Rates of ReactionAQA GCSE Chemistry: Revision notes
Section 1
What is rate of reaction and how do we calculate it?
Rate of reaction is a measure of how quickly a chemical reaction occurs. It is calculated using the formula:
Rate = quantity of reactant used (or product formed) ÷ time
The quantity can be measured in:
- Mass (in grams)
- Volume (in cm³ for gases)
- Concentration (in mol/dm³)
- Number of moles
The time is measured in seconds (s) or minutes (min).
Units of rate depend on what is being measured:
- If using mass: g/s or g/min
- If using volume: cm³/s or cm³/min
- If using concentration: mol/(dm³·s) or mol/(dm³·min)
A faster reaction has a larger rate value; a slower reaction has a smaller rate value.
A reaction produces 8 g of product in 40 seconds. Rate = 8 ÷ 40 = 0.2 g/s. If another reaction produces 12 g in 60 seconds, rate = 12 ÷ 60 = 0.2 g/s – both reactions have the same rate despite different quantities.
Always include units in your final answer. Examiners reward correct units as marks, and forgetting them is a common error.
Section 2
How can we measure the rate of a reaction experimentally?
There are several methods to monitor reaction progress and measure rate:
| Method | Measures | Use Case | Example |
|---|---|---|---|
| Change in mass | Decrease in mass of reactants (or gas lost) | Reactions producing gas | Decomposition of calcium carbonate with acid |
| Volume of gas collected | Volume of gas produced over time | Gas-producing reactions | Reaction of metal with dilute acid |
| Colour change | Change in colour intensity using absorbance | Reactions with coloured products or reactants | Iodine being decolourised in starch-iodine reaction |
| Precipitation | Formation of solid precipitate (cross experiment) | Reactions forming insoluble products | Sodium thiosulfate reacting with acid producing sulphur |
The cross experiment is used to measure rate when a precipitate forms. A cross is drawn on paper beneath a beaker containing the reaction mixture. A timer is started, and the time taken for the cross to become obscured by the opaque precipitate is recorded. This measures how quickly the precipitate forms.
Rate–time graphs show how rate changes during a reaction:
- Steeper gradient = faster rate
- Shallower gradient = slower rate
- Gradients become less steep over time as reactants are consumed
For the cross experiment, examiners expect you to explain that you record the time when the cross disappears. State clearly that this measures how quickly the precipitate forms, which indicates reaction rate.
Students often confuse concentration–time graphs with rate–time graphs. Remember: concentration–time shows the amount of substance remaining, whilst rate–time shows how fast the reaction is happening at each moment.
Section 3
How does concentration affect the rate of reaction?
Increasing the concentration of reactants increases the rate of reaction.
This is explained using collision theory:
- When concentration increases, there are more particles of reactant in the same volume
- This means particles are closer together and collide more frequently
- More frequent collisions = more successful reactions per unit time
- Therefore, the rate of reaction increases
On a concentration–time graph:
- Higher initial concentration shows a steeper gradient at the start
- The curve descends more steeply
- The reaction reaches completion faster
Important: Increasing concentration does NOT change the activation energy of the reaction – it only increases how often particles collide with sufficient energy to react.
This relationship is particularly important in bimolecular reactions where the rate depends on the concentrations of two reactants. If both concentrations are doubled, the collision frequency increases significantly.
Think of concentration like a crowd in a room: more people (higher concentration) means more people bump into each other (more collisions), so interactions happen faster.
If you increase the concentration of a reactant from 0.5 mol/dm³ to 1.0 mol/dm³ (doubling it), the particle density doubles, so collisions occur approximately twice as frequently, and the reaction rate increases.
Section 4
How do temperature and surface area affect the rate of reaction?
Temperature increases the rate of reaction through two mechanisms:
- Increased collision frequency: Particles move faster at higher temperatures, so they collide more often
- Increased proportion of particles with sufficient energy: Only particles with energy ≥ activation energy can react. Higher temperature means more particles exceed the activation energy threshold. This is the more important effect at GCSE level.
On a concentration–time graph:
- A higher temperature shows a steeper initial gradient
- The curve descends faster
- The reaction is completed in a shorter time
Surface area affects rate when one reactant is a solid:
- Increasing surface area increases the rate of reaction
- When a solid is broken into smaller pieces, its total surface area increases
- More surface area exposed = more sites where reactant particles can collide with the solid
- More frequent collisions between liquid/gas particles and the solid surface
- Powdered solids react faster than large lumps of the same solid
Examples:
- Marble chips (large lumps) react slowly with acid; marble powder reacts much faster
- Zinc granules react faster than zinc foil with the same acid concentration
For higher tier, remember that temperature's effect on the proportion of particles exceeding activation energy is more significant than its effect on collision frequency. Examiners reward understanding this distinction.
Calcium carbonate and acid: A 10 cm lump takes 60 seconds to react. The same mass ground to powder might react in 15 seconds because the surface area increases dramatically, creating many more collision sites.
Section 5
How do catalysts affect rate, and what about higher tier rate calculations?
Catalysts increase the rate of reaction without being consumed or chemically changed:
- A catalyst lowers the activation energy of the reaction
- With a lower activation energy, more particles have sufficient energy to react at any given temperature
- This increases the proportion of successful collisions
- The reaction proceeds faster without altering reactants or products
Catalysts are used in industry to speed up reactions and reduce costs (e.g., iron catalyst in the Haber process).
Higher Tier: Calculating rate from concentration–time graphs
When a concentration–time graph is curved, the rate changes over time. To find the rate at a specific moment, use the tangent method:
- Identify the point on the curve where you need to find the rate
- Draw a tangent line (a straight line that just touches the curve without crossing it at that point)
- Calculate the gradient of the tangent:
- Gradient = vertical change (Δ concentration in mol/dm³) ÷ horizontal change (Δ time in s)
- Gradient = (y₂ − y₁) ÷ (t₂ − t₁)
- The gradient equals the rate at that moment
Important: The gradient of a tangent to a concentration–time graph gives rate in mol/(dm³·s) or similar concentration units per time unit. The steeper the tangent, the faster the rate at that point.
On a concentration–time graph, at 10 seconds the concentration is 0.8 mol/dm³. A tangent drawn at this point shows: from 8s (0.9 mol/dm³) to 12s (0.7 mol/dm³). Gradient = (0.7 − 0.9) ÷ (12 − 8) = −0.2 ÷ 4 = −0.05 mol/(dm³·s). The negative sign shows concentration decreasing; the rate of reaction is 0.05 mol/(dm³·s).
When calculating tangent gradients, use a ruler and extend the tangent line across a wide section of your graph to minimise errors. Examiners reward clear working and accurate gradient calculations with method marks.
Must Know
- Rate = quantity of reactant used (or product formed) ÷ time; includes correct units (g/s, cm³/s, mol/(dm³·s))
- Collision theory: reactions occur when particles collide with sufficient energy; faster reactions have more frequent collisions and/or higher collision energy
- Concentration: increasing concentration increases collision frequency and rate (no change to activation energy)
- Temperature: increases rate by increasing both collision frequency AND the proportion of particles with energy ≥ activation energy
- Surface area: increasing the exposed surface area of a solid increases collision sites and rate
- Catalysts: lower activation energy, allowing more particles to react, without being consumed
- Rate-time graphs: steeper gradient = faster rate; can measure rate at any time by drawing a tangent to a concentration–time curve and calculating its gradient
That's the notes covered.
Carry on to the next subtopic.