OCR A level Chemistry Paper 3 2025 Ques

OCR A level Chemistry Paper 3 2025 Question paper Potassium manganate and iron ions [or (COOH)2] titrations Potassium manganate reduced (burette) Iron solution + excess dilute H2SO4 (provides H+ ions) added to conical flask (colourless) doesnt need indicator (self indicating) end point = permanent pink colour Fe2+ -- Fe3+ + e- MnO4- + 8H+ + 5e- -- Mn2+ + 4H2O MnO4- + 8H+ + 5Fe2+ -- Mn2+ + 4H2O + 4Fe3+ Iodine and sodium thiosulfate titrations Na2S2O3 oxidised (burette) oxidisng agent + excess KI produces iodine in conical flask (yellow-brown) t itration -- pale straw colour add starch indicator (end point= blue-black -- colourless) I2 + 2e- -- 2I- 2S2O3 2- -- S4O6 2- + 2e- 2S2O3 2- + I2 -- S4O6 2- + 2I- ClO- content in bleach ClO- reacts w/ I- and H+ to form I2, then titrated Cu2+ content in copper (II) compounds 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 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 Contain species present in redox half-equation Metal/metal ion half cells its aq ion Equilibrium set up at phase boundary where metal is in contact w/ Forwards reaction shows reduction, reverse shows oxidation In isolated half cell, theres no net transfer of electrons into or out of cell Zn2+ (aq) | Zn (s) Cu2+ (aq) | Cu (s) Ion/ion half cell Ions of the same element in different oxidations tates Inert metal electrode used (Pt) Fe3+ (aq) + e- -- Fe2+ (reversible) Relative tendency Negative electrode= more reactive, loses electrons (oxidised) Positive electrode= less reactive, gains electrons (reduced) Standard electrode potential The tendency to be reduced and gain electrons Use half cell w/ H2 (g) and H+ solution w/ Pt electrode Standard electrode potential (E°) definition the emf of a half cell compared with a standard hydrogen half cell under standard conditions (298K, 1 moldm-3, 100kPa) Measuring E° electrode of half cell connected to standard hydrogen electrode by wire to allow connected flow of electrons two solutions are connected w/ salt bridge that allows ions to flow more neg E° value= greater tendency to lose electrons + undergo oxidation more pos E° value= greater tendency to gain electrons + undergo reduction Salt bridge Soak piece of filter paper in aq solution of KNO3 or NH4NO4 Contains conc. solution of an electrolyte that doesnt react w/ either solution Measuring standard cell potentials Prepare two standard half cells under standard conditions (for ion/ion half cells, have same conc) Connect to voltmeter using wires Prepare salt bridge + connect to solutions Record standard cell potential from voltmeter ELECTRONS FLOW FROM MORE NEG TO LESS NEG HALF CELL E° cell= E° (positive electrode) - E°(negative electrode) Predictions from electrode potentials likely to be oxidised Strongest reducing agent= most negative + most Strongest oxidising. agent= most positive + most likely to be reduced when writing over equations, keep red equation same way round + oxidation one reversed Limitations of predictions Reactions can have very high a.e so very slow reaction rates Value of electrode potential differ from standard value if conc isnt 1moldm-3 If conc 1 eqm shifts left, more electrons in system, E more neg