GCSEChemistry: Combined ScienceOCR GatewayExam Questions3. Chemical Reactions3.4 Electrolysis

 Electrolysis (OCR Gateway GCSE Chemistry: Combined Science)

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3.4 Electrolysis

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3.4 Electrolysis

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1a2 marks
a)

Look at the diagram.

It shows the apparatus used for the electrolysis of some molten compounds.

q17a-paper-1-specimen-2016-ocr-gcse-chemistry

The table shows the products at each electrode during the electrolysis of two molten compounds.

Complete the table.

Molten compound Formula Product at negative electrode (cathode) Product at positive electrode (anode)
sodium chloride NaCl …………………… chlorine
lead bromide PbBr2 lead ……………………

[2]

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1b2 marks
b)
Copper sulfate solution can be electrolysed using non-inert copper electrodes.
Describe what happens at the negative copper electrode and the positive copper electrode.

Negative electrode: ..............................................................

Positive electrode: .................................................................

[2]

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1c2 marks
c)
A student is electrolysing a solution of sodium chloride, NaCl, in water, H2O.

Complete the list of ions present in sodium chloride solution.

Positive ions (cations) Negative ions (anions)
Na+ ……………………
…………………… OH–

[2]

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2a5 marks

Electrolysis can be used to separate the elements in some compounds using electricity.

a)

i)
Look at the diagram of an electrolysis experiment.

Complete the diagram using the words in the list.

You may use each word once, more than once or not at all.

q18ai-paper-1-nov-2020-ocr-gcse-chem

[2]

ii)
A teacher demonstrates the electrolysis of molten lead bromide.

Predict the products made at each electrode.

Positive electrode ......................................................

Negative electrode ....................................................

[2]

iii)

Molten lead bromide contains lead ions, Pb2+, and bromide ions, Br−.

What is the formula for lead bromide?

Tick (✓) one box.

PbBr      square
PbBr2 square
Pb2Br  square
Pb2Br2 square

[1]

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2b
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b)
The student investigates the mass of copper made during the electrolysis of aqueous copper chloride.

The student varies the electric current and passes the current for the same time in each experiment. Here is a table of their results.

Current (A) Mass of copper produced (g)
0.2 0.6
0.4 1.3
0.6 1.8
0.8 2.5
1.0 3.1

i)
Plot a graph of the student’s results and draw a line of best fit.
q18bi-paper-1-nov-2020-ocr-gcse-chem

[3]

ii)
Use your graph to estimate the current needed to make 2.25 g of copper.

Current = ....................................................... A

[1]

iii)
Use your graph, and a calculation, to find the mass of copper that would be produced using 15 A.

Give your answer to 2 significant figures.

Mass of copper produced = ....................................................... g

[2]

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3a2 marks

A student electrolyses dilute sulfuric acid.

q20-paper-1-june-2018-ocr-gcse-chem

Hydrogen gas is made at the cathode.

The student measures the volume of hydrogen made at the cathode every 2 minutes for 10 minutes.

Look at his results.

Time
(minutes)		 Volume of hydrogen
(cm3)
0 0.0
2 4.0
4 8.0
6 14.0
8 16.0
10 20.0

a)
Plot the results on the grid. The first 3 points have been done for you.

Draw a line of best fit.
[2]
q20a-paper-1-june-2018-ocr-gcse-chem

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3b1 mark
b)
One of the results is anomalous.

Circle the anomalous result on the graph.
[1]

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3c1 mark
c)
Sulfuric acid contains these particles.

H+                   OH-                   H2O               SO42-

Which particles are attracted to the anode?
[1]

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3d2 marks
d)
The student also investigates the electrolysis of some molten (liquid) salts.

Complete the table.

Molten salt Formula Product at cathode Product at anode
Potassium chloride KCl Potassium ..............................
Lead iodide

PbI2

 .............................. Iodine

[2]

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4a1 mark

A student electrolyses copper sulfate using copper electrodes.

Look at the diagram. It shows the apparatus she uses.

q23-paper-3-specimen-2016-ocr-gcse-chem

She investigates the change in mass at each electrode before and after the electrolysis.

Look at her method.

1.
Using a balance, measure the mass of the copper cathode and copper anode.
2.
Set up the apparatus and run the electrolysis for 30 seconds.
3.
Remove the copper cathode and the copper anode and immediately place them on the balance and measure their masses again.

a)

What improvements could you make to the student’s experiment?

Explain your answers.

[4]

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4b2 marks
b)

The student finds that:

  • the cathode gains mass
  • the anode loses mass.

Explain these observations in terms of the reactions at each electrode.

[2]

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5a2 marks

A student investigates the electrolysis of aqueous solutions of ionic compounds

Aqueous solution Product at cathode Product at anode
Copper sulfate Copper Oxygen
Zinc bromide Hydrogen Bromine
Copper chloride Copper Chlorine
Sulfuric acid Hydrogen Oxygen

a)
Write the formulae of the ions that are present in aqueous copper sulfate solution.

[2]

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5b1 mark
b)
Why is it important that the investigation is done with inert electrodes?

[1]

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5c3 marks
c)
 Electroplating is used to cover a metal with another metal.

i)
Which aqueous solution would you use to electroplate a metal spoon with copper using a safe method?
 
Tick (✓) one box.
 
Copper sulfate square
Zinc bromide square
Copper chloride square
Sulfuric acid square

[1]

ii)
Give two reasons for your answer to (c)(i).

[2]

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5d4 marks

d)

i)
Predict the product made at the anode when sodium sulfate solution is electrolysed.
 

[1]

ii)
Hydrogen gas is made at the cathode instead of sodium metal.

Explain why.

[1]

iii)
Write the balanced half equation for the formation of hydrogen gas.

Use e− to represent an electron.

[2]

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5e1 mark
e)
The electrolysis products of ionic compounds can be different in the molten or aqueous states.

Suggest why.

[1]

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6a2 marks

A student is investigating the electrolysis of copper sulfate solution.

q24-paper-3-june-2018-ocr-gcse-chem

He does two experiments.

Experiment 1 uses platinum electrodes. Experiment 2 uses copper electrodes.

a)
Complete the table to show the products at each electrode.

Experiment What happens at cathode (–) What happens at anode (+)
1 ........................................... Oxygen made
2 Copper deposited ...........................................

[2]

How did you do?

6b1 mark
b)

Copper electrodes are non-inert electrodes.

What is meant by non-inert electrodes?

[1]

How did you do?

6c2 marks

HIgher Only

c)
Look at the results for experiment 2 using copper electrodes.

At the cathode copper ions, Cu2+ gain electrons to make copper atoms.

Write the half equation for this reaction.

Use e–  to represent an electron.

[2]

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6d2 marks
d)
The student also electrolyses sodium chloride solution using platinum (inert) electrodes.

At the cathode, hydrogen gas is made rather than sodium metal.

Explain why.

[2]

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1a4 marks

Sodium chloride, NaCl, is an ionic compound.

Sodium chloride forms a giant ionic lattice that can be represented using different models.

Look at the diagrams. They show two models of sodium chloride.

q21-paper-3-june-2019-ocr-gcse-chem

i)
A scientist thinks the ball-and-stick model should be used to model ionic compounds.

Describe two limitations of using the ball-and-stick model for ionic compounds.

[2]

ii)
Ionic compounds can also be modelled using a dot-and-cross diagram.

Draw a dot and cross diagram to show the ions in sodium chloride.

[2]

How did you do?

1b6 marks

A student investigates the electrolysis of potassium bromide solution.

q21b-paper-3-june-2019-ocr-gcse-chem

He notices that different products are formed at each electrode.

Explain the formation of the products during the electrolysis of potassium bromide solution.

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