A student follows a reaction by measuring the concentration of a dissolved reactant. The concentration decreases as the reaction proceeds. The readings are shown in the table.
| Time / min | Concentration of reactant / mol dm−3^{-3}−3 |
|---|---|
| 12 | 0.80 |
| 42 | 0.20 |
The student wants to determine how quickly the reactant is used up during this interval. Which calculation should be used to find the mean rate?
0.80 mol dm−3−0.20 mol dm−342 min\frac{0.80\,\mathrm{mol\,dm^{-3}}-0.20\,\mathrm{mol\,dm^{-3}}}{42\,\mathrm{min}}42min0.80moldm−3−0.20moldm−3
0.80 mol dm−3(42−12) min\frac{0.80\,\mathrm{mol\,dm^{-3}}}{(42-12)\,\mathrm{min}}(42−12)min0.80moldm−3
0.20 mol dm−3−0.80 mol dm−3(42−12) min\frac{0.20\,\mathrm{mol\,dm^{-3}}-0.80\,\mathrm{mol\,dm^{-3}}}{(42-12)\,\mathrm{min}}(42−12)min0.20moldm−3−0.80moldm−3
0.80 mol dm−3−0.20 mol dm−3(42−12) min\frac{0.80\,\mathrm{mol\,dm^{-3}}-0.20\,\mathrm{mol\,dm^{-3}}}{(42-12)\,\mathrm{min}}(42−12)min0.80moldm−3−0.20moldm−3
122 exam-style questions on Edexcel GCSE Chemistry 8.1 Rates of reaction, covering 8.1.1 Measuring and interpreting rates of reaction, 8.1.2 Collision theory and the factors affecting rate, and 8.1.3 Catalysts and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.