Enthalpy diagrams

Limitations of mean bond enthalpies

• They are averages over a range of compounds

• Bond enthalpies are always based on the compound being a gas, water is a liquid in its standard state

  • The value of formation for water and steam will be different

Enthalpy diagrams

• As well as Hess’ law, another type of diagram can be used to calculate a range of different enthalpy changes.

  • These are called enthalpy diagrams

• They show the level of enthalpy on the vertical scale

• The enthalpies of all elements in their standard states are taken as 0.

  • Eg H₂(g), C(s, graphite)

    • Carbon has different forms so graphite is used

• What is the enthalpy change for the following reaction?

• CH₃OCH₃(g) → C₂H₅OH(l)

• Given the enthalpies of formation of

  • CH₃OCH₃(g) -184kJmol^-1

  • C₂H₅OH(l) -277kJmol^-1

• Draw a line at level 0 to represent the elements

• Add the values of enthalpy of formation, remembering negative values go downwards, positive values upwards

• Find the difference in levels between the two compounds

• Use an enthalpy diagram to determine the enthalpy change for the following reaction

• NH₃(g) + HCl(g) → NH₄Cl(s)

• Given the enthalpies of formation of

  • NH₃(g) -46kJmol^-1

  • HCl(g) -92kJmol^-1

  • NH₄Cl(s) -314kKmol^-1