AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2024: Question 5
10 marks · Standard Demand difficulty · Short Answer
Describe the formation of calcium chloride, explain why solid calcium chloride cannot be electrolysed, identify electrolysis products, select the correct half-equation for the positive electrode, and explain the movement of ions during the electrolysis of copper chromate.
Practise this questionQuestion
Question text
05 This question is about ionic compounds and electrolysis.
Calcium chloride is an ionic compound.
05.1 Calcium and chlorine react to produce calcium chloride.
Describe what happens to calcium atoms and chlorine atoms when the ionic
compound calcium chloride is formed.
[4 marks]
05.2 Solid calcium chloride cannot be electrolysed.
Give one reason why.
[1 mark]
05.3 Name the product formed at the negative electrode when aqueous calcium chloride
solution is electrolysed.
[1 mark]
05.4 What is the half equation for the reaction at the positive electrode when aqueous
calcium chloride solution is electrolysed?
[1 mark]
Tick ( ) one box.
2 Cl¯ → Cl + 2 e–
Cl + 2 e– → 2 Cl¯
4 OH¯ → O + 2 H O + 4 e–
O + 2 H O + 4 e– → 4 OH¯
22 16
05.5 A student investigated the electrolysis of green copper chromate solution.
Figure 4 shows the apparatus.
Figure 4
Figure 5 shows the results.
Figure 5
Copper chromate solution contains the ions Cu2+ and CrO 2–
Explain the results shown in Figure 5.
[3 marks]
Mark scheme
Show the mark scheme
Question 5
AO /
Question Answers Extra information Mark
Spec. Ref.
05.1 each calcium atom loses two 1 AO2
electrons 4.2.1.2
allow 1 mark for calcium
atoms lose electrons and
(and) each chlorine atom gains chlorine atoms gain electrons 1
one electron
(so) one calcium atom reacts 1
with two chlorine atoms
(to form) Ca2+ ions and Cl- ions 1
or
(to form) calcium ion(s) and allow (to form) ions with full
chloride ion(s) outer shells
allow energy level for shell
AO /
Spec. Ref.
05.2 the ions cannot move allow the ions are in fixed 1 AO1
positions 4.2.2.3
4.4.3.1
4.4.3.2
AO /
Spec. Ref.
05.3 hydrogen allow H2 1 AO2
4.4.3.4
RPA3
AO /
Spec. Ref.
05.4 2 Cl– → Cl + 2 e– 1 AO2
4.1.1.1
4.4.3.4
4.4.3.5
– – – RPA3
AO /
16 Spec. Ref.
allow cathode for negative
05.5 electrode AO3
allow anode for positive 4.4.3.1
electrode
allow attraction for movement
Cu2+ / copper ions are blue 1
and
CrO 2– / chromate ions are
yellow
(because) Cu2+ / copper ions 1
move to the negative electrode
(and also) CrO 2– / chromate 1
ions move to the positive
electrode
Total Question 5 10
How to answer it
Ionic Bonding and Electrolysis Essentials
What this question tests
This question assesses your ability to describe ionic bonding via electron transfer, explain electrical conductivity in different states, apply the rules of aqueous electrolysis, and explain ion migration based on charge and colour.
Formation of Calcium Chloride
Describing ionic bonding (4 Marks)
The Perfect Answer
- Calcium atoms lose two electrons.
- Each chlorine atom gains one electron.
- Two chlorine atoms react with every one calcium atom.
- This forms Ca²⁺ and Cl⁻ ions (with full outer shells).
Exam Technique
When asked to describe bonding, always mention:
- Direction: Who loses and who gains?
- Number: Exactly how many electrons are moved?
- Ratio: How many of each atom are needed to balance the charge?
Common Error: "Sharing"
Never use the word "share" when describing calcium chloride. It is ionic. Sharing only happens in covalent bonding. If you mention sharing, you will likely lose the marks for electron transfer.
Conductivity of Solids
Why can't solid calcium chloride be electrolysed? (1 Mark)
Key Knowledge
For electrolysis to happen, the electrolyte must contain mobile ions to carry the charge.
Correct Answer
The ions are in fixed positions (or the ions cannot move).
Common Error: Moving Electrons
Students often say "electrons cannot move." In ionic compounds, conductivity is about ions moving, not electrons. Metals conduct via electrons; ionic liquids conduct via ions.
Aqueous Electrolysis Rules
Predicting products and half equations
The "Aqueous Rules"
At the Negative Electrode (-): Hydrogen is produced IF the metal is more reactive than hydrogen (e.g., Calcium).
At the Positive Electrode (+): A Halogen is produced IF halide ions (Cl⁻, Br⁻, I⁻) are present.
Answers
05.3: Hydrogen (H₂)
05.4: 2Cl⁻ → Cl₂ + 2e⁻
Half Equation Tip
The positive electrode is where oxidation (loss of electrons) happens. The electrons must be on the right-hand side of the arrow. 2Cl⁻ → Cl₂ + 2e⁻ shows two negative ions losing their extra electrons to become a neutral molecule.
Ion Migration (Copper Chromate)
Explaining the movement of colours (3 Marks)
How to structure this 3-mark answer
You must link the colour to the ion, and then the ion to the electrode.
Step-by-Step Explanation
- Identify Colours: Cu²⁺ ions are blue and CrO₄²⁻ ions are yellow.
- Negative Electrode: Blue Cu²⁺ ions move to the negative electrode because they are positively charged (opposite charges attract).
- Positive Electrode: Yellow CrO₄²⁻ ions move to the positive electrode because they are negatively charged.
Visualizing the Result
The green drop "splits." The blue part moves left (-), and the yellow part moves right (+). This proves that the compound is made of charged particles (ions) that are free to move in the solution.
Topics
Chemistry · C2: Bonding, Structure and the Properties of Matter · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.