Derive the integrated rate equation for a zero order reaction. Draw the concentration-time graph and give one example.

1. 1 · Write the rate law. For R → P: −d[R]/dt = k[R]⁰ = k.
2. 2 · Separate and integrate. d[R] = −k dt, so [R] = −kt + I.
3. 3 · Find the constant. At t = 0, [R] = [R]₀, so I = [R]₀. Hence [R] = −kt + [R]₀.
4. 4 · Draw the graph. [R] on the y-axis, t on the x-axis; straight line from [R]₀ downwards, labelled slope = −k.
5. 5 · Give an example. 2NH₃ → N₂ + 3H₂ on hot platinum at high pressure.
6. 6 · Close in one line. So k = ([R]₀ − [R])/t, with unit mol L⁻¹ s⁻¹.

Scoring tip: Show the step where you put t = 0 to find the constant; it carries a mark.