Required practical 11 · Chemistry · Chemistry Paper 2, both tiers

Concentration and rate of reaction

Investigate how changing the concentration affects the rate of reaction, by measuring the volume of gas produced and by a change in colour or turbidity.

The exam trap. Higher concentration means more frequent collisions, not faster particles. Fitting the bung quickly reduces gas loss; it does not stop it.

Variables

Independent (you change): Concentration of the acid (or of sodium thiosulfate)
Dependent (you measure): Volume of gas over time, or time for the cross to disappear
Control (you keep the same): Temperature
Mass and size of marble chips (or volume of thiosulfate and total volume)
Volume of acid
The same cross and viewing position

Apparatus

Conical flask, bung and delivery tube
Gas syringe (or upturned measuring cylinder in water)
Stopwatch
Hydrochloric acid of different concentrations
Sodium thiosulfate solution and paper marked with a cross
Measuring cylinders

Method, and why each step matters

Gas method: put a set volume of acid in the flask, add the marble chips, fit the bung at once and start the stopwatch.
Why The reaction starts on contact, so fit the bung quickly to lose as little gas as possible.
Record the volume of gas every 10 seconds until it stops increasing.
Why The readings give a graph of volume against time.
Repeat with acid of different concentrations.
Why Concentration is the independent variable.
Turbidity method: put the thiosulfate in a flask on a cross, add acid, start timing and stop when the cross disappears.
Why The sulfur precipitate makes the mixture cloudy; the end point is when the cross is hidden.
Dilute the thiosulfate with water to change its concentration, keeping the total volume the same.
Why Only the concentration should change, not the depth of liquid.
Calculate mean rate = volume ÷ time, or rate = 1 ÷ time.
Why These let concentrations be compared.

Risks

Hazard	How to reduce the risk
Acid is an irritant.	Wear eye protection.
The thiosulfate reaction releases sulfur dioxide.	Ventilate the room and pour the mixture away promptly; people with asthma should take care.

A worked set of results

Time in s	Volume of gas in cm³ (1.0 mol/dm³)
0	0
10	18
20	32
30	42
40	48
50	50
60	50

Mean rate for the first 20 s = 32 ÷ 20 = 1.6 cm³/s.
The curve is steepest at the start and levels off when a reactant is used up.
Higher concentration: more particles in the same volume, so more frequent collisions.

Mistakes that cost marks

Claiming that fitting the bung quickly stops all gas escaping.
Instead It reduces the early loss; it does not remove it.
Explaining higher concentration with faster-moving particles.
Instead Particles are closer together, so they collide more often; their speed depends on temperature.
Drawing a straight line of best fit through curved data.
Instead Draw a smooth curve.