Edexcel A Level Chemistry Paper 3 Mark Scheme June 2023 Questions And Answers

State what is meant by a Brønsted-Lowry acid. - correct answer A proton doner Write the expression that defines the pH of a solution. - correct answer pH=-log[H+] Calculate the concentration of hydrogen ions, in mol dm-3, in a solution with a pH of 2.76 - correct answer [H+] = 10 ^-pH =10^-2.76 = 1.74 x 10^-3 ^-3 Explain why the pH of a 1 × 10-8 mol dm-3 solution of nitric acid, HNO3 , is not 8. [Ionic product of water, Kw = 1.00 × 10-14 mol2 dm-6] - correct answer The water contains some H+ ions Concentration of hydrogen ions from water is negligible/ cannot be ignored Aqueous sodium carbonate and aqueous sodium sulfate are both colourless solutions. (a) Give the reagent and the observation to show the presence of carbonate ions. - correct answer addition of dilute hydrochloric acid and effervescence/bubbling Give the reagent and the observation to show the presence of sulfate ions. - correct answer Addition of barium chloride solution (BaCl2) White precipitate forms This question is about flame tests for Group 1 and 2 metal ions. (a) (i) State the colour of the flame produced by separate samples of potassium and strontium ions. - correct answer Potassium ions - Lilac Strontium ions - Crimson red Give a reason why carrying out a flame test on a mixture of potassium chloride and strontium chloride does not clearly show that two different metal ionsare present. - correct answer The crimson red colour will hide the Lilac colour because it is darker and it will therefore be harder to distinguish between the two In the first stage of the flame test, the nichrome wire is dipped into concentrated hydrochloric acid and then heated in a Bunsen flame. In the second stage, this nichrome wire is dipped into fresh hydrochloric acid and then into the metal salt to be tested before being reheated in the Bunsen flame. (i) Give two reasons why the wire is made of nichrome and not iron. - correct answer Nichrome produces no colour and is inert Iron can produce a colour and react with oxygen Give a reason why the wire is dipped into acid and then heated in the first stage. - correct answer It is heated to remove the residue of the previous sample being tested State why fresh concentrated hydrochloric acid is used in the second stage of the flame test. - correct answer The acid can become contaminated with residue from previous tests (which can produce incorrect results) State why hydrochloric acid is used in the second stage of the flame test. - correct answer Concentrated hydrochloric acid produces volatile chlorides Explain, with reference to electron transitions, the formation of the colour in a flame test. - correct answer Electrons are excited and are promoted to higher energy levels Electrons fall from the excited state to lower energy levels Electrons release energy/photons as visible light Write the formula of potassium chlorate(V). - correct answer KClo3 Write the equation for the first electron affinity of chlorine. Include state symbols. - correct answer Cl(g) + e- -----> Cl-(g) The standard electrode potential for the chlorine / chloride ion half-cell is 1⁄2Cl (aq) + e− ⇌ Cl−(aq) EO = +1.36V (i) Identify an oxidising agent from the Data Booklet that will convert chloride ions into chlorine under standard conditions. - correct answer acidified potassium manganate Analysis shows that a compound has the molecular formula C4H8O2. A student suggests that the compound could be either A or B. A - CH3CH2CH2COOH or B - HOCH2CH=CHCH2OH (a) Deduce a chemical test which would give a positive result for A but not for B. Include the reagent and observation. - correct answer NaHCO3 effervescence Deduce a chemical test which would give a positive result for B but not for A. Include the reagent and observation. - correct answer bromine water orange/yellow/brown solution This question is about aluminium chloride. (a) Complete the electronic configuration of an aluminium atom. - correct answer 1s2 2s2 2p6 3s2 3p1 This question is about the Ag+(aq)|Ag(s) half-cell. (a) A student was asked to plan an experiment to measure the standard electrode potential of the Ag+(aq)|Ag(s) half-cell.