3.2 Rates of ReactionEdexcel IGCSE Chemistry: Revision notes
Section 1
How do you investigate the factors affecting rate of reaction?
Rate of reaction can be investigated by monitoring how quickly a reactant is used up or a product is formed. Common methods:
- Measuring gas volume produced over time (e.g. marble chips + hydrochloric acid producing CO2, collected in a gas syringe).
- Measuring mass loss over time (e.g. gas escaping from an open flask on a balance).
- Timing a colour change or precipitate forming (e.g. sodium thiosulfate + hydrochloric acid — timing how long it takes for a cross viewed through the solution to disappear as a sulfur precipitate forms).
To investigate a single factor (e.g. concentration), all other variables (temperature, volumes, mass of solid, surface area) must be kept constant.
Section 2
How do surface area, concentration, pressure and temperature affect rate?
| Factor | Effect on rate |
|---|---|
| Increasing surface area (smaller pieces) | Increases rate |
| Increasing concentration | Increases rate |
| Increasing pressure (of a gas) | Increases rate |
| Increasing temperature | Increases rate |
| Adding a catalyst | Increases rate |
Specific investigations:
- Changing the surface area of marble chips (powder reacts faster than large lumps) and concentration of hydrochloric acid — measure the volume of gas produced over time for each condition and compare rates.
- Investigating different solids as catalysts for the decomposition of hydrogen peroxide — compare the rate of oxygen gas production with different solid catalysts (e.g. manganese(IV) oxide) added.
When describing a rate graph, state that a steeper initial gradient means a faster rate, and the reaction finishes when the line becomes horizontal (flat).
Section 3
How does collision theory explain these effects?
Collision theory states that for a reaction to occur, particles must collide with sufficient energy (at least the activation energy) in the correct orientation.
- Surface area: smaller pieces expose more surface area, so more particles are available to collide, increasing collision frequency.
- Concentration/pressure: more particles in a given volume means particles are closer together, increasing collision frequency.
- Temperature: particles move faster and have more kinetic energy, so collisions are both more frequent AND more particles have energy ≥ activation energy, so a greater proportion of collisions are successful.
All of these increase the frequency of successful collisions, increasing the rate of reaction.
For temperature, don't just say 'particles move faster' — you must also say a greater proportion of collisions have energy exceeding the activation energy, which is the key reason the rate increases so much.
Section 4
What is a catalyst and how does it work?
A catalyst is a substance that increases the rate of a reaction but is chemically unchanged at the end of the reaction (it is not used up).
A catalyst works by providing an alternative reaction pathway with a lower activation energy. This means a greater proportion of colliding particles now have enough energy to react, increasing the rate without needing more energy input (e.g. higher temperature).
On a reaction profile diagram, adding a catalyst is shown by a lower activation energy 'hump' between reactants and products, while the overall energy change (ΔH) between reactants and products stays exactly the same.
Must Know
- Rate can be measured by gas volume produced, mass loss, or time for a colour change/precipitate to obscure a mark.
- Increasing surface area, concentration, pressure (of gases) or temperature all increase the rate of reaction; so does adding a catalyst.
- Collision theory: reactions need collisions with energy ≥ activation energy in the correct orientation.
- Increasing surface area/concentration/pressure increases collision frequency; increasing temperature increases both collision frequency and the proportion of successful (sufficiently energetic) collisions.
- A catalyst speeds up a reaction by providing a lower activation energy pathway, without being chemically changed itself.
- On a reaction profile, a catalyst lowers the activation energy hump but does not change ΔH.
That's the notes covered.
Carry on to the next subtopic.