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Enthalpy is a convenient way to describe energy and heat transfer in a system.

It is defined as the internal energy + the pressure * volume, which is a constant force. (Pressure = Force/ Volume)

So, H = E + PV, the convenient thing about enthalpy is that the change in enthalpy is also the heat gained or lost in a reaction.

Hess's Law is an important law dealing with enthalpy says that the total enthalpy change in a system doesn't depend on the pathway it takes, only on the initial and final states. So, as long as you start with the same reactants, and end with the same products, the total enthalpy change is the same.

Standard enthalpy of formation is defined as the amount of heat lost or gained when one mole of a compound is formed from its constituent elements.

So this is the formula for Hess's Law:

Total change of Enthalpy =  SUM(moles * SEF(compounds))_products 
- SUM(moles * SEF(compounds))_reactants

The reason you multiply by moles is because the SEF (standard enthalpy of formation) is per mole.= 

For example, Fe2O3, or rust has a standard enthalpy of reaction of -824.4 kJ (looked it up). So, the chemical equation of a hand warmer is 4FE + 3O2 -> 2FE2O3.

The SEF of O2 and FE are both 0, and Fe2O3 is -824.4 kJ.

Now, we just plug it in 2 * -824.4 - (4 * 0 + 3 * 0) = -1648.8 kJ. 

So, if the enthalpy is negative, it means it released heat, so the reaction released 1648.8 kJ of energy.

Overall, enthalpy is very important in thermodynamics and is a very useful term.

David Witten

Chapter 6 Notes