How do I find the rate at a specific time from a graph?

Draw a tangent to the curve at that time and calculate its gradient.

Why does rate decrease as a reaction proceeds?

Reactant concentration falls, so collisions become less frequent.

ChemistryGCSE, IGCSE & A Level

What is rate of reaction?

The rate of reaction is how quickly reactants are used up or products are formed, usually measured as a change in amount per unit time.

Rate of reaction explained

Collision theory explains every rate factor: reactions speed up when particles collide more frequently or with more energy than the activation energy.

Concentration, pressure, surface area, temperature and catalysts are the standard factors, and each needs an explanation in collision terms for full marks.

Key formula

\text{rate} = \dfrac{\Delta\text{amount}}{\Delta\text{time}}

Worked example

Collecting

48\,\text{cm}^{3}

of gas in

30\,\text{s}

gives a mean rate of

1.6\,\text{cm}^{3}\,\text{s}^{-1}

Examiner tip

Temperature increases rate mainly because more particles exceed the activation energy, not just because they move faster — state both, lead with the first.

Rate of reaction: common questions

How do I find the rate at a specific time from a graph?: Draw a tangent to the curve at that time and calculate its gradient.
Why does rate decrease as a reaction proceeds?: Reactant concentration falls, so collisions become less frequent.

Related terms

Catalyst

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, without being used up itself.

Dynamic equilibrium

A dynamic equilibrium is reached in a closed system when the forward and reverse reactions occur at the same rate, so concentrations stay constant.

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