The Enthalpies

What is Enthalpy?

Usually there is energy change involved in a chemical reaction somehow. In chemistry this change in heat energy is represented as enthalpy change using the following symbol:

So that this figure is the same everywhere and fair, it is taken at standard conditions which are 298K and 100 kPa pressure. Here are two reactions, each with a different enthalpy change.

For the first rection, the enthalpy change is negative this is because the reaction is exothermic meaning it gives out heat. The reason it is negative is energy is leaving the reaction.
The second reaction has a positive enthalypy change, this is because it is endothermic and heat is entering the system.

Enthalpy of Reaction, Combustion, and Formation

These are three different types of situation that an energy change may take place. On the top image x is used to show where a letter is written to indicate what type of enthalpy is shown: r for reaction and so on.
The easiest one to get your head around is standard enthalpy of reaction this is merely the energy change of a reaction in the quantities that are expressed.
The standard enthalpy of combustion is the enthalpy change when 1 mole of substance is completely burned in excess oxygen under standard conditions, all reactants in their standard states. The enthalpies of combustion can be determined using a calorimeter which is explained below; it uses the symbol c.
The standard enthalpy of formation is the enthalpy change when 1 mole of compound is produced from its elements in standard conditions, all products and reactants in the standard state. The standard state is important, it means the way that the element is, at standard conditions (see above). So you would have H₂(g) NOT H on its own. Therefore you sometimes need to do fractions of elements, as with the standard enthalpy of formation of ethanol:

C (s) + 2H₂ (g) + ½O₂ (g) ® CH₃OH (l)

other enthalpies....

Enthalpy	Definition
Atomisation ΔHθa	The standard enthalpy change when 1mol of gaseous atoms are formed. Depending on the standard state of the element, this could also be called the enthalpy of sublimation if going from solid to gas.
Bond dissociation, ΔHθdiss	The standard enthalpy change when 1mol of gaseous covalently bonded molecule is broken to form 2 radicals. So the enthalpy change for A (g) ® X. (g) + Y..
Electron Affinity, ΔHθea	The standard molar enthalpy change when an electron is added to an atom in the gas phase. For example: Cl (g) + e- ® Cl- (g)
Lattice Enthalpy, ΔHθL	The standard enthalpy change accompanying dissociation: the seperation of 1 mol of solid ionic lattice into gaseous ions; formation the creation of 1 mol of solid from gas ions. Note that formation enthalpies are always negative and dissociation positive.
Hydration, ΔHθhyd	The standard molar enthalpy change for the process: X+/- (g)® X+/- (aq).
Solution, ΔHθsol	The standard enthalpy change when 1mole of ionic solid dissolves in enough water that none of the dissolved ions interact with each other