There's a few ways we can use this enthalpy, so let's try some applications right now.
One thing we could ask is 'how much energy is required to form a given compound?'
So for example, I have H2O down here, the water molecule.
How much energy would be required to form this compound, H2O, from its elemental forms?
We have to be very careful here because in the elemental forms that we're talking about,
we're talking about the oxygen in its natural form as it typically appears in nature
which is an O2 molecule, and then the two hydrogen molecules.
The amount of energy we have to add is called the enthalpy of formation.
So this standard enthalpy of formation is written as delta H,
representing the change in enthalpy or how much energy we have to add,
and then we have an "f" and a little circle at the top with line through it.
All of this are representing the change in the enthalpy or energy we have to add
of formation and it's the standard enthalpy or energy of formation.
This number, the enthalpy or energy of formation to form a particular compound
like the one we have here,
will be positive if we have to add energy to create our compound
and negative if energy is released in the creation of this compound.
The word 'standard' in the standard enthalpy of formation means
how much energy do we have to add at this particular pressure and temperature.
Where the particular values are one atmosphere of pressure,
so standard atmoshperic pressure, and the temperature of 25 degrees Celsius.
There's a few reasons why this enthalpy of reaction is so useful.
One is that we can imagine a reaction occuring, like the one we just discussed,
either you start with an oxygen atom and you have two hydrogen atoms
and you put them together, in which case they might release energy,
or maybe they started together, and in order to break them apart
you have to add some sort of energy.
In the one case we have energy leaving your system,
in that case, we have a negative standard enthalpy of formation.
In the other case we have energy being put into the system in order to break this apart.
In this case, you have a positive change in the enthalpy.
So when the change in the enthalpy is a positive number,
we say that this is an endothermic reaction.
So this would correspond to the lower case that you see here,
in which case we had to take in energy from our environment
in order to break this H2O molecule apart.
So this endothermic reaction is taking -- thermal energy is taking energy into itself
which in fact means that the environment around it will become colder
because it stole, if you will, some energy from that environment in order to undergo its reaction.
On the other hand, as the reaction occurs
you might be releasing energy, and that would be a minus or a negative change in the enthalpy.
In that case, we call this an exothermic reaction.
And these reactions release heat energy to their environment.
If we wanted to talk about one last quantity,
so we're just gonna briefly discuss this
to make sure we know that it's different from the enthalpy,
we could ask for any molecule at all -- like this O2 molecule here --
how much energy do have to supply to this O2 molecule to break it apart, to break apart the bond?
And that would be some energy called the dissociation energy,
which is the energy required to break this object apart.
This is different from the standard enthalpy of formation or enthralpy change
in the reaction, because here we're simply talking about how much energy
we need to add to a system in order to dissociate these two.
And we can be talking about absolutely any sort of molecule or any sized molecule
when we discuss the dissociation energy for that object.
And again, this is different from the enthalpy of formation.
So I go back to the standard enthalpy of formation definition,
and make sure you understand the key, sort of tricky aspects and the subtleties of that
as opposed to the dissociation energy
which is just the energy required to break some object apart.