IGCSE Chemistry — Chemical Energetics: Notes, Practice Questions & Worked Solutions

IGCSE Chemistry 0620 · Topic 5: Chemical Energetics

Chemical Energetics

Every chemical reaction involves breaking bonds in the reactants and making new bonds in the products. Whether a reaction releases or absorbs energy overall depends on which of these two processes “wins.”

1. Exothermic and Endothermic Reactions

	Exothermic	Endothermic
Energy	Released to surroundings	Absorbed from surroundings
Temperature	Increases	Decreases
Example	Combustion, neutralisation	Thermal decomposition, photosynthesis

Bond breaking always absorbs energy (endothermic); bond making always releases energy (exothermic). The overall reaction is exothermic if more energy is released making bonds than was absorbed breaking them, and endothermic if the reverse is true.

Worked example — calculating ΔH from bond energies Calculate the enthalpy change for H₂ + Cl₂ → 2HCl, given bond energies: H−H = 436 kJ/mol, Cl−Cl = 242 kJ/mol, H−Cl = 431 kJ/mol.
Energy to break bonds (reactants): 436+242 = 678 kJ/mol.
Energy released making bonds (products): 2×431 = 862 kJ/mol.
ΔH = energy in − energy out = 678 − 862 = −184 kJ/mol (exothermic)

Worked example — calculating heat energy from temperature change 100 g of water rises in temperature by 10°C. Calculate the heat energy released (specific heat capacity of water = 4.2 J/g°C).
q = mass × specific heat capacity × temperature change = 100 × 4.2 × 10 = 4200 J = 4.2 kJ

2. Reaction Pathway Diagrams

A reaction pathway (energy level) diagram shows the energy of reactants and products, and the activation energy — the minimum energy needed for a reaction to start. In an exothermic reaction, products sit lower than reactants; in an endothermic reaction, products sit higher.

Common mistakes to watch for

Mixing up which process is endothermic — bond BREAKING always needs energy IN (endothermic), regardless of whether the overall reaction is exothermic.
Forgetting units — bond energies are usually in kJ/mol, but q=mcΔT gives an answer in J unless you convert.
Confusing activation energy with the overall enthalpy change (ΔH) — activation energy is always positive (a barrier to overcome), even in an exothermic reaction.

Key Vocabulary Recap

Exothermic — a reaction that releases energy to the surroundings

Endothermic — a reaction that absorbs energy from the surroundings

Activation energy — the minimum energy needed for a reaction to occur

Syllabus Reference

This note covers IGCSE Chemistry 0620 Topic 5, Chemical Energetics: exothermic and endothermic reactions, activation energy, reaction pathway diagrams, bond breaking/making, and calculating enthalpy changes using bond energies.

Practice questions

IGCSE Chemistry 0620 · Topic 5: Chemical Energetics — Practice Question Bank (no answers)

Chemical Energetics — Practice

Covers: exothermic/endothermic reactions, bond energy calculations, and heat energy calculations. 10 questions.

Q1 Core

Define exothermic and endothermic reactions, in terms of energy transfer.

Q2 Core

State whether bond breaking is exothermic or endothermic, and whether bond making is exothermic or endothermic.

Q3 Core

Classify each as exothermic or endothermic: (a) combustion, (b) thermal decomposition of calcium carbonate, (c) neutralisation.

Q4 Core

Define activation energy.

Q5 Extended

Calculate the enthalpy change for H₂ + Cl₂ → 2HCl, given bond energies: H−H = 436 kJ/mol, Cl−Cl = 242 kJ/mol, H−Cl = 431 kJ/mol.

Q6 Extended

150 g of water rises in temperature by 8°C. Calculate the heat energy absorbed (specific heat capacity of water = 4.2 J/g°C).

Q7 Core

Sketch (in words) how a reaction pathway diagram for an exothermic reaction differs from one for an endothermic reaction.

Q8 Core

Explain why a catalyst does not change the overall enthalpy change (ΔH) of a reaction.

Q9 Core

A student mixes an acid and an alkali and notices the temperature of the mixture rises. State whether this reaction is exothermic or endothermic, and explain your reasoning.

Q10 Extended

Explain why an overall exothermic reaction must release more energy making bonds than it absorbs breaking them.

Worked solutions

Show worked solutions

IGCSE Chemistry 0620 · Topic 5: Chemical Energetics — Solutions

Chemical Energetics — Practice: Solutions

Define exothermic/endothermic.

✓Exothermic: a reaction that releases (transfers) energy to the surroundings, usually shown by a temperature rise. Endothermic: a reaction that absorbs energy from the surroundings, usually shown by a temperature fall.

Bond breaking/making: exo or endo?

✓Bond breaking is endothermic (absorbs energy). Bond making is exothermic (releases energy).

Classify: combustion, thermal decomposition of CaCO₃, neutralisation.

✓(a) Combustion — exothermic. (b) Thermal decomposition of calcium carbonate — endothermic (requires continuous heating). (c) Neutralisation — exothermic.

Define activation energy.

✓Activation energy is the minimum energy that colliding particles must have for a reaction to occur.

ΔH for H₂+Cl₂→2HCl.

Energy to break bonds (reactants)

436+242=678 kJ/mol

Energy released making bonds (products)

2×431=862 kJ/mol

✓ΔH = 678−862 = −184 kJ/mol (exothermic)

Heat absorbed: 150g, 8°C rise.

q = mass × c × ΔT

150×4.2×8

✓5040 J = 5.04 kJ

Compare exothermic vs endothermic pathway diagrams.

✓In an exothermic reaction diagram, the products are drawn at a lower energy level than the reactants (energy released overall). In an endothermic reaction diagram, the products are drawn at a higher energy level than the reactants (energy absorbed overall). Both show an activation energy “hump” above the reactants’ energy level.

Why doesn’t a catalyst change ΔH?

✓A catalyst provides an alternative reaction pathway with a lower activation energy, speeding up the rate of reaction — but it does not change the energy of the reactants or products themselves, so the overall enthalpy change (the difference between them) stays the same.

Acid+alkali, temperature rises. Exo or endo?

✓Exothermic — a temperature rise means energy is being released to the surroundings (the reaction mixture), which is the definition of an exothermic reaction.

Q10

Why must an exothermic reaction release more than it absorbs?

✓The overall enthalpy change equals (energy absorbed breaking bonds) − (energy released making bonds). For this to be negative (exothermic, energy released overall), the energy released making the new bonds in the products must be greater than the energy absorbed breaking the bonds in the reactants.

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