Q1.
You may find the following electrode potential data helpful when answering this question.
 
Electrode half-equation	Eϴ / V	
Cr2O72–(aq)  + 14H+(aq)  + 6e–  2Cr3+(aq)  + 7H2O(I)	+1.33	
           O2(g)  + 4H+(aq)  + 4e–   2H2O(I)	+1.23	
                           Cr3+(aq)  + e–   Cr2+(aq)	–0.44	
                         Zn2+(aq)  + 2e–   Zn(s)	–0.76	
                         Cr2+(aq)  + 2e–   Cr(s)	–0.91	
(a)     Describe the colour changes that you would observe when an excess of zinc is added to an acidified solution of potassium dichromate(VI) in the absence of air.
For each colour change, identify the coloured ions responsible and write an equation for each reaction that occurs with zinc.
In the equations, you should represent the ions in their simplest form, for example Cr3+.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(Extra space) ________________________________________________________
___________________________________________________________________
(5)

(b)     Describe what you would observe when dilute aqueous sodium hydroxide is added, dropwise until in excess, to a dilute aqueous solution containing chromium(III) ions.
Write two equations to illustrate your observations.
In these equations you should give the full formula of each of the complexes, for example [Cr(H2O)6]3+.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(Extra space) ________________________________________________________
___________________________________________________________________
___________________________________________________________________
(4)
(c)     When an aqueous solution containing [Cr(H2O)6]3+ ions is warmed in the presence of Cl– ions, [Cr(H2O)5Cl]2+ ions are formed and the colour of the solution changes.
Name this type of reaction.
Suggest, in terms of electrons, why the colours of the complex ions are different.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(Extra space) ________________________________________________________
___________________________________________________________________
(3)
(d)     The chromium(II) ion [Cr(H2O)6]2+ has different properties from the [Cr(H2O)6]3+ ion.
Use data from the table above to explain why, in an open container, [Cr(H2O)6]2+(aq) ions change into [Cr(H2O)6]3+(aq) ions.
Suggest the identity of the products formed in each case when sodium carbonate solution is added to separate solutions containing [Cr(H2O)6]2+(aq) ions and [Cr(H2O)6]3+(aq) ions.
Explain why the [Cr(H2O)6]3+(aq) ions behave differently from the [Cr(H2O)6]2+(aq) ions.
In your answer to this part of the question, equations are not required.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(Extra space) ________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(7)
(Total 19 marks)

Mark schemes
Q1.
(a)     Orange dichromate
Allow max 2 for three correct colours not identified to species but in correct order
1
Changes to purple / green / ruby / red-violet / violet Chromium(III)
(Note green complex can be [Cr(H2O)5Cl]2+ etc)
Do not allow green with another colour
1
That changes further to blue Chromium(II)
Allow max 1 for two correct colours not identified but in correct order
1
[Cr2O7]2- + 14H+ + 3Zn → 2Cr3+ + 3Zn2+ + 7H2O
2Cr3+ + Zn → 2Cr2+ + Zn2+ /
Ignore any further reduction of Cr2+
1
[Cr2O7]2- + 14H+ + 4Zn → 2Cr2+ + 4Zn2+ + 7H2O
Ignore additional steps e.g. formation of CrO42-
1
(b)     Green precipitate
1
(Dissolves to form a) green solution
Solution can be implied if ‘dissolvesߣ stated
1
[Cr(H2O)6]3+ + 3OH– → Cr(H2O)3(OH)3 + 3H2O
Penalise Cr(OH)3 once only
1
Cr(H2O)3(OH)3 + 3OH– → [Cr(OH)6]3- + 3H2O
Allow [Cr(H2O)6]3+ + 6OH– → [Cr(OH)6]3- + 6H2O
Allow formation of [Cr(H2O)2(OH)4]– and [Cr(H2O)(OH)5]2- in balanced equations
Ignore state symbols, mark independently
1
(c)     (ligand) substitution / replacement / exchange
Allow nucleophilic substitution
1
The energy levels/gaps of the d electrons are different (for each complex)
Ignore any reference to emission of light
1
So a different wavelength/frequency/colour/energy of light is absorbed (when d 
electrons are excited)
OR light is absorbed and a different wavelength/frequency/colour/energy (of light) is transmitted/reflected
1
(d)     E O2 (/ H2O) > E Cr3+ (/ Cr2+) / e.m.f = 1.67 V
Allow E(cell) = 1.67
1
So Cr2+ ions are oxidised by oxygen/air
Allow any equation of the form:
Cr2+ + O2→ Cr3+
1
With [Cr(H2O)6]2+ get CrCO3
If named must be chromium(II) carbonate
1
with [Cr(H2O)6]3+ get Cr(H2O)3(OH)3 / Cr(OH)3
Allow 0 to 3 waters in the complex
1
and CO2
Can score M3, M4, M5 in equations even if unbalanced
1
Cr(III) differs from Cr(II) because it is acidic / forms H+ ions
1
because Cr3+ ion polarises water
Ignore charge/size ratio and mass/charge
1
[19]