5.2

Bond energies

10 flashcards to master Bond energies

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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).

About Bond energies (5.2)

These 10 flashcards cover everything you need to know about Bond energies for your Cambridge IGCSE Chemistry (0620) exam. Each card is designed based on the official syllabus requirements.

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