Rates of Reaction — Measuring and Calculating | AQA

The rate of reaction tells you how fast a reaction proceeds — how quickly reactants are converted into products. Understanding how to measure and calculate rates is essential for the AQA GCSE Combined Science exam and provides the foundation for understanding collision theory and the factors that affect rate.

Defining Rate of Reaction

$\text{Rate of reaction} = \frac{\text{Amount of reactant used or product formed}}{\text{Time taken}}$

Rate can be measured in terms of:

Mass of product formed per unit time (g/s)
Volume of gas produced per unit time (cm³/s)
Concentration change per unit time (g/dm³/s or mol/dm³/s)

Units

What You Measure	Unit of Rate
Mass per second	g/s
Volume per second	cm³/s
Concentration per second	mol/dm³/s

Methods for Measuring Rate

Method 1 — Measuring Mass Lost (Gas Given Off)

Place the reaction on a balance. As gas escapes, the mass decreases.

Step	Action
1	Place flask with reactants on a balance
2	Start the reaction and start the timer
3	Record mass at regular time intervals
4	Plot mass lost vs time

Advantage: Continuous data, easy to record. Disadvantage: Very light gases (e.g. H₂) may not be detected accurately by the balance.

Method 2 — Measuring Volume of Gas Collected

Collect gas using a gas syringe or over water in an inverted measuring cylinder.

Step	Action
1	Connect flask to gas syringe
2	Start reaction and timer
3	Record volume at regular intervals
4	Plot volume vs time

Advantage: Works for all gases; accurate volume measurement. Disadvantage: Gas syringe may stick if it gets wet.

Method 3 — Timing a Colour Change (Disappearing Cross)

Used for reactions that produce a precipitate (e.g. sodium thiosulfate + HCl → sulfur precipitate).

Step	Action
1	Place a flask over a paper cross
2	Add reactants and start timer
3	Stop timer when cross disappears from view
4	Rate ∝ 1/time

Advantage: Simple and quick. Disadvantage: Subjective — different people may judge the endpoint differently.

flowchart TD
    A["Choose method"] --> B{"What is produced?"}
    B -->|"A gas"| C["Gas syringe or\nmass loss method"]
    B -->|"A precipitate"| D["Disappearing cross\nmethod"]
    B -->|"Colour change"| E["Colorimeter or\nvisual observation"]
    style A fill:#3b82f6,color:#fff,stroke:#2563eb
    style C fill:#10b981,color:#fff,stroke:#059669
    style D fill:#f59e0b,color:#000,stroke:#d97706
    style E fill:#a855f7,color:#fff,stroke:#7c3aed

Interpreting Rate Graphs

Volume of Gas vs Time

A typical graph shows a curve that:

Is steep at the start (reaction is fast — lots of reactant available).
Gets less steep over time (reactant is being used up).
Becomes flat when the reaction is complete (no more gas is produced).

Key Features

Feature	Meaning
Steep gradient	Fast rate
Shallow gradient	Slow rate
Flat line (horizontal)	Reaction has finished
Total height of curve	Total amount of product formed

Calculating Rate from a Graph

Average rate = total product ÷ total time

Rate at a specific time = gradient of the tangent to the curve at that point

Worked Example 1 — Average Rate

A reaction produces 60 cm³ of gas in 120 seconds.

$\text{Average rate} = \frac{60}{120} = 0.5 \text{ cm}^3\text{/s}$

Worked Example 2 — Rate at a Point

From a graph, you draw a tangent at t = 30 s. The tangent passes through (20, 10) and (40, 30).

$\text{Gradient} = \frac{30 - 10}{40 - 20} = \frac{20}{20} = 1.0 \text{ cm}^3\text{/s}$

Worked Example 3 — Mean Rate in First 10 Seconds

12 cm³ of gas is collected in the first 10 seconds.

$\text{Mean rate} = \frac{12}{10} = 1.2 \text{ cm}^3\text{/s}$

This is higher than the overall average because the reaction is fastest at the start.

Calculating Rate from Concentration Data

Worked Example 4

The concentration of hydrochloric acid decreases from 1.0 mol/dm³ to 0.4 mol/dm³ in 30 seconds.

$\text{Rate} = \frac{1.0 - 0.4}{30} = \frac{0.6}{30} = 0.02 \text{ mol/dm}^3\text{/s}$

Using 1/Time as a Measure of Rate

In the disappearing cross experiment, rate is inversely proportional to time:

$\text{Rate} \propto \frac{1}{t}$

Experiment	Time (s)	1/time (s⁻¹)	Relative Rate
A	20	0.050	Fastest
B	40	0.025	Medium
C	80	0.0125	Slowest

Common Exam Mistakes

Mistake	Correction
Confusing "rate" with "total product"	Rate is product per unit time, not the total amount
Drawing a tangent that crosses the curve	A tangent must touch the curve at exactly one point and not cross it
Forgetting units on rate	Always include g/s, cm³/s, or mol/dm³/s
Saying "the reaction speeds up at the end"	The curve flattens because the reaction slows down as reactant is used up
Mixing up rate at a point with average rate	Average = total ÷ total time; point rate = gradient of tangent
Not recognising a flat line means "reaction complete"	A flat line does NOT mean the rate is constant — it means the rate is zero

Summary

Rate of reaction = amount of product formed (or reactant used) per unit time.
Three main methods: mass loss, gas syringe, disappearing cross.
On a graph: steep = fast, flat = finished.
Average rate = total change ÷ total time.
Instantaneous rate = gradient of the tangent.
1/time is used as a relative measure of rate in the disappearing cross method.

Worked Example 5 — Reading Rate from a Data Table

Time (s)	Volume of Gas (cm³)
0	0
10	18
20	30
30	38
40	43
50	46
60	48
80	48

Average rate = 48 ÷ 60 = 0.8 cm³/s (using the time when the reaction finishes, not the last reading time).

Initial rate (first 10 s) = 18 ÷ 10 = 1.8 cm³/s — much faster than the average because lots of reactant is available.

Rate between 30 and 40 s = (43 − 38) ÷ (40 − 30) = 5 ÷ 10 = 0.5 cm³/s — slower because reactant is being used up.

After 60 s: rate = 0 cm³/s — the graph is flat, so the reaction has finished.

Practical Tips for Rate Experiments

Tip	Reason
Use a cotton wool plug (not a bung) if measuring mass loss	Allows gas to escape but prevents liquid spray
Record mass every 30 seconds, not just at the start and end	More data points give a better graph
Use a stopwatch with 0.01 s resolution	Reduces random error in timing
Do a trial run first	Helps you choose sensible time intervals
Repeat each experiment at least 3 times	Identifies anomalies and improves reliability

Rates of Reaction — Measuring and Calculating