Chemistry · Lesson 10

Rates of reaction

JAMBWAECNECO

What the syllabus asks

JAMB: Rates of chemical reaction: elementary treatment of factors affecting rate, collision theory, activation energy, catalysts, reaction rate curves

WAEC: Rates of chemical reaction: meaning and measurement of rate; factors affecting rate (nature of reactants, concentration, pressure, temperature, surface area, catalysts, light); collision theory and activation energy; energy profile diagrams; interpretation of rate curves

NECO: Rates of chemical reaction: meaning and measurement of rate; factors affecting rate (nature of reactants, concentration, pressure, temperature, surface area, catalysts, light); collision theory and activation energy; energy profile diagrams; interpretation of rate curves

What you will learn

Define rate of reaction and calculate average rates from data.
Describe experiments to measure rate.
Explain the effects of concentration, pressure, temperature, surface area, catalysts and light using collision theory.
Draw and interpret energy profile diagrams with and without a catalyst.
Interpret and sketch rate curves for different conditions.
Apply rate ideas to everyday and industrial situations.

Key points

Rate = change in amount ÷ time; units such as cm³/s, g/s, mol dm⁻³ s⁻¹.
Measure by gas volume, mass loss, or time for colour/precipitate (rate ∝ 1/time).
Collision theory: particles must collide with energy ≥ Ea and correct orientation.
Concentration and pressure ↑ → more frequent collisions.
Temperature ↑ → more frequent collisions AND more particles ≥ Ea; ~10 °C doubles rate.
Smaller particles → larger surface area → faster.
Catalysts lower Ea via an alternative path; unchanged at the end; ΔH unchanged.
Light speeds photochemical reactions; enzymes are biological catalysts with optimum temperature and pH.
Slope = rate; final level = amount of limiting reactant.

Start here: What is rate of reaction?

Some reactions are very fast: a match flares in an instant, and gunpowder or a gas cooker explosion happens in a fraction of a second. Others are slow: iron roofing sheets rust over months, and palm wine slowly turns sour over a day or two. Chemists describe how fast a reaction goes by its rate.

Rate of reaction = change in amount of reactant or product ÷ time taken.

Rate can be measured as the amount of reactant used up per second, or product formed per second. Units depend on what is measured: cm³/s for gas, g/s for mass, mol dm⁻³ s⁻¹ for concentration.

Rate is like speed of a car. Just as speed = distance ÷ time, rate = change ÷ time. A fast reaction makes a lot of product in a short time.

Worked example 10.1

Calcium carbonate reacts with acid and 120 cm³ of carbon dioxide is collected in 60 s. Find the average rate.

Rate = volume ÷ time. We divide because rate is "per second".
120 ÷ 60 = 2 cm³/s.

2 cm³ s⁻¹

Worked example 10.2

A flask loses 0.12 g of mass in the first minute as gas escapes. Find the average rate in g/s.

Convert time: 1 minute = 60 s.
0.12 ÷ 60 = 0.002 g/s.

0.002 g s⁻¹

Try it yourself. 45 cm³ of hydrogen is produced in 15 s. Find the average rate.

Show solution

45 ÷ 15 = 3 cm³/s.

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Practice questions

1. Which factor does NOT increase the rate of a reaction?

Decreasing temperature
Using a catalyst
Increasing concentration
Powdering the solid

Show answer

A: Decreasing temperature
Lower temperature slows collisions.

2. A catalyst increases rate by

increasing the yield at equilibrium
raising the temperature
changing ΔH
lowering the activation energy

Show answer

D: lowering the activation energy
A catalyst gives an alternative path with lower activation energy; equilibrium position is unchanged.

3. Which factor does NOT generally increase the rate of a reaction?

Adding a catalyst
Increasing concentration
Increasing particle size
Raising the temperature

Show answer

C: Increasing particle size
Larger particles have less surface area.

4. How does a catalyst speed up a reaction?

It increases the concentration
It provides a route with lower activation energy
It raises the temperature
It increases ΔH

Show answer

B: It provides a route with lower activation energy
An alternative pathway.

5. Powdered marble reacts faster with acid than lumps because it has

a larger surface area
more mass
a higher temperature
a different formula

Show answer

A: a larger surface area
More particles are exposed.

Key words

Rate of reaction: Change in quantity of reactant or product per unit time.
Collision theory: Reaction occurs only when particles collide with enough energy and correct orientation.
Activation energy (Ea): Minimum energy colliding particles need to react.
Effective collision: A collision that leads to reaction.
Catalyst: A substance that changes the rate of a reaction but is chemically unchanged at the end.
Inhibitor (negative catalyst): A substance that slows a reaction.
Enzyme: A biological catalyst made of protein.
Energy profile diagram: A graph of energy against reaction path showing Ea and ΔH.
Activated complex: The high-energy arrangement at the peak of the energy profile.
Limiting reactant: The reactant that is used up first and controls the amount of product.
Gradient (slope): Steepness of a graph; on a rate curve, the rate.
Photochemical reaction: A reaction started or sped up by light.

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