Question

Calculate the standard enthalpy change for the combustion of methane (CH4) at constant pressure, given that the standard enthalpies of formation are -74.87 kJ/mol for methane and -393.51 kJ/mol for carbon dioxide (CO2), and 2H2(g) + O2(g) →  2H2O(l) has a standard enthalpy change of -571.66 kJ/mol.

Answer 1

To calculate the standard enthalpy change for the combustion of methane, we first need to write the balanced chemical equation for the combustion reaction:CH4 g  + 2O2 g   CO2 g  + 2H2O l Next, we will use Hess's Law, which states that the enthalpy change for a reaction is the sum of the enthalpy changes for the formation of the products minus the sum of the enthalpy changes for the formation of the reactants.H_combustion = [H_f CO2  + 2H_f H2O ] - [H_f CH4  + 2H_f O2 ]Since O2 is in its standard state, its enthalpy of formation is 0 kJ/mol. Therefore, the equation becomes:H_combustion = [H_f CO2  + 2H_f H2O ] - [H_f CH4 ]Now, we can plug in the given values for the standard enthalpies of formation:H_combustion = [ -393.51 kJ/mol  + 2 -571.66 kJ/mol ] - [ -74.87 kJ/mol ]H_combustion = [-393.51 + 2 -571.66  + 74.87] kJ/molH_combustion = [-393.51 - 1143.32 + 74.87] kJ/molH_combustion = [-1461.96] kJ/molTherefore, the standard enthalpy change for the combustion of methane at constant pressure is -1461.96 kJ/mol.