Calculating energy changes

Chemical bonds

The forces that hold together atoms in a molecule are called bonds. During a chemical reaction the bonds within the reactants break. This requires the transfer of energy from the surroundings. This is called the activation energy.

The formation of bonds within the products is the opposite process. Energy is released back to the surroundings.

If the energy required to break bonds is greater than the energy released when new bonds are formed then overall energy is taken in from the surroundings. This reaction is endothermic.

If the energy required to break bonds is less than the energy released to the surroundings when new bonds form then overall energy is released back into the surroundings. This reaction is exothermic.

Calculating energy change - Higher

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:

StepAction
1Write the balanced chemical equation for the reaction.
2Look up the bond energy of each bond that breaks and each bond that forms.
3Use the chemical equation to work out how many of each type of bond must break.
4Calculate the total energy of all the bonds that must break - this is the 'energy in'.
5Use the chemical equation to work out how many of each type of bond must form.
6Calculate the total energy of all the bonds that must break - this is the 'energy out'.
7Calculate the overall energy change (energy in - energy out).

If the answer is negative, more energy was released by bond formation than was needed to break the bonds. The reaction is exothermic.

If the answer is positive, the reaction is endothermic.

Example

Hydrogen and chlorine react to form hydrogen chloride gas. What is the energy change of this reaction?

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

BondBond energy (kJ mol-1)
H-H436
Cl-Cl243
H-Cl432
StepResult
1H2 + Cl2 → 2HCl
2Bond energies are given in the question.
3Bonds that must break: 1 × H-H, 1 × Cl-Cl
4Energy in = 436 + 243 = 679 kJ mol-1
5Bonds that must form: 2 × H-Cl
6Energy out = (2 × 432) = 864 kJ mol-1
7Overall energy change = 679 - 864 = -185 kJ mol-1

The energy change is negative showing that the reaction is exothermic.

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 (kJ mol-1)
H-H436
O=O498
O-H464

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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