How fast is the reaction?

The rate of reaction tells you how quickly reactants are used up or products are made. To measure it you must track a quantity that changes over time:

• Volume of gas made — collect it in a gas syringe (e.g. hydrochloric acid + marble chips).
• Loss of mass — stand the flask on a balance as a gas escapes.
• Colour change / turbidity — time how long a cross under the flask takes to disappear (sodium thiosulfate + hydrochloric acid).

Edexcel Core Practical: investigate how changing the conditions changes the rate, by (a) measuring the gas produced from acid + marble chips, and (b) observing the colour change / cloudiness from sodium thiosulfate + acid.

Pick the method

Mean rate of reaction

The mean (average) rate over a period of time is simply the amount of reactant used or product made, divided by the time taken:

Units depend on what you measured — for example cm³/s (gas volume), g/s (mass) or mol/s.

Your turn — mean rate

Reading a rate graph

Plot the volume of gas (or mass, or concentration) against time. The curve is steepest at the start — that's when the reaction is fastest — then it levels off when a reactant runs out.

Watch out: a steeper line means a faster reaction — but a steeper graph does not mean more product overall. Two reactions can reach the same final volume; the faster one just gets there sooner and its curve flattens earlier.

Your turn — rate from a graph

Why reactions happen

Particles must collide to react — but not every collision works. For a reaction the particles must collide:

• often enough — a higher frequency of collisions, and
• hard enough — with at least the activation energy, the minimum energy needed to react.

So a reaction speeds up whenever you increase the frequency and/or the energy of collisions.

What changes the rate

Edexcel wants you to explain each factor in terms of collisions:

• Temperature ↑ — particles move faster, so collisions are more frequent and more energetic (more reach the activation energy).
• Concentration ↑ — more particles in the same volume → collisions more frequent.
• Pressure ↑ (gases) — squeezes particles closer → collisions more frequent.
• Surface area : volume ratio ↑ (smaller pieces / powder) — more particles exposed → collisions more frequent.
• Catalyst — provides a lower-energy pathway (next screen).

Watch out: raising the temperature is special — it increases BOTH the frequency and the energy of collisions. Concentration, pressure and surface area only change the frequency.

Will the rate increase?

Tap the change that would make the reaction go faster.

A faster, lower-energy route

A catalyst speeds up a reaction without being used up — it is chemically unchanged and has the same mass at the end, and it does not change the products.

It works by giving the reaction an alternative pathway with a lower activation energy, so a larger fraction of collisions now has enough energy to react.

Watch out: a catalyst lowers the activation energy and is not used up. It does not change how much product forms, and it does not change the overall energy released or taken in.

What a catalyst does

Exothermic and endothermic

Chemical reactions transfer heat energy to or from the surroundings:

• Exothermic — heat is given out, so the surroundings warm up (temperature rises). E.g. neutralisation, displacement, most combustion.
• Endothermic — heat is taken in, so the surroundings cool down (temperature falls). E.g. some salts dissolving, thermal decomposition.

Suggested practical: measure the temperature change for salts dissolving, neutralisation, displacement and precipitation reactions in solution to classify each as exothermic or endothermic.

Watch out: "exothermic" means energy exits to the surroundings → the thermometer reading goes up. Endothermic takes energy in → the reading goes down.

Exothermic or endothermic?

Tap a change, then tap the box it belongs in.

Drawing the energy story

A reaction profile plots energy against the progress of the reaction. The hump is the activation energy (Ea) — the minimum energy to start reacting. The difference between reactants and products is the overall energy change.

Reading a profile

Energy from breaking & making bonds

Every reaction first breaks bonds in the reactants, then makes new bonds in the products:

• Breaking bonds is endothermic — it takes in energy.
• Making bonds is exothermic — it releases energy.

The overall energy change is found from bond energies (in kJ/mol):

Your turn — bond-energy ΔH

Enzymes

Enzymes are biological catalysts — protein molecules made by living things that speed up reactions in the same way: by lowering the activation energy.

Edexcel asks you to recall that enzymes are used in the production of alcoholic drinks — yeast enzymes catalyse fermentation, turning sugar (glucose) into ethanol and carbon dioxide.

Remember: like any catalyst, an enzyme is not used up and gives a lower-activation-energy pathway — it just does it inside living systems.

Catalysts in living things

The key ideas to know

You've covered the whole of Edexcel Topic 7 — rates of reaction, collision theory, catalysts and energy changes. Press Finish to see your score.