Question

Calculate the standard enthalpy change (∆H°) of combustion for methane gas (CH4) given that the heat of formation (∆Hf°) for methane is -74.87 kJ/mol and the heat of formation (∆Hf°) for carbon dioxide (CO2) is -393.51 kJ/mol, and the heat of formation (∆Hf°) for water (H2O) is -285.83 kJ/mol.

Answer 1

To calculate the standard enthalpy change  H  of combustion for methane gas  CH4 , we need to consider the balanced chemical equation for the combustion of methane:CH4 g  + 2 O2 g   CO2 g  + 2 H2O l Now, we can use Hess's Law, which states that the enthalpy change of a reaction is the sum of the enthalpy changes of formation of the products minus the sum of the enthalpy changes of formation of the reactants.H = [Hf CO2  + 2Hf H2O ] - [Hf CH4  + 2Hf O2 ]Since the heat of formation for elemental oxygen  O2  is zero, the equation becomes:H = [-393.51 kJ/mol + 2 -285.83 kJ/mol ] -  -74.87 kJ/mol H =  -393.51 - 571.66 + 74.87  kJ/molH = -890.3 kJ/molThe standard enthalpy change  H  of combustion for methane gas  CH4  is -890.3 kJ/mol.