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Calculating Enthalpy Changes from Bond Enthalpies

   
bondmean bond enthalpy kJmol-1
N≡N            945
H-H            436
N-H            391
N2(g) + 3H2(g) → 2NH3(g)The enthalpy change is calculated from
total bond enthalpies reactants - total bond enthalpies products.bonds broken:
1x N≡N 945 kjmol-1
3x H-H 436 kjmol-1bonds formed:
6x N-H 391kjmol-1The enthalpy change of the reaction is
945+3(436)-6(391) = - 93kJmol-1

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Specification: OCR Chemistry A (H032), AS-level, 3.2.1 Enthalpy changes. Topic: Enthalpy.

Key terms in this lesson

Enthalpy
A measure of the total energy of a chemical system.
Enthalpy change
The heat energy that is transferred between the system and its surroundings during a process carried out at constant pressure.

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