6.1.5Chemical Reactions
Investigating The Rate of a Reaction — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 10 Unit quiz
Section 1 of 7Measuring a rate
What can be measured
- What can be measured The rate is how fast reactants are used up or how fast products are made.
- We choose a property that changes steadily as the reaction goes on.
Mass loss
- Mass loss Where a gas escapes, the flask can stand on a balance and the mass is recorded at intervals.
- A loose plug of cotton wool lets the gas out but stops liquid spraying away.
- This is unsuitable for hydrogen because the mass lost is too small to measure.
Gas volume
- Gas volume The gas passes along a delivery tube into an upturned measuring cylinder over water or into a gas syringe,
- and the volume is noted at regular times.
Colour or clarity
- Colour or clarity The time taken for a visible change is measured, such as a precipitate hiding a cross drawn beneath the flask.
Section 2 of 7Concentration: sodium thiosulfate and acid
1
Measure 50 cm3 of sodium thiosulfate solution into a conical flask and place it over a cross drawn on paper.
2
Measure 5 cm3 of dilute hydrochloric acid in a measuring cylinder.
3
Add the acid to the flask and start the stopwatch at once.
4
Looking down from above, stop the clock when the cross can no longer be seen through the cloudy sulfur.
5
Repeat with weaker solutions made by mixing the thiosulfate with water, always keeping the total volume at 50 cm3.
6
Compare 1 ÷ time for each concentration; a shorter time means a faster rate. Take care, because sulfur dioxide gas is made, so work in a ventilated room.
Section 3 of 7Other single-variable investigations
| Variable | Method | Measured | Controlled |
|---|---|---|---|
| Surface area | Acid with magnesium ribbon cut into different piece sizes, gas collected over water | Gas volume in a fixed time | Same mass of magnesium, acid volume, concentration and temperature |
| Temperature | Acid heated in a water bath to a set temperature, then magnesium added | Time for the magnesium to disappear | Same acid, volume, concentration and mass of magnesium |
| Catalyst | Hydrogen peroxide with and without manganese(IV) oxide, gas collected | Gas volume in a fixed time | Same volume and concentration of peroxide, same temperature |
Section 4 of 7Planning a fair test
One variable at a time
Independent variable
- Independent variable The factor you deliberately change, for example the temperature.
Dependent variable
- Dependent variable The quantity you measure to find the rate, such as the volume of gas or the time.
Control variables
- Control variables Everything else is kept the same so that only the independent variable can explain a change in rate.
Repeats
- Repeats Repeat each run and calculate a mean to reduce the effect of random errors, ignoring any anomalies.
Section 5 of 7Evaluating and devising methods (HT only)
Devise a method
- Devise a method You may be asked to design your own investigation, choosing the apparatus, the range of values, the measurements and the safety precautions.
Evaluate
- Evaluate Typical sources of error are gas lost before the bung is replaced, a gas syringe sticking,
- and a temperature that drifts in a water bath.
- Suggest sensible improvements such as using a data logger or more repeats.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | The change in the amount of a reactant or product per unit time. |
| Independent variable | The variable deliberately changed in an investigation. |
| Dependent variable | The variable measured as the outcome of the investigation. |
| Control variable | A variable kept constant to make the test fair. |
| Gas syringe | A calibrated syringe that collects a gas and measures its volume. |
Section 7 of 7Exam technique
1
Changing more than one variable at once, for example using a different mass of magnesium as well as different piece sizes.
2
Collecting hydrogen by mass loss, where the change is too small to be detected.
3
Losing some gas while the bung is put back, which makes the early volumes too low.
4
Reading the time as a rate; a longer time means a slower reaction, so calculate 1 ÷ time or volume ÷ time.
Last updated: