- [Instructor] The change in enthalpy
for a chemical reaction delta H,
we could even write delta H of reaction in here
is equal to the heat transferred
during a chemical reaction at constant pressure.
So delta H is equal to qp.
Let's say we are performing a chemical reaction,
an aqueous solution under constant atmospheric pressure.
The reactants and products of that chemical reaction
make up the system and everything else makes
up the surroundings.
When heat flows from the surroundings to the system,
the system or the reaction absorbs heat
and therefore the change in enthalpy
is positive for the reaction.
This is called an endothermic reaction.
If heat flows from the system to the surroundings,
the reaction gave off energy.
Therefore the change in enthalpy
for the reaction is negative
and this is called an exothermic reaction.
As an example of a reaction, let's look at the decomposition
of hydrogen peroxide to form liquid water and oxygen gas.
The change in the enthalpy for this reaction
is equal to negative 196 kilojoules.
The negative sign means the reaction is exothermic.
And for the units, sometimes you might see kilojoules.
Sometimes you might see kilojoules per mole,
and sometimes you might see kilojoules per mole of reaction.
What kilojoules per mole of reaction
is referring to is how the equation is written.
So if we look at this balanced equation,
there's a two as a coefficient in front of hydrogen peroxide
and therefore two moles of hydrogen peroxide
are decomposing to form two moles of water
and one mole of oxygen gas.
So when two moles of hydrogen peroxide decompose,
196 kilojoules of energy are given off.
Next, let's calculate how much heat is released
when 5.00 grams of hydrogen peroxide decomposes
at a constant pressure.
The first step is to find out how many moles
of hydrogen peroxide that we have.
So we take the mass of hydrogen peroxide
which is five grams
and we divide that by the molar mass of hydrogen peroxide
which is 34.0 grams per mole.
Grams cancels out and this gives us 0.147
moles of hydrogen peroxide.
Next, we take our negative 196 kilojoules
per mole of reaction
and we're gonna multiply this by a conversion factor.
When we look at the balanced equation for how it's written,
there are two moles of hydrogen peroxide.
So for our conversion factor
for every one mole of reaction as it is written,
there are two moles of hydrogen peroxide.
So two moles of H2O2.
Now the of reaction will cancel out
and this gives us negative 98.0 kilojoules
per one mole of H2O2.
So two moles of hydrogen peroxide
would give off 196 kilojoules of energy.
And one mole of hydrogen peroxide would give
off half that amount or 98.0 kilojoules of energy.
Next, we take our 0.147 moles of hydrogen peroxide.
So let me just go ahead
and write this down here really quickly.
So we have 0.147 moles of H202.
And remember, we're trying to calculate,
we're trying to calculate the amount of heat
that was released.
So next we multiply that by negative 98.0 kilojoules
per mole of H202, and moles of H2O2 will cancel out
and this gives us our final answer.
So the heat that was released when 5.00 grams
of hydrogen peroxide decompose at constant pressure,
this turns out to be equal to negative 14.4 kilojoules.