WJEC Chemistry for A2: Study and Rev Guide

Tom’s answer Seren’s answer (i) EMF = 0.77 - 1.33 = –0.56 V ✗  (ii) Pt | Cr 2 O 7 2− | Cr 3+ | | Fe 2+ | Fe 3+ | Pt ✗  (iii) The iron ion is the reducing agent in this case because it has reduced the chromium from oxidation state +6 to +3. ✓  (iv) The dichromate ions are weaker oxidising agents than the peroxodisulfate ions so they cannot oxidise sulfate ions to peroxodisulfate. ✓  (i) EMF = 1.33 – 0.77 = 0.56 V ✓  (ii) Pt | Cr 2 O 7 2− , Cr 3+ | Fe 2+ , Fe 3+ | Pt ✗  (iii) The Fe 2+ is the reducing agent in this case ✓ because it is being oxidised from +2 to +3, and reducing agents are oxidised as they reduce something else. ✓  (iv) The reaction of dichromate ions with sulfate to make peroxodisulfate would have an EMF of: EMF = E reduction − E oxidation = 1.33−2.01 = −0.68V ✓ A negative EMF means that this reaction is not feasible. ✓  Examiner commentary  The EMF should be positive, so no mark is awarded.  Tom does not gain credit here as the ions in solution should be separated by a comma not a line. (Cr 2 O 7 2− , Cr 3+ , and Fe 2+ , Fe 3+ )  He gains a mark for the reason but ‘iron ion’ is not speci c enough to gain a mark.  Tom identi es that the reaction cannot occur and gives a reason so he gains a mark. He does not gain the second mark as the question speci es the use of the standard electrode potentials. Tom achieves 2 marks out of 6. Examiner commentary  Is correct and gains 1 mark.  Seren does not gain a mark as she drew the salt bridge as one line.  Seren gains 2 marks as she identi es the reducing agent and gives an appropriate reason.  Includes a correct identi cation and reason, and so gains 2 marks. Seren achieves 5 marks out of 6. Q & A 87 Questions and answers Using standard electrode potentials 2 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) + 14 H + (aq) + 6 Fe 2+ (aq) 2 Cr 3+ (aq) + 7 H 2 O (l) + 6 Fe 3+ (aq) This reaction can be used as the basis of an electrochemical cell involving the rst two of the half-reactions shown below: E θ /V Fe 3+ (aq) + e – Fe 2+ (aq) +0.77 Cr 2 O 7 2– (aq) + 14 H + (aq) + 6e – 2 Cr 3+ (aq) + 7 H 2 O (l) +1.33 S 2 O 8 2– (aq) + 2e − 2 SO 4 2– (aq) +2.01 (i) Calculate the EMF of the cell. [1] (ii) Give the cell diagram that represents this cell. [1] (iii) Identify the reducing agent in the reaction above. Give a reason for your answer. [2] (iv) Using the standard electrode potentials above, state and explain whether dichromate(VI) ions can oxidise sulfate ions to peroxodisulfate ions, S 2 O 8 2− (aq). [2]