AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2023: Question 4

12 marks · Standard Demand difficulty · Extended Answer

This multi-part question assesses understanding of bond energy calculations, reaction profiles, covalent bonding, and the relationship between molecular size and intermolecular forces.

Practise this question

Question

A multi-part chemistry question starting with a reaction equation for hydrogen and chlorine, followed by a bond energy table. Subsequent parts ask to identify the correct energy change expression, explain why the reaction is exothermic, complete a reaction profile diagram, draw a dot-and-cross diagram for HCl, and explain the difference in physical states between methane and poly(ethene) based on intermolecular forces.
Question text

04 This question is about hydrogen and compounds of hydrogen.

Figure 3 shows the displayed formulae for the reaction between hydrogen and

chlorine.

Figure 3

Table 4 shows the bond energies.

Table 4

Bond

Bond energy in

436 346 432

kJ/mol

04.1 Which expression shows how to calculate the overall energy change for the reaction

in Figure 3?

Use Table 4.

[1 mark]

Tick ( ) one box.

436 + 346 + 432 kJ/mol

436 + 346 + (2 × 432) kJ/mol

436 + 346 − 432 kJ/mol

436 + 346 − (2 × 432) kJ/mol 11

The reaction between hydrogen and chlorine is exothermic.

04.2 Explain why this reaction releases energy to the surroundings.

[2 marks]

04.3 Figure 4 shows part of a reaction profile for the reaction between hydrogen

and chlorine.

Complete the reaction profile in Figure 4.

You should:

• label the activation energy

• label the overall energy change.

[3 marks]

Figure 4

04.4 Draw a dot and cross diagram for a molecule of hydrogen chloride (HCl).

Show the outer shell electrons only.

[2 marks]

04.5 Figure 5 represents molecules of methane and of poly(ethene).

Figure 5

Methane is a gas at room temperature but poly(ethene) is a solid at room

temperature.

Explain why methane and poly(ethene) exist in different states at room temperature.

[4 marks]

Mark scheme

Show the mark scheme The mark scheme provides the correct mathematical expression for bond energy, the explanation for exothermic reactions involving bond breaking and forming, a completed reaction profile diagram, the dot-and-cross structure for HCl, and the explanation for the physical states of methane and poly(ethene) focusing on intermolecular forces and molecular size.

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 436 + 346 − (2 × 432) kJ/mol 1 AO2

4.5.1.3

AO /

Question Answers Extra Mark

Spec. Ref.

04.2 AO1

energy is needed to break 1 4.5.1.3

bonds

and

energy is released when bonds

form

(and) the energy released is allow the energy transferred in 1

greater than the energy needed bond making is greater than the

energy transferred in bond

breaking

allow 2 x 432 (kJ/mol) is greater

than 436 + 346 (kJ/mol)

allow the overall energy change–HEMISTRY– –

is negative

AO /

Spec. Ref.

04.3 ignore arrow heads

profile completed with product 1 AO1

energy below reactant energy

activation energy labelled from 1

reactant energy to top of curve AO1

overall energy change labelled 1

from reactant energy to product AO2

energy

4.5.1.2

an answer of

scores 3 marks – HEMISTRY – –

AO /

Spec. Ref.

04.4 allow any combination of x, o, AO1

e(−), • for electrons 4.2.1.4

do not accept molecules

containing more than 2 atoms

bonded pair of electrons in the 1

overlap

chlorine with 6 non-bonded do not accept if extra electrons 1

electrons on H

an answer of

scores 2 marks

or

an answer of

scores 2–marks HEMISTRY – –

AO /

Spec. Ref.

04.5 (methane) AO1

4.2.2.4

methane has (much) smaller 1 4.2.2.5

molecules

(so) has weaker intermolecular do not accept reference to 1

forces weak(er) covalent bonds

(so the intermolecular forces) do not accept reference to 1

need less energy to overcome breaking covalent bonds

14 (so) the boiling / melting point is 1

lower (and methane is a gas)

OR

(poly(ethene))

poly(ethene) has (much) larger

molecules (1)

(so) has stronger intermolecular do not accept reference to

forces (1) weak(er) covalent bonds

(so the intermolecular forces) do not accept reference to

need more energy to break (1) breaking covalent bonds

(so) the melting / boiling point is

higher (and poly(ethene) is a

solid) (1)

– HEMISTRY – –

Total Question 4 12

Question 5

How to answer it

Hydrogen, Bonding, and Energy Changes

What this question tests

This question assesses your ability to calculate energy changes using bond energies, explain the theory of exothermic reactions, draw reaction profiles, represent covalent bonding with dot-and-cross diagrams, and compare physical properties based on intermolecular forces.

Part 4.1

Bond Energy Calculations

Step-by-Step Calculation

  1. Identify bonds broken (Reactants): 1 × H-H (436) and 1 × Cl-Cl (346).
  2. Identify bonds made (Products): 2 × H-Cl (2 × 432).
  3. Apply the formula: Energy Change = (Bonds Broken) - (Bonds Made).

Correct Expression

436 + 346 - (2 × 432) kJ/mol

1 Mark: For selecting the correct mathematical expression.
Part 4.2

Exothermic Reaction Theory

Key Knowledge

  • Bond breaking: Endothermic (requires energy).
  • Bond making: Exothermic (releases energy).

How to score 2/2

You must mention both breaking and making. An exothermic reaction happens when the energy released from making bonds is greater than the energy needed to break them.

Common Error

Students often lose marks by simply saying "energy is released." You must explicitly compare the energy of making vs breaking to get full marks.

Part 4.3

Reaction Profiles

Drawing the Profile

  • Product Level: Draw a horizontal line for 2HCl lower than the reactant line (because it is exothermic).
  • Activation Energy (Eₐ): Draw a vertical arrow from the reactant line to the peak of the curve.
  • Overall Energy Change (ΔH): Draw a vertical arrow from the reactant line down to the product line.

Examiner Tip

Ensure your arrows are clear. The activation energy arrow must start at the reactants, not the bottom of the graph! Arrow heads are not strictly required by the mark scheme, but they help show direction.

Part 4.4

Dot and Cross Diagrams

What to draw

Draw two overlapping circles (H and Cl):

  • Overlap: One dot and one cross (1 shared pair).
  • Chlorine Shell: 6 more electrons (3 pairs) around the rest of the Cl circle.
  • Hydrogen Shell: No other electrons (it only has the shared pair).

Common Pitfall

Forgetting the lone pairs on Chlorine. Chlorine is in Group 7; it uses 1 electron to bond, so 6 "non-bonded" electrons must still be shown on its outer shell.

Part 4.5

Intermolecular Forces (4 Marks)

The Comparison Logic

This is a classic "State and Explain" question. Use this structure:

  1. Size: Poly(ethene) has much larger molecules than methane.
  2. Force: Poly(ethene) has stronger intermolecular forces.
  3. Energy: More energy is required to overcome these forces in poly(ethene).
  4. Result: Therefore, poly(ethene) has a higher boiling point (solid), while methane has a lower one (gas).

The "Covalent Bond" Trap

NEVER say that covalent bonds break when a substance melts or boils. Covalent bonds are very strong and stay intact. It is only the weak intermolecular forces between the molecules that are overcome.

Topics

Chemistry · C2: Bonding, Structure and the Properties of Matter · C5: Energy Changes

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.