Measuring Reaction Rate
Choose a fixed end-point, gas-syringe or mass-loss method, plan a fair rate comparison and recognise measurement limitations.
On this page
A useful rate investigation needs a change you can measure reliably. Start with the reaction: does it produce gas, lose mass as gas escapes, or reach a clear visible end-point? Choose the method before deciding which apparatus to use.
1. Definition
The rate of reaction is the amount of reactant used up or product formed per unit time. If you need a reminder of average rates and units, revisit Speed of Reaction.
2. Key Ideas
- Choose a measurable change that tracks the reaction, and record time as well.
- For continuous measurements, record readings at regular intervals from the start.
- For a fixed end-point, compare the time to the same change in each trial.
- To investigate one factor, change that factor and control the other relevant conditions. Repeat trials to judge the consistency of the result.
3. Detailed Explanations
A. Method 1: Time taken for a fixed change
Use a clear end-point, such as the complete disappearance of a measured magnesium ribbon. Use the same mass and dimensions of ribbon in every trial. If acid concentration is the factor you change, keep acid volume and initial temperature constant and ensure the acid is in excess in every trial.
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
Start the stopwatch when the reactants are mixed and stop at the agreed end-point. Repeat for each concentration, keeping the mixing procedure the same. Shorter time means a greater average rate for this same fixed change.
1/t in s⁻¹ is a relative-rate comparison for the same end-point. It is not a gas-volume rate in cm³ s⁻¹ and is not the general definition of reaction rate.
B. Method 2: Volume of gas produced vs time (gas syringe)
Marble reacts with dilute hydrochloric acid to release carbon dioxide:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Reaction: marble chips and dilute hydrochloric acid release carbon dioxide.
Gas path: gas passes from the flask headspace through an airtight tube into the syringe, pushing its piston outwards.
Measurement: clamp the syringe, allow its plunger to move freely and record volume changes at regular times.
Swipe or scroll sideways to inspect the complete overview.
- Use a gas syringe with enough capacity for the expected gas volume and a freely moving plunger. Secure it in a clamp.
- Record the initial syringe reading. Mix the reactants, start the stopwatch and fit the bung promptly. Minimise gas lost before the flask is sealed and use the same starting procedure each time.
- Record the syringe reading at regular intervals until it stops increasing. The rise from the initial reading is the volume collected.
- Repeat trials. A leak can give a falsely small volume; a stuck plunger can delay a reading. Control temperature because gas volume changes with temperature as well as amount.
A flat reading means no further gas is being collected. In a working, leak-free setup for this reaction, a reactant may have been used up; a flat reading alone does not identify which reactant it was.
Collect carbon dioxide from marble and acid and record the gas volume at regular times.
0.20 g of marble as small chips in 20 cm³ of 1.00 mol/dm³ hydrochloric acid at 25 °C. After 0 s the gas syringe reads 0 cm³.
- Volume of gas
- 0 cm³
- Rate at the tangent
- — cm³/s
- Half-life
- — s
Try this
0 of 4 doneDuring a run, drag the tangent back to the start of the curve to find the initial rate. (not done yet)
The rate is the gradient of the volume–time graph. It is greatest at the start, when the reactants are most concentrated, and falls to zero.
Use two different acid concentrations, with the marble used up both times. (not done yet)
More concentrated acid gives a steeper curve: acid particles hit the marble more often. The final volume is the same, because the same mass of marble reacts.
Compare large chips with powder of the same mass. (not done yet)
Powder has a much larger surface area, so many more acid particles collide with the marble each second.
Decompose H₂O₂ with no MnO₂, then with some MnO₂. (not done yet)
MnO₂ gives the reaction a pathway with a lower activation energy. It is a catalyst: the same mass of MnO₂ is left at the end.
Your readings
| # | t / s | V / cm³ | Remove |
|---|---|---|---|
| No readings yet. Set up a measurement, then record it. | |||
C. Method 3: Mass loss vs time (gas escapes)
Use a balance to measure the total mass of the flask and contents as gas escapes. A loose cotton-wool plug reduces loss of droplets while allowing gas out.
Reaction: marble chips and dilute hydrochloric acid release carbon dioxide.
Gas path: carbon dioxide escapes through a loose cotton-wool plug. The flask is not sealed.
Measurement: record the total mass of the flask and contents at regular times. The decrease is the gas mass lost if other mass losses are negligible.
Swipe or scroll sideways to inspect the complete overview.
- Record the initial mass, mix the reactants and start the stopwatch promptly.
- Record mass at regular intervals, leaving the apparatus on the balance. The positive mass loss is initial mass − later mass.
- Repeat trials. Keep the balance away from draughts and avoid splashing: losing liquid would make the measured loss greater than the gas mass.
This method needs a balance sensitive enough for the expected change. Hydrogen is very light, so its mass loss can be difficult to resolve on a school balance; collecting its volume may be more useful. The method also assumes other mass losses, such as evaporation, are negligible.
Use dilute acids as instructed. In the mass-loss method, the cotton wool must not seal the flask. In the gas-syringe method, the plunger must be free to move.
D. Choosing the best method (quick guide)
| Reaction and measurement | Useful method | What to record | Main limitation |
|---|---|---|---|
| A gas can be collected | Gas syringe | gas volume and time | leaks or a stuck plunger |
| Escaping gas causes a measurable mass loss | Balance | total mass and time | small changes, draughts, splashing or evaporation |
| A clear, repeatable visible change | Fixed end-point | time to the same change | judgement of the end-point and timing delay |
For a concentration comparison with marble and acid, keep the marble mass and chip size, acid volume and initial temperature constant. Change acid concentration only. Use the same time interval or the same fixed change to compare trials. Different concentrations can also change the final gas amount if acid is limiting; use marble as the limiting reactant in each trial when you want the same final amount.
4. Common Mistakes
- Changing acid concentration and marble mass together, then attributing the result to concentration alone.
- Treating bubble size or bubbling intensity as an accurate gas-volume measurement.
- Comparing final gas amounts without comparing their time intervals.
- Ignoring a leak, loss of liquid or a change too small for the instrument to measure.
5. Exam Tips
A plan should name the changed variable, the controlled conditions, the measurement apparatus, what is recorded and when, and how the trials are repeated. Explain why the method tracks this reaction. For graph calculations, continue to Reading Reaction-Rate Graphs.
6. Worked Examples
Modelled example 1
Comparing rates using time
Problem
Study the worked solution
Check that the endpoint is comparable
Method
Use complete disappearance of identical magnesium pieces as the same fixed change.Reason
Time alone compares rate only when the experiments reach an equivalent endpoint with the same reactant amount.Working
Both trials use identical ribbon and the same endpoint.Compare the times
Method
Choose Acid A because it reaches the endpoint sooner.Reason
For the same change, a shorter time means a greater change per unit time.Working
25 s < 35 s, so A is faster.Confirm with relative rate
Method
Compare reciprocal times.Reason
1/t is a valid relative-rate tool for this controlled fixed endpoint.Working
A: 1/25 = 0.040 s⁻¹; B: 1/35 = 0.029 s⁻¹.
Guided practice 2
Plan a concentration comparison
Problem
Investigate how hydrochloric acid concentration affects the rate of reaction with marble. Both acid portions are in excess. Use gas volume readings to compare the initial reaction rates.
Plan before revealing the answer
Hints
Hint 1: track a product
The reaction produces carbon dioxide.
Hint 2: isolate concentration
Which solid and solution conditions must be unchanged?
View solution step by step
Measure a timed product change
Method
Connect an airtight flask to a freely moving, clamped gas syringe.
Reason
The volume increase tracks the carbon dioxide formed.
Working
Record the initial volume and readings at short, regular intervals from mixing.
Control and repeat
Method
Use equal masses of the same-size marble chips, equal acid volumes and the same initial temperature. Repeat each concentration.
Reason
These controls help isolate acid concentration; repeats show consistency.
Working
Compare gas-volume changes over the same short initial interval, or initial gradients of the volume–time graphs.
Average rate from data
This calculation now has its own graph and practice in Reading Reaction-Rate Graphs.
Interpreting a gas-volume graph
This calculation now has its own graph and practice in Reading Reaction-Rate Graphs.
Instantaneous rate from a tangent
This calculation now has its own graph and practice in Reading Reaction-Rate Graphs.
7. Mind Stretchers
Mind stretcher 1: Choose a measurable product changeExtension
A school balance can resolve changes of 0.1 g. A magnesium–acid reaction is expected to release 0.004 g of hydrogen, corresponding to a measurable gas volume. Explain which method you would choose and one condition needed for reliable readings.
Show Answer
Choose a gas syringe: the expected hydrogen mass is much smaller than a 0.1 g balance step, so a mass-loss comparison would be unreliable. Use an airtight connection and a freely moving syringe with enough capacity. Record volume changes with time; a gas volume alone is not a rate.
Tangent trap
The independent Tangent trap now follows the graph lesson.
Practise and check
Use the Rate of Reactions topic check to practise method selection and fair investigations.
Open the topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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