Required Practical: Rates of Reaction

This lesson covers the AQA GCSE Chemistry Required Practical on investigating rates of reaction. You must know how to investigate the effect of changing the concentration of acid on the rate of a reaction between hydrochloric acid and marble chips (calcium carbonate). The specification also expects you to be able to apply this method to other rate investigations. This is one of the most commonly examined practicals, so thorough preparation is essential.

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Overview of the Required Practical

The required practical investigates how the concentration of hydrochloric acid affects the rate of reaction with calcium carbonate (marble chips).

Reaction:

CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + H2O(l) + CO2(g)

Method of measuring rate: Collect the carbon dioxide gas produced and measure its volume over time using a gas syringe.

Variable	Type	Details
Concentration of HCl	Independent (changed)	Use different concentrations, e.g. 0.5, 1.0, 1.5, 2.0 mol/dm3
Volume of CO2 produced	Dependent (measured)	Measured in cm3 using a gas syringe at regular time intervals
Mass of marble chips	Control	Keep the same mass (e.g. 5 g) for each experiment
Size of marble chips	Control	Use the same size chips (e.g. small chips, 2-4 mm)
Volume of HCl	Control	Use the same volume (e.g. 50 cm3) each time
Temperature	Control	Carry out all experiments at room temperature

Exam Tip: In the exam, you may be asked to plan this experiment from scratch. Make sure you can clearly state the independent variable, the dependent variable and at least three control variables. Naming specific values (e.g. "5 g of marble chips") shows the examiner you understand practical work.

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Apparatus

The following apparatus is needed:

Apparatus	Purpose
Conical flask	Contains the marble chips and acid
Gas syringe	Collects and measures the volume of CO2 gas produced
Delivery tube with bung	Connects the flask to the gas syringe; the bung prevents gas escaping
Balance	Weighs the marble chips to ensure the same mass is used
Measuring cylinder	Measures the volume of hydrochloric acid accurately
Stopwatch	Times the reaction at regular intervals
Marble chips (CaCO3)	The solid reactant
Hydrochloric acid (HCl)	The solution reactant at different concentrations
Safety goggles	Eye protection (HCl is corrosive/irritant)

graph TD
    A[Apparatus Setup] --> B[Conical flask containing marble chips]
    B --> C[Add measured volume of HCl]
    C --> D[Quickly attach bung with delivery tube]
    D --> E[Gas travels through delivery tube]
    E --> F[CO2 collected in gas syringe]
    F --> G[Read volume at regular intervals]
    G --> H[Record results in a table]

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Method: Step by Step

1. Weigh 5 g of marble chips using a balance. Use chips of the same size.
2. Measure 50 cm3 of hydrochloric acid at the chosen concentration using a measuring cylinder.
3. Place the marble chips into a clean, dry conical flask.
4. Pour the acid into the flask and immediately start the stopwatch and attach the bung connected to the delivery tube and gas syringe.
5. Record the volume of gas in the syringe every 10 seconds for 2-3 minutes (or until the reaction is complete).
6. Repeat the experiment at least three times for each concentration to calculate a mean.
7. Repeat the entire procedure for each different concentration of HCl.
8. Record all results in a table.

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Recording Results

A suitable results table:

Time (s)	Volume of CO2 (cm3) — 0.5 mol/dm3	Volume of CO2 (cm3) — 1.0 mol/dm3	Volume of CO2 (cm3) — 2.0 mol/dm3
0	0	0	0
10	2	5	12
20	5	12	24
30	8	18	34
40	11	23	42
50	13	27	48
60	15	30	50
90	18	34	50
120	20	36	50

Exam Tip: You may be asked to identify anomalous results. An anomalous result is one that does not fit the overall pattern. If you spot one, do not include it in your mean calculation. Circle it in the table and state why you have excluded it.

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Plotting and Interpreting the Graph

Plot a graph of volume of CO2 (cm3) on the y-axis against time (s) on the x-axis. Draw a separate curve for each concentration.

Expected Observations

Observation	Explanation
Higher concentration has a steeper initial gradient	More acid particles per volume, so more frequent collisions with marble chips — faster initial rate
All curves level off at the same final volume	The same mass of marble chips (limiting reactant) was used, so the same total amount of CO2 is produced
Higher concentration curves level off sooner	The reaction finishes faster because the rate was higher throughout

If a different mass of marble chips is used, the final volume of gas will be different.

graph TD
    A[Higher acid concentration] --> B[More HCl particles per cm3]
    B --> C[More frequent collisions with CaCO3]
    C --> D[More successful collisions per second]
    D --> E[Faster rate of reaction]
    E --> F[Steeper curve on the graph]
    E --> G[Reaction finishes sooner]

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Calculating Rate from the Graph

Mean Rate

mean rate = total volume of gas produced / total time taken

Example: If 50 cm3 of gas is produced in 60 s, mean rate = 50/60 = 0.83 cm3/s.

Instantaneous Rate (Using a Tangent)

1. Choose a time point on the x-axis.
2. Draw a tangent to the curve at that point.
3. Calculate the gradient of the tangent.
4. The gradient = the rate at that instant.

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Alternative Methods of Measuring Rate

The gas syringe method is the most common, but AQA may ask about other methods:

Method	How It Works	Advantage	Disadvantage
Gas syringe	Collects gas directly; read volume from syringe scale	Accurate; measures volume directly	Syringe can stick if gas production is fast
Mass loss (balance)	Place flask on balance; record mass at intervals as CO2 escapes	Continuous data; easy to set up	Small mass changes may be hard to detect; gas must escape
Inverted measuring cylinder over water	Collect gas by displacement of water	Simple apparatus	Gas dissolves in water (CO2 is slightly soluble)
Disappearing cross	Time how long a precipitate takes to obscure a mark	Very simple; no complex apparatus	Subjective — different people may judge the endpoint differently; only gives one time value per experiment

Exam Tip: If asked to evaluate a method, always consider: accuracy (how close to the true value?), precision (how close are repeat readings?), resolution (smallest change the measuring device can detect), and whether the method gives continuous data or just one reading.

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