Solutions R and S react to form an insoluble solid, so the mixture gradually turns cloudy. A water-treatment laboratory tests how the concentration of solution R affects how fast the solid forms.
A student mixed solution R with an excess of solution S in a flask standing on a black cross, and timed how long it took for the cross to disappear from view. The student planned to keep all other conditions the same. For this reaction, the rate is directly proportional to the concentration of solution R.
| Concentration of solution R (mol/dm3\text{mol/dm}^3mol/dm3) | Time for the cross to disappear (s) |
|---|---|
| 0.15 | 192 |
| 0.60 | ? |
Calculate how long the cross should take to disappear when the concentration of solution R is 0.60 mol/dm3.
When the student carried out the test with 0.60 mol/dm3 solution R, the cross took 56 s to disappear. This is longer than the expected time.
Suggest one reason for the difference.
The student made the 0.15 mol/dm3 solution by diluting 10 cm3 of the 0.60 mol/dm3 solution with water to a total volume of 40 cm3.
Explain, in terms of particles, why the rate of reaction with the diluted solution is one quarter of the rate with the 0.60 mol/dm3 solution.
127 exam-style questions on AQA GCSE Chemistry 6.1 Rate of reaction, covering 6.1.1a Calculating rates of reactions, 6.1.1b Gradient of a tangent as rate, 6.1.2 Factors which affect the rates of chemical reactions, 6.1.3 Collision theory and activation energy, and 6.1.4 Catalysts. Each one has a worked solution and a mark scheme showing where the marks go.