S6 Chemistry Practice Book · Unit 12: Factors that affect the rate of reactions

Unit 12 Factors that affect the rate of reactions

Key unit competence: Analyse the factors that affect the rate of a chemical reaction and the quantitative relationship between rate and these factors.

Key ideas

Rate of reaction = change in concentration (or amount) of a reactant or product per unit time: rate = −Δ[reactant] ÷ Δt = Δ[product] ÷ Δt; units mol dm⁻³ s⁻¹. The rate at a given moment is the gradient of the tangent to the concentration–time curve.
Collision theory: particles must collide with energy at least equal to the activation energy (E_a) and in the correct orientation. Anything that increases the frequency of effective collisions increases the rate.
Concentration (and pressure for gases): more particles per unit volume, more frequent collisions. Surface area: smaller pieces of solid expose more surface (powder reacts faster than lumps; dust explosions in flour mills and mines).
Temperature: particles move faster and, more importantly, a larger fraction has energy ≥ E_a (Maxwell–Boltzmann distribution). As a rough rule a 10 °C rise doubles the rate of many reactions.
Catalysts provide an alternative pathway with lower activation energy and are unchanged at the end. Homogeneous (same phase, e.g. H⁺ in esterification, Fe²⁺ in the S₂O₈²⁻/I⁻ reaction) and heterogeneous (different phase, e.g. Fe in the Haber process, Pt/Rh in catalytic converters). Enzymes are biological catalysts. Light speeds up photochemical reactions (photosynthesis, H₂ + Cl₂, silver halides in photography).
Rates are followed by measuring gas volume (gas syringe), loss of mass (gas escaping), colour change (colorimeter), conductivity or pH change, or by sampling and titrating (quenching). Example: CaCO₃ + HCl, Mg + HCl, sodium thiosulfate + HCl (cross disappears: rate ∝ 1/time).

Common misconceptions

"Temperature speeds up a reaction only because particles collide more often." No: The main effect is that many more particles have energy above E_a.
"A catalyst is used up in the reaction." No: A catalyst takes part but is regenerated, unchanged in mass and chemical nature.
"A catalyst gives more product." No: It speeds up the reaction; the amount of product (yield) is not changed.

Worked examples

Example 1. Describe an experiment to show how temperature affects the rate of the reaction between sodium thiosulfate and hydrochloric acid.

Solution

Put 50 cm³ of sodium thiosulfate solution in a conical flask on a paper marked with a cross; warm to a measured temperature (e.g. 20, 30, 40, 50 °C).

Add 5 cm³ of HCl, start the clock: Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O; the sulfur makes the mixture cloudy.

Record the time for the cross to disappear; rate ∝ 1/time.

Keep concentrations and volumes constant (fair test); plot 1/t against temperature: the rate rises steeply with temperature.

Example 2. Explain, using the Maxwell–Boltzmann distribution, why a catalyst increases the rate.

Solution

The curve shows the number of particles with each energy; only those beyond E_a can react.

A catalyst does not change the curve (same temperature).

It provides a route with a lower activation energy, E_cat, further to the left on the energy axis.

The area under the curve beyond E_cat is larger: many more particles have enough energy, so the rate increases.

Practice questions

Define activation energy.
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The minimum energy colliding particles need for a reaction to take place.
Common mistake: Saying it is the energy released.
State two conditions for a collision to be effective.
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Energy ≥ E_a and correct orientation.
Common mistake: Saying 'the particles must be hot'.
Explain why the rate of a reaction falls as it proceeds.
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Reactant concentration decreases, so collisions become less frequent.
Common mistake: Saying the catalyst wears out.
Name two methods of following the rate of a reaction.
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Gas volume, mass loss, colorimetry, conductivity, titration of samples (any two).
Common mistake: Naming 'tasting the mixture'.
Why do enzymes stop working at high temperatures?
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They are proteins that are denatured: their active site changes shape.
Common mistake: Saying they melt.
Give an everyday example of the effect of surface area on rate.
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Chopped vegetables cook faster; kindling burns faster than logs; powdered medicine acts faster (any one).
Common mistake: An example about temperature.
What is a heterogeneous catalyst? Give an example.
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A catalyst in a different phase from the reactants, e.g. iron in the Haber process.
Common mistake: Giving H⁺ in esterification.
Why does a catalyst affect forward and backward reactions equally?
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It lowers the activation energy of both directions by the same amount.
Common mistake: Saying it only speeds the forward reaction.
Why are reactions of ions in solution usually very fast?
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Oppositely charged ions attract and need little activation energy.
Common mistake: Saying ions are heavy.
How does a catalytic converter reduce pollution?
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Pt/Rh catalysts speed up 2CO + 2NO → 2CO₂ + N₂ and oxidation of unburnt fuel.
Common mistake: Saying it filters out smoke.

Matching and drag-and-drop

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. Flour mills can explode because fine flour dust has:	a. more particles have energy ≥ E_a
2. Food is kept in a refrigerator to:	b. Hydrogen with chlorine
3. Which reaction is sped up by light?	c. slow down the reactions that spoil it
4. A rise in temperature increases the rate mainly because:	d. a very large surface area

Show answer

1 → d (a very large surface area) · 2 → c (slow down the reactions that spoil it) · 3 → b (Hydrogen with chlorine) · 4 → a (more particles have energy ≥ E_a)

Very fast combustion.
Lower temperature, slower rate.
Photochemical reaction.
Maxwell–Boltzmann.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
heterogeneous catalyst · activation energy · produces a gas · forms a precipitate only
1. A gas syringe is used to follow the rate of a reaction that .
2. Iron in the Haber process is a .
3. For particles to react, they must collide with energy at least equal to the .
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1: produces a gas · 2: heterogeneous catalyst · 3: activation energy
- Measure volume against time.
- Solid catalyst, gaseous reactants.
- And correct orientation.

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. A catalyst increases the rate by:	a. reactant concentration falls
2. The units of rate of reaction are usually:	b. mol dm⁻³ s⁻¹
3. The rate of a reaction usually decreases with time because:	c. providing a pathway with lower activation energy
4. In the thiosulfate–acid experiment, rate is taken as proportional to:	d. 1 ÷ time for the cross to disappear

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1 → c (providing a pathway with lower activation energy) · 2 → b (mol dm⁻³ s⁻¹) · 3 → a (reactant concentration falls) · 4 → d (1 ÷ time for the cross to disappear)

Alternative route.
Concentration per time.
Fewer collisions.
Shorter time, faster rate.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
the gas particles are closer together · the reaction becomes reversible · collisions become more frequent · platinum and rhodium
1. Increasing the concentration of a reactant increases the rate because .
2. Increasing the pressure of a gaseous reaction increases its rate because .
3. A catalytic converter in a car uses .
Show answer
1: collisions become more frequent · 2: the gas particles are closer together · 3: platinum and rhodium
- More particles per unit volume.
- Higher concentration.
- Converts CO and NO to CO₂ and N₂.

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