Edexcel IGCSE Chemistry · Spec 3.15-3.16
Investigating Rates of Reaction
Required practical: investigating how conditions affect the rate of a reaction.
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Investigating Rates of Reaction
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Explained
Measuring a rate, and reading the graph
To measure a rate you follow something that changes as the reaction proceeds, and record it against time. Which quantity you follow depends on what the reaction actually does.
Three methods
Collecting a gas. If a gas is produced, collect it in a gas syringe or an inverted measuring cylinder over water, and record the volume at fixed intervals. This is the most precise of the three and works for magnesium with hydrochloric acid.
Loss of mass. Stand the flask on a balance with a cotton wool plug, and record the mass at intervals as the gas escapes. Better for carbon dioxide, which is dense enough to weigh usefully, and no good for hydrogen, which is too light to register.
The disappearing cross. For a reaction that turns cloudy, such as sodium thiosulfate with hydrochloric acid, draw a cross on paper under the flask and time how long it takes to disappear from view.
That last one gives a single time rather than a curve, so the rate is taken as one divided by the time. It is also subjective, because different people judge the disappearance differently, and saying so is the standard evaluation point. Using a light sensor and a data logger removes that judgement.
Changing one condition
Whichever method you choose, only one factor changes: concentration, temperature, surface area or the presence of a catalyst.
Everything else stays the same, including the volume and concentration of the other reactant, the mass and form of the solid, the total volume of solution and the temperature. Keeping the total volume constant when you dilute an acid means adding water to make up the difference, and that step is a mark of its own.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for a gas volume graph awards two marks with a dependency between them: one for a smooth curve starting at the origin, and one for it levelling off horizontally at a constant volume, with the second available only if the line rises first.
Both features carry meaning. The curve starts at the origin because no gas has been produced at time zero. It levels off because a reactant has been used up and the reaction has stopped, not because it has slowed down.
On measuring a rate from such a graph, another mark scheme is blunt. Where the question says to draw a tangent, not drawing one loses two marks straight away. The same note records a response that found the gradient correctly but wrote the unit as g/m instead of g/min, and lost a mark for it.
The steepest part of the curve is at the start, because the reactants are at their most concentrated then, so collisions are most frequent. The gradient falls as they are used up.
Comparing two runs on one graph
A faster reaction gives a steeper initial gradient and reaches its final level sooner.
The height it levels off at depends only on how much reactant there was, not on how fast it reacted. So two curves that finish at the same height used the same amount of reactant, and a catalyst or a higher temperature changes the steepness but not the final level.
If one curve levels off lower than another, something about the quantity changed, not just the conditions. That distinction, between the shape and the endpoint, answers most graph comparison questions on its own.
Spec 3.15-3.16
What you need to know
- Describe ways to measure the rate of a reaction
- Describe a rate experiment
- Interpret rate graphs
Active recall
Quick check
Answer each question before opening the answer.
Give two ways of measuring the rate of a reaction.
Measure the volume of gas produced over time, or time how long a cross/precipitate takes to become obscured.
On a rate graph, what does a steeper line show?
A faster rate of reaction.
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