Calculating energy changes - Higher

Using bond energies

The energy change in a reaction can be calculated using bond energies. A bond energy is the amount of energy needed to break one mole of a particular covalent bond.

Different bonds have different bond energies. These are given when they are needed for calculations.

To calculate an energy change for a reaction:

  • add together the bond energies for all the bonds in the reactants - this is the 'energy in'
  • add together the bond energies for all the bonds in the products - this is the 'energy out'
  • energy change = energy in - energy out

Worked example

Hydrogen and chlorine react to form hydrogen chloride gas:

H2 + Cl2 → 2HCl

H−H + Cl−Cl → 2 × (H−Cl)

Use the bond energies in the table to calculate the energy change for this reaction.

BondBond energy
H−H436 kJ/mol-1
Cl−Cl243 kJ/mol-1
H−Cl 432 kJ/mol-1

Energy in = 436 + 243 = 679 kJ/mol-1

Energy out = (2 × 432) = 864 kJ/mol-1

Energy change = in - out

= 679 - 864

= -185 kJ/mol-1

The energy change is negative. This shows that the reaction is exothermic.

Worked example

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr → H2 + Br2

2 × (H−Br) → H−H + Br-Br

Use the bond energies in the table to calculate the energy change for this reaction.

BondBond energy
H−Br366 kJ/mol-1
H−H436 kJ/mol-1
Br−Br193 kJ/mol-1

Energy in = 2 × 366 = 732 kJ/mol-1

Energy out = 436 + 193 = 629 kJ/mol-1

Energy change = in - out

= 732 - 629

= +103 kJ/mol-1

The energy change is positive. This shows that the reaction is endothermic.

Question

Hydrogen reacts with oxygen to form water:

H2 + O2 → 2H2O

2 × (H−H) + O=O → 2 × (H−O–H)

Use the bond energies in the table to calculate the energy change for this reaction.

BondBond energy
H−H436 kJ/mol-1
O=O498 kJ/mol-1
O–H464 kJ/mol-1

Energy in = (2 × 436) + 498

= 872 + 498

= 1370 kJ/mol-1

Energy out = 2 × 2 × 464

= 1856 kJ/mol-1 (there are two O–H bonds in each water molecule)

Energy change = in - out

= 1370 – 1856

= -486 kJ/mol-1

The energy change is negative. This shows that the reaction is exothermic.

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