Chemical Reactions and Reaction Rates
This domain tests how fast reactions go and why. You must explain rates using collision theory, interpret rate graphs and describe how concentration, temperature, surface area and catalysts change the rate without changing the final yield.
Collision theory
Particles react only when they collide with enough energy and in the correct orientation. The minimum energy needed is the activation energy. Anything that increases the frequency of successful collisions increases the rate.
Factors affecting rate
- Concentration or pressure: more particles in a given volume means more frequent collisions.
- Temperature: particles move faster, so collisions are more frequent and more of them exceed the activation energy.
- Surface area: a powder exposes more solid to the reactant than large lumps, so collisions are more frequent.
- Catalyst: provides an alternative route with lower activation energy. It is not used up and does not change the final amount of product.
Worked example
Equal masses of marble chips and marble powder are added to the same volume and concentration of hydrochloric acid at the same temperature. Explain which reacts faster and whether the final gas volume differs. The powder reacts faster because it has a much larger surface area. More acid particles can collide with the calcium carbonate at the same time, so the rate of successful collisions is higher. The final volume of carbon dioxide is the same because it depends on the number of moles of calcium carbonate, and equal masses contain equal moles. On a graph of gas volume against time, the powder curve is steeper but levels off at the same height.
Traps that cost marks
- Saying a catalyst increases the yield; it only changes the rate.
- Saying temperature increases rate only because there are more collisions; more collisions also exceed the activation energy.
- Confusing rate with yield.
- Forgetting that surface area affects rate but not the final amount of product.