Calculating energy changes using bond energy values - Higher

Breaking and making bonds

Energy is transferred when chemical bonds are broken or are formed.

Bond brokenBond formed
Type of processEndothermicExothermic
Heat energy transferredGiven outTaken in

During a chemical reaction:

The difference between the energy needed to break bonds, and the energy released when new bonds form, determines the type of reaction. A reaction is:

  • exothermic, if more heat energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants
  • endothermic, if less heat energy is released in forming bonds in the products than is taken in when breaking bonds in the reactants

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

The 'energy in' is an endothermic change, as the energy is being used to break bonds. The 'energy out' is an exothermic change, as the energy is released as new bonds are formed. Therefore the energy change is:

  • 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.

BondEnergy
H−H436 kJ
Cl−Cl243 kJ
H−Cl432 kJ

Energy in = 436 + 243 = 679 kJ

Energy out = (2 × 432) = 864 kJ

Energy change = in - out

= 679 - 864

= -185 kJ

The energy change is negative. This shows that the reaction is exothermic, as the 'energy out' is larger than the 'energy in'.

Example

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.

BondEnergy
H-Br366 kJ
H-H436 kJ
Br-Br193 kJ

Energy in = 2 × 366 = 732 kJ

Energy out = 436 + 193 = 629 kJ

Energy change = in - out

= 732 - 629

= +103 kJ

The energy change is positive. This shows that the reaction is endothermic, as the 'energy out' is smaller than the 'energy in'.

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.

BondEnergy
H-H436 kJ
O=O498 kJ
O-H464 kJ

Energy in = (2 × 436) + 498

= 872 + 498

= 1370 kJ

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

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

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