R.2 Energy Changes & Rates of Reaction

UNIT SYLLABUS

MYP

Some reactions blaze (combustion), some chill (instant cold packs) — the difference is the energy balance between breaking old bonds and making new ones. Exothermic reactions release energy to the surroundings; endothermic reactions absorb it. Meanwhile, collision theory explains reaction speed: particles must collide with enough energy to react, so anything that increases collision frequency or energy — concentration, temperature, surface area, or a catalyst lowering the energy barrier — speeds the reaction up. These two ideas power every industrial chemistry decision from fertiliser plants to hand warmers.

Guiding Questions

? Why do some reactions give out heat while others take it in?
? What can we change to make a reaction go faster — and why does it work?

✓ What the IB expects you to master

Classify reactions as exothermic (temperature of surroundings rises) or endothermic (falls), with everyday examples.
Explain energy changes through bond breaking (absorbs energy) and bond making (releases energy).
Sketch and interpret energy level diagrams, including activation energy.
State collision theory: reactions need collisions with sufficient energy.
Explain how concentration, temperature, surface area and catalysts affect reaction rate.
Measure rate experimentally: gas volume against time, or mass loss against time, and interpret the graphs.
Describe catalysts as substances that speed reactions without being used up, by lowering activation energy.

1 Key Formulas

Mean rate of reaction

\text{rate} = \frac{\text{quantity of product formed}}{\text{time taken}}

2 Exam Preparation & Topic Explanations

Rates investigations — the Criterion B & C goldmine

Rates is the most common MYP chemistry investigation: it offers clean variables (concentration, temperature, surface area), measurable outcomes (gas volume, mass loss, time for a cross to disappear), and graph analysis. Know the method, the fair-test controls, and how to read initial rate from the steepest part of the curve.

For explain marks, ALWAYS answer through collision theory: frequency of collisions, energy of collisions, or both.

Pro Exam Strategy

Steeper initial gradient = faster rate; same final plateau = same amount of product.
"More collisions" is not enough — say more collisions PER SECOND (frequency).
Temperature is the double-action factor: collisions are both more frequent AND more energetic.
Exothermic examples: combustion, neutralisation, respiration. Endothermic: photosynthesis, thermal decomposition, cold packs.

3 MCQ Practice

Q1. A reaction mixture in a beaker gets colder as the reaction proceeds. The reaction is:

Exothermic, because energy is released
Endothermic, because energy is absorbed from the surroundings
Catalytic, because no energy is involved
Reversible, because temperature changed

Q2. Powdered marble reacts faster with acid than the same mass of marble chips because the powder has:

More mass
A larger surface area
A higher temperature
A higher concentration

Q3. A catalyst speeds up a reaction by:

Increasing the temperature of the mixture
Increasing the energy of the collisions
Providing a pathway with lower activation energy
Being consumed to release extra energy

4 Short Answer Questions

PDF

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All questions from this topic + answer key — free, printable.

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MYP Physics Guide →

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Criterion B Guide →

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