8.1.3—Experimental data to calculate the rate

Average rate = change in concentration divided by time. For a reactant, use the decrease in concentration as a positive rate magnitude; for a product, use the increase.

Read the correct interval and units from the data. Tangent gradients give an instantaneous rate at one point, whereas a secant over two readings gives an average rate.

If a product rises from 0.10 to 0.34 mol dm⁻³ in 60 s, average rate = (0.34−0.10)/60 = 4.0×10⁻³ mol dm⁻³ s⁻¹.

Do not divide by the final time when the interval begins later, and do not confuse concentration change with total amount when volume changes.