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Calculating hydrogen bond energy
Figure 7 shows the displayed formulae equation for the reaction of nitrogen with hydrogen.
In the reaction the energy released forming new bonds is 93 kJ/mol greater than the energy needed to break existing bonds.
| Bond | N≡N | H—H | N—H |
|---|---|---|---|
| Bond energy in kJ/mol | 945 | X | 391 |
Calculate the bond energy X for the H—H bond. Use Figure 7 and Table 3.
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Marking points
- 1 Counts one N≡N bond and three H—H bonds broken and six N—H bonds formed.
- 2 Uses the stated 93 kJ/mol difference with the correct energy-change relationship.
- 3 Gives X = 436 kJ/mol.
Full-mark answer
Bonds broken = 945 + 3X. Bonds formed = 6 × 391 = 2346 kJ/mol. Since the energy released is 93 kJ/mol greater, 93 = 2346 − (945 + 3X). Therefore 3X = 1308 and X = 436 kJ/mol.
Why this answer loses marks
I added all the bond energies and divided by the number of unknown bonds.
Broken and made bond energies must be totalled separately and linked by the signed energy-change equation before the unknown is isolated.