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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).
$$ \text{Mean rate} = \frac{\text{Amount of product formed (or reactant used)}}{\text{Time taken}} $$
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 |
A reaction produced 48 cm³ of gas in 120 seconds. Calculate the mean rate.
$$ \text{Mean rate} = \frac{48}{120} = 0.40 \text{ cm}^3\text{/s} $$
A flask lost 1.8 g in mass during a 90 s reaction. Calculate the mean rate of reaction.
$$ \text{Mean rate} = \frac{1.8}{90} = 0.020 \text{ g/s} $$
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.
$$ \text{Rate at a point} = \frac{\text{change in } y}{\text{change in } x} $$
Exam 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.