What is enthalpy?

is a thermodynamic state function that describes (in quantity) changes in heat under constant pressure.

This is really neat because most reactions usually occur understand constant pressure because everyday we experience conditions which don't fluctuate wildly.

Enthalpy is a state function which means that it is only dependent on change between initial and final state. That means' the reaction doesn't matter. Enthalpy change is what's important.

Enthalpy is calculated by the difference between enthalpy of products and enthalpy of reactants

From enthalpy you can determine whether you're reaction releases heat (exothermic) or if it absorbs heat (endothermic). Exothermic is denoted by a negative enthalpy and Endothermic is denoted by a positive enthalpy.

Usually it is impossible to calculate enthalpy directly. However a change in enthalpy in reactions can be measured.

Change in enthalpy for a reaction = (sum of the products' standard heat of formation - sum of the reactants' standard heat of formation)

Standard heat of formation (H(f)) is just the enthalpy change which would occur if 1 mol of the compound is formed from its constituent [elements] (http://socratic.org/chemistry/a-first-introduction-to-matter/elements) in their standard states under standard conditions

Standard enthalpy change of combustion - the enthalpy change of a substance when one mole of a substance is burned completely in oxygen under standard conditions

Source:- https://answers.yahoo.com

Don't forget to check this if you want more about enthalpy.

Here is a video which discusses how to calculate the enthalpy change when 0.13g of butane is burned. Video from: Noel Pauller