND an INSOLUBLE IONIC COMPOUND is formed. • If both possible products are soluble there is NO REACTION • Write a balanced chemical equation for the reaction of a sodium chloride solution with a lead(II) nitrate solution • Complete the following equation: NaCl(aq) + KNO3(aq) → No reaction because both possible products are soluble Precipitation.MOV 2 2/17/2009 Figure 4.6: Reaction of Magnesium Chloride and Silver Nitrate Copyright © Houghton Mifflin Company. All rights reserved. Presentation of Lecture Outlines, 4a–5 Figure 4.5: Limestone Formations Source: Royalty-Free/Corbis. Copyright © Houghton Mifflin Company. All rights reserved. Presentation of Lecture Outlines, 4a–6 3 2/17/2009 Lecture 12: Solubility Rules, Ionic Equations, Acids and Bases • Readings for next class – 4.5 Oxidation-Reduction Reactions Solubility of Ionic Compounds • Solubility can be measured as the concentration of a SATURATED SOLUTION (i.e. when no more solid will dissolve) • SOLUBLE COMPOUNDS (> 5 g will dissolve in 1 L of water) – All NITRATES – All ACETATES – All AMMONIUM SALTS – All GROUP 1 METAL SALTS (Li, Na, K, Rb, Cs) – Most CHLORIDES AND SUFATES • INSOLUBLE COMPOUNDS (< 5 g will dissolve in 1 L of water) – Most CARBONATES – Most HYDROXIDES – Most SULFIDES – AgCl, Hg2Cl2, PbCl2, SrSO4, BaSO4, PbSO4 • See Appendix 1 of Lab manual for more details 4 2/17/2009 Ionic Equations • A NET IONIC EQUATION shows only the ions that participate in a reaction. Other ions are called SPECTATOR IONS • For example: Pb2+(aq) + 2Cl-(aq) → PbCl2(s) This is a general ionic equation for reaction of any Pb2+ containing solution with any Cl- containing solution to form a precipitate of PbCl2 Procedure for Writing Ionic Equations • Write the “molecular equation” as if all substances were molecular. E.g. for NaCl(aq) + Pb(NO3)2(aq) → • Write DISSOLVED ELECTROLYTES as dissociated ions – This gives the TOTAL IONIC EQUATION • Cancel ions that appear in equal numbers on each side – These are the SPECTATOR IONS – This gives the NET IONIC EQUATION – See previous side for the example above 5 2/17/2009 Ionic Equations • Ionic equations must be balanced for mass and charge – Checking this will help you spot mistakes • Net ionic equations are more general than “molecular equations” – Each one describes many possible examples with different spectator ions Acids and Bases • Arrhenius definition: – Acid - forms H+ (protons) in water • E.g. HNO3(aq) → H+(aq) + NO3-(aq) – Base - forms OH- in water • e.g. NaOH(s) → Na+(aq) + OH-(aq) (strong base) nitric acid (strong acid) sodium hydroxide • Bronsted and Lowry definition: – Acid – donates a proton to another species – Base – accepts a proton from another species • Eg. NH3(aq) + H2O(l) → NH4+(aq)+ OH-(aq) base acid – Ionization of an acid is best described as transfer of a proton to water, which acts as a base • HCl(aq) + H2O(l) → H3O+(aq) + Cl-(aq) Hydronium ion acid base 6 2/17/2009 Strong and Weak Acids and Bases • Strong acids and bases ionize completely – E.g. HCl(aq) solution contains NO HCl molecules – Also HNO3(aq), H2SO4(aq), HClO4(aq), HBr(aq), HI(aq) – Group IA and IIA hydroxides (except Be(OH)2) • E.g. KOH(aq) contains no KOH units • Weak acids and bases ionize only partly – Ammonia – solution mainly contains NH3 molecules, with some NH4-(aq) – Also: organic acids, HF(aq), HNO2(aq), H3PO4(aq) – Write the chemical equation for ionization of aqueous acetic acid. Which form predominates in the solution? Lecture 13: Neutralization Reactions; OxidationReduction (Redox) reactions • Readings for next class – 4.6 Balancing Simple Oxidation-Reduction Equations – 4.7 Molar Concentration – 4.8 Diluting Solutions 7 2/17/2009 Neutralization Reactions • Reaction of an acid with a base to produce an ionic compound (called a SALT) – Water is usually produced as well • For a strong acid and strong base, the NET IONIC EQUATION is: H+(aq) + OH-(aq) → H2O(l) – The salt is formed from the spectator ions – Write “molecular”, total, and net ionic equations for the neutralization of aqueous sulphuric acid with potassium hydroxide • Neutralization causes weak acids and bases to ionize and react completely, but they are left in molecular form in ionic equations: – Write “molecular”, total, and net ionic equations for the neutralization of aqueous acetic acid with sodium hydroxide Polyprotic acids • Have more than on ionizable proton • React in multiple steps – Write “molecular equations” for the reaction of aqueous sulfuric acid with NaOH(aq) (a) in two steps (b) for the overall reaction 8 2/17/2009 Formation of a gas • Carbonates, sulfites, and sulfides react with acids to produce gases – Carbonates produce CO2(g) – Sulfites produce SO2(g) – Sulfides produce H2S(g) • Write “molecular”, total, and net ionic equations for the reaction of aqueous sodium carbonate with HCl(aq) – Note that gases produced are molecular (i.e. not ionized) • Write “molecular”, total, and net ionic equations for the reaction of solid CuS with HCl(aq) – Note the CuS is insoluble, and therefore not ionized Oxidation and Reduction • OXIDATION – loss of electrons from a substance • REDUCTION – Gain of electrons by a substance • A reaction that involves oxidation and reduction is called a REDOX REACTION • Write the molecular, total ionic and net ionic equations for the reaction of zinc with hydrochloric acid to produce hydrogen and a solution of zinc chloride – Identify the OXIDIZING AGENT (OXIDANT) and REDUCING AGENT (REDUCTANT) – Write HALF EQUATIONS for the oxidation and reduction 9 2/17/2009 Lecture 14: Oxidation Numbers; Reaction Types; Balancing Redox Equations • Readings for next class – None – Study material from chapters 1, 2, and 3 for the test on Thursday – Attempt the following problem: • Combustion of a 0.173 g sample of an organic solid containing only C, H and O produced 0.437 g of CO2 and 0.077 g of H2O. Calculate the empirical formula of the compound. OXIDATION NUMBER OXIDATION NUMBER, ON - charge on atom if all bonding is considered to be ionic Used to identify redox reactions and the oxidation and reduction processes Rules 1. ON = 0 for all elements. e.g. for H in H2 2. ON = charge of a monotomic ion. e.g. +1 for Na+, -2 for S23. ON = +1 for hydrogen (H) in all compounds except ionic hydrides (e.g. NaH) 4. ON = -2 for oxygen (O) in all compounds except H2O2 (ONO = -1) and other peroxides 5. Sum of oxidation numbers must be (a) zero for molecules (b) ionic charge for ions Determine the oxidation number of each element in AlCl3, NH3 and ClO4- . 10 2/17/2009 Use of Oxidation Numbers • OXIDATION – ON increases by the number of electrons involved (n) • REDUCTION – ON decreases by the number of electrons involved (n) • Identify the oxidation and reduction processes in the following reaction: 6KI(aq) + KClO3(aq) + 6HCl(aq) → 3I2(s) + 7KCl(aq) +3H2O(l) How many electrons are involved in the oxidation? How many electrons are involved in the reduction? Types of Reactions • COMBINATION REACTIONS – Two substances combine to make a new substance Al and Br2.MOV – When elements combine, a redox process occurs Write a chemical equation for the combination of hydrogen and carbon to form methane (CH4). Identify the oxidant and reductant. • DECOMPOSITION REACTIONS Oxidation of Zn.MOV – One substance reacts to give two or more substances – Usually requires heat or a catalyst – Carbonates produce CO2, perchlorates produce O2 Write a chemical equation for the thermal decomposition of sodium perchorate to produce NaCl and oxygen gas. Identify any changes in oxidation states. Write a chemical equation for the thermal decomposition of sodium carbonate to produce sodium oxide and carbon dioxide. Identify any changes in oxidation states. 11 2/17/2009 Lecture 15: Combustion Reactions; Displacement Reactions; Balancing Redox Equations • Readings for next class – 5.1 Gas Pressure – 5.2 Empirical Gas Laws Important Notice • Assignments are due at 1:00pm in your A tutorial, and NOT in the dropboxes outside of C-4009. These will only be used in the event of a storm • There are 10 different versions of each assignment. You are only required to submit 1. • Assignments will only be marked if they are handed in on the original assignment sheet. Photocopies or separate sheets are NOT acceptable. If you lose or misplace an assignment sheet, please come to C-4009 to get a new version. • On the top of your assignment, please indicate when your lab is, and what bench number you sit at in the lab. This is for our sorting and recording purposes. 12 2/17/2009 Types of redox reactions • COMBUSTION REACTIONS – A substance reacts rapidly with oxygen (often produces a flame) – Always redox reactions – One or more oxides are produced – Organic compounds produce CO2 and H2O Write a balanced chemical equation for the combustion of C6H12O6(s). Write a balanced chemical equation for the combustion of aluminum. Identify changes in oxidation state in the above reactions. • DISPLACEMENT REACTIONS – An element reacts with a compound to give a new element and new compound – Always redox reactions Write a chemical equation for the displacement of hydrogen from water by calcium metal. Thermite.mov An activity series for predicting the occurrence of displacement reactions Table 4.6 • Elements only displace other element that are below them in the series • Complete the following: Zn(s) + CuCl2(aq) → Cu(s) + ZnCl2(aq) → Ag+ and Cu.MOV Zn in acid and Cu2+.MOV You need to learn this order 13 2/17/2009 Balancing Redox Equations • Use the Half-Reaction Method – Write the reduction and oxidation half-equations with the correct number of electrons to account for the change in oxidation state – Multiply the complete half-equations, as needed to have equal numbers of electrons for the reduction and oxidation – Add the two half-equations and cancel the electrons Balance the following equation by the half-reaction method Al(s) + ZnCl2(aq) → AlCl3(aq) + Zn(s) Lecture 16: Solution Concentrations; Dilution; Quantitative Analysis • Readings for next class – 5.3 The Ideal gas law 14 2/17/2009 SOLUTION CONCENTRATION • In a SOLUTION, a SOLUTE is dissolved in a SOLVENT • MOLARITY (Molar Concentration) - moles of solute per litre of solution MOLARITY = moles of solute/litres of solution • UNITS are mol L-1 or M • A VOLUMETRIC FLASK is used to make solutions of known concentration • Example: What concentration of CuCl2is obtained by dissolving 17.87 g of CuCl2 in 500.00 mL of water? Dilution Concentrated solution + solvent ⇒ dilute solution • MOLES OF SOLUTE DOESN’T CHANGE • Initial moles of solute (MiVi) = final moles of solute (MfVf) • What volume of 2.63 M NaOH(aq) is required to prepare 500.0 mL of 0.100 M NaOH(aq)? Answer: 1.90 x 10-2 L 15 2/17/2009 Quantitative Analysis Determination of the amount of a species in a sample • Gravimetric Analysis – the amount of a species is determined from the mass of a product – Precipitation is often used • Volumetric Analysis (TITRATION) - the amount of a species is determined from the volume of solution needed for complete reaction – Most commonly and acid-base reaction (neutralization) is used Problems The silver in a 10.0 g sample of ore was oxidized and dissolved to give 25.0 mL of Ag+(aq) solution. The Ag+ was then quantitatively precipitated as AgCl(s). If the mass of AgCl obtained was 0.549 g, what was the concentration of Ag+ in the solution? Answer: 0.153 M 21.20 mL of 0.1000 M NaOH were required to neutralize 10.00 mL of H2SO4(aq). What was the initial H2SO4 concentration? Answer: 0.1060 M 16 2/17/2009 Stoichiometry for Solution Reactions Given volume and molarity, or mass Mol of substance Chemical equation Mol required Mass, molarity or volume required 17