Required Practical: Rate of Reaction

This lesson covers the AQA GCSE Combined Science Trilogy (8464) required practical on investigating how changing concentration (or temperature) affects the rate of reaction. You must be able to describe the method, identify variables, interpret results, and evaluate the experiment. This practical is commonly examined on both papers.

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Overview of the Practical

There are two common versions of this required practical:

1. Sodium thiosulfate and hydrochloric acid (disappearing cross) — investigating the effect of concentration or temperature.
2. Marble chips (calcium carbonate) and hydrochloric acid — investigating the effect of concentration using gas collection.

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Practical 1: Sodium Thiosulfate and Hydrochloric Acid

The Reaction

$\text{Na}_2\text{S}_2\text{O}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{S(s)} + \text{SO}_2\text{(g)} + \text{H}_2\text{O(l)}$Na2​S2​O3​(aq)+2HCl(aq)→2NaCl(aq)+S(s)+SO2​(g)+H2​O(l)

The sulfur produced is a yellow precipitate that makes the solution go cloudy.

Method (Investigating Concentration)

1. Draw a cross on a piece of white paper.
2. Place a conical flask on top of the cross.
3. Measure 50 cm³ of sodium thiosulfate solution at a known concentration.
4. Add 5 cm³ of dilute hydrochloric acid to the flask and start the stopwatch.
5. Look down through the solution at the cross.
6. Stop the stopwatch when the cross can no longer be seen.
7. Repeat with different concentrations of sodium thiosulfate (diluted with distilled water to keep total volume constant).
8. Repeat each concentration at least three times and calculate a mean.

Variables

Variable	Details
Independent	Concentration of sodium thiosulfate
Dependent	Time taken for the cross to disappear
Control	Volume of acid, volume of total solution, temperature, same cross and flask

Using 1/time as a Measure of Rate

The time taken for the cross to disappear is inversely proportional to the rate:

$\text{rate} \propto \frac{1}{\text{time}}$rate∝time1​

A shorter time means a faster rate. Calculating 1/time (in s⁻¹) gives a value proportional to the rate, which can be plotted on a graph.

Exam Tip: If the question says "calculate the rate," use 1/time. If it asks you to "calculate the mean rate," use quantity/time. Read the question carefully.

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Practical 2: Marble Chips and Hydrochloric Acid

The Reaction

$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$CaCO3​(s)+2HCl(aq)→CaCl2​(aq)+H2​O(l)+CO2​(g)

Method (Investigating Concentration)

1. Place a known mass of marble chips (e.g. 5 g) into a conical flask.
2. Add 50 cm³ of hydrochloric acid at a known concentration.
3. Immediately connect a gas syringe and start the timer.
4. Record the volume of CO₂ gas collected every 10 seconds.
5. Repeat with different concentrations of acid.
6. Plot volume of gas against time for each concentration.

flowchart TD
    A["Weigh marble chips"] --> B["Add measured HCl"]
    B --> C["Connect gas syringe and start timer"]
    C --> D["Record gas volume every 10 s"]
    D --> E{"Reaction finished?"}
    E -- No --> D
    E -- Yes --> F["Repeat with different concentration"]
    F --> G["Plot results and compare"]

Variables

Variable	Details
Independent	Concentration of hydrochloric acid
Dependent	Volume of CO₂ collected at each time interval
Control	Mass and size of marble chips, volume of acid, temperature

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Interpreting Results

For the Disappearing Cross Experiment

Concentration (mol/dm³)	Time (s)	1/time (s⁻¹)
0.10	120	0.0083
0.08	150	0.0067
0.06	200	0.0050
0.04	300	0.0033
0.02	600	0.0017

A graph of 1/time against concentration gives a straight line through the origin, showing that rate is directly proportional to concentration.

For the Gas Collection Experiment

• Higher concentration → steeper initial gradient → faster rate.
• All concentrations reach the same final volume if marble chips are the limiting reactant.
• The reaction stops when all the marble chips have reacted.

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Evaluating the Practical

Strength	Limitation
Gas syringe gives quantitative, continuous data	Gas can leak if syringe sticks
Disappearing cross is quick and easy to set up	Endpoint is subjective — different people may stop at different times
Repeats improve reliability	Temperature may vary slightly between experiments

Improving Reliability

• Repeat each experiment at least three times and calculate a mean.
• Discard anomalous results before calculating the mean.
• Use the same person to judge the disappearing cross endpoint.

Improving Accuracy

• Use a gas syringe rather than collecting over water (some CO₂ dissolves).
• Use a data logger with a light sensor instead of the human eye for the disappearing cross.
• Use an accurate measuring cylinder or burette for solution volumes.

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Worked Example: Calculating Rate from Results

Question: In an experiment, 45 cm³ of gas was collected in 90 seconds. Calculate the mean rate of reaction.

$\text{mean rate} = \frac{45\text{ cm}^3}{90\text{ s}} = 0.50\text{ cm}^3\text{/s}$mean rate=90 s45 cm3​=0.50 cm3/s

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Common Mistakes

• Not keeping total volume constant when diluting sodium thiosulfate (you must add water to replace the reduced thiosulfate volume).
• Confusing independent and dependent variables.
• Not controlling temperature — if investigating concentration, temperature must be kept the same.
• Forgetting to state that repeats should be done and a mean calculated.

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Summary

• The required practical investigates how concentration (or temperature) affects rate
• Disappearing cross: measure time for cross to disappear; rate ∝ 1/time
• Gas collection: measure volume of gas at intervals; plot volume vs time
• Control variables must be identified and maintained
• Results should be repeated and means calculated
• Evaluate strengths and limitations of each method

Exam Tip: AQA will often set questions asking you to describe the method, identify variables, plot or interpret graphs, and suggest improvements. Practise all of these skills.

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Extended Practical Skills and Worked Analysis

Worked Example 1: Processing Disappearing-Cross Data

Question: In a sodium thiosulfate + hydrochloric acid experiment, the time for the cross to disappear at three concentrations of thiosulfate was recorded: