Question

Calculate the enthalpy change for the combustion of methane gas if the standard enthalpies of formation of CH4(g) and H2O(l) are -74.8 kJ/mol and -285.8 kJ/mol, respectively.

Answer 1

To calculate the enthalpy change for the combustion of methane gas, we need to consider the balanced chemical equation for the combustion reaction:CH4 g  + 2 O2 g   CO2 g  + 2 H2O l Next, we need to find the standard enthalpies of formation for all the reactants and products involved in the reaction. We are given the standard enthalpies of formation for CH4 g  and H2O l , but we also need the values for O2 g  and CO2 g . The standard enthalpy of formation for O2 g  is 0 kJ/mol  since it is in its elemental form , and the standard enthalpy of formation for CO2 g  is -393.5 kJ/mol.Now we can use Hess's Law to calculate the enthalpy change for the reaction:H =  [Hf products ] -  [Hf reactants ]H = [1   -393.5 kJ/mol  + 2   -285.8 kJ/mol ] - [1   -74.8 kJ/mol  + 2  0 kJ/mol]H =  -393.5 - 571.6 + 74.8  kJ/molH = -890.3 kJ/molThe enthalpy change for the combustion of methane gas is -890.3 kJ/mol.