Measuring rates of reaction

The rate of reaction is how quickly reactants are used up or products are made.

mean rate = quantity of reactant used ÷ time taken

mean rate = quantity of product formed ÷ time taken

Units depend on what you measure: g/s for mass, cm³/s for gas volume.

Example: 60 cm³ of hydrogen is collected in 30 s.

mean rate = 60 ÷ 30 = 2.0 cm³/s

You can measure it by:

• the mass lost as a gas escapes (flask on a balance, with cotton wool in the neck)
• the volume of gas made (gas syringe or upturned measuring cylinder)
• the time for a solution to go cloudy (disappearing cross)

Plotting the amount of product against time gives a curve:

• The steeper the line, the faster the reaction.
• The line is steepest at the start, when there are most reactant particles.
• It gets less steep as reactants are used up.
• It goes flat when the reaction has finished.

A faster reaction finishes sooner. If the same amounts of reactants are used, both curves level off at the same height, because the same amount of product is made.

To find the rate at a particular time, draw a tangent — a straight line that just touches the curve at that time — and work out its gradient:

gradient = change in y ÷ change in x

Example: at 20 s, a tangent rises by 30 cm³ over 15 s.

rate at 20 s = 30 ÷ 15 = 2.0 cm³/s

Rates can also be given in mol/s. If 0.06 mol of gas forms in 30 s, the mean rate is 0.06 ÷ 30 = 0.002 mol/s.

1. Measure 50 cm³ of dilute hydrochloric acid into a conical flask.
2. Connect a gas syringe with a bung and delivery tube.
3. Add a fixed length of magnesium ribbon, quickly replace the bung and start a stopwatch.
4. Record the volume of hydrogen every 10 seconds until it stops increasing.
5. Repeat with different concentrations of acid, keeping the volume and the magnesium the same.
6. Plot volume against time for each concentration and compare the gradients.

Hazard: hydrochloric acid is an irritant — wear eye protection; hydrogen is flammable, so keep flames away.

Better accuracy: replace the bung straight away so no gas escapes.

Sodium thiosulfate reacts with hydrochloric acid to make a precipitate of sulfur, which turns the solution cloudy:

Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O

1. Draw a black cross on paper and stand a conical flask on it.
2. Add 10 cm³ of sodium thiosulfate solution and 40 cm³ of water.
3. Add 10 cm³ of dilute hydrochloric acid, swirl and start a stopwatch.
4. Look down through the flask and stop timing when the cross disappears.
5. Repeat with different concentrations of sodium thiosulfate (more of it and less water, keeping the total volume the same).

A shorter time means a faster rate (rate ∝ 1 ÷ time).

Hazard: sulfur dioxide is made, which can trigger asthma — use a well-ventilated room.

Better accuracy: have the same person judge every run.