When 1 mole of hydrogen gas (H2) reacts with excess oxygen to form water at a constant pressure, 241.8 KJ of energy is released as heat. Calculate ΔH for a process in which 21.9 g sample of hydrogen gas (H2) reacts with excess oxygen at constant pressure.

Explanation:

1. Gather all the information in one place

Mᵣ:   2.016  

          H₂ + ½O₂ ⟶ H₂O + 148.1 kJ

m/g:  21.9

2. Moles of H₂

3. Calculate ΔH

Treat the heat AS IF it were a reactant or product. In this case, the reaction is exothermic, so the heat is a product.

In effect, you have 148.1 mol of "kJ"s for each mole of hydrogen and

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"