Rate of Reaction

Particles react only when they collide with at least the activation energy. Rate increases with concentration (or gas pressure) and surface area, because collisions become more frequent; with temperature, because particles collide more often and more collisions have enough energy; and with a catalyst, which provides a route with lower activation energy. Rate is the gradient of a product-against-time graph.

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Section 1 of 14

Learning objectives

State and explain the factors that affect rate.
Use collision theory.
Describe experiments to measure rate and interpret the graphs.

Pause and think

Why do we keep food in a fridge, and why does powdered sugar dissolve faster than a sugar lump?

Section 2 of 14

Core idea

Start hereExplosions and rust

An explosion is over in milliseconds; rusting takes years. Both are chemical reactions. What controls how fast particles react?

The big idea

Reactions happen when particles collide with at least the activation energy. Anything that makes successful collisions more frequent speeds up the reaction.

1. Collision theory

How each factor works

Drawing diagram…

Temperature acts in two ways; a catalyst lowers the barrier.

2. Measuring rate

Worked exampleRate from a tangent

On a gas-volume graph, the tangent at $t=20$ s passes through $(0,\ 10)$ and $(40,\ 70)$ ( $\mathrm{cm^3}$ ).

Goal: Rate at $20$ s.
Strategy: Gradient of the tangent.

$\frac{70-10}{40-0}$
Rise over run.
$=1.5\ \mathrm{cm^3/s}$
Evaluate.

Answer

1.5\ \mathrm{cm^3/s}

✓ CheckLower than the initial rate, as reactants are used up ✓.

Exam tipPrecise language Write "more frequent successful collisions" or "more collisions per second", not just "more collisions".

Key takeawayEnzymes Enzymes are biological catalysts; above about

40\,^\circ\mathrm{C}

they denature and the rate falls.

Pause and think

Why does a log burn slowly but sawdust can explode?

Summary

Rate = frequency of successful collisions.
Temperature: two effects.
Rate = gradient.

Rate of reaction: Change in amount of reactant or product per unit time.
Catalyst: Speeds up a reaction without being used up.
Collision theory: Particles must collide with enough energy to react.

HistoryBerzelius again. Berzelius coined the word "catalysis" in 1835, from Greek for "loosening", after noticing that some substances speed reactions without being consumed.

Fun factFlour explosions. Flour is harmless in a bag, but a cloud of fine flour dust has such a huge surface area that a spark can make it explode. Flour mills take strict precautions.

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