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CHEMICAL REACTIONS Topic 5: Preliminary course Key Points Mass is conserved Atoms are conserved Phosphate PO4 3- Carbonate CO3 2- Sulfate SO4 2- Nitrate NO3 1- Hydroxide OH 1- Ammonium NH4 1+ Key Terms Syllabus What is Gay-Lussac’s Law? Radicals Name 1. 2. 3. Symbol Valency Synthesis: Cu + S –> CuS Decomposition: ZnCO3 –> ZnO + CO2 Combustion: Ethanol Prac-work Series of chemical reactions 8. C2H5OH + O2 –> 2CO2 + 9. 3H2O 4. 7. Displacement: Iron wool + copper sulphate –> copper + Iron sulphate Neutralisation: Acid + Base + Universal indicator –> H2O + NaCl. Hydrogen swaps. Precipitation: Silver Nitrate + NaCl –> AgCl + NaNO3. Silver chloride = photographic. Corrosion: Fe +O2 –> Fe2O3. Mg is more reactive to Fe and so is sacrificed. It looses New substance formed Energy change (up or down), 5. 6. Chemistry : Page 1 Sat 12 February 2000 electrons and becomes ions floating in the solution. Stops Fe from losing electrons. Exothermic: KMnO3 + C3H8O3 (glycerol) –> + energy Endothermic: Sodium Bicarbonate + Citric acid (with water) –> absorbs heat. Reacting Gas Volumes Nitric Oxide + Oxygen –> Nitrogen dioxide (brown gas, soluble in water). NO2 dissolves in water, leaving unreacted NO. How do we tell of a chemical reaction has occurred? sound, light, temp. Colour change (change in state) change in physical/chemical Molar Weight properties of product compares to reactants different volume (gasses) difficult to reverse. is the same as the atomic weight. Eg. 1 mol of Sulphur weighs 32.1 grams. (Mr = 32.1) Law of conservation of mass Total number of atoms in reactants = The weight of the substance in grams Amount of substance (n) Amount of substance = mass of substance(m) molar mass (Mr or FM) total number of atoms in products. (Mass (and atoms) are conserved) n = Gay-Lussac’s law of combining gas volumes (1808) The law of combing gas volumes states that the ratio of the volume of gasses involved in a chemical reaction are expressed by small, whole numbers. 2H2 (20ml) + O2 (10ml) –> 2H20 (at 125C) m M n = amount of substance m = mass of compound M = Molar mass Mole Theory and Chemical Reactions Balanced chemical equations show the number of moles of REACTANTS and Avogadro’s Law states that equal PRODUCTS needed and produced. This is particularly useful to chemists as they can calculate the masses of reactants and products in the reaction without performing the experiment. volumes of gasses, at the same temperature and pressure, contain equal numbers of molecules. Example: Have 160g Methane. How much O2 is needed to react with this. Ratio: 2:1:2 Avogadro’s Law METHANE + OXYGEN –> CARBON DIOXIDE + WATER 1. Write a balanced chemical equation CH4 + 2O2 –> CO2 + 2H2O 1. Write amount of Moles Moles 1 :2 :1 :2 1. Work out weight Chemistry : Page 2 Sat 12 February 2000 Methane = CH4. Formula mass = 12 + 4 = 16. 1 mol = 16 g 10 mol = 160 g 10 mol CH4 needs 20 mol O2 1 mol O2 = 32 g 20 mol (2 x 10) = 640 g 20 mol H2O = 2 + 16 = 18 x 2 = How much H2O and CO2 are 360 g produced from the reaction? 160 g + 640 g –> ? + ? Moles:10 20 10 20 10 mol CO2 = 12 + 2(32)= 440g 1. Write balanced Chemical Equation 2H2 + O2 –> 2H2O 1. In terms of Moles 2 mol : 1 mol –> 2 mol 1. 1. 2. 3. 4. 5. In terms of weights 2H2 = 4, O2 = 32, 2H2O = 36 4g : 32 g –> 36 g General Equation Chemical Equation Moles Relative weights (F.M.) Weights Example 4: What mass of calcium metal would be needed to react with Example 3: What mass H2O would be produced from the complete combustion of 20 grams H2 gas. (20/4 = 5. Therefore all x 5) 20 g : 160 g –> 180 g Example 3: What mass of magnesium chloride would be produced from the reaction of 2.4 g of magnesium with excess hydrochloric acid. conditions. 25 C 1 atm 298 K 101.3 kPa Molar Volume (Vm) - Volume of one mole of gas under specified conditions. S.T.P Vm = 22.4 L/mol OC S.L.C. Vm = 24.5 L/mol 298K an excess of sulphuric acid to produce 0.5 mol calcium sulphate. n = Example 4: What masses of CO2, NaSO4 and H2O would be produced from the reaction of 50 g of NaCO3 with excess sulphuric acid? Molar Volume S.T.P.: Standard Temperature and Pressure. 0 C 1 atm 273 K 101.3 kPa S.L.C.: Standard Laboratory Chemistry : Page 3 Sat 12 February 2000 V Vm n = number of moles V = Volume of gas Vm = Molar Volume Example: No. of moles of gas in 100L at SLC. n = V 100 , n = = 4.08 mol Vm 24.5 Example 2: Volume of CO2 produced when 24.47 L of methane combusted at SLC.1 Example 3: 2.4 g of a compound of C, H and O give on combustion 3.52 g of CO2 and 1.44 g H2O. The relative molar mass of the compound was 60. 1 24.47 L CO2 Chemistry : Page 4 Sat 12 February 2000 a) What volume of CO2 is produced? b) What are the EF and MF of the compound General Equation 4. HYDROCARBON + OXYGEN –> WATER + c) compound 4. 5. CARBON DIOXIDE 1. 2. 3. Balanced Chemical Equation Work out moles of each Weights of each Recall that solute + solvent –> solution. (Volume of final solution) The concentrations of a solution is defined as the quantity of solute in a given volume of solution (at a fixed temperature and pressure) concentration = amount of solute Concentrations expressed as mol/L are called molarities (M). ie. a 1 M solution contains 1 mol of solvent per litre of solution. Experiment: exact concentration of acid. Titration (see book). Mol Ratio. Gases Volume of gas at pressure n = PV RT n = amount, mol P = Pressure V = Volume T = Temperature R = gas constant (8.314 J/K/mol) Chemistry : Page 5 Sat 12 February 2000 What are the masses of C, H and O in 2.4 grams of the Empirical formula Using molar mass work out MF. Molarity Volume of solution c = n V c = concentration n = no. of moles V = volume, L General Equations Synthesis - Forms more complex compound METAL + ACID –> SALT + HYDROGEN Decomposition - Forms elements or simpler compounds ACID + BASE –> SALT + WATER CARBONATE + ACID –> CO2 + SALT + WAT ER Calculations involving excess or limiting reagents (make sure can be done) Steps HYDROCARBON + OXYGEN –> CO2 + WAT ER 1. 2. 3. 4. 5. General Equation Balanced Chemical equation Work out number of moles Write all information below each formula Do calculations (amount of mols = sodium in water petrol and oxygen Rust Temperature of the reactants (higher temp, higher rate) More molecules above activation energy at higher temps (moving fate so more successful collisions). Concentration - More concentrated - faster reactions, and more volume of product (1 M –> 6 M). Surface Area - only the exposed surface can react. Chemistry : Page 6 Sat 12 February 2000 concentration x Volume) (n = c x V) Rates of Reactions Reactions vary greatly in rate Fast Slow The rate of a reaction depends on: Higher SA, faster reaction Catalyst - A substance which increases the rate of a chemical reaction. Works by reacting with one of the initial compounds so the chemical bonds can be broken more easily (and with less energy). The catalyst alters the pathway by which a reaction takes place. They lower the activation energy at which a reaction can take place. (MnO2) catalyst is not used up 2H2O2 –>MnO2 O2 + 2H2O Activation Energy - the minimum energy required to start a chemical reaction. Chemistry : Page 7 Sat 12 February 2000