Grade 12 Physical Sciences · Chemical Change · Paper 2 (Chemistry)

Rates of Reaction Grade 12

Real Rates of Reaction past paper questions and answers from 2024 to 2025, with each memo split into the ticks examiners give. Try one, mark yourself, and see exactly which skill cost you marks.

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Question 5.1 · November 2025 · Paper 2 · National (DBE) · 2 marks

5.1 Define the term reaction rate.(2)

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More Rates of Reaction Grade 12 questions and answers

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4. Question 4.2.1 · November 2025 · Paper 1 · National (DBE) · 3 marks
Cricketers at an academy conduct an experiment to determine the relationship between the contact time and the final momentum of a ball for a constant average net force.

A cricket ball of mass $150$ g is thrown horizontally with a certain initial velocity and is struck by a bat so that the ball moves in the opposite direction, as shown in the diagram below. The average net force acting on the ball is horizontal.

The experiment is repeated using the same ball. The average net force and the initial velocity of the ball are kept constant. The contact time between the bat and the ball is changed EACH time. The results obtained are shown in the sketch graph below.
4.2 Calculate the:
4.2.1 Average net force acting on the ball(3)
Show the memo (3 ticks)Memo
1. ✓
  METHOD 1: Change in final momentum (numerator)Gradient $= F_{n e t} = \frac{Δ p _{f}}{Δ ( Δ t )}$
  $= \frac{4 , 5 - 0}{\dots}$
  1 mark · Read a graph of results
2. ✓
  METHOD 1: Change in contact time (denominator) $F_{n e t} = \frac{4 , 5 - 0}{0 , 03 - 0 , 01} = 225$ N
  1 mark · Read a graph of results
3. ✓
  METHOD 1: Answer with direction $F_{n e t} = 225$ N to the right / opposite to the original direction of the ball
  1 mark · Read a graph of results
5. Question 10.3.2 · May/June 2025 · Paper 1 · National (DBE) · 4 marks
Light of different frequencies is incident on a metal plate. The sketch graph below shows the relationship between the maximum kinetic energy, $E_{k (ma x)}$ , of the photoelectrons and the energy of the incident photons.
10.3 Calculate the:
10.3.2 Value of Y(4)
Show the memo (4 ticks)Memo
1. ✓
  METHOD 1: Formula $E = W_{0} + E_{k (ma x)}$ (or $h f = W_{0} + E_{k (ma x)}$ )
  1 mark · Use the photoelectric equation
2. ✓
  METHOD 1: Work function $= 3, 68 \times 1 0^{- 19} + \dots$
  1 mark · Use the photoelectric equation
3. ✓
  METHOD 1: Kinetic energy $= 3, 68 \times 1 0^{- 19} + 2, 99 \times 1 0^{- 19}$
  1 mark · Use the photoelectric equation
4. ✓
  METHOD 1: Answer $E = 6, 67 \times 1 0^{- 19}$ (J)
  1 mark · Use the photoelectric equation
6. Question 6.3 · November 2025 · Paper 1 · National (DBE) · 2 marks
An investigation is performed to determine the relationship between the velocity of a moving sound source and the frequency of the sound detected.

The siren of an ambulance produces sound with a constant frequency. The ambulance, with its siren on, moves towards a stationary sound detector (Part A) and away from the stationary sound detector (Part B) at constant velocities. The experiment is repeated for different velocities of the ambulance. The detected frequency is measured for EACH velocity. The results obtained are shown in the graph below.
6.3 What conclusion can be made for Part B of this experiment?(2)
Show the memo (1 tick)Memo
1. ✓
  Conclusion for Part BAs the velocity of the ambulance increases, the detected frequency decreases.
  2 marks · Write investigative question or conclusion
7. Question 1.4 · May/June 2025 · Paper 2 · National (DBE) · 2 marks
1.4 Two gases are added into each of four empty containers at the same temperature. The diagrams below show the molecules of the gases and the volumes of the containers at the start of the reaction.

In which ONE of the following containers is the initial reaction rate the highest?(2)
1. A Container A
2. B Container B
3. C Container C
4. D Container D
Show the memo (1 tick)Memo
1. ✓
  Correct optionCount the molecules per volume: A has 7 molecules in $5$ cm $^{3}$ , B has 5 in $5$ cm $^{3}$ , C has 7 in $10$ cm $^{3}$ and D has 14 in $20$ cm $^{3}$ . A has the most particles per unit volume (highest concentration), so the most effective collisions per unit time.
  
  Answer: A
  2 marks · Explain rate using collision theory
8. Question 4.1.3 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
4.1 In an experiment, a test tube containing methanol, propanoic acid and a catalyst is heated in a water bath, as shown below.

Write down:
4.1.3 TWO reasons why the use of a water bath is preferred in this experiment(2)
Show the memo (2 ticks)Memo
1. ✓
  First reasonAlcohol/methanol/reactant is flammable/catches fire easily.
  1 mark · Describe and evaluate an experiment
2. ✓
  Second reasonTo heat evenly / a steady, controlled, gradual increase in temperature. (OR: Alcohol/methanol will evaporate too quickly/is volatile.)
  1 mark · Describe and evaluate an experiment
9. Question 5.1.1 · 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.
5.1.1 Is the reaction between Aℓ(s) and dilute $H_{2} SO_{4}$ (aq) ENDOTHERMIC or EXOTHERMIC? Give a reason for the answer by referring to the above diagrams.(2)
Show the memo (2 ticks)Memo
1. ✓
  ExothermicExothermic
  1 mark · Interpret the energy change of reactions
2. ✓
  ReasonLower (potential) energy of the products than reactants. / $Δ H < 0$ / $Δ H$ negative / $Δ H = - 121, 7$ kJ / More energy is released than absorbed.
  1 mark · Interpret the energy change of reactions
10. Question 5.1.2 · November 2024 · Paper 2 · National (DBE) · 6 marks
5.1 The reaction between pure aluminium, Aℓ(s), and EXCESS hydrochloric acid, HCℓ(aq), is used to investigate the factors that affect the rate of a reaction.

The balanced equation for the reaction is:

$2 A ℓ (s) + 6 HC ℓ (aq) \to 2 A ℓ C ℓ_{3} (aq) + 3 H_{2} (g)$

Answer the questions that follow.
EXPERIMENT I

In this experiment, $1$ mol·dm $^{- 3}$ HCℓ solution reacts with a $0, 5$ g Aℓ strip from an aluminium roll at room temperature.

The graph of volume $H_{2}$ (g) versus time for this experiment, not drawn to scale, is shown below.
5.1.2 For the time interval t = 0 to t = 5 minutes, the average reaction rate for the formation of $H_{2}$ (g) is $0, 033$ dm $^{3}$ ·min $^{- 1}$ .

Calculate the mass of Aℓ present in the container at t = 5 minutes.

Take the molar gas volume as $24, 5$ dm $^{3}$ ·mol $^{- 1}$ .(6)
Show the memo (6 ticks)Memo
1. ✓
  (a) Substitute 0,033 and 5 in rate formulaRate $= \frac{Δ V ( H _{2} )}{Δ t}$
  $0, 033 = \frac{Δ V ( H _{2} )}{5}$
  $V (H_{2}) = 0, 165$ dm $^{3}$
  1 mark · Calculate reaction rate
2. ✓
  (b) Substitute 24,5 in $n = \frac{V}{V _{m}}$ $n (H_{2}) = \frac{V}{V _{m}} = \frac{0 , 165}{24 , 5} = 6, 74 \times 1 0^{- 3}$ mol
  1 mark · Calculate number of moles
3. ✓
  (c) Use mol ratio n(Aℓ) : n(H $_{2}$ ) = 2 : 3 $n (A ℓ) = \frac{2}{3} n (H_{2}) = \frac{2}{3} (6, 74 \times 1 0^{- 3}) = 4, 49 \times 1 0^{- 3}$ mol
  1 mark · Use the mole ratio
4. ✓
  (d) Substitute 27 g·mol $^{- 1}$ in $n = \frac{m}{M}$ $n (A ℓ) = \frac{m}{M}$
  $4, 49 \times 1 0^{- 3} = \frac{m ( A ℓ )}{27}$
  $m (A ℓ) = 0, 12$ g (0,121) used
  1 mark · Calculate number of moles
5. ✓
  (e) Subtract m(Aℓ) used from m(Aℓ) initial $m (A ℓ) = 0, 5 - 0, 12$
  1 mark · Identify the limiting reagent
6. ✓
  (f) Final correct answer $m (A ℓ) = 0, 38$ g (0,379 g)
  1 mark · Identify the limiting reagent

Rates of Reaction Grade 12 videos

Short lessons from South African teachers first. Videos load only when you tap play, to save your data.

Energy and Chemical Change grade 11: Activation energy, heat of reaction, catalyst and MORE!Miss Martins Maths and Science

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

Collision Theory grade 12: Part 1Kevinmathscience

Rate and extent of reaction: HOW TO CALCULATE the rate of reaction Grade 12 ChemistryMiss Martins Maths and Science

Rate of Reaction Graphs Grade 12 ChemistryMiss Martins Maths and Science

Maxwell Boltzmann Distribution Curve grade 12Kevinmathscience

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