Physical chemistry -- Calculations of enthalpy of reaction
3) Calculation based on the enthalpies of formation of reagents and products
Basically, e just have to plug the values of the enthalpies of formation of all substances in the reaction whose enthalpy we eant to discover.
The table shows the enthalpies of formation of some oxides produced in high energy mixtures.
Aluminium in particular releases a lot of energy when oxidised, and that is why it is used in various high energy mixtures: explosives, fireworks, rocket fuel, etc... The space shuttles (like challenger and others) used aluminium as solid fuel. The liquid fuel was oxygen and hydrogen, which combine to form water also releasing a large amount of energy.
Magnesium is also commonly added to explosives, because of the high ΔHf of its oxide.
The other component needed in a high energy mixture is the oxidiser. That is the compound that will provide the oxygens needed for the reaction. Compounds like chlorates, nitrates and hydrogen peroxide are normally used for this purpose.
-Example: calculate de enthalpy of the following reaction:
Photoflash : mixing Mg with KClO4 (oxidiser)
KClO4 + 4 Mg --> KCl + 4 MgO
the corresponding ΔHf 's are :
-432.2 + 0 --> -436.4 + 4* (-601.1)
(Observe that ΔHf for Mg is zero because it is in the standard state)
Calculating:
ΔHreaction = [-436.4 + 4* (-601.1)] - [ -432.2]
= -2408.5 kJ /mol of KClO4
| Compound | ΔHf (kJ / mol) |
| Al203 | -1674.1 |
| B2O3 | -1271.5 |
| Cr2O3 | -1138.6 |
| MgO | -601.1 |
| NaO | -413.8 |
Other ways of calculating enthalpy of reaction: