Calculate the enthalpy change (∆H) for the combustion reaction of methane gas (CH4) using Hess’s Law, when the enthalpy changes for the following reactions are given: Reaction 1: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l); ∆H = -891 kJ/mol Reaction 2: C (s) + O2 (g) → CO2 (g); ∆H = -393.5 kJ/mol Reaction 3: 2H2 (g) + O2 (g) → 2H2O (l); ∆H = -484 kJ/mol

Question

Calculate the enthalpy change (∆H) for the combustion reaction of methane gas (CH4) using Hess’s Law, when the enthalpy changes for the following reactions are given: Reaction 1: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l); ∆H = -891 kJ/mol Reaction 2: C (s) + O2 (g) → CO2 (g); ∆H = -393.

Answer 1

To use Hess's Law, we need to manipulate the given reactions so that they add up to the combustion reaction of methane gas  CH . The target reaction is:CH  g  + 2O  g   CO  g  + 2HO  l Reaction 1 is already the target reaction, so we don't need to manipulate it. The enthalpy change for the combustion reaction of methane gas is the same as the enthalpy change for Reaction 1:H = -891 kJ/mol