Calculating reaction rate from data

4 minAS Level8.1.3

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end, you can calculate an average reaction rate from experimental data and choose a unit that matches the data.

Rate is change divided by time

Experimental results often give a starting value, a later value and the time between them.

Find the change first, then divide by the time:

$\text{average rate} = \dfrac{\text{change in measured quantity}}{\text{time}}$

The measured quantity might be concentration, mass or gas volume. The unit follows automatically.

Worked example: Concentration of a reactant

The concentration of a reactant falls from $0.80$ to $0.50\ \text{mol dm}^{-3}$ in $15\ \text{s}$ . Calculate the average rate of disappearance.

Have a go first?

First calculation

CalculateQuestion 1 · Apply

The concentration of a reactant decreases from $0.72$ to $0.42\ \text{mol dm}^{-3}$ in $10\ \text{s}$ . Calculate the average rate of disappearance in $\text{mol dm}^{-3}\text{ s}^{-1}$ .

mol dm⁻³ s⁻¹

Answer the 4 questions above to continue.

Previous lessonConcentration and pressure: more collisions, more chances Next lessonReaction rate: counting useful collisions

CalculateQuestion 2 · Recall

A reaction produces $75\ \text{cm}^3$ of gas in $25\ \text{s}$ . Calculate the average rate in $\text{cm}^3\text{ s}^{-1}$ .

cm³ s⁻¹

CalculateQuestion 3 · Apply

The mass of a product rises from $1.20\ \text{g}$ to $2.00\ \text{g}$ in $40\ \text{s}$ . Calculate the average rate of mass formation in $\text{g s}^{-1}$ .

g s⁻¹

Spot the errorQuestion 4 · Stretch

Find the first incorrect line in this calculation of a reactant's average rate of disappearance.

Which line contains the first error?

1A reactant concentration falls from 0.60 to 0.20 mol dm⁻³ in 20 s.2Change in concentration = 0.20 − 0.60 = −0.40 mol dm⁻³.3Average rate of disappearance = 0.40 ÷ 20 = 0.020 mol dm⁻³ s⁻¹.4The negative sign shows the rate is impossible.