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This lesson covers the definition of rate of reaction, methods for measuring rate, and how to calculate mean rate as required by the Edexcel GCSE Combined Science specification (1SC0). Understanding rate of reaction is essential — it underpins every topic in this unit.
The rate of reaction is a measure of how quickly reactants are turned into products. It can be defined as:
Rate of reaction = the amount of reactant used up or product formed per unit time.
A fast reaction is one that is completed in a short time — an explosion, for example, is over in a fraction of a second. A slow reaction may take minutes, hours or even years — the rusting of iron is a familiar example.
We cannot measure rate of reaction directly. Instead we measure a quantity that changes over time — such as the volume of gas produced, the mass of the reaction mixture, or a colour change — and use that data to calculate rate.
If one of the products is a gas, we can collect it and measure its volume at regular time intervals using a gas syringe or by collecting over water in an inverted measuring cylinder.
| Feature | Detail |
|---|---|
| Apparatus | Conical flask, delivery tube, gas syringe (or water trough and measuring cylinder) |
| What you record | Volume of gas collected (cm³) at regular time intervals |
| Advantages | Accurate, continuous measurement; gas syringe gives easy readings |
| Disadvantages | Some gas may escape before the bung is fitted; not suitable if the gas is very soluble in water |
graph LR
A["Conical flask<br/>(reactants)"] -->|"delivery<br/>tube"| B["Gas syringe"]
B --> C["Read volume<br/>at intervals"]
style A fill:#2980b9,color:#fff
style B fill:#27ae60,color:#fff
style C fill:#8e44ad,color:#fff
If a gas is produced and escapes from an open flask on a balance, the total mass of the flask decreases over time. We record the mass at regular intervals.
| Feature | Detail |
|---|---|
| Apparatus | Conical flask on a top-pan balance, cotton wool plug (to stop liquid splashing out while allowing gas to escape) |
| What you record | Mass of flask + contents (g) at regular time intervals |
| Advantages | Simple, continuous; balance gives precise readings |
| Disadvantages | If the gas produced is very light (e.g. hydrogen) the mass changes are very small and hard to measure accurately |
Exam Tip: A cotton wool plug lets gas escape but stops liquid or solid from splashing out. If a bung is used instead, the mass will not change because no gas can leave.
In the reaction between sodium thiosulfate and hydrochloric acid, a yellow precipitate of sulfur forms, making the solution go cloudy. A cross drawn on paper beneath the flask gradually disappears as the precipitate builds up. We time how long it takes for the cross to vanish.
| Feature | Detail |
|---|---|
| Apparatus | Conical flask, paper with a cross, stopwatch |
| What you record | Time for the cross to disappear (s) |
| Advantages | Very simple, minimal apparatus |
| Disadvantages | Subjective — different people may judge the disappearing point differently; gives only one measurement per experiment (not continuous) |
When we plot the amount of product formed (or reactant used) against time, we get a characteristic curve.
graph LR
subgraph "Typical Rate Graph"
direction LR
A["Time →"] --- B["Volume of gas ↑"]
end
| Feature | Meaning |
|---|---|
| Steep gradient at the start | The reaction is fastest at the beginning when reactant concentrations are highest |
| Gradient decreases over time | The reaction slows down as reactants are used up |
| Curve levels off (plateau) | The reaction has finished — all of the limiting reactant has been used up |
| Total product formed | Read from the final (plateau) value on the y-axis |
Mean rate is the average rate over the entire reaction (or over a measured time period).
The units depend on what you are measuring:
| Quantity measured | Mean rate units |
|---|---|
| Volume of gas (cm³) | cm³/s |
| Mass lost (g) | g/s |
| 1/time (s⁻¹) | s⁻¹ |
Mean rate=12048=0.40 cm3/sA reaction produced 48 cm³ of gas in 120 seconds. Calculate the mean rate.
Mean rate=901.8=0.020 g/sA flask lost 1.8 g in mass during a 90 s reaction. Calculate the mean rate of reaction.
Exam Tip: When calculating mean rate from a graph, pick two points on the straight (initial) part of the curve to find the initial rate, or use the total values for the mean rate across the whole reaction.
To find the instantaneous rate at a particular moment, draw a tangent to the curve at that point and calculate the gradient of the tangent line.
Rate at a point=change in xchange in yExam Tip: A tangent is a straight line that just touches the curve at one point. Use a ruler and make the triangle as large as possible for accuracy.
A student collects hydrogen gas from the reaction of magnesium with dilute hydrochloric acid. At 20 s, the volume is 14 cm³. At 80 s, the volume is 56 cm³. Calculate the mean rate between 20 s and 80 s.
Step 1 — change in volume: 56 − 14 = 42 cm³. Step 2 — change in time: 80 − 20 = 60 s. Step 3 — mean rate: 42 ÷ 60 = 0.70 cm³/s.
Exam Tip: "Mean rate between two points" means use only the change between those two points, not the totals. Candidates often lose a mark by dividing by 80 s instead of by 60 s.
Initial rate=25−060−0=2.4 cm3/sA tangent is drawn at the origin of a curve showing volume of CO₂ (cm³) against time (s). The tangent passes through (0, 0) and (25, 60). Calculate the initial rate.
Exam Tip: The initial rate is always the gradient at t = 0. It is the fastest rate of the whole reaction because reactant concentrations are highest at the start.
A reaction produced 0.24 g of magnesium chloride in 2.0 minutes. Express the mean rate in g/s.
Step 1 — convert time: 2.0 min × 60 = 120 s. Step 2 — mean rate: 0.24 ÷ 120 = 0.0020 g/s (or 2.0 × 10⁻³ g/s).
Common mistake callout: Students forget to convert minutes to seconds. Always check that the units for time match the units asked for in the answer.
| Method | Best for | Measuring instrument | Continuous? | Precision | Limitation |
|---|---|---|---|---|---|
| Gas syringe | Reactions producing insoluble gases | Gas syringe (cm³) | Yes | High | Gas may escape before bung is fitted |
| Collection over water | Insoluble gases (not CO₂, SO₂) | Measuring cylinder | Yes (at intervals) | Moderate | Soluble gases dissolve — underestimate rate |
| Mass loss | Gas products, heavy gases | Top-pan balance (g) | Yes | High (for heavy gases) | Poor for H₂ (too light) |
| Disappearing cross | Precipitate-forming reactions | Stopwatch + cross | No — single time | Low (subjective) | Human judgement of endpoint |
| Colorimetry | Reactions with colour change | Colorimeter | Yes | Very high | Requires specialist equipment |
graph TD
A["What is produced?"] -->|"Gas"| B{"Is the gas heavy<br/>or very soluble?"}
A -->|"Precipitate / cloudy"| C["Disappearing cross"]
A -->|"Colour change"| D["Colorimeter"]
B -->|"Heavy, e.g. CO2"| E["Mass loss"]
B -->|"Light, e.g. H2"| F["Gas syringe"]
B -->|"Insoluble"| F
style C fill:#8e44ad,color:#fff
style D fill:#2980b9,color:#fff
style E fill:#27ae60,color:#fff
style F fill:#27ae60,color:#fff
Common mistake 1: Reporting rate with no units. Always include units (cm³/s, g/s, or s⁻¹).
Common mistake 2: Confusing mean rate and initial rate. Mean rate uses the entire reaction; initial rate is the gradient at t = 0.
Common mistake 3: Drawing a tangent that is too short. Make the triangle as large as possible — small triangles magnify reading errors.
Common mistake 4: Writing "the reaction stops" at the plateau. The correct wording is "the reaction has finished because the limiting reactant has been used up."
The curve bends because the rate is continuously changing. As the reaction proceeds:
| Time region | Gradient | What is happening |
|---|---|---|
| t = 0 | Steepest | Highest concentration of reactants |
| Middle | Decreasing | Reactants being used up |
| Late | Near zero | Very few reactant particles remain |
| Plateau | Zero | Limiting reactant fully consumed |
| Grade band | Expected depth of answer |
|---|---|
| Grade 3–4 | State that rate of reaction measures how fast products form. Quote the formula: rate = amount ÷ time. Name one measurement method (e.g. gas syringe). |
| Grade 5–6 | Use the precise term mean rate and calculate with correct units. Describe how a rate–time curve is steep at the start and levels off. Explain qualitatively how activation energy means only energetic collisions succeed. |
| Grade 7–9 | Calculate instantaneous rate from a tangent and justify the choice of measurement method. Link the falling gradient to the fall in reactant concentration and explain, using activation energy, why only a fraction of collisions are successful at any moment. Use precise terms: rate, activation energy, catalyst, exothermic, endothermic, bond energy. |
Question 1: A student collects 75 cm³ of gas in 150 s. Calculate the mean rate, with units.
Answer: 75 ÷ 150 = 0.50 cm³/s.
Question 2: A tangent drawn at t = 30 s on a curve passes through (10, 12) and (50, 60). Calculate the instantaneous rate at t = 30 s.
Answer: gradient = (60 − 12) / (50 − 10) = 48/40 = 1.2 cm³/s.
Question 3: A reaction is faster at the start. Explain in terms of collision theory.
Answer: At the start, reactant concentrations are at their highest, so collisions are most frequent and more successful collisions per second occur. As the reaction proceeds, reactants are used up and collision frequency falls, so the rate decreases.
Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Chemistry Topic 6 Rates of reaction / Topic 7 Energy changes — specifically CC14 Rates of reaction (measuring rate, calculating mean rate, interpreting rate–time graphs). Assessed on Chemistry Paper 2.