Bond Energy Calculations

Section 1 of 10Bond breaking and making

Bond breaking

• Bond breaking Breaking bonds takes in energy from the surroundings, so it is endothermic.

Bond making

• Bond making Forming bonds releases energy to the surroundings, so it is exothermic.

Bond energy

• Bond energy Each type of bond has a bond energy, in kJ/mol: the energy to break it, or released when it forms.

Overall result

• Overall result If more energy is released making bonds than is needed to break them, the reaction is exothermic;
  • otherwise it is endothermic.

Section 2 of 10Calculation method

1

Draw the displayed formulae of all reactants and products so every bond can be counted.

2

Add together the bond energies of all the bonds broken in the reactants: this is the energy in.

3

Add together the bond energies of all the bonds formed in the products: this is the energy out.

4

Calculate: energy change = energy in − energy out.

5

A negative answer means exothermic; a positive answer means endothermic.

Section 3 of 10Worked example: H2 + Cl2 → 2HCl

Step	Working	Value
Bonds broken	1 × H–H (436) + 1 × Cl–Cl (243)	679 kJ/mol
Bonds formed	2 × H–Cl (432)	864 kJ/mol
Energy change	679 − 864	−185 kJ/mol
Conclusion	Negative value	Exothermic

Section 4 of 10Reverse reaction

Decomposition of 2HBr → H₂ + Br₂

Bonds broken

• Bonds broken 2 × H–Br at 366 each gives 732 kJ/mol.

Bonds formed

• Bonds formed H–H (436) + Br–Br (193) gives 629 kJ/mol.

Energy change

• Energy change 732 − 629 = +103 kJ/mol, so the decomposition is endothermic.

Section 5 of 10Some average bond energies

Bond	Energy (kJ/mol)
H–H	436
Cl–Cl	243
H–Cl	432
C–H	413
O=O	498
C=O	805
O–H	464

Section 6 of 10Worked example: burning methane

1

The equation is CH₄ + 2O₂ → CO₂ + 2H₂O.

2

Bonds broken: 4 × C–H (413) + 2 × O=O (498) = 1652 + 996 = 2648 kJ/mol.

3

Bonds formed: 2 × C=O (805) + 4 × O–H (464) = 1610 + 1856 = 3466 kJ/mol.

4

Energy change = 2648 − 3466 = −818 kJ/mol.

5

The value is negative, so the combustion of methane is exothermic, as expected for a fuel.

Section 7 of 10Limitations of the method

Average values

• Average values Bond energies are averages taken over many compounds, so calculated energy changes are only approximate.

Gaseous state

• Gaseous state The method assumes all substances are gases, so it ignores the energy needed for changes of state.

Section 8 of 10Linking to energy profiles

1Reactants

• Reactants The reactants start at a certain energy level.

2Bonds break

• Bonds break Energy is taken in, shown by the line rising to the activation energy.

3Bonds form

• Bonds form Energy is released as the products are made, shown by the line falling.

4Overall change

• Overall change If the products end lower than the reactants the reaction is exothermic;
  • if higher it is endothermic.

Section 9 of 10Key terms

Term	Meaning
Bond energy	The energy needed to break one mole of a particular bond, or released when it is formed.
Exothermic	A reaction that transfers energy to the surroundings; the energy change is negative.
Endothermic	A reaction that takes in energy from the surroundings; the energy change is positive.
Energy change	Energy taken in to break bonds minus energy given out when bonds form.
kJ/mol	Kilojoules per mole, the usual unit for bond energies.

Section 10 of 10Exam technique

1

Subtracting in the wrong orderuse energy in (bonds broken) minus energy out (bonds formed).

2

Forgetting to multiply the bond energy by the number of bonds, such as 2 × H–Cl.

3

Counting the bonds from the formula without drawing it, for instance missing the double bonds in CO₂.

4

Leaving the sign off the answer; the sign shows whether the reaction is exothermic or endothermic.