Enthalpies of Reactions

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Since bond energies are given, we use the monoatomic gases as the reference level in this calculation. The energy level diagram shown below illustrates the principle of conservation of energy, and you are expected to have the skill to draw such a diagram.

------2 H(g) + 2 F(g)-------
                        
|                        ||436+158 kJ
|                        ||
|+2*568 kJ          ---H₂ + F₂---
|                        |
|                        | \(\Delta H\) = -542 kJ/equation
|                        ¯
--------2 HF(g)------------

This diagram is very similar to those of the Born-Haber cycle used to evaluate lattice energy.

Calculate Enthalpy of Reaction from Enthalpy of Formation

A similar cycle can be devised to calculate energy of a reaction when the standard enthalpies of formation are given. We illustrate this cycle by an an example.

.

---2 C(graphite) + 3 H₂ + 3.5 O₂(g)---
 |                                  |
 |                                  | -228 kJ
 |                                  ¯
 |                  ---C₂H₅OH + 3 O₂---
 |-394*2 -286*3                     |
 |                                  |
 |                                  | \(\Delta H\) = -394*2 - 286*3 - (-228)
 |                                  |   = -1418 kJ
 ¯                              ¯
-------------2 CO₂ + 3 H₂O------------

above. Using the diagrams shown indicates clearly what should be the sign of the results. When the reaction is from a higher level to a lower level, the enthalpy of reaction should be negative (downward arrow). If you reverse the direction of the reaction, you also change the sign of the enthalpy of the reaction.

Contributors and Attributions

Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)