Eduqas Chemistry for A Level Year 2: Student Bk

92 Eduqas A Level Chemistry Year 2: Component 1 9 Iron metal can corrode in water in the presence of air by a redox reaction. 2Fe (s) + O 2 (g) + 2H 2 O (l) 2Fe 2+ (aq) + 4OH – (aq) (a) Use the changes in oxidation numbers (states) to determine which reactant is being oxidised and which reduced in the above equation. [2] (b) An equivalent corrosion reaction could be written for silver, Ag: 4Ag (s) + O 2 (g) + 2H 2 O (l) 4Ag + (aq) + 4OH – (aq) Given the standard electrode potentials below, explain why the corrosion reaction for iron occurs but silver does not corrode. [2] Fe 2+ (aq) + 2e – Fe (s) E = – 0·44V O 2 (g) + 2H 2 O (l) + 4e – 4OH – (aq) E = +0·40V Ag + (aq) + e – Ag (s) E = +0·80V 10 Hydrogen fuel cells are used in some vehicles and these are advertised as being good for the environment. Give a reason why the use of fuel cells is good for the environment and one environmental disadvantage of their use. [2] 11 Dichromate(VI) ions, Cr 2 O 7 2– , react with iron(II) ions in acid solution according to the following equation. Cr 2 O 7 2– (aq) + 14H + (aq) + 6Fe 2+ (aq) 2Cr 3+ (aq) + 7H 2 O (l) + 6Fe 3+ (aq) This reaction can be used as the basis of an electrochemical cell where the standard electrode potentials of the two half-reactions are shown below. Fe 3+ (aq) + e – Fe 2+ (aq) +0.77V Cr 2 O 7 2– (aq) + 14H + (aq) + 6e – 2Cr 3+ (aq) + 7H 2 O (l) +1.33V (a) Calculate the EMF of the cell. [1] (b) Platinum electrodes are used in the cell. Explain the purpose of these platinum electrodes. [1] (c) Draw a labelled diagram to show a possible construction for the cell. Your labels should include: • The apparatus used • The reagents • The positive and negative electrodes • The direction of electron flow in the circuit. [5] 12 Vanadium(V)(aq), as VO 3 – , is yellow and can be reduced by zinc and aqueous acid producing a series of coloured solutions until the reduction stops with the formation of a violet solution. The reducing agent involves the Zn 2+ (aq) / Zn(s) equilibrium. Oxidation state of vanadium at start of reaction Reaction E θ / V +5 VO 3 – (aq) + 4H + (aq) + e VO 2+ (aq) + 2H 2 O (l) +1.00 +4 VO 2+ (aq) + 2H + (aq) + e V 3+ (aq) + H 2 O (l) +0.34 +3 V 3+ (aq) + e V 2+ (aq) –0.26 +2 V 2+ (aq) + 2e V (s) –1.13 Zn 2+ (aq) + 2e Zn (s) –0.76 Cu 2+ (aq) + 2e Cu (s) +0.34