Define the rate of a reaction. Distinguish between average and instantaneous rate, and show how each is found from a concentration-time graph. Write the rate expression for 2HI → H₂ + I₂.

1. 1 · Define in one line. Rate of reaction = change in concentration of a reactant or product per unit time; unit mol L⁻¹ s⁻¹.
2. 2 · Average rate. r(av) = −Δ[R]/Δt over an interval; on the graph, the slope of the chord joining two points.
3. 3 · Instantaneous rate. r(inst) = −d[R]/dt at time t; on the graph, minus the slope of the tangent at t.
4. 4 · Draw the graph. [R] (mol L⁻¹) on the y-axis, time on the x-axis, a falling curve, one chord labelled Δ[R] and Δt, and one tangent labelled.
5. 5 · Write the rate expression. Rate = −½ d[HI]/dt = d[H₂]/dt = d[I₂]/dt: each term divided by its coefficient.
6. 6 · Close in one line. So the rate falls with time because reactant concentration falls.

Scoring tip: Put the minus sign only before reactant terms, and never forget the 1/coefficient.