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Rate of Reaction — Measuring and Calculating
Rate of Reaction — Measuring and Calculating
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.
What Is the Rate of Reaction?
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:
- The speed at which reactants are used up
- The speed at which products are formed
| 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.
Methods of Measuring Rate
There are several practical methods for measuring the rate of a reaction. The method you choose depends on the type of reaction taking place.
1. Measuring Change in Mass Over Time
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).
2. Measuring Volume of Gas Collected Over Time
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.
3. The Disappearing Cross Method
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.
4. Colour Change
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 |
Calculating Mean Rate of Reaction
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 |
Worked Example 1
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
Worked Example 2
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.
Reading and Interpreting Rate Graphs
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.
Key Features of a Typical Rate Graph
- Steep gradient at the start — the reaction is fastest at the beginning because the concentration of reactants is highest.
- Gradient decreases — the rate slows as reactants are used up and the concentration falls.
- Line levels off (plateau) — the reaction is complete; no more product is being formed. All the limiting reactant has been used up.
What Does the Gradient Mean?
The gradient (slope) of the line at any point on the graph equals the rate of reaction at that moment.
- A steeper gradient = a faster rate of reaction
- A shallower gradient = a slower rate of reaction
- A horizontal line (zero gradient) = the reaction has stopped
Calculating Rate from a Graph
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)
The 1/t Method
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.
Comparing Reactions on the Same Graph
When two experiments are plotted on the same axes, you can compare them:
- A curve that rises more steeply represents the faster reaction
- If both curves level off at the same height, the same total amount of product was formed (the same amount of reactant was used)
- If one curve levels off at a lower height, less product was formed (perhaps less reactant was used or a different amount was present)
Summary
- The rate of reaction measures how quickly reactants are converted into products
- Rate can be measured by change in mass, volume of gas collected, disappearing cross or colour change
- Mean rate = amount of product formed ÷ time taken (or amount of reactant used ÷ time)
- Common units include g/s, cm³/s and mol/s
- On a graph of product vs time, the gradient equals the rate
- The rate is fastest at the start and decreases as reactants are used up
- The 1/t method is used when a single time measurement is taken (e.g. disappearing cross)
- A steeper curve on a rate graph means a faster reaction