Lesson focus: surface areaThe total area of the outside of an object, where things can touch or pass through. and concentrationThe amount of solute dissolved in a given volume of solution, such as in g/dm³; a higher concentration has more solute particles in the same volume. change reaction rate by changing how often reacting particles can meet, without necessarily changing final yield.
Explain and investigate surface-area and concentration effects using collision theory and rate graphs.
Build on earlier learning
Collision theory and temperatureTemperature changed successful-collision frequency. This lesson applies the same framework to contact area and particle number per volume.
Practical
Surface area, concentration and rate
- React marble chips with acid in a flask on a balance.
- Record the mass every 30 seconds as carbon dioxide escapes.
- Repeat with powdered marble, then with a more concentrated acid.
Record: Mass-loss graphs for each condition on the same axes.
Safety: Wear eye protection.
Collision theory
Particles react only when they collide with enough energy (the activation energy). Heating gives more particles that energy; a higher concentration gives more collisions.
Reactant A Reactant B Enough energy to react
Controls
Speeds are exaggerated so the effect of temperature shows within a minute.
You will be successful when you can…
- Explain concentration effects.
- Explain surface-area effects.
- Interpret slope and plateau.
- Design a controlled rate comparison.
Words that carry the science
- concentration
- Amount of dissolved substance per unit volume of solution.
- surface area
- The total area of a solid exposed to surrounding reactant.
- initial rate
- Reaction rate near the start, represented by the initial graph gradient.
- limiting reactant
- The reactant used up first, limiting the amount of product.
- plateau
- A graph region where the measured product amount no longer changes.
- gradient
- Change in vertical quantity divided by change in horizontal quantity.
Change collision opportunities while separating rate from yield
01Higher concentration increases collision frequency
At higher concentrationThe amount of solute dissolved in a given volume of solution, such as in g/dm³; a higher concentration has more solute particles in the same volume., more reacting particles occupy each unit volume. They collide with the other reactant more frequently, increasing successful collisions per second.
Concentration does not make each particle intrinsically more reactive. Temperature controls energy distribution; concentration mainly changes encounter frequency.
02Smaller pieces expose more surface
Breaking a fixed mass of solid into smaller pieces increases total surface areaThe total area of the outside of an object, where things can touch or pass through.. More particles are exposed at the boundary, so more collisions can occur at once. Powder commonly reacts faster than large chips.
Particle size also affects hazards: combustible dusts may burn or explode rapidly because of their large contact area with oxygen.
03Graph shape separates rate from product amount
A steeper initial gradientHow steep a line on a graph is. means a faster initial rateHow fast a reaction goes right at the start.. If the same amount of limiting reactantThe reactant that gets used up first and so stops the reaction. is used, curves can reach the same plateauA flat part of a graph, where the value has stopped changing. even when concentration or particle size changes the time taken.
If increasing concentration also increases the amount of limiting reactant, both rate and final product may change. State exactly which quantities are held constant.
Compare early graph rates
Curve A reaches 30 cm³ in 20 s; curve B reaches 30 cm³ in 50 s near the start.
- Use average early rate = gas volume ÷ time.
- A: 30 ÷ 20 = 1.5 cm³ s⁻¹.
- B: 30 ÷ 50 = 0.60 cm³ s⁻¹.
- A is 2.5 times the early average rate of B.
How to make the reasoning visible: Use the same interval and call this an early average unless a tangent gives instantaneous rate.
Distinguish rate and yield changes
Calcium carbonate reacts with hydrochloric acid; compare one change at a time.
| Trial change | Initial slope | Expected plateau | Collision explanation | Key control |
|---|---|---|---|---|
| Powder instead of chips | Steeper | Same if amounts unchanged | More exposed surface | Same CaCO₃ mass |
| 2.0 instead of 1.0 mol dm⁻³ acid | Steeper | Same if CaCO₃ limits | More acid particles per volume | Same acid volume |
| Double CaCO₃; acid excess | May change with area | Higher | More solid available | Match particle size |
| Higher temperature | Steeper | Usually same theoretical amount | More energetic collisions | Control concentration |
Graph skills: SLAPUK and choosing a scalePlotting rate results? SLAPUK and the scale rules.
Why might powder show a lower measured plateau in a leaky apparatus?
Why must concentration and volume both be reported?
Check your understanding
Answer from memory, then check against the suggested answer.
How does higher concentration increase rate?
Why does powder react faster than chips?
What does a plateau show?
A catalyst makes a curve steeper but the plateau is unchanged. Explain.
Explain why doubling concentration does not guarantee exactly double rate.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Equal masses of calcium carbonate chips and powder react with excess acid.
Explain why the powder reacts faster.
(4)Explain why both produce the same final volume of carbon dioxide.
(1)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., use surface → collisions → rate, then a separate limiting-reactant argument for yield.