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Chemistry · Glossary

What is Bond dissociation enthalpy, mean bond enthalpy?

Also known as: bond dissociation enthalpy · mean bond enthalpy

Definition 1.14 University Chemistry — Year 2 · Chapter 1 — Enthalpies of Reaction

The bond dissociation enthalpy of a bond A–B in a gaseous molecule is the standard reaction enthalpy of the breaking of that bond into two gaseous fragments, AB(g)→A(g)+B(g)\ce{AB(g) -> A(g) + B(g)} for a diatomic molecule. When a molecule has nn equal bonds, its mean bond enthalpy is the standard enthalpy of its complete atomisation into gaseous atoms divided by nn.

bondenthalpy (kJ/mol\mathrm{kJ}/\mathrm{mol})obtained from
H–H436HX2\ce{H2}, dissociation
O=O498OX2\ce{O2}, dissociation
N≡\equivN945NX2\ce{N2}, dissociation
Cl–Cl243ClX2\ce{Cl2}, dissociation
H–Cl432HCl\ce{HCl}, dissociation
C–H416CHX4\ce{CH4}, mean of four
N–H391NHX3\ce{NH3}, mean of three
O–H464HX2O\ce{H2O}, mean of two
C=O804COX2\ce{CO2}, mean of two
C–C330CX2HX6\ce{C2H6}, with C–H taken from methane
C=C589CX2HX4\ce{C2H4}, with C–H taken from methane
Bond enthalpies at 298.15 K298.15\,\mathrm{K}, computed from the standard enthalpies of formation of the gaseous atoms and molecules. The last two lines show the limit of the method: a C–H bond of ethane is not exactly one of methane, and the C–C value inherits the difference.

Examples

Example 1.16 (Hydrogenation of ethene)

In CX2HX4(g)+HX2(g)→CX2HX6(g)\ce{C2H4(g) + H2(g) -> C2H6(g)}, one C=C and one H–H are broken and replaced by one C–C and two C–H. With the table: 589+436−330−2×416=−137 kJ/mol589 + 436 - 330 - 2 \times 416 = -137\,\mathrm{kJ}/\mathrm{mol}; from formation enthalpies the value is −136.3 kJ/mol-136.3\,\mathrm{kJ}/\mathrm{mol}. The agreement is good here because the table’s C–C and C=C were obtained from these very molecules; for a molecule unlike those of the table, an error of 10 to 30 kJ/mol30\,\mathrm{kJ}/\mathrm{mol} is common.

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