Bond Breaking and Bond Forming (HT Only)

Section 1 of 7Two opposite processes

Breaking bonds

• Breaking bonds Energy must be taken in from the surroundings to pull atoms apart, so bond breaking is always endothermic.
• Every bond in the reactant molecules has to be broken (or at least loosened) before a reaction can occur.

Making bonds

• Making bonds When atoms join together, energy is released to the surroundings, so bond making is always exothermic.
• The energy released when a bond forms equals the energy needed to break the same bond.

The balance decides the type

• The balance decides the type A reaction does both at once.
  • Whether it is exothermic or endothermic depends on which of the two energy amounts is larger.

Section 2 of 7Bond energy

Bond energy (kJ/mol)

Definition

• Definition The energy needed to break one mole of a particular covalent bond, or released when one mole of that bond forms.

Specific to each bond

• Specific to each bond A strong bond such as H–Cl (431 kJ/mol) has a higher value than a weaker bond such as I–I (151 kJ/mol).

Given in the exam

• Given in the exam Values are normally supplied in a table, but you must choose the correct bonds and count them using the balanced equation.

Section 3 of 7Calculating the energy change (HT only)

1

Draw out every molecule so that each bond can be seen, for example H–H and Cl–Cl on the left, and two H–Cl on the right.

2

Add up the bond energies for all bonds in the reactants. This is the energy in.

3

Add up the bond energies for all bonds in the products. This is the energy out.

4

Use: energy change = energy in − energy out.

5

A negative answer means more energy was released than absorbed, so the reaction is exothermic. A positive answer means it is endothermic.

6

Give the unit, kJ/mol, and state your conclusion in words.

Section 4 of 7Worked example: H2 + Cl2 → 2HCl

Stage	Working	Result
Energy in (bonds broken)	H–H 436 + Cl–Cl 242	678 kJ/mol
Energy out (bonds made)	2 × H–Cl 431	862 kJ/mol
Energy change	678 − 862	−184 kJ/mol
Conclusion	Negative value, so more energy released than taken in	Exothermic

Section 5 of 7Link to energy changes

1More energy out than in

• More energy out than in The new bonds are stronger overall than the old ones.

2Products lower in energy

• Products lower in energy The products store less chemical energy than the reactants.

3Energy change is negative

• Energy change is negative The surplus is transferred to the surroundings as heat, so the temperature rises.

Section 6 of 7Key terms

Term	Meaning
Bond energy	The energy needed to break one mole of a given covalent bond; the same amount is released when it forms.
Endothermic	A process that takes in energy from the surroundings; the overall energy change is positive.
Exothermic	A process that releases energy to the surroundings; the overall energy change is negative.
Energy in	The total of the bond energies of all bonds broken in the reactants.
Energy out	The total of the bond energies of all bonds made in the products.

Section 7 of 7Exam technique

1

Mixing up which side is whichbonds are broken in the reactants and made in the products.

2

Forgetting to multiply by the number of molecules in the balanced equation, for example 2HCl contains two H–Cl bonds.

3

Subtracting the wrong way roundthe formula is energy in minus energy out, not the reverse.

4

Saying that bond breaking releases energy. Breaking bonds always needs energy to be supplied.

5

Leaving out the signthe minus sign is what shows the reaction is exothermic.