Rate of reaction and collision theory

6 minAS Level8.1.18.1.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can explain what the rate of a reaction means, distinguish effective from non-effective collisions, and explain how changing concentration or pressure changes the rate.

How fast, not how far

Some reactions are over in a flash: a firework, a neutralisation. Others take years: iron rusting, limestone weathering.

The rate of reaction measures how quickly reactants are used up, or products are formed, per unit time. It says nothing about how much product you eventually get. A reaction can be fast and still give a small yield.

Rate is usually measured as a change in concentration per second, so its units are $\text{mol dm}^{-3}\,\text{s}^{-1}$ . If you measure a volume of gas, the rate may be in $\text{cm}^3\,\text{s}^{-1}$ ; if you measure mass loss, in $\text{g s}^{-1}$ .

Definition

Rate of reaction

The change in concentration of a reactant or product per unit time.

Collisions, and why most of them fail

For a reaction to happen, particles must collide. But most collisions do nothing.

A collision is effective only if:

the particles collide with at least the activation energy, and
they are oriented in a way that lets bonds break and form.

A collision that lacks the energy, or has the wrong orientation, is non-effective: the particles bounce apart unchanged.

The frequency of collisions is the number of collisions per second. More collisions per second means more chances of an effective collision, so a faster rate, provided the fraction that are effective stays the same.

Worked example: Squeezing the same reaction

Marble chips react with dilute hydrochloric acid. Explain why using more concentrated acid increases the rate, even though the chips are unchanged.

Have a go first?

Pressure is concentration in disguise

For gases, increasing the pressure squeezes the same number of particles into a smaller volume. That is exactly the same as increasing the concentration: more particles per unit volume.

So the explanation is identical. Higher pressure means gas particles are closer together, collide more frequently, and produce more effective collisions per second, so the rate increases.

Pressure only matters for reactions involving gases. Squeezing a solid or a liquid changes their volume hardly at all.

Common mistakes

Every collision between reactant particles causes a reaction.

Only effective collisions do. Most collisions do not have enough energy, or the particles are badly oriented, so nothing happens.

Increasing concentration increases the activation energy.

Concentration changes how often particles collide, not how much energy each collision needs. The activation energy is a property of the reaction.

Increasing pressure always increases the rate.

It increases the rate of reactions involving gases, because it increases their concentration. A reaction between two solids or two liquids is essentially unaffected.

A faster reaction always gives more product.

In the exam

Define rate precisely: change in concentration per unit time. "How fast a reaction goes" is too vague for a definition mark.
When explaining concentration or pressure, use the chain: more particles per unit volume, so more frequent collisions, so more effective collisions per second, so a faster rate.
Say effective collisions, not just collisions. Examiners specifically reward the distinction.
For pressure, state that it applies to gases and that it works by changing the concentration.
Do not say the particles "move faster" when you increase concentration. Temperature does that. Concentration changes how crowded the particles are.

Key takeaways

Rate is the change in concentration of a reactant or product per unit time.
Collisions must have at least the activation energy and a suitable orientation to be effective.
Most collisions are non-effective.
Higher concentration means more particles per unit volume, more frequent collisions and a faster rate.
For gases, higher pressure has the same effect as higher concentration.

Definitions

Multiple choiceQuestion 1 · Recall

DefinitionQuestion 2 · Recall

Multiple choiceQuestion 3 · Recall

Answer the 3 questions above to continue.