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.
'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.
Questions this Bond energies deck will help you answer
- › Is bond breaking endothermic or exothermic, and why?
- › Is bond formation endothermic or exothermic, and why?
- › In an exothermic reaction, are the products at a higher or lower energy level than the reactants?
- › In an endothermic reaction, are the products at a higher or lower energy level than the reactants?
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.
Is bond breaking endothermic or exothermic, and why?
Bond breaking is always endothermic. Energy must be *supplied* to overcome the attractive forces holding the atoms together in the bond.
Is bond formation endothermic or exothermic, and why?
Bond formation is always exothermic. Energy is *released* when atoms form a bond because they are moving to a more stable, lower energy state.
How is the enthalpy change (ΔH) of a reaction calculated using bond energies?
ΔH = Energy required to break bonds (reactants) - Energy released when bonds are formed (products). A negative ΔH indicates an exothermic reaction.
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.
ΔH = [(1 x H-H) + (1 x Cl-Cl)] - [2 x H-Cl] = [436 + 243] - [2 x 432] = 679 - 864 = -185 kJ/mol.
In an exothermic reaction, are the products at a higher or lower energy level than the reactants?
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.
In an endothermic reaction, are the products at a higher or lower energy level than the reactants?
In an endothermic reaction, the products are at a *higher* energy level than the reactants because energy is absorbed from the surroundings.
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).
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.
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.
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.
Related Chemistry guides
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