26.1.3—Show understanding that the half-life

For a first-order reaction, t½ = 0.693/k, so every successive half-life is the same at fixed temperature, regardless of the initial concentration.

Use repeated halving to estimate remaining concentration and use the equation to calculate k. The independence is a diagnostic of first-order behaviour, not a universal property of all reactions.

If 0.80 mol dm⁻³ falls to 0.40 in 10 s and then 0.20 in another 10 s, t½=10 s and k=0.0693 s⁻¹.

A constant half-life does not mean a constant rate; the rate decreases as concentration falls.