Chemistry · University
Half-life of a first-order reaction
This formula applies to a first-order rate law and not generally to zero- or second-order reactions.
Method and conditions
This formula applies to a first-order rate law and not generally to zero- or second-order reactions.
t½ = ln 2/k
Worked examples
A first-order reaction has k=0.6 min⁻¹. Find its half-life. Round the final result to 4 decimal places if needed. Answer in min.
Set the remaining fraction exp(−kt) to 1/2 and solve t=ln 2/k.
1.1552 min
A first-order reaction has k=0.25 min⁻¹. Find its half-life. Round the final result to 4 decimal places if needed. Answer in min.
Set the remaining fraction exp(−kt) to 1/2 and solve t=ln 2/k.
2.7726 min
Try it
A first-order reaction has k=0.49 min⁻¹. Find its half-life. Round the final result to 4 decimal places if needed. Answer in min.
A first-order reaction has k=0.44 min⁻¹. Find its half-life. Round the final result to 4 decimal places if needed. Answer in min.
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Answers and marking notes
- 1.4146 min
Set the remaining fraction exp(−kt) to 1/2 and solve t=ln 2/k.
- 1.5753 min
Set the remaining fraction exp(−kt) to 1/2 and solve t=ln 2/k.
Further reading and review references
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