Question

Determine total H for bonds broken and formed, the overall change in H, and the final answer with units. Is it ENDOthermic or EXOthermic?
Reaction:
2H2(g) +O2(g) -> 2H2O^(g)
Given:
H-H 436
O+O 499
O-H 463

Answer 1

• E(Bonds broken) = 1371 kJ/mol reaction
• E(Bonds formed) = 1852 kJ/mol reaction

• ΔH = -481 kJ/mol.
• The reaction is exothermic.

Explanation

2 H-H + O=O → 2 H-O-H

There are two moles of H-H bonds and one mole of O=O bonds in one mole of reactants. All of them will break in the reaction. That will absorb

• E(Bonds broken) = 2 × 436 + 499 = 1371 kJ/mol reaction.
• ΔH(Breaking bonds) = +1371 kJ/mol

Each mole of the reaction will form two moles of water molecules. Each mole of H₂O molecules have two moles O-H bonds. Two moles of the molecule will have four moles of O-H bonds. Forming all those bond will release

• E(Bonds formed) = 2 × 2 × 463 = 1852 kJ/mol reaction.
• ΔH(Forming bonds) = - 1852 kJ/mol

Heat of the reaction:

• $\Delta H_{\text{rxn}} = \Delta H(\text{Breaking bonds}) + \Delta H(\text{Forming bonds})\\\phantom{ \Delta H_{\text{rxn}}} = +1371 + (-1852) \\\phantom{ \Delta H_{\text{rxn}}} = -481 \; \text{kJ} / \text{mol}$

$\Delta H_{\text{rxn}}$ is negative. As a result, the reaction is exothermic.