Question

Calculate the standard enthalpy change for the complete combustion of methane (CH4) at constant pressure and 298 K, given that the standard enthalpies of formation for CH4(g) and H2O(l) are -74.8 kJ/mol and -285.8 kJ/mol, respectively.

Answer 1

To calculate the standard enthalpy change for the complete combustion of methane, we need to use the following equation:H_combustion =  H_f products  -  H_f reactants The balanced chemical equation for the complete combustion of methane is:CH4 g  + 2 O2 g   CO2 g  + 2 H2O l Now, we need to find the standard enthalpies of formation  H_f  for all the reactants and products involved in the reaction. We are given the H_f values for CH4 g  and H2O l , but we need to find the H_f values for O2 g  and CO2 g .For O2 g , since it is an element in its standard state, its H_f value is 0 kJ/mol.For CO2 g , the standard enthalpy of formation is -393.5 kJ/mol  you can find this value in a standard thermodynamic table or chemistry reference .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 for the complete combustion of methane at constant pressure and 298 K is -890.3 kJ/mol.