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The rate of reaction is one of the most important ideas in the Edexcel GCSE Chemistry specification (1CH0). Understanding how to measure, calculate and interpret rates of reaction will help you tackle a wide range of exam questions. In this lesson you will learn what rate of reaction means, how it is measured experimentally, how to calculate the mean rate, and how to read and interpret rate-time graphs.
The rate of reaction is a measure of how quickly reactants are converted into products during a chemical reaction. Some reactions are very fast (such as an explosion), while others are very slow (such as rusting).
Rate of reaction can be thought of in two equivalent ways:
| Reaction | Approximate Time Scale | Rate |
|---|---|---|
| Explosion of gunpowder | Fractions of a second | Very fast |
| Burning magnesium ribbon | A few seconds | Fast |
| Marble chips reacting with acid | Several minutes | Moderate |
| Rusting of iron | Weeks to months | Slow |
| Weathering of limestone | Centuries | Very slow |
Exam tip: Always use the term "rate of reaction" rather than "speed of reaction" in your answers. The Edexcel mark scheme rewards correct scientific terminology.
There are several practical methods for measuring the rate of a reaction. The method you choose depends on the type of reaction taking place.
If a reaction produces a gas that escapes from an open container, the mass of the container and its contents will decrease over time. By recording the mass at regular intervals using a balance, you can track the progress of the reaction.
Advantages: Continuous data, easy to read from a balance. Disadvantages: Small changes in mass can be hard to detect; gas must escape (not suitable for toxic gases without a fume cupboard).
The gas produced can be collected using a gas syringe or by displacement of water. Recording the volume of gas at regular time intervals gives rate data.
Advantages: Accurate volume readings, continuous data. Disadvantages: Gas syringes can stick; water displacement is unsuitable if the gas is soluble in water.
A cross is drawn on a piece of paper. The reaction vessel (a conical flask) is placed on top. When a reaction produces a precipitate (such as sulfur), the solution becomes cloudy. The time taken for the cross to disappear from view is recorded.
Advantages: Simple to set up. Disadvantages: Subjective — different people judge the end point differently; gives only a single time value rather than continuous data.
A colorimeter can be used to measure changes in the colour (and therefore concentration) of a solution over time. Alternatively, the time taken for a colour change to occur can be recorded.
| Method | What is measured | Typical units |
|---|---|---|
| Mass loss | Decrease in mass over time | g |
| Gas syringe | Volume of gas over time | cm³ |
| Disappearing cross | Time for mark to disappear | s |
| Colour change / colorimeter | Light absorbance over time | arbitrary units |
The mean rate of reaction is calculated using the formula:
mean rate of reaction = amount of product formed ÷ time taken
or equivalently:
mean rate of reaction = amount of reactant used up ÷ time taken
The units of rate depend on the units of the quantity measured and the units of time:
| Quantity | Time | Rate unit |
|---|---|---|
| g | s | g/s |
| cm³ | s | cm³/s |
| mol | s | mol/s |
| g | min | g/min |
| cm³ | min | cm³/min |
Question: In an experiment, 60 cm³ of gas was collected in 2 minutes. Calculate the mean rate of reaction. Give your answer in cm³/s.
Solution:
Step 1 — Convert time to seconds: 2 minutes = 2 × 60 = 120 s
Step 2 — Apply the formula:
mean rate = volume of gas ÷ time taken
mean rate = 60 cm³ ÷ 120 s
mean rate = 0.50 cm³/s
Question: A flask containing marble chips and acid lost 1.44 g of mass over 6 minutes. Calculate the mean rate of reaction in g/s.
Solution:
Step 1 — Convert time to seconds: 6 minutes = 6 × 60 = 360 s
Step 2 — Apply the formula:
mean rate = mass lost ÷ time taken
mean rate = 1.44 g ÷ 360 s
mean rate = 0.004 g/s
Exam tip: Always check the units the question asks for. If the question says "give your answer in g/s" but the time is given in minutes, you must convert minutes to seconds before dividing. Forgetting this unit conversion is one of the most common mistakes in the exam.
When you plot the amount of product formed (y-axis) against time (x-axis), you get a curve that provides a wealth of information about the reaction.
The gradient (slope) of the line at any point on the graph equals the rate of reaction at that moment.
To calculate the mean rate over an interval, pick two points on the curve and use:
rate = change in amount of product ÷ change in time
Example data:
| Time (s) | Volume of gas (cm³) |
|---|---|
| 0 | 0 |
| 10 | 18 |
| 20 | 30 |
| 30 | 38 |
| 40 | 42 |
| 50 | 44 |
| 60 | 44 |
Rate in the first 10 seconds = 18 ÷ 10 = 1.8 cm³/s
Rate between 30 and 40 seconds = (42 − 38) ÷ (40 − 30) = 4 ÷ 10 = 0.4 cm³/s
Rate between 50 and 60 seconds = (44 − 44) ÷ (60 − 50) = 0 ÷ 10 = 0 cm³/s (reaction complete)
For experiments where a single time measurement is taken (such as the disappearing cross), the rate can be expressed as:
rate = 1 / time
This gives a value proportional to the rate. A shorter time means a faster reaction and a larger value of 1/t.
| Experiment | Time for cross to disappear (s) | Rate (1/t in s⁻¹) |
|---|---|---|
| A | 25 | 0.040 |
| B | 50 | 0.020 |
| C | 100 | 0.010 |
| D | 12.5 | 0.080 |
Experiment D has the fastest rate because 1/t is the largest value.
Exam tip: A shorter time does not mean a slower reaction. A shorter time means the reaction happened more quickly, so the rate is higher. This is a very common error in exams — make sure you are clear on this.
When two experiments are plotted on the same axes, you can compare them:
There is an important distinction between two types of rate that examiners like to test:
flowchart LR
A[Rate question] --> B{Single interval or single moment?}
B -->|Interval| C[Mean rate = change / time]
B -->|Moment| D[Instantaneous rate = gradient of tangent]
C --> E[Use two data points]
D --> F[Draw tangent, measure Δy and Δx]
A student collects gas during a reaction between magnesium and hydrochloric acid. At t = 20 s, she draws a tangent to the curve. The tangent rises by 15 cm³ over a time span of 10 s.
instantaneous rate at 20 s = 15 ÷ 10 = 1.5 cm³/s
Exam tip: When drawing a tangent, use a clear ruler, make the tangent line long (fill most of the graph width) and pick whole-number coordinates from the line — not from the curve. Picking points from the curve is the single most common marking error at GCSE.
Using the example data table earlier (volume of gas against time):
The initial rate is more than double the overall mean rate. This confirms that the reaction is fastest at the start — when reactant concentration is highest — and slows as reactants are consumed.
| Quantity | Value (cm³/s) | What it tells us |
|---|---|---|
| Initial rate (0–10 s) | 1.80 | Fastest moment of reaction |
| Mean rate (0–60 s) | 0.73 | Overall average |
| Rate 50–60 s | 0.00 | Reaction has finished |
| Mistake | Correction |
|---|---|
| Forgetting to convert minutes to seconds | Always check the required units first |
| Reading the gradient from curve instead of tangent | Draw a long, clear tangent using a ruler |
| Saying "shorter time = slower reaction" | Shorter time means faster rate (1/t is larger) |
| Using the final volume as "the rate" | Rate is change per unit time, not total product |
| Confusing mean rate and instantaneous rate | Mean uses two end points; instantaneous uses a tangent |
Exam tip: If the examiner provides graph paper, a ruler and the phrase "determine the rate at t = X," you are being asked for the instantaneous rate via a tangent. The word "determine" is a strong hint that working must be shown on the graph.
| Grade band | Expected response |
|---|---|
| Grades 3–4 (Foundation) | State that the rate of reaction is how fast reactants turn into products. Use rate = change/time with correct units (g/s or cm³/s). Recognise that a steeper graph line means a faster reaction. |
| Grades 5–6 (core) | Calculate mean rate accurately with unit conversions. Explain that rate is fastest at the start because collision theory predicts more frequent collisions when reactant concentration is highest. Interpret a rate-time curve, including the plateau. |
| Grades 7–9 (Higher) | Calculate instantaneous rate by drawing a tangent and using gradient = Δy/Δx. Use the 1/t method to analyse disappearing-cross data. Compare mean vs instantaneous rate quantitatively, link back to the activation energy and collision theory, and evaluate accuracy of different measurement methods (gas syringe vs mass loss vs colorimeter). |
Edexcel alignment: This content is aligned with Edexcel GCSE Chemistry (1CH0) specification Topic 6 Rate and extent of chemical change — specifically 6.1 Measuring rate (calculating mean rate of reaction, interpreting graphs, the 1/t method) and 6.2 Factors affecting rate (introduction). Assessed on Paper 2.