Grade 11 Physical Sciences · Chemical Change

Interpret the energy change of reactions

Decide if a reaction is exothermic or endothermic from ΔH or an energy graph, and calculate ΔH or activation energy (burning releases heat, so ΔH < 0).

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Energy and chemical change Heat Of Reaction Grade 11 & 12 ChemistryMiss Martins Maths and Science

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Question 1.6 · November 2025 · Paper 2 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.

1.6 A hypothetical endothermic reaction is given below.

A(g) + B(g) ⇌ AB(g)

The activation energy for the reverse reaction is

50

kJ·mol

^{- 1}

.

Which ONE of the following is possible for the reaction above?(2)

A The heat of the reaction is +70 kJ·mol $^{- 1}$ .
B The activation energy for the forward reaction is 50 kJ·mol $^{- 1}$ .
C The energy of the activated complex is 40 kJ·mol $^{- 1}$ .
D The activation energy for the forward reaction is 40 kJ·mol $^{- 1}$ .

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: interpret the energy change of reactions

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3. Question 5.1.3 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
The reaction between aluminium and EXCESS sulphuric acid is used to investigate factors affecting rates of reactions.

$2 A ℓ (s) + 3 H_{2} SO_{4} (aq) \to A ℓ_{2} (SO_{4})_{3} (aq) + 3 H_{2} (g)$
5.1 INVESTIGATION I

The effect of a catalyst on the rate of reaction is determined.

Aluminium powder of mass 5 g reacts with excess $0, 1$ mol·dm $^{- 3}$ $H_{2} SO_{4}$ at $60$ °C.

Consider the following energy diagrams (not drawn to scale) for this investigation. X and Y in diagram B represent the activation energies.

Answer the questions that follow.
5.1.3 Determine the numerical value represented by the letter X on diagram B.(2)
Show the memo (2 ticks)Memo
1. ✓
  Subtraction $240, 8 - 208, 2$
  1 mark · Interpret the energy change of reactions
2. ✓
  Answer $= 32, 6$ (kJ)
  1 mark · Interpret the energy change of reactions
4. Question 6.5.1 · November 2023 · Paper 2 · National (DBE) · 3 marks
Consider the following hypothetical reaction reaching equilibrium in a $4$ dm $^{3}$ closed container at $150$ °C.

$2 AB(g) ⇌ A_{2} (g) + B_{2} (g)$

The graph below shows the changes in the amounts of reactants and products over time.
6.5 At t = $130$ s the temperature of the system is decreased to $100$ °C.
6.5.1 Draw a potential energy diagram for this reaction.(3)
Show the memo (2 ticks)Memo
1. ✓
  Both axes correctly labelled and shape of Ep curveVertical axis: Potential energy. Horizontal axis: Course of reaction. A curve that rises to a peak (activated complex) and then falls.
  1 mark · Interpret the energy change of reactions
2. ✓
  Shape of Ep curve for an endothermic reactionThe products end at a HIGHER potential energy than the reactants (endothermic curve).
  2 marks · Interpret the energy change of reactions
5. Question 1.5 · May/June 2023 · Paper 2 · National (DBE) · 2 marks
1.5 The potential energy diagram for a chemical reaction is shown below.

Which ONE of the following combinations is CORRECT for the FORWARD reaction?(2)
1. A $Δ H$ : Y − X; Activation energy: Z + Y; Potential energy of the activated complex: Z
2. B $Δ H$ : Y − X; Activation energy: Z − Y; Potential energy of the activated complex: Z + Y
3. C $Δ H$ : X − Y; Activation energy: Z − Y; Potential energy of the activated complex: Z
4. D $Δ H$ : X − Y; Activation energy: Z; Potential energy of the activated complex: Z − Y
Show the memo (1 tick)Memo
1. ✓
  Read ΔH, Ea and the activated complex from the profileReactants are at Y, products at X, the peak at Z.
  $Δ H = E_{products} - E_{reactants} = X - Y$ (negative: exothermic).
  $E_{a}$ (forward) = peak − reactants = $Z - Y$ .
  The activated complex sits at the top of the curve, so its potential energy is Z.
  
  Answer: C
  2 marks · Interpret the energy change of reactions
6. Question 1.5 · May/June 2022 · Paper 2 · National (DBE) · 2 marks
1.5 The equation below represents a hypothetical reaction.

$A(g) + B(g) ⇌ C(g)$ $Δ H = - 50$ kJ·mol $^{- 1}$

The activation energy for the REVERSE reaction is $110$ kJ·mol $^{- 1}$ .

Which ONE of the following is the activation energy (in kJ·mol $^{- 1}$ ) for the FORWARD reaction?(2)
1. A 50
2. B 60
3. C 110
4. D 160
Show the memo (1 tick)Memo
1. ✓
  Ea(forward) = ΔH + Ea(reverse) $Δ H = E_{A (forward)} - E_{A (reverse)}$
  $- 50 = E_{A (forward)} - 110$
  $E_{A (forward)} = 60$ kJ·mol $^{- 1}$
  
  Answer: B
  2 marks · Interpret the energy change of reactions

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