Question

Calculate the heat of formation for the compound methane (CH4) given that the enthalpy change for the reaction of carbon (C) with hydrogen gas (H2) to form methane gas (CH4) is -74.8 kJ/mol.

Answer 1

The heat of formation  Hf  of a compound is the change in enthalpy during the formation of 1 mole of the substance from its constituent elements in their standard states. For methane  CH4 , the reaction can be represented as:C  graphite  + 2 H2  g   CH4  g The given enthalpy change for this reaction is -74.8 kJ/mol. Since this reaction represents the formation of 1 mole of methane from its constituent elements in their standard states, the heat of formation for methane  Hf  is equal to the enthalpy change for this reaction:Hf  CH4  = -74.8 kJ/mol