Disclaimer:
The exact reagents, reactions, processes and equipment used in practical activities may vary between schools, colleges, and exam board exemplar methods. The essential techniques, skills, and learning objectives remain the same. Always follow the instructions, risk assessments, and safety guidance provided by your teacher or centre.

PRACTICAL 3 – Investigating How the Rate of Reaction Changes with Temperature

Aim:
To investigate how the rate of the reaction between sodium thiosulfate and hydrochloric acid changes with temperature.

Overview:
The rate is monitored by timing how long it takes for a black cross beneath the reaction container to disappear due to the sulfur precipitate.

Reaction

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s)

Sulfur is produced as a precipitate, which makes the solution cloudy.

By placing the reaction mixture over a white cross and timing how long it takes for the cross to disappear from view we can compare rates for different concentrations of reactants or conditions.

AQA A-Level Chemistry disappearing cross setup showing a vessel over a black cross used to time the rate as sulfur precipitate forms

Apparatus and Chemicals

Apparatus:

Thermometer
400 cm³ beaker (for use as water bath)
Plastic container with lid
Two glass tubes (12–14 cm³ capacity)
10 cm³ measuring cylinder
Plastic graduated pipette
Stopwatch
Graph paper

Chemicals:

0.05 mol dm⁻³ sodium thiosulfate solution
1.0 mol dm⁻³ hydrochloric acid (or 0.5 mol dm⁻³ sulfuric acid)

Method Summary

Add 10 cm³ of 1 mol dm⁻³ hydrochloric acid to a clean test tube. Place in container (without cross underneath).
Note - a conical flask may also be used and this makes the observations easier, however larger volumes of reactants may be required if conical flask used.
Add 10 cm³ of sodium thiosulfate to another test tube. Place this over the cross. Insert thermometer.
Record start temperature. Quickly add 1 cm³ of acid to thiosulfate. Start timer.
Observe from above. Stop timing when the cross is no longer visible.
Record final temperature of the reaction mixture.
Discard into stop bath (sodium carbonate).
For next temperature, warm thiosulfate tube using hot water bath (not over 55 °C).
Repeat experiment for at least five different temperatures.
Repeat steps to ensure reproducibility.

Data Analysis (AS Level)

Calculate mean temperature of each reaction.
For each temperature, calculate 1/t where t is time taken for cross to disappear.
Plot graph of 1/t (rate) vs temperature (°C).

A-Level Analysis

See Arrhenius Equation for more detail.

Rate constant k varies with temperature using Arrhenius equation:
k = A e^−E_a/RT

Since rate ∝ 1/t, we substitute:
ln(1/t) = −E_a/R × (1/T) + constant

Plot graph of ln(1/t) vs 1/T (T in K).

AQA A-Level Chemistry Arrhenius plots showing ln(1/t) versus 1/T with gradient equal to minus Ea over R

Gradient = −E_a/R
Use R = 8.31 J K⁻¹ mol⁻¹ to calculate activation energy (E_a) in kJ mol⁻¹.

Safety Notes

Hydrochloric acid is corrosive – wear goggles and handle carefully.
Dispose of contents in stop bath (sodium carbonate neutralises acid).
Avoid exceeding 55 °C to prevent risk of burns or breakages.

Sources of Error and Improvements

Subjective end point (cross disappearing): use same observer, consistent lighting.
Temperature fluctuation: take initial and final temperature to use average.
Inconsistent mixing: ensure thorough mixing immediately after acid addition.
Heat loss: minimise by using a lid on the container.