>> Welcome to a Screencast on an Introduction to Enthalpy.
When a chemical reaction occurs, energy is either given off or released by the reaction
or energy is taken in or absorbed by the reaction.
For example, in an acetylene torch, acetylene combusts, it produces a lot of energy during
that combustion process and, in fact, it produces so much energy
that it can be used to weld railroad ties together.
A chemical cold pack containing ammonium nitrate, that dissolves, actually takes in
or absorbs energy and this can actually be used to cool down
or ice an injury and reduce swelling.
Now, chemists use something called enthalpy, which we symbolize by the letter H,
to follow or to track energy changes during chemical reactions.
We should note that technically, enthalpy is heat flow at constant pressure,
but most of the time, we do reactions at constant pressure so we can normally just think
of enthalpy of reaction as the heat or energy of reaction.
And it's important to note that- enthalpies are as integral a part of a chemical reaction
as the reactants and products, and they're treated quantitatively in the same way
as the amounts of the substances involved.
And we'll see this in an example shortly.
Okay, so let's start by considering the thermal chemical equation
for the complete combustion of acetylene.
And here is the chemical equation for the complete combustion of acetylene.
And acetylene is C2H2 reacts with oxygen to make CO2 and H2O
and this equation is correctly balanced,
but it doesn't show the energy that's involved in this reaction.
So, it turns out that 2600 kilojoules of energy are given off when 2 moles of acetylene reacts
with 5 moles of oxygen and makes 4 moles of carbon dioxide and 2 moles of water.
So, this is now a thermochemical equation and it shows that heat is a product of the reaction,
and it shows explicitly how much heat is the- is a product of the reaction.
Now, there's another way we can write this and that is to write the chemical equation
as we did before, but instead of showing the energy as an explicit product,
we write the enthalpy change, delta H. And the delta H, the enthalpy change
for this reaction is negative 2600 kilojoules and the reason
for the negative sign is it indicates that energy is given off.
This is what's called an exothermic reaction and all exothermic reactions give off energy
and all like exothermic reactions have a negative value for delta H.
Okay, now let's see how we can use this quantitatively
and let's calculate the enthalpy change when 0.473 moles
of acetylene burns and see what it means.
Well, here is the thermochemical equation.
And if we have 0.473 moles of acetylene,
we note that what the thermochemical equation tells us is that 2 moles
of acetylene will produce 2600 kilojoules of energy.
We put a negative sign on the energy, or the enthalpy change, because energy is given off
and then note, this conversion factor converts from moles event-
of acetylene into kilojoules of energy.
And, if we do the calculation, we get -615 kilojoules for the enthalpy change.
Six hundred fifteen is the quantity, 615 kilojoules,
and the negative simply means energy is given off.
So, in words, we would say what this means is that this amount of acetylene, when it burns,
gives off or produces 615 kilojoules of heat.
Okay, let's also do a thermochemical equation for the ammonium nitrate dissolving
in water in a chemical cold pack.
Here is the chemical equation.
It's actually just a dissolving process, but solid ammonium nitrate when it is dissolved
in water produces ammonium ions and nitrate ions and it turns out that actually takes in energy
and it takes in 25.7 kilojoules of energy, or heat,
when 1 mole of ammonium nitrate solid dissolves.
So, here's one way of writing the thermochemical equation for this process
and the other way involves using enthalpy, or enthalpy change like we did previously,
and so we write the chemical equation, NH4NO3 solid makes NH4 plus and NO3 minus aqueous,
and then we tack on the delta H, the enthalpy change value at the end,
and here 25.7 kilojoules of enthalpy is the change, but it has a positive sign because,
in this case, energy or heat is taken in during the process.
And this is an example of what's called an endothermic reaction.
All endothermic reactions take in energy
and all endothermic reactions have a positive delta H value.
And let's do a calculation with this, what's the enthalpy change when 40.0 grams
of ammonium nitrate dissolves and what does this mean?
Well, there's the thermochemical equation.
And again note, the chemical equation or thermochemical equations,
like chemical equations, are based upon moles, so we have to convert the 40 grams
of ammonium nitrate, I'm using that as a shorthand for it, using the molar mass
of ammonium nitrate to get the moles.
And now, we know this is a simple stoichiometry, for every 1 mole of ammonium nitrate
that dissolves, 25.7 kilojoules of energy are absorbed, so that's positive.
That converts from moles ammonium nitrate into enthalpy change and that turns
out to be 12.8 kilojoules, and it's positive, so what this means is when 40 grams
of ammonium nitrate dissolves, 12.8 kilojoules of energy or heat is absorbed.
And that's it for an Introduction to Enthalpy.