Bond Enthalpy
The energy required to break a chemical bond in the gas phase at constant pressure.
Endothermic always !
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Actual values at 298 K
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Average Values
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ΔHorxn
Measure of relative bond strengths of reactants and products
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Break
Make |
bonds of reactants
bonds of products |
Energy IN
Energy OUT |
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Can estimate ΔHorxn
as sum of all reactant bonds broken minus all product bonds formed (another use of Hess's Law).
ΔHorxn
= Σ nBE
(reactants) – Σ nBE
(products)
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If
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Reactant bonds stronger than product bonds
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1. Endothermic
2. Exothermic |
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Example: CO2(g) + 4H2(g) –––> CH4(g) + 2H2O(g)
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| ΔHorxn ~ 2 BE(C=O) + 4 BE(H–H) – 4 BE(C–H) – 4 BE(O–H) | ||
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