A Level OCR A -Chemistry Paper 3 MARKING SCHEME

Amount of substance equations - CORRECT ANSWERS-mass = mr x moles mol = vol x conc vol(gas) = mol x 24 (dm) PV=nRT (pressure, Pa, volume, m3, moles, -, 8.314, temp, k) Empirical formula - CORRECT ANSWERS-The simplest whole number ratio of atoms of each element in a compound Molecular formula - CORRECT ANSWERS-The number and type of atoms of each element in a molecule Celsius to Kelvin - CORRECT ANSWERSK=C+273 % yield and atom economy - CORRECT ANSWERS-% yield = (actual/ theoretical) x 100 atom economy = (desired products/all reactants) x 100 Oxidising agent - CORRECT ANSWERS-it itself is reduced, causes another species to be oxidised Reducing agent - CORRECT ANSWERS-it itself is oxidised, causes another species to be reduced Oxidation - CORRECT ANSWERS-Loss of electrons, increased oxidation number Reduction - CORRECT ANSWERS-Gain in electrons, decreased oxidation number Balancing half equations CORRECT ANSWERS-Na2S2O3 oxidised (burette) oxidisng agent + excess KI produces iodine in conical flask (yellow-brown) titration -- pale straw colour add starch indicator (end point= blue-black -- colourless) I2 + 2e- -- 2I- 2S2O3 2- -- S4O6 2- + 2e2S2O3 2- + I2 -- S4O6 2- + 2I- ClO- content in bleach - CORRECT ANSWERS-ClO- reacts w/ I- and H+ to form I2, then titrated Cu2+ content in copper (II) compounds - CORRECT ANSWERS-For copper (II) salts, dissolve in water to form Cu2+ ions For insoluble copper (II) compounds, react w/ acid acid to form Cu2+ For copper alloys (e.g. brass or bronze), dissolve in conc. nitric acid then neutralise to form Cu2+ Cu2+ + I- -- I2 + CuI (overall brown coloured mixture) Voltaic cell - CORRECT ANSWERS-Converts chemical energy to electrical energy Made by connecting 2 different half cells, allowing electrons to flow DONT MIX OR HEAT ENERGY MADE Half cell - CORRECT ANSWERS-Contain species present in redox half-equation Metal/metal ion half cells - CORRECT ANSWERSEquilibrium set up at phase boundary where m