Rates of ReactionEdexcel GCSE Chemistry: Revision notes
Section 1
How can the rate of a reaction be measured?
The rate of a reaction describes how quickly reactants are used up or products are formed. Practical methods to determine rate include:
- Measuring the volume of gas produced over time (e.g. using a gas syringe) — used for the reaction between hydrochloric acid and marble chips (calcium carbonate), which produces CO2
- Measuring the mass lost over time (e.g. on a balance, as gas escapes)
- Observing a colour change — used for the reaction between sodium thiosulfate and hydrochloric acid, which produces a cloudy precipitate of sulfur; the reaction can be timed by watching a mark placed under the flask disappear from view as the solution becomes cloudy
These two reactions are the specified Core Practical methods for investigating how changing conditions affects rate.
For the sodium thiosulfate practical, state clearly that a fixed cross/mark is viewed from above through the solution, and the time taken for it to disappear is measured — the shorter the time, the faster the rate.
Section 2
Why do reactions happen, and what is collision theory?
Reactions occur when reactant particles collide with each other with sufficient energy (at least the activation energy). This is collision theory.
The rate of reaction increases when the frequency of collisions and/or the energy of collisions between particles increases.
Section 3
How do temperature, concentration, surface area and pressure affect rate?
| Factor | Effect on rate | Explanation |
|---|---|---|
| Increasing temperature | Increases rate | Particles move faster, so collide more frequently AND with more energy |
| Increasing concentration (solutions) | Increases rate | More particles in the same volume, so collisions occur more frequently |
| Increasing pressure (gases) | Increases rate | Particles are closer together, so collisions occur more frequently |
| Increasing surface area to volume ratio (breaking a solid into smaller pieces) | Increases rate | More particles are exposed at the surface, so collisions occur more frequently |
Interpreting graphs: on a graph of mass, volume or concentration against time, a steeper initial gradient means a faster rate; the graph levels off (flattens) once the reaction has finished (a reactant is used up).
Only temperature increases BOTH the frequency and the energy of collisions — concentration, pressure and surface area only increase collision frequency.
Section 4
How do catalysts speed up reactions?
A catalyst is a substance that speeds up the rate of a reaction without being used up or altering the products — it is chemically unchanged and has the same mass at the end of the reaction.
A catalyst works by providing an alternative reaction pathway with a lower activation energy. Because more particles have enough energy to react at this lower activation energy, more successful collisions occur per second, increasing the rate.
Enzymes are biological catalysts — for example, enzymes in yeast are used in the production of alcoholic drinks (fermentation).
Explain catalysts in terms of activation energy: 'provides an alternative pathway with a lower activation energy', not just 'speeds things up'.
Must Know
- Rate can be measured by volume of gas produced (marble chips + HCl) or timing a colour change/precipitate appearing (sodium thiosulfate + HCl)
- Reactions happen when particles collide with enough energy (collision theory)
- Temperature increases both the frequency and energy of collisions; concentration, pressure and surface area increase only the frequency of collisions
- On a rate graph, a steeper gradient = faster rate; the graph flattens when the reaction ends
- A catalyst speeds up a reaction by providing a pathway with lower activation energy, without being used up or changed
- Enzymes are biological catalysts, e.g. used by yeast to make alcoholic drinks
That's the notes covered.
Carry on to the next subtopic.