Question

Given the following data and using Hess’ Law, Calculate change in Enthalpy for the reaction

Overall reaction : CH4(g) + 2 O2 (g) ------> CO2(g) + 2H2O(l)

2H2O(l) ------> 2H2O(g) ∆H = 88 kJ

CH4(g) + 2 O2 (g) ------> CO2(g) + 2H2O(g) ------> 2O(g) ∆H = -802 kJ

Answer 1

Reactions must add up to overall reaction. You may need to manipulate reaction for this to happen. Addition of reaction will give overall.

lets see the original steps

1.     2H2O(l) ------> 2H2O(g)   ∆H = 88 kJ

2.     CH4(g) + 2 O2 (g) ------> CO2(g) + 2H2O(g) ------> 2O(g)      ∆H = -802 kJ

rearrenge the these steps to get the oroginal reaction so to get the ∆H ?

1. reverse step 1.

2H2O(g) ------> 2H2O(l)     ∆H = -88 kJ ( as u reverse the reaction the sign changes and becomes - ve )

CH4(g) + 2 O2 (g) ------> CO2(g) + 2H2O(g) ------> 2O(g)    ∆H = -802 kJ

                                                                                                                  

add the reactions and cancel the similar groups ( 2H2O(g) gets canceled )

CH4(g) + 2 O2 (g) ------> CO2(g) + 2H2O(l)   ( -88 k J + -802 kJ) = -890 kJ

∆H = -890 kJ