Question

Calculate the enthalpy change for the combustion of methane gas (CH4) using Hess's Law, given the following equations and enthalpy changes: C(s) + O2(g) → CO2(g), ΔH = -393.5 kJ/mol2H2(g) + O2(g) → 2H2O(l), ΔH = -571.6 kJ/mol CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔH = ?

Answer 1

To calculate the enthalpy change for the combustion of methane gas  CH4  using Hess's Law, we need to manipulate the given equations to match the target equation:Target equation: CH4 g  + 2O2 g   CO2 g  + 2H2O l First, we need to form CH4 from its elements, C s  and H2 g . We can do this by reversing the first equation and dividing the second equation by 2:1. C s  + O2 g   CO2 g , H = -393.5 kJ/mol  reverse this equation    C s  + 2H2 g   CH4 g , H = ?2. 2H2 g  + O2 g   2H2O l , H = -571.6 kJ/mol  divide this equation by 2    H2 g  + 1/2 O2 g   H2O l , H = -285.8 kJ/molNow, we can add the manipulated equations together:1. C s  + O2 g   CO2 g , H = 393.5 kJ/mol  reversed    C s  + 2H2 g   CH4 g , H = ?2. H2 g  + 1/2 O2 g   H2O l , H = -285.8 kJ/mol  multiply this equation by 2    2H2 g  + O2 g   2H2O l , H = -571.6 kJ/molAdd equations 1 and 2 together:C s  + 2H2 g  + 2O2 g   CH4 g  + CO2 g  + 2H2O l H = 393.5 kJ/mol - 571.6 kJ/molNow, subtract the enthalpy changes:H = -178.1 kJ/molSo, the enthalpy change for the combustion of methane gas  CH4  is -178.1 kJ/mol.