Using Hess's law

Hess's law states that the enthalpy change in converting reactants into products is the same regardless of the route taken.

So the enthalpy change for two routes from the same reactants to the same products will be equal.

Example:

Hess's Law states that:

H_x = H₁ + H₂

If all but one of the above enthalpy changes are known then the remaining enthalpy change can be calculated.

Example:

The formation of ethane can be represented by the following equation:

2C + 3H₂→C₂H₆

Calculate the enthalpy change for the reaction using the enthalpies of combustion of carbon, hydrogen and ethane given in the SQA data book.

H₁ = 2 × enthalpy of combustion of carbon (2 × -394)

H₂ = 3 × enthalpy of combustion of hydrogen (3 × -286)

H₃ = reverse of enthalpy of combustion of ethane. The enthalpy of combustion of ethane has a negative value so the reverse will have a positive value (+1560).

Using Hess's Law:

H_x = H₁ + H₂ + H₃

H_x = -788 + (-858) + 1560

H_x = -86 kJ mol ^-1

Answer: The enthalpy change for the reaction is -86 kJ mol^-1

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