CatalystsAQA GCSE Chemistry: Revision notes
Section 1
How Do We Measure the Rate of a Reaction?
The mean rate of reaction can be calculated from how much reactant is used up, or how much product is formed, in a given time:
mean rate of reaction = quantity of reactant used (or product formed) ÷ time taken
This can be measured, for example, in g/s or cm³/s depending on what is being tracked.
Section 2
How Do We Read Rate From a Graph?
Rate of reaction can be shown as a graph of the quantity of product formed (or reactant used up) against time.
- A steeper slope means a faster rate
- The curve typically flattens out as the reaction slows down and eventually stops (when a reactant is used up)
- A tangent drawn to the curve at a particular point, and its slope calculated, gives the rate of reaction at that specific moment in time
To read a rate from a graph, always look at the steepness of the line, not just how high it has reached.
Section 3
What Factors Change Reaction Rate?
The rate of a chemical reaction is affected by:
- Concentration of reactants in solution
- Pressure of reacting gases
- Surface area of solid reactants
- Temperature
- The presence of a catalyst
Using collision theory: increasing concentration, pressure or surface area increases the frequency of collisions between particles, so the rate increases. Increasing temperature increases both the frequency and the energy of collisions, so it has an even bigger effect on rate.
Section 4
What Is a Catalyst and How Does It Speed Up a Reaction?
A catalyst is a substance that changes the rate of a chemical reaction without being used up itself in the reaction.
- Different reactions need different catalysts
- Enzymes are biological catalysts (found in living organisms)
- Catalysts increase reaction rate by providing an alternative reaction pathway with a lower activation energy — more particles then have enough energy to react when they collide
- Because a catalyst is not used up, it does not appear in the chemical equation for the reaction, and this is one way to identify it from a description of an experiment
A catalyst is like a mountain pass through a range of hills — it doesn't remove the need to climb, but it offers a much lower route over the barrier (activation energy), so more walkers (particles) can get across.
Don't say a catalyst 'lowers the energy of the reactants or products' — it lowers the activation energy barrier between them, not the energy levels themselves.
Section 5
How Do We Spot a Catalyst in an Experiment?
You can identify that a substance is acting as a catalyst if:
- The reaction happens faster when it is present, compared with when it is absent (under otherwise identical conditions)
- Its mass and chemical identity are unchanged at the end of the reaction
- It does not appear as a reactant or product in the balanced chemical equation
In the decomposition of hydrogen peroxide, manganese(IV) oxide speeds up the reaction but is recovered unchanged at the end — showing it is a catalyst, not a reactant.
Must Know
- Mean rate of reaction = quantity of reactant used (or product formed) ÷ time taken
- A steeper graph slope, or a tangent's gradient at a point, shows the rate of reaction
- Rate is increased by higher concentration, pressure, surface area, temperature, or by a catalyst
- Collision theory: more frequent and/or more energetic collisions mean a faster rate
- A catalyst speeds up a reaction without being used up, by providing a lower activation energy pathway
- Enzymes are biological catalysts; catalysts never appear in the reaction's balanced equation
That's the notes covered.
Carry on to the next subtopic.