Question

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.

Answer 1

Answer:

$\text{1540 kJ}$

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

$n = \text{21.9 g H}_{2} \times \dfrac{\text{1 mol H}_{2}}{\text{2.016 g H}_{2}} = \text{10.86 mol H}_{2}$

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

$\Delta \text{H} = \text{10.86 mol H}_{2} \times \dfrac{\text{141.8 kJ}}{\text{1 mol H}_{2}} = \textbf{1540 kJ}$