Bond Breaking & Bond Forming (Cambridge (CIE) O Level Chemistry) : Revision Note

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Bond Breaking & Bond Forming

  • Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed

  • Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds

  • Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed

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 products have less energy than the reactants

  • Therefore an exothermic reaction has a negative ΔH value

bond-making-exothermic-reaction

Making new chemical bonds releases energy which radiates outwards from the reaction to the surroundings in the form of heat

Endothermic reactions

  • If more energy is absorbed to break bonds than is released to form new bonds, this reaction is endothermic overall

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

  • The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol for enthaply, which is a measure of the total heat of reaction of a chemical reaction

  • Therefore an endothermic reaction has a positive ΔH value, which is shown on the reaction pathway diagrams and in calculations

bond-breaking-endothermic-reaction

Breaking chemical bonds requires energy which is taken in from the surroundings in the form of heat

Bond Energy Calculations

Energy of reaction calculations

  • Each chemical bond has specific bond energy associated with it

  • This is the amount of energy required to break the bond or the 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

Method

  • Write a balanced equation if none is present already

  • Optional - draw the displayed formula in order to identify the type and number of bonds more easily

  • 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 enthalpy change:

Enthalpy change (ΔH)  = Energy taken in - Energy given out

Worked Example

Hydrogen and chlorine react to form hydrogen chloride gas:

H2  + Cl2 ⟶ 2HCl

The table below shows the bond energies.

Calculating energy Changes WE Table 1, downloadable IGCSE & GCSE Chemistry revision notes

Calculate the enthalpy change, ΔH, for the reaction and deduce whether it is exothermic or endothermic.

Answer

5-1-3-calculating-enthalpy-change-hcl

Worked Example

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr  ⟶ H2  + Br2

The table below shows the bond energies.

Calculating energy Changes WE Table 2, downloadable IGCSE & GCSE Chemistry revision notes

Calculate the enthalpy change, ΔH,  for the reaction and deduce whether it is exothermic or endothermic.

Answer

5-1-3-calcualting-enthalpy-change-hbr

Examiner Tips and Tricks

When answering questions to calculate the enthalpy change using bond energies,, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes. The reaction thus becomes: H-H + Cl-Cl → H-Cl + H-Cl

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