AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2020: Question 3
8 marks · Low Demand difficulty · Short Answer
Complete a word equation, label an exothermic reaction profile diagram, identify the function of a fuel cell, and determine which pair of metals produces the greatest voltage in a chemical cell.
Practise this questionQuestion
Question text
03 This question is about chemical reactions and energy.
Hydrogen reacts with oxygen to produce water.
This reaction releases energy.
03.1 Complete the word equation for the reaction.
[1 mark]
hydrogen + oxygen → …
03.2 Figure 3 shows a reaction profile for the reaction between hydrogen and oxygen.
Figure 3
What do the labels W, X, Y and Z represent?
Choose answers from the box.
[4 marks]
activation energy energy overall energy change
products progress of reaction reactants
W
X
Y
Z
03.3 The reaction between hydrogen and oxygen is used in a hydrogen fuel cell.
*10* What is the reason for using this reaction in a fuel cell?
[1 mark]
Tick ( ) one box.
To produce a change of state
To produce a potential difference
To produce a temperature change
03.4 A student investigated the voltage produced by a chemical cell.
The student used different metals as the electrodes in the cell.
The metals used were:
• copper
• iron
• magnesium.
Which two metal electrodes would produce the greatest voltage when used in the
chemical cell?
Give one reason for your answer.
[2 marks]
Metals and
Reason
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 water allow H2O 1 AO1
4.1.1.1
do not accept energy 4.5.2.2
03.2 W = energy 1 AO1
4.5.1.2
X = activation energy 1
Y = overall energy change 1
Z = progress of reaction 1
03.3 to produce a potential difference 1 AO1
4.5.2.2
03.4 magnesium and copper 1 AO2
4.4.1.2
4.5.2.1
(the metals) have the largest 1
difference in reactivity
Total 8
How to answer it
Reaction Profiles, Fuel Cells & Chemical Cells
This question evaluates your foundational knowledge of AQA GCSE Chemistry Paper 1 (Energy Changes & Chemical Changes):
- Recalling products of the reaction between hydrogen and oxygen.
- Correctly identifying and labelling key features of an exothermic reaction profile (axes, activation energy, overall energy change).
- Understanding the primary practical purpose of a hydrogen fuel cell.
- Predicting and explaining how electrode reactivity dictates the voltage generated in a chemical cell.
Word Equation for Hydrogen and Oxygen
Completing the chemical word equation [1 mark]
✅ Correct Answer
water (or H₂O)
hydrogen + oxygen → water
❌ Common Errors
- Writing "energy" or "heat" – energy is released, but it is not a chemical substance. The mark scheme explicitly states: "do not accept energy".
- Writing "hydrogen oxide" – although chemically descriptive, standard naming requires water.
Labelling the Reaction Profile
Identifying key features of Figure 3 [4 marks]
✅ Correct Labels
- W: energy
- X: activation energy
- Y: overall energy change
- Z: progress of reaction
💡 Key Knowledge
- Activation energy (X): The minimum energy required by colliding particles to react (measured from the reactants line to the highest peak).
- Overall energy change (Y): The difference in energy between reactants and products. Because products are lower than reactants, this reaction is exothermic.
- Axes: Vertical axis (W) is always energy; horizontal axis (Z) is progress of reaction (or time).
🧠 Exam Technique: Reading Reaction Profiles
Always draw mental horizontal guide lines:
- Arrow from reactant baseline to peak = Activation energy.
- Arrow between reactant baseline and product baseline = Overall energy change.
❌ Common Errors
- Mixing up W and Z on the axes.
- Confusing X and Y (e.g. thinking X is overall change because it is taller).
- Choosing "reactants" or "products" for X/Y – reactants and products are the horizontal lines, not the vertical measurement arrows!
Function of a Hydrogen Fuel Cell
Identifying the purpose of the reaction [1 mark]
✅ Correct Answer
Tick: To produce a potential difference
💡 Key Knowledge
A fuel cell converts the chemical energy of a fuel (hydrogen) and an oxidant (oxygen) directly into electrical energy (a potential difference / voltage).
- It does not exist primarily to produce heat or a change of state – its intended function in vehicles and devices is providing electricity.
Electrode Potential in a Chemical Cell
Choosing metals for maximum voltage [2 marks]
✅ Correct Answer
Metals: magnesium and copper
Reason: (They) have the largest difference in reactivity
• 1 mark for naming both magnesium and copper.
• 1 mark for identifying the greatest difference/gap in reactivity.
📐 Step-by-Step Rule: Cell Voltage
- Rank the metals by reactivity:
Magnesium (most reactive) > Iron > Copper (least reactive) - Apply the voltage rule:
The voltage produced by a simple chemical cell depends on the difference in reactivity between the two metal electrodes. - Select the extremes:
Pick the most reactive metal and the least reactive metal to achieve the largest gap and highest potential difference.
❌ Common Errors & Examiner Traps
- Incomplete reason: Writing only "magnesium is the most reactive" loses the second mark. The voltage depends on the difference between both metals.
- Vague statements: Saying "they have different reactivities" is not enough – you must state they have the largest / greatest difference.
Topics
Chemistry · C4: Chemical Changes · C5: Energy Changes
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.