Define the half-life of a reaction.

The half-life is the time taken for the concentration of a reactant to fall to half of its value at the start of that interval.

Which order of reaction has a half-life that is independent of concentration?

A first-order reaction has a constant half-life.

How can successive half-lives be used as evidence for first-order kinetics?

If each successive half-life read from a concentration–time graph is the same, the reaction is first order in that reactant.

How do successive half-lives behave for a zero-order reaction?

Each successive half-life is half as long as the one before, because the concentration falls at a constant rate.

How do successive half-lives behave for a second-order reaction?

Each successive half-life is twice as long as the one before.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

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Elements of Life

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The Chemical Industry

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Colour by Design

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Chemical Literacy

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Mathematical Skills in Chemistry

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The Chemical Industry

Half-lives

4 things to know
Components 01, 02 and 03

Overview

Recall questions on which order has a constant half-life, how successive half-lives behave for each order, and k = ln 2 / t½.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Which order has a half-life independent of concentration, how do successive half-lives show first-order kinetics, and how do they behave for zero order?

Numbers

How do successive half-lives behave for second order, and how are the half-life and the rate constant related for a first-order reaction?

Evaluation

What concentration remains 150 s after a first-order reaction starts at 0.80 mol dm⁻³ with t½ = 50 s, why should more than one half-life be measured, and how long until 87.5% of a first-order reactant with t½ = 4.0 min has gone?

Key term

What is the symbol used for half-life?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the half-life of a reaction.
Model answer
The half-life is the time taken for the concentration of a reactant to fall to half of its value at the start of that interval.
2
What is the symbol used for half-life?
Model answer
The symbol is t½.
3
Which order of reaction has a half-life that is independent of concentration?
Model answer
A first-order reaction has a constant half-life.
4
How can successive half-lives be used as evidence for first-order kinetics?
Model answer
If each successive half-life read from a concentration–time graph is the same, the reaction is first order in that reactant.
5
How do successive half-lives behave for a zero-order reaction?
Model answer
Each successive half-life is half as long as the one before, because the concentration falls at a constant rate.
6
How do successive half-lives behave for a second-order reaction?
Model answer
Each successive half-life is twice as long as the one before.

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