Some reactions are over in a flash, others take years. Five factors control the speed: concentration, pressure, surface area, temperature and catalysts. Measure gas given off or mass lost against time and you can see each one at work on a graph.
The rate of a reaction increases when the concentration of a solution or the pressure of a gas increases, when a solid is broken into smaller pieces (larger surface area), when the temperature increases, and when a catalyst is added. A catalyst increases the rate of a reaction and is unchanged at the end. Rate is measured by following a product or reactant against time, for example the volume of gas given off or the loss in mass.
Run the magnesium and acid experiment in the simulator. Record the volume of hydrogen every 10 seconds and watch the graph build up.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which change increases the rate of reaction between marble chips and hydrochloric acid?
A catalyst is a substance that:
On a graph of gas volume against time, how can you tell when the reaction has finished?
Which apparatus is used to measure the volume of gas produced in a reaction?
Change one factor at a time in the simulator, keeping everything else the same, and compare the curves.
| Change | Effect on rate | Example |
|---|---|---|
| increase concentration of a solution | faster | 2 mol/dm3 acid reacts faster than 1 mol/dm3 |
| increase pressure of a gas | faster | gas reactions in industry run at high pressure |
| increase surface area (smaller pieces) | faster | powdered marble fizzes faster than large chips |
| increase temperature | faster | food lasts longer in a fridge |
| add a catalyst | faster; catalyst unchanged at the end | manganese(IV) oxide with hydrogen peroxide |
Changing the rate does not change the amount of product if the same amounts of reactants are used. On the graph, a faster reaction is steeper at the start but levels off at the same final volume. Only changing the amount of the limiting reactant changes the final volume.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Magnesium reacts with excess hydrochloric acid. The experiment is repeated with the same mass of magnesium but at a higher temperature. How does the graph of hydrogen volume against time change?
A reaction produces 45 cm3 of gas in the first 15 s. What is the mean rate over this time?
Calcium carbonate reacts with acid in a flask on a balance. Why is a cotton wool plug put in the neck of the flask?
Hydrogen peroxide decomposes slowly: 2H2O2 → 2H2O + O2. Adding manganese(IV) oxide makes it fizz rapidly, and the same mass of manganese(IV) oxide is recovered afterwards. What does this show?
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