WJEC Chemistry for A2: Study and Rev Guide

Tom’s answer Seren’s answer (i) orange green ✗  (ii) The oxidation state of Cr in Cr 2 O 7 2− is +12 and in CrO 4 2− is 2 × +6 = +12. ✗ This isn’t a redox reaction because the oxidation states don’t change. ✓  (iii) The standard electrode potential is the voltage you would measure if you connected a half-cell to the standard hydrogen electrode (S.H.E.) under standard conditions ( 1 mol dm –3 concentrations, 1 atm pressure for gases and a temperature of 298 ✓ ✗ ). The S.H.E. has a platinum electrode dipping in a solution containing H + (aq) ions, with hydrogen gas bubbled over it. ✓  (i) It turns yellow. ✗  (ii) Chromium in dichromate = +6; chromium in chromate = +6. ✓ This is not a redox reaction because the oxidation state stays the same. ✓  (iii) The standard electrode potential is the value measured on the high-resistance voltmeter when a half-cell is connected to the standard hydrogen electrode.  Q & A 14 Reduction and oxidation Acidi ed potassium dichromate is a common oxidising agent, but adding sodium hydroxide solution causes the reaction below: Cr 2 O 7 2− + 2 OH − → 2 CrO 4 2− + H 2 O (i) Give the colour change seen during this reaction. [1] (ii) Use the oxidation states of chromium to show that this is not a redox reaction. [2] (iii) The standard electrode potential for acidi ed potassium dichromate when used as an oxidising agent is +1.33V. Explain what is meant by the term standard electrode potential and describe how this value would be measured. [6] Tom’s answer Seren’s ans er (i) orange → green ✓  (ii) The oxidation state of Cr in Cr 2 O 7 2− is +12 and CrO 4 2− is 2 × +6 = +12. ✗ This isn’t a redox reaction because the oxidation states don’t change. ✓  (iii) The standard electrode potential is the voltage you would measure if you connected a half-cell to th t ard hydrogen el ctrode (S.H.E.) under stand rd co ditions ( 1 mol dm –3 concentrations, 1 atm pressure for gases and a temperature of 298 ✓ ✗ ). The S.H.E. has a platinum electrode dipping in a solution containing H + (aq) ions, with hydrogen gas bubbled over it. ✓  (i) It turns yellow. ✗  (ii) Chromium in dichromate = +6; chromium in chromate = +6. ✓ This is not a redox reaction because the oxidation state stays the same. ✓  (iii) The standard electrode potential is the value measured on the high resistance voltmeter when a half-cell is conne t d to the sta dard hydrog n electrode.  Examiner commentary  The colour change given occurs when dichromate is reduced, and not in this reaction.  He has doubled the oxidation state, but has gained a mark for his reasoning.  This answer is not precise. He receives marks for giving the substances in the S.H.E. and another for giving two standard conditions. He loses a second mark for conditions by missing the K from 298K, and loses several marks by not discussing the second half-cell or how they are connected. Tom achieves 4 marks out of 9. Examiner commentary  No mark as the initial colour of the solution is missing.  Seren gains both marks here.  The labelled diagram provides three marks for the substances in each cell, the platinum electrodes. She also gains a mark for the high resistance voltmeter and salt bridge. She failed to give standard conditions so doesn’t gain these marks. Seren achieves 6 marks out of 9. V platinum electrodes salt bridge H 2 gas in H + (aq) high-resistance voltmeter a mixture of Cr 3+ (aq) and Cr 2 0 7 2 – (aq) Al Al Cl Cl Cl Cl Cl Cl Al Al Cl Cl Cl Cl Cl Cl ✓✓✓✓ A2 Chemistry: Study and Revision Guide Examiner commentary  The colour change given occurs when dichromate is reduced, and not in this reaction.  He has doubled the oxidation state, but has gained a mark for his reasoning.  This answer is not precise. He receives marks for giving the substances in the S.H.E. and another for giving two standard conditions. He loses a second mark for conditions by missing the K from 298K, and loses several marks by not discussing the second half-cell or how they are connected. Tom achieves 3 marks out of 9. Examiner commentary  No mark as the initial colour of the solution is missing.  Seren gains both marks here.  The lab lled diagr m provid s three marks for the substances in each cell, the platinum electrodes. She also gains a mark for the high-resistance voltmeter and salt bridge. She failed to give standard conditions so doesn’t gain these marks. Seren achieves 6 marks out of 9. Unit 3 Physical and inorganic chemistry Q & A A2 Chemistry: Study and Revision Guide 86 Reduction and oxidation 1 Acidi ed potassium dichromate is a common oxidising agent, but adding sodium hydroxide solution causes the reaction below: Cr 2 O 7 2− + 2 OH – 2 CrO 4 2− + H 2 O (i) Give the colour change seen during this reaction. [1] (ii) Use the oxi ation s ates of chromium to show that this is not a redox reaction. [2] (iii) The standard electrode potential for acidi ed potassium dichromate when used as an oxidising agent is +1.33V. Explain what is meant by the term standard electrode potential and describe how this value would be measured. [6]