5.2

Bond energies

Cambridge IGCSE Chemistry (0620)  · Unit 5: Chemical energetics  · 10 flashcards

Bond energies is topic 5.2 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 5 — Chemical energetics , alongside Exothermic and endothermic reactions.  In one line: Bond energy is the energy required to break one mole of a particular bond in the gaseous phase. It's usually an average value, as the energy can vary depending on the molecule.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).  It is a Supplement (Extended-tier) topic, so it appears only on the Extended-tier papers.

The deck below contains 10 flashcards — 4 definitions and 4 key concepts — covering the precise wording mark schemes reward.  Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

'bond energy'

Bond energy is the energy required to break one mole of a particular bond in the gaseous phase. It's usually an average value, as the energy can vary depending on the molecule.

What the Cambridge 0620 syllabus says

2026-2028 syllabus, in our words

What the syllabus expects you to be able to do in this topic, in plain English. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks. For the exact wording, see the official syllabus (PDF).

  1. State State that in an exothermic reaction thermal energy is transferred to the surroundings, so the surroundings' temperature goes up.
  2. State State that in an endothermic reaction thermal energy is taken in from the surroundings, so the surroundings' temperature falls.
  3. Interpret Interpret reaction pathway diagrams for both exothermic and endothermic reactions.
  4. State State that the enthalpy change of a reaction, ΔH, is the thermal energy transferred during it, and that ΔH has a negative sign for exothermic reactions and a positive sign for endothermic ones. Supplement
  5. Define Define activation energy, Eₐ: the least energy that particles need when they collide in order to react. Supplement
  6. Draw Draw reaction pathway diagrams and label them, from information provided, for both endothermic and exothermic reactions, marking: (a) the reactants; (b) the products; (c) the reaction's enthalpy change, ΔH; (d) the activation energy, Eₐ. Supplement
  7. State State that breaking bonds is endothermic and forming bonds is exothermic, and explain a reaction's enthalpy change in terms of the bonds broken and the bonds made. Supplement
  8. Calculate Calculate a reaction's enthalpy change from bond energies. Supplement
Definition Flip

Define 'bond energy'.

Answer Flip

Bond energy is the energy required to break one mole of a particular bond in the gaseous phase. It's usually an average value, as the energy can vary depending on the molecule.

Key Concept Flip

Is bond breaking endothermic or exothermic, and why?

Answer Flip

Bond breaking is always endothermic. Energy must be *supplied* to overcome the attractive forces holding the atoms together in the bond.

Key Concept Flip

Is bond formation endothermic or exothermic, and why?

Answer Flip

Bond formation is always exothermic. Energy is *released* when atoms form a bond because they are moving to a more stable, lower energy state.

Key Concept Flip

How is the enthalpy change (ΔH) of a reaction calculated using bond energies?

Answer Flip

ΔH = Energy required to break bonds (reactants) - Energy released when bonds are formed (products). A negative ΔH indicates an exothermic reaction.

Key Concept Flip

Calculate the enthalpy change (ΔH) for the reaction H₂(g) + Cl₂(g) → 2HCl(g), given the following bond energies: H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, H-Cl = 432 kJ/mol.

Answer Flip

ΔH = [(1 x H-H) + (1 x Cl-Cl)] - [2 x H-Cl] = [436 + 243] - [2 x 432] = 679 - 864 = -185 kJ/mol.

Key Concept Flip

In an exothermic reaction, are the products at a higher or lower energy level than the reactants?

Answer Flip

In an exothermic reaction, the products are at a *lower* energy level than the reactants. This is because energy has been released to the surroundings.

Key Concept Flip

In an endothermic reaction, are the products at a higher or lower energy level than the reactants?

Answer Flip

In an endothermic reaction, the products are at a *higher* energy level than the reactants because energy is absorbed from the surroundings.

Definition Flip

If a reaction has a positive ΔH, is it endothermic or exothermic?

Answer Flip

A positive ΔH indicates an *endothermic* reaction, meaning heat is absorbed from the surroundings.

Definition Flip

If a reaction has a negative ΔH, is it endothermic or exothermic?

Answer Flip

A negative ΔH indicates an *exothermic* reaction, meaning heat is released to the surroundings.

Definition Flip

What does 'Ea' represent on an energy profile diagram?

Answer Flip

'Ea' represents the activation energy. It is the minimum energy required for reactants to start a reaction and reach the transition state (peak of the curve).

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5.1 Exothermic and endothermic reactions 6.1 Rate of reaction

Key Questions: Bond energies

Define 'bond energy'.

Bond energy is the energy required to break one mole of a particular bond in the gaseous phase. It's usually an average value, as the energy can vary depending on the molecule.

If a reaction has a positive ΔH, is it endothermic or exothermic?

A positive ΔH indicates an *endothermic* reaction, meaning heat is absorbed from the surroundings.

If a reaction has a negative ΔH, is it endothermic or exothermic?

A negative ΔH indicates an *exothermic* reaction, meaning heat is released to the surroundings.

What does 'Ea' represent on an energy profile diagram?

'Ea' represents the activation energy. It is the minimum energy required for reactants to start a reaction and reach the transition state (peak of the curve).

More topics in Unit 5 — Chemical energetics

Bond energies sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

5.1 Exothermic and endothermic reactions

12 flashcards

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

bond energy bond breaking bond making enthalpy change kJ/mol energy calculation

Key terms covered in this Bond energies deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

'bond energy'
If a reaction has a positive ΔH, is it endothermic or exothermic
If a reaction has a negative ΔH, is it endothermic or exothermic
What does 'Ea' represent on an energy profile diagram

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