Rate of reaction is one of the most predictable topics in O-Level Combined Science (Chemistry). Almost every year there is a graph of gas volume or mass loss against time, followed by a sketch, a calculation and an explanation. The good news is that the marks follow a pattern. Once you know the pattern, this becomes one of the easiest sections to secure full marks in.
What the graph is actually telling you
Most rate graphs plot one of these on the y-axis:
- Volume of gas produced (cm³)
- Mass of the flask (g), which decreases as gas escapes
- Mass of product formed (g)
Two features carry almost all the marks:
- Steepness (gradient) tells you the rate. Steeper curve means faster reaction.
- The plateau (horizontal section) tells you the reaction has stopped because one reactant has been completely used up. The height of the plateau tells you the total amount of product formed.
That is the whole idea. Steepness equals speed, plateau height equals quantity. Keep those two separate in your head, because examiners deliberately test whether you can tell them apart.
The three-step method for every graph question
Step 1: Read the axes and the conditions
Check the units, then check what changed between Experiment 1 and Experiment 2. It is usually one of: concentration, temperature, particle size or surface area, a catalyst, or the amount of reactant used.
Step 2: Decide if the plateau moves
Ask yourself: has the number of moles of the limiting reactant changed?
- Higher temperature, catalyst, smaller lumps, or the same moles in a more concentrated solution but smaller volume: the reaction is faster, but the final volume of gas stays the same.
- More reactant used, or a more concentrated acid where the acid is limiting and the volume is unchanged: faster and a higher plateau.
This single check is where most marks are lost in Paper 2.
Step 3: Explain using collision theory in the right order
Use this structure and you will hit the mark points:
More particles per unit volume, so more frequent collisions, so more effective collisions per second, so the rate increases.
For temperature, add energy:
The particles gain more kinetic energy and move faster, so collisions are more frequent and a greater proportion of collisions have energy equal to or greater than the activation energy.
For surface area:
Smaller pieces have a larger total surface area, so more particles of the solid are exposed to the acid, giving more frequent effective collisions.
For a catalyst:
The catalyst provides an alternative pathway of lower activation energy, so a greater proportion of collisions are successful.
Avoid vague phrases like "the particles react more" or "there is more energy". Examiners want frequency of collisions and activation energy.
Worked example
Excess magnesium ribbon is added to 50 cm³ of 0.5 mol/dm³ hydrochloric acid. The volume of hydrogen collected is 60 cm³ after 100 s, and the curve flattens.
(a) Calculate the average rate of reaction in the first 40 s, given 36 cm³ was collected.
Average rate = change in volume ÷ time = 36 ÷ 40 = 0.9 cm³/s. Always write the unit.
(b) Why does the curve become less steep after 40 s?
As the reaction proceeds, the concentration of hydrochloric acid decreases, so there are fewer acid particles per unit volume, so collisions are less frequent and the rate decreases.
(c) Sketch the curve if the experiment is repeated with 50 cm³ of 1.0 mol/dm³ acid, magnesium still in excess.
The acid is limiting and you have doubled the moles of acid. So the new curve is steeper at the start and plateaus at 120 cm³, double the original.
Sketching rules that examiners look for
- Start at the origin. A curve that begins above zero loses a mark.
- The steepest part is always at t = 0, then it curves and flattens. Never draw a straight diagonal line.
- If the plateau is the same, make your line clearly join or run alongside the original at the top.
- Label your curve, for example "higher temperature".
Common mistakes to avoid
- Saying a catalyst produces more product. It does not, it only speeds things up.
- Writing "more collisions" without "per second" or "more frequent".
- Forgetting that temperature increases rate through both frequency and activation energy.
- Mixing up "the reaction stops because the acid is used up" when the acid is actually in excess. Read the question for the word "excess".
Practise by taking any past-year rate graph and covering the answers, then writing your explanation in exactly three linked sentences. Speed and precision come from repetition.
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