Measuring the rate of a reaction

The rate of a reaction is how quickly reactants are used up or products are made. To measure it we follow something that changes as the reaction goes — Eduqas wants you to be able to suggest practical methods:

• Gas collection — measure the volume of gas given off over time (gas syringe), e.g. a metal + acid making hydrogen.
• Loss of mass — stand the flask on a balance; as gas escapes, the mass falls.
• Colour change / turbidity (precipitation) — time how long until a cross under the flask disappears as a precipitate clouds the mixture.

Data-logging apparatus (e.g. a pressure or mass sensor) lets you record many readings automatically.

Mean rate of reaction

The mean (average) rate over a period of time is simply the amount that changed divided by the time it took:

Bigger number = faster reaction. A reaction that makes 48 cm³ of gas in 60 s has a mean rate of 0.8 cm³/s.

Your turn — mean rate

Rate–time graphs & the tangent Higher

Plotting gas volume against time gives a curve. It is steepest at the start (fastest rate, most reactant) and flattens off when a reactant runs out and the reaction stops.

Watch out: a graph that finishes higher up just made more product. It is the steepness (gradient) — not the final height — that tells you how fast the reaction went.

Reading the graph

Five things that change the rate

Eduqas expects you to describe how each of these changes the rate of a reaction:

• Concentration (and, for gases, pressure Higher) — more crowded particles.
• Surface area — smaller pieces of a solid expose more surface.
• Temperature — hotter particles move faster and carry more energy.
• Catalysts — speed things up without being used up.

Why the factors work

Particles can only react when they collide with enough energy. That minimum energy is the activation energy (E_a). A faster rate means either more frequent collisions, or collisions with more energy, or both.

• Concentration / pressure ↑ → particles closer together → collisions are more frequent.
• Surface area ↑ → more solid surface exposed → more frequent collisions.
• Temperature ↑ → particles move faster (more frequent collisions) and have more energy (more collisions exceed E_a) Higher.

Heating it up

Catalysts speed things up

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy. Crucially, a catalyst is not used up — it is chemically unchanged at the end, so a tiny amount can be used over and over.

• It does not appear in the overall equation and does not change the amount of product.
• Different catalysts work for different reactions (you should be able to identify a catalyst in a reaction).
• Enzymes are biological catalysts — proteins that speed up reactions in living things.

Watch out: a catalyst lowers the activation energy and is not consumed — it does not make the products more energetic, and it does not shift how much product forms.

What does a catalyst do?

Investigating rate in the lab

Topic 9 has three specified practicals. Each changes one factor while keeping the others constant:

• SP9A — effect of one factor on rate using a gas collection method (e.g. acid + a metal or carbonate, collecting gas in a syringe).
• SP9B — the "disappearing cross" reaction of dilute hydrochloric acid + sodium thiosulfate; time how long until a precipitate of sulfur clouds the mixture (turbidity).
• SP9C — effect of various catalysts on the decomposition of hydrogen peroxide (e.g. manganese(IV) oxide), measuring oxygen given off.

In a non-enclosed system, mass falls as gas escapes — explain this with the particle model: gas particles leave the flask, so the mass on the balance drops.

Will this speed it up?

Tap whether each change makes the reaction go faster or slower.

Reactions that go both ways

Some reactions are reversible — the products can react to re-form the reactants. We show this with the ⇌ symbol instead of a one-way arrow:

The energy change is equal but opposite in the two directions. If the forward reaction is exothermic (gives out heat), the reverse is endothermic (takes in the same amount of heat) — for example, hydrated and anhydrous copper(II) sulfate.

Eduqas: some reactions may be reversed by altering the reaction conditions (such as temperature).

A balanced, busy state

In a closed system (nothing in or out), a reversible reaction reaches dynamic equilibrium: the rate of the forward reaction equals the rate of the reverse reaction, so the amounts of reactants and products stay constant.

Watch out: "dynamic" means the reactions have not stopped — they continue in both directions at equal rates. The amounts are constant, not zero.

What is dynamic equilibrium?

Changing the conditions Higher

If you disturb a system at equilibrium, the position of equilibrium shifts to oppose the change (Le Chatelier's idea). You should be able to predict the effect of changing concentration, temperature and pressure:

• Concentration: add more of a substance → equilibrium shifts to the other side to use it up.
• Temperature: heat → shifts in the endothermic direction (to absorb the heat); cool → shifts the exothermic way.
• Pressure (gases): increase pressure → shifts towards the side with fewer gas molecules.

Industrial link: in the Haber process (N₂ + 3H₂ ⇌ 2NH₃, studied in Topic 11), conditions are chosen to push the equilibrium towards ammonia and balance rate against yield. The forward reaction is exothermic, so a lower temperature favours more ammonia — but too low and the rate is too slow.

Watch out: Le Chatelier always opposes the change you make — add a reactant and the system removes some of it; heat an exothermic equilibrium and it shifts back the other way.

Your turn — shifting equilibrium

Which way does it shift?

Tap a change on the left, then the matching shift on the right. (Reaction is exothermic forward, fewer gas molecules on the product side.)

The key ideas to know

You've covered the whole of Eduqas Topic 9 — rate, the factors and their collision-theory explanations, catalysts and the required practicals, and reversible reactions reaching dynamic equilibrium. Press Finish to see your score.