5.1.3Monitoring and Controlling Chemical Reactions
Change Rates — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 4 of 7
Section 1 of 7Collision theory
Particles must collide
- Particles must collide A reaction can only happen when reactant particles collide with each other.
- Most collisions do nothing. The particles simply bounce apart.
Activation energy
- Activation energy The activation energy is the minimum energy colliding particles need for a reaction to occur.
- Every reaction has its own value.
- A collision that has at least this energy is a successful collision.
Rate depends on two things
- Rate depends on two things The rate depends on how often particles collide and on how many of those collisions have enough energy.
- To speed up a reaction, increase the number of successful collisions per second.
Section 2 of 7The four factors and their explanations
| Factor | Change | Effect on rate | Explanation |
|---|---|---|---|
| Concentration (solutions) | Increase | Faster | More particles in the same volume, so more frequent collisions |
| Pressure (gases) | Increase | Faster | Same number of particles squeezed into a smaller space, so more frequent collisions |
| Temperature | Increase | Faster | Particles move faster and have more energy, so collisions are more frequent and a greater proportion exceed the activation energy |
| Surface area of a solid | Increase (smaller pieces) | Faster | More of the solid is exposed to the other reactant, so more collisions per second |
Section 3 of 7Temperature, concentration and surface area in more detail
1Raise the temperature
- Raise the temperature Particles gain kinetic energy and move faster.
2More collisions
- More collisions Fast particles meet each other more often every second.
3More energetic collisions
- More energetic collisions A larger share of collisions now reach or exceed the activation energy.
4Rate increases strongly
- Rate increases strongly A small rise in temperature gives a large rise in rate.
- As a rough rule, a 10 K rise roughly doubles the rate.
Section 4 of 7Surface area and particle size
Surface area
Only the outside reacts
- Only the outside reacts In a solid, only particles at the surface can collide with the other reactant.
- Particles inside a lump are protected.
Cutting up a cube
- Cutting up a cube A cube of side 2 cm has a surface area of 6 × 2 × 2 = 24 cm2.
- Cutting it into eight 1 cm cubes gives 8 × 6 = 48 cm2, which doubles the surface area.
- Smaller pieces and powders have a larger surface area to volume ratio.
Proportionality
- Proportionality Doubling the surface area doubles the number of collisions per second, as more of the solid is available to react.
Section 5 of 7Reading rate graphs
1
Plot the amount of product (or reactant remaining) against time for each set of conditions.
2
The steeper the line at the start, the faster the reaction. The gradient at any point equals the rate at that moment.
3
A faster reaction levels off sooner, because the limiting reactant is used up in less time.
4
If only the rate is changed and the amounts of reactant are the same, every curve levels off at the same final height. The same total product is made.
5
To compare two conditions, draw a tangent on each curve at the same time, such as t = 0, and compare gradients.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Collision theory | The idea that reactions happen only when particles collide with enough energy. |
| Activation energy | The minimum energy that colliding particles must have in order to react. |
| Successful collision | A collision in which the particles have at least the activation energy and so react. |
| Surface area to volume ratio | The surface area of a solid divided by its volume; larger for small pieces than for big lumps. |
| Gradient | The steepness of a graph line, found by change in y divided by change in x. |
Section 7 of 7Exam technique
1
Saying that a higher temperature only makes particles collide more. It also means more collisions have enough energy to react.
2
Stating that smaller pieces have a smaller surface area to volume ratio. The ratio is larger for smaller pieces.
3
Thinking that a faster reaction produces more product. The final amount is set by the reactants, not the rate.
4
Forgetting that increasing the pressure only affects reactions that involve gases.
5
Writing 'the particles collide more' without 'more often' or 'more frequently', which is the idea examiners need.
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