Question

What is the standard enthalpy change of combustion for methane gas (CH4) at 298 K? Given that the standard enthalpy of formation of CO2(g) and H2O(l) are -393.5 kJ/mol and -285.8 kJ/mol, respectively.

Answer 1

To calculate the standard enthalpy change of combustion for methane gas  CH4 , we need to use the following equation:H_combustion =  H_f products  -  H_f reactants The balanced combustion reaction for methane is:CH4 g  + 2 O2 g   CO2 g  + 2 H2O l Now, we need to find the standard enthalpy of formation  H_f  for each substance involved in the reaction. We are given the values for CO2 g  and H2O l , but we also need the values for CH4 g  and O2 g . The standard enthalpy of formation for O2 g  is 0 kJ/mol since it is in its elemental form. The standard enthalpy of formation for CH4 g  is -74.8 kJ/mol.Now, we can plug these values into the equation:H_combustion = [1   -393.5 kJ/mol  + 2   -285.8 kJ/mol ] - [1   -74.8 kJ/mol  + 2   0 kJ/mol ]H_combustion =  -393.5 - 571.6  -  -74.8 H_combustion = -965.1 + 74.8H_combustion = -890.3 kJ/molSo, the standard enthalpy change of combustion for methane gas  CH4  at 298 K is -890.3 kJ/mol.