Rates of Reaction — Measuring and Calculating

Understanding rates of reaction is a core part of the AQA GCSE Combined Science Trilogy (8464) chemistry specification. In this lesson you will learn how to measure the rate of a chemical reaction using different experimental methods, how to calculate the mean rate, and how to interpret graphical data including drawing tangents for instantaneous rate.

What Is the Rate of Reaction?

The rate of reaction measures how quickly reactants are used up or how quickly products are formed during a chemical reaction.

A fast reaction (such as an explosion) has a high rate.
A slow reaction (such as rusting) has a low rate.

Reaction Example	Approximate Timescale	Rate Description
Explosion of hydrogen and oxygen	Fractions of a second	Very fast
Burning magnesium ribbon	A few seconds	Fast
Marble chips reacting with dilute acid	A few minutes	Moderate
Rusting of iron	Days to months	Slow
Weathering of limestone	Years to centuries	Very slow

Exam Tip (AQA 8464): The specification uses the term "rate of reaction" rather than "speed of reaction." Always use the correct terminology in your written answers.

Calculating Mean Rate of Reaction

The mean rate of reaction is calculated using:

$$\text{mean rate of reaction} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}$$

The quantity measured depends on the experimental method used:

Measurement	Units	Method
Volume of gas produced	cm³	Gas syringe or collection over water
Mass lost	g	Measuring decrease in mass on a balance
Time for a precipitate to obscure a mark	s	Disappearing cross experiment
Colour change	s	Timing a colour change (colorimeter)

The units of rate depend on the quantities involved:

Quantity Unit	Time Unit	Rate Unit
cm³	s	cm³/s
g	s	g/s
mol	s	mol/s
cm³	min	cm³/min

Exam Tip: Always include units with your calculated rate. A numerical answer without units will lose marks.

Worked Example 1: Mean Rate from Volume

Question: In an experiment, 60 cm³ of gas was collected in 150 seconds. Calculate the mean rate of reaction.

Solution:

$$\text{mean rate} = \frac{\text{volume of gas}}{\text{time taken}}$$

$$\text{mean rate} = \frac{60\text{ cm}^3}{150\text{ s}}$$

$$\text{mean rate} = 0.40\text{ cm}^3\text{/s}$$

Worked Example 2: Mean Rate from Mass Loss

Question: A reaction caused the mass of a flask to decrease by 0.90 g over 3 minutes. Calculate the mean rate of reaction in g/s.

Solution:

First convert minutes to seconds: 3 min = 180 s

$$\text{mean rate} = \frac{0.90\text{ g}}{180\text{ s}} = 0.005\text{ g/s}$$

Common Mistake: Forgetting to convert minutes to seconds when the question asks for a rate in g/s or cm³/s. Always check the units required in the question.

Methods for Measuring Rate

1. Gas Syringe Method

A gas syringe collects the gas produced and measures its volume at regular intervals. This is accurate for gases that do not dissolve in water.

2. Mass Loss Method

Place the reaction flask on a balance. As gas escapes, the mass decreases. Record mass at regular time intervals.

3. Disappearing Cross Method

A cross is drawn on paper beneath a conical flask. When the reaction produces a precipitate, the solution becomes opaque. The time taken for the cross to disappear is recorded.

Method	Advantages	Disadvantages
Gas syringe	Accurate volume readings; easy to plot	Gas can leak if syringe sticks
Mass loss	Continuous data; simple equipment	Small mass changes hard to detect; drafts affect readings
Disappearing cross	Quick and simple	Subjective endpoint; only gives one time reading

Interpreting Rate Graphs

A typical graph of product formed (y-axis) against time (x-axis) shows:

graph TD
    A["Start: reactant concentration highest"] --> B["Steep curve — fast rate"]
    B --> C["Curve becomes less steep — rate slows"]
    C --> D["Curve levels off — reaction complete"]
    D --> E["Flat line — no more product formed"]

Feature	Meaning
Steep gradient at start	Rate is fastest when reactant concentration is highest
Decreasing gradient	Rate slows as reactants are used up
Horizontal line	Reaction has finished — limiting reactant used up
Final y-value	Total amount of product formed

Finding Instantaneous Rate: Drawing a Tangent

The mean rate gives the average over the whole reaction. The instantaneous rate at a particular time is found by drawing a tangent to the curve at that point.

Steps to Draw a Tangent

Identify the time point on the x-axis.
Place a ruler so it just touches the curve at the corresponding point without cutting through it.
Draw a straight line — this is the tangent.
Choose two points on the tangent that are far apart.
Calculate the gradient:

$$\text{gradient} = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}$$

The gradient equals the instantaneous rate at that time.

Worked Example: Tangent Method

A graph shows volume of gas (cm³) against time (s). A tangent drawn at t = 40 s passes through (10, 8) and (70, 56).

$$\text{gradient} = \frac{56 - 8}{70 - 10} = \frac{48}{60} = 0.80\text{ cm}^3\text{/s}$$

The instantaneous rate at 40 seconds is 0.80 cm³/s.

Exam Tip: When drawing a tangent in an exam, make it as long as possible across the graph. Longer lines give more accurate gradient calculations because small errors in reading coordinates have less impact.

Summary

Rate of reaction = quantity of reactant used or product formed ÷ time taken
Common methods: gas syringe, mass loss, disappearing cross
Graphs of product vs time show rate changes: steep = fast, flat = finished
Instantaneous rate is found by drawing a tangent and calculating its gradient
Always include units and watch for unit conversions (minutes to seconds)

Exam Tip: Rate calculation questions appear frequently on AQA 8464 papers. Practise with different unit combinations and always show your working clearly.