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

Example

Hydrogen and chlorine react to form hydrogen chloride gas:

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.

Example 2

Hydrogen bromide decomposes to form hydrogen and bromine:

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:

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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