Calculating Energy Changes (OCR GCSE Chemistry A (Gateway))

Calculating Energy Changes

Higher Tier Only

Bond Energies

• Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process

• The opposite occurs for forming bonds as it releases energy back to the surroundings in an exothermic process

• Both processes occur in the same chemical reaction, for example, in the production of ammonia:

N₂ + 3H₂ ⟶ 2NH₃

• The bonds in the N-N and H-H molecules must be broken which requires energy while the bonds in the NH₃ molecule are formed which releases energy

• Most reactions occur in a number of steps including steps that are exothermic and steps that are endothermic Whether a reaction is overall endothermic or exothermic depends on the difference between the sum of the exothermic steps and the sum of the endothermic steps

Endothermic reactions

• If more energy is absorbed than is released, this reaction is endothermic

• More energy is required to break the bonds than that gained from making the new bonds

• The change in energy is positive since the products have more energy than the reactants

• Therefore an endothermic reaction has a positive change in energy

Energy must be absorbed from the surroundings for bonds to be broken 

Exothermic Reactions

• If more energy is released than is absorbed, then the reaction is exothermic

• More energy is released when new bonds are formed than energy required to break the bonds in the reactants

• The change in energy is negative since the reactants have more energy than the products

• Therefore an exothermic reaction has a negative change in energy

Making new bonds gives off heat from the reaction to the surroundings

Bond Energy Calculations

• Each chemical bond has a specific bond energy associated with it

• This is the mean amount of energy required to break the bond or the mean amount of energy given out when the bond is formed

• This energy can be used to calculate how much heat would be released or absorbed in a reaction

• To do this it is necessary to know the bonds present in both the reactants and products

• We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved

• Add together all 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’

• Calculate the energy change using the equation:

Energy change = Energy taken in - Energy given out