CIE AS Chemistry (9701) 2019-2021

Revision Notes

1.5.3 Enthalpy Change of Reaction

Enthalpy Change of Reaction

Enthalpy change

  • The total chemical energy inside a substance is called the enthalpy (or heat content)
  • When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
  • An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
  • An enthalpy change can be positive or negative

Standard conditions

  • To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions
  • These standard conditions are:
    • A pressure of 101 kPa
    • A temperature of 298 K (25 oC)
    • Each substance involved in the reaction is in its normal physical state (solid, gas or liquid)
  • To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used
    • Eg. ΔH = the standard enthalpy change

Standard Enthalpies

Chemical Energetics Standard Enthalpies (1), downloadable AS & A Level Chemistry revision notes
Chemical Energetics Standard Enthalpies (2), downloadable AS & A Level Chemistry revision notes

 

Bond energies

  • During a reaction, enthalpy changes take place because bonds are being broken and formed
  • Energy (in the form of heat) is needed to overcome attractive forces between atoms
  • Bond breaking is therefore endothermic
  • Energy is released from the reaction to the surroundings (in the form of heat) when new bonds are formed
  • Bond forming is therefore exothermic

Exam Tip

It is important to specify the physical states of each species in an equation when dealing with enthalpy changes, as any changes in state can cause very large changes of enthalpy.

For example:

Na+Cl (s) → Na+ (aq) + Cl (aq)   ΔH = +4 kJ mol-1

Na+Cl (g) → Na+ (g) + Cl (g)   ΔH = + 500 kJ mol-1

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