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Understanding how to calculate the rate of a chemical reaction is a fundamental skill in the AQA GCSE Chemistry specification. The rate of reaction tells us how fast reactants are being used up or how fast products are being formed. In this lesson you will learn the key formula for rate, how to read and interpret rate data from experiments, and how to draw and use tangents on graphs to find the rate at a particular point in time.
The rate of reaction is a measure of how quickly a chemical reaction takes place. It can be defined in two equivalent ways:
A fast reaction (such as an explosion) has a high rate. A slow reaction (such as rusting) has a low rate.
| Reaction | Approximate Time | Rate Description |
|---|---|---|
| Explosion of hydrogen and oxygen | Fractions of a second | Very fast |
| Burning of magnesium ribbon | A few seconds | Fast |
| Reaction of marble chips with dilute acid | A few minutes | Moderate |
| Rusting of iron | Days to months | Slow |
| Weathering of limestone buildings | Years to centuries | Very slow |
Exam Tip: The AQA specification uses the term "rate of reaction" rather than "speed of reaction." Always use the correct terminology in your answers to pick up marks for correct use of scientific language.
The mean (average) rate of reaction is calculated using:
mean rate of reaction = quantity of reactant used or product formed / time taken
The quantity measured depends on the experiment. Common measurements include:
| Measurement | Units | Example Method |
|---|---|---|
| Volume of gas produced | cm3 | Collecting gas in a syringe or over water |
| Mass lost | g | Measuring the decrease in mass as gas escapes |
| Time for a precipitate to obscure a mark | s | Disappearing cross experiment |
| Colour change | s (time taken) | Timing a colour change using a colorimeter |
The units of rate depend on the units of the quantity and time:
| 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 answer when calculating rate. A common mistake is to give a numerical value without units, which will cost you a mark. Check that the units of your answer match the units in the question.
Question: In an experiment, 48 cm3 of gas was collected in 120 seconds. Calculate the mean rate of reaction.
Solution:
mean rate = volume of gas / time taken
mean rate = 48 cm3 / 120 s
mean rate = 0.40 cm3/s
Question: A reaction caused the mass of a flask to decrease by 0.72 g over 3 minutes. Calculate the mean rate of reaction.
Solution:
mean rate = mass lost / time taken
mean rate = 0.72 g / 180 s
mean rate = 0.004 g/s
Exam Tip: Watch out for unit conversions. If time is given in minutes but the answer requires seconds, you must convert: 1 minute = 60 seconds. Similarly, if mass is given in grams and you need kilograms, divide by 1000.
In many experiments, measurements are taken at regular intervals. When you plot these results on a graph, you get a curve that shows how the reaction progresses over time.
A typical graph of product formed against time shows:
graph TD
A[Start of Reaction] --> 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 | What It Tells You |
|---|---|
| Steep gradient at the start | The rate of reaction is fastest at the beginning, when reactant concentration is highest |
| Gradient decreasing over time | The rate is slowing down as reactants are used up |
| Horizontal (flat) line | The reaction has finished — all of the limiting reactant has been used up |
| Total amount of product | Read from the y-axis where the graph levels off |
The mean rate tells you the average rate over the entire reaction. But sometimes you need to find the instantaneous rate — the rate at one particular moment in time. To do this, you draw a tangent to the curve at that point.
| Step | Action |
|---|---|
| 1 | Mark the time point on the x-axis |
| 2 | Draw a tangent to the curve at the corresponding point |
| 3 | Read off two sets of coordinates from the tangent line |
| 4 | Calculate gradient = (y2 - y1) / (x2 - x1) |
| 5 | State the rate with correct units |
Exam Tip: When drawing a tangent in an exam, make it as long as possible across the graph. Longer tangent lines give more accurate gradient calculations because small errors in reading coordinates matter less when the values are large.
A graph shows volume of gas (cm3) on the y-axis and time (s) on the x-axis. A tangent drawn at t = 30 s passes through the coordinates (10, 5) and (50, 45).
gradient = (45 - 5) / (50 - 10) = 40 / 40 = 1.0 cm3/s
The instantaneous rate of reaction at 30 seconds is 1.0 cm3/s.
When two or more experiments are plotted on the same axes, you can compare their rates by looking at the steepness of the curves.
| Observation | Interpretation |
|---|---|
| Steeper initial curve | Faster initial rate |
| Same final volume of gas | Same total amount of product (same limiting reactant used) |
| Different final volume | Different amount of limiting reactant was used |
| Curve reaches plateau sooner | Reaction finished faster |
graph LR
A[Compare Experiments] --> B[Look at initial gradient]
A --> C[Look at final amount of product]
A --> D[Look at time to reach completion]
B --> B1[Steeper = faster rate]
C --> C1[Same height = same amount of reactant]
D --> D1[Reaches plateau sooner = faster overall]
It is important to understand that you can express the rate in two ways:
Both are valid ways of expressing the rate. On a graph:
| Graph Type | y-axis | Gradient |
|---|---|---|
| Product vs time | Amount of product | Positive gradient (increasing) |
| Reactant vs time | Amount of reactant | Negative gradient (decreasing) |
In both cases, the steepness of the gradient tells you the rate of reaction.
Sometimes data is presented in a table rather than a graph. You can still calculate the rate between any two time points.
Example:
| Time (s) | Volume of gas (cm3) |
|---|---|
| 0 | 0 |
| 10 | 14 |
| 20 | 24 |
| 30 | 31 |
| 40 | 35 |
| 50 | 37 |
| 60 | 37 |
Rate in the first 10 seconds = 14 / 10 = 1.4 cm3/s
Rate between 20 and 30 seconds = (31 - 24) / (30 - 20) = 7 / 10 = 0.7 cm3/s
Notice how the rate decreases as the reaction progresses.
For reactions where you time how long it takes for a change to occur (such as a precipitate forming), the rate is often expressed as:
rate = 1 / time
This gives a value that is proportional to the rate. A shorter time means a faster reaction, and 1/t is larger.
| Experiment | Time for cross to disappear (s) | Rate (1/t in s^-1) |
|---|---|---|
| A | 20 | 0.050 |
| B | 40 | 0.025 |
| C | 80 | 0.013 |
| D | 10 | 0.100 |
Experiment D has the fastest rate because 1/t is the largest.
Exam Tip: When using the 1/t method, remember that a shorter time does NOT mean a slower reaction. Shorter time = faster reaction = higher rate. This is a common source of confusion in exams.
Exam Tip: Graph questions on rate are extremely common in AQA GCSE Chemistry. Practise drawing tangents on printed graphs until you are confident. In the exam, use a sharp pencil and ruler, and show your working clearly — even if your tangent is slightly off, you can still gain method marks for correct gradient calculations.