Rates of reaction
1.8 · Chemical change
Rates of reaction
By the end of this section:
- I can find the rate of a simple acid-base reaction
- I can name and describe the factors that change the rate of a reaction: concentration, temperature and surface area
- I can use a cause-and-effect model to make a detailed prediction that can be tested
- I can draw a diagram of an experiment that shows the set-up, the equipment and the key variables
- I can calculate a rate of reaction from the change in volume per unit of time
- I can explain why the average of several measurements is more accurate than one
- I can identify the independent and dependent variables and state their values
- I can say what a ‘fair test’ is
- I can identify the control variables in an experiment and say how to keep them the same
- I can spot hazards in a lab and suggest precautions that reduce the risk (a risk assessment)
- I can collect data and record it by hand in a table
- I can use software (Sheets, Excel, etc.) to organise data in columns, work out averages and present it as a table
- I can plot a scatter graph by hand showing how a dependent variable changes with an independent variable
- I can use software (Sheets, LoggerPro, Excel, etc.) to plot a scatter graph showing how a dependent variable changes with an independent variable
- I can spot and describe trends in data: increasing, decreasing, curved, linear and so on
- I can explain trends in data
1 · Read
The notes, the key idea and the words to knowThe rate of a reaction is how fast it happens — defined as the change in concentration of a reactant or product over time. A faster reaction uses up reactants and makes products more quickly.
Collision theory explains rate. For a reaction to occur, particles must collide, and the collision must be successful — the particles need enough energy to react. Anything that makes successful collisions happen more often speeds the reaction up.
Reading a rate graph: a steeper line means a faster rate. The slope is steepest at the start, when concentration is highest, and the line levels off once a reactant runs out and the reaction stops.
The main factors are concentration, temperature, surface area and catalysts.
- Higher concentration: more particles in the same space → more frequent collisions.
- Higher temperature: particles move faster and carry more energy → more collisions, and more of them successful.
- Larger surface area (e.g. a powder): more particles exposed at the surface → more collisions per second.
- A catalyst: provides an easier path needing less energy → more collisions succeed. It is not used up.
Reactant A (solid) Reactant B Product
- Collisions
- 0
- Successful
- 0
- Product made
- 0
A model: real reactions involve far more particles.
Why each factor works — all through collisionsTap to open
Every factor changes the rate for the same underlying reason: it changes how often successful collisions happen.
This also explains the shape of the graph. At the start, reactant concentration is highest, so collisions are most frequent and the line is steepest. As reactants are consumed, collisions become less frequent, the line flattens, and when a reactant runs out the reaction stops.
Why is the reaction fastest at the very start? At the start the reactant concentration is at its maximum, so particles collide most often. The most successful collisions per second means the fastest rate — which is why the graph is steepest there and then gradually levels off.
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Not thisA catalyst is used up in the reaction.
ThisNo — a catalyst speeds the reaction up but is recovered unchanged at the end, so a little goes a long way.
-
Not thisAny collision causes a reaction.
ThisOnly successful collisions do — the particles must have enough energy. Most collisions simply bounce off.
-
Not thisA flat line on the graph means the reaction is fast.
ThisA flat line means the reaction has slowed or stopped. A steep line means it is fast.
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Not thisMore concentration means bigger particles.
ThisConcentration is about how many particles are packed into a space, not their size — more particles means more frequent collisions.
- rate of reaction
- the change in concentration of a reactant or product over time
- collision theory
- particles must collide with enough energy (a successful collision) to react
- successful collision
- a collision with enough energy for a reaction to occur
- catalyst
- a substance that speeds up a reaction without being used up
- surface area
- the amount of a solid’s surface exposed to react; greater for a powder than a lump
2 · Watch
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Videos in other languagesDeutsch · Português · 中文
We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.
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- Booklet pp. 35–38
- Knowledge Book pp. 31–33
- In your class workbook: Chapter 1.10 Rates of reaction & the investigation (pp. 53–57) · Revise this section
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