Exam questions kerboodle redox answers PDF

Preview

Summary

a level exam questions kerboodle redox mark scheme....

Description

7 Oxidation, reduction, and redox reactions Exam-style mark scheme

AQA Chemistry Question number

Answer

Marks

1a

3CuS(s) + 8HNO3(aq) → 3CuSO4(aq) + 8NO(g) + 4H2O(l)

1

1b

(+) 5 (+) 2

1

1c

Guidance

1 −

−

4H+ + NO3 + 3e → 2H2O + NO

1

Ignore state symbols. Credit multiples of this equation only. Ignore absence of charge on the electron

1d

S

2−

+ 4H2O → SO4

2−

−

+ 8e + 8H+

1

Ignore state symbols. Credit multiples of this equation only. Ignore absence of charge on the electron.

2ai

–

H2O2 + 2H+ + 2e → 2H2O –

1

–

2 a ii

2I → I 2 +2e

2 a iii

H2O2 + 2H+ + 2I → I2 + 2H2O

2bi

Cl2 + 2e → 2Cl

2 b ii

+1

2 b iii

Cl2 + 2H2O → 2HClO + 2H+ + 2e

1

3ai

MnO2 (+) 4

1

3 a ii

MnO2 + 4H+ + 2e → Mn + 2H2O

1 –

1

–

1

–

1 –

–

2+

1

–

Allow ‘2HI’ in place of 2H+ + 2I

Allow 1 / 1+ / Cl+

Or multiples Ignore state symbols Credit electrons subtracted from RHS Ignore absence of charge on e฀

3 a iii

Iodide ion(s) is / are oxidised because they have lost electron(s)

3b

2Br → Br2 + 2e

–

+

4H + SO4

2–

– –

+ 2e → SO2 + 2H2O

1

Do not penalise reference to iodine; the mark is for electron loss

1

NOT multiples

1

Ignore state symbols Credit electrons subtracted from incorrect side

OR +

–

2H + H2SO4 + 2e → SO 2 + 2H2O 4a

Oxidation

Ignore absence of charge on e 1

OR Oxidised ONLY 4b

NO2

© Oxford University Press 2015

(+) 4

1

www.oxfordsecondary.co.uk/acknowledgements

This resource sheet may have been changed from the original

1

7 Oxidation, reduction, and redox reactions Exam-style mark scheme

AQA Chemistry Question number

Answer NO3

–

HNO2

Marks

(+) 5

1

(+) 3

1

5ai

Loss (of electrons)

1

5 a ii

Oxidation state of nitrogen is NO is +2

1

Oxidation state of nitrogen in NH4 + is –3

1

I2

1

5 a iii 5bi

–

Cl2 + 2e → 2Cl

–

1

Guidance

Ignore state symbols Allow multiples

5 b ii

SO 2 + 2H2O → SO4

2–

+

–

1

+ 4H + 2e

Ignore state symbols Allow multiples

5 b iii

SO 2 + 2H2O + Cl2 → SO4

2–

–

+ 2Cl + 4H

+

1

Ignore state symbols Allow multiples

5c

Use small quantities of solutions e.g. , 2–5 cm

3

Use a fume cupboard since chlorine and SO2 are toxic 6a

–

2+

1 1

MnO2 + 4H+ + 2e → Mn + 2H 2O

1

OR multiples

An oxidising agent is an electron acceptor OR receives / accepts / gains electrons

1

Ignore state symbols

1

NOT an ‘electron pair acceptor’

MnO2 is the oxidising agent 6b

Formation of SO2 and Br2 (could be in an equation) Balanced equation (several possible equations) 2KBr + 3H2SO 4 → 2KHSO4 + Br2 + SO2 + 2H2O OR

Ignore ‘takes electrons’ or ‘takes away electrons’ 1 1 1

–

1 1

2KBr + 2H2SO 4 → K2SO4 + Br2 + SO 2 + 2H2O 2KBr + Cl2 → 2KCl + Br2 % atom economy of bromine

Br2 = ×100 2KBr + CI2 =

=

2 × 79.9 × 100 238.0 + 71.0

159.8 309.0

Could be ionic equation with or without K+

× 100 = 51.72% OR 51.7% OR 52%

2Br + 6H+ + 3SO4 + SO2 + 2H2O

2–

–

→ Br2 + 2HSO4

(3H2SO4) –

2Br + 4H+ + SO4 2H2O

2–

→ Br2 + SO2 +

(2HBr + H2SO4) Accept HBr and H2SO4 in these equations as shown or mixed variants that balance. Ignore equations for KBr reacting to produce HBr Could be ionic equation with or without K+ –

–

2Br + Cl2 → 2Cl + Br2

One from:

Ignore greater number of significant © Oxford University Press 2015

www.oxfordsecondary.co.uk/acknowledgements

This resource sheet may have been changed from the original

2

7 Oxidation, reduction, and redox reactions Exam-style mark scheme

AQA Chemistry Question number

6c

Answer

Marks

Guidance

•

High atom economy

figures

•

Less waste products

Ignore reference to cost

•

Cl2 is available on a large scale

Ignore reference to yield

•

No SO2 produced

•

Does not use concentrated H 2SO4

•

(Aqueous) KBr or bromide (ion) in seawater

•

Process 3 is simple(st) or easiest to carry out

HBr

–1

1

HBrO

+1

1

Equilibrium will shift to the right

1

OR Favours forward reaction OR Produces more HBrO To oppose the loss of HBrO

1

OR Replaces (or implied) the HBrO (that has been used up)

© Oxford University Press 2015

www.oxfordsecondary.co.uk/acknowledgements

This resource sheet may have been changed from the original

3...