Question

Calculate the ΔHrxn for the combustion of methane gas (CH4) if the enthalpies of formation for methane, carbon dioxide, and water are -74.8, -393.5, and -285.8 kJ/mol respectively, using Hess's Law.

Answer 1

To calculate the Hrxn for the combustion of methane gas  CH4  using Hess's Law, we first need to write the balanced chemical equation for the combustion reaction:CH4 g  + 2O2 g   CO2 g  + 2H2O l Now, we can use Hess's Law, which states that the enthalpy change of a reaction  Hrxn  is equal to the sum of the enthalpy changes of formation  Hf  of the products minus the sum of the enthalpy changes of formation of the reactants:Hrxn = [Hf CO2  + 2Hf H2O ] - [Hf CH4  + 2Hf O2 ]Since O2 is an element in its standard state, its enthalpy of formation is zero:Hf O2  = 0Now, we can plug in the given enthalpy values:Hrxn = [ -393.5  + 2 -285.8 ] - [ -74.8  + 2 0 ]Hrxn =  -393.5 - 571.6  -  -74.8 Hrxn = -965.1 + 74.8Hrxn = -890.3 kJ/molSo, the Hrxn for the combustion of methane gas  CH4  is -890.3 kJ/mol.