First Order Reaction Half-life
- The half-life of a first-order reaction is independent of the concentration of reactants
- This means that despite the concentrations of the reactants decreasing during the reaction
- The amount of time taken for the concentrations of the reactants to halve will remain the same throughout the reaction
- The graph is a straight line going downwards
- The rearrangement of the methyl group (CH3) in ethanenitrile (CH3CN) is an example of a first-order reaction with rate equation rate = k [CH3CN]
CH3CN (g) → CH3NC (g)
Rearrangement of the CH3 group in CH3CN
- Experimental data of the changes in concentration over time suggests that the half-life is constant
- Even if the half-lives are slightly different from each other, they can still be considered to remain constant
- This means that no matter what the original concentration of the CH3CN is, the half-life will always be around 10.0 minutes
Half-life table
| Change in [CH3CN] (mol dm-3) | Half-life (minutes) |
| 8.00 - 4.00 | 10.0 |
| 4.00 - 2.00 | 9.50 |
| 2.00 - 1.00 | 9.25 |
In a first-order reaction, the time taken for the concentration to halve remains constant
Worked Example: Using the half-life of first-order reactions in calculations
Answer
- Step 1: Plot the concentration-time graph using appropriate scales and labels for the axis
- Step 2: Find the first and second half-lives by determining when the concentrations halve using the graph
Step 2 table
- Step 3: Determine the reaction order It is a first-order reaction as the successive half-lives remain reasonably constant (around 450 seconds) throughout the reaction
