- [Instructor] Let's say we run an experiment
to determine if a reaction is endo or exothermic.
For our hypothetical reaction, A reaction would B to form C.
And let's say this reaction takes place in aqueous solution
in a beaker.
We can define our system as the reactants and products
that make up our chemical reaction,
and everything else is part of the surroundings,
for example, the water and also the beaker
the reaction is taking place in.
Let's say we run the reaction
and we put our hand on the beaker
and we feel that the beaker is warm.
If the beaker is warm
since the beaker is part of the surroundings,
energy must've been transferred from the system
to the surrounding.
So heat flowed from the system to the surroundings.
And that's an example of an exothermic reaction.
So Delta H is negative.
We can determine the amount of energy that flowed
from the system to the surroundings
by looking at the energy profile
for our hypothetical reaction.
In an energy profile, potential energy is in the y-axis
in kilojoules per mole and reaction to progress
is on the x-axis.
So as we move to the right on the x-axis,
the reaction is occurring.
Our reactants, which are A and B
have a certain amount of potential energy.
So that's right here on our energy profile.
So that part represents the energy of our reactants.
Our reactants react together to form our product
which is C and that's at the very end.
So over here, this line represents the energy
the potential energy of our products.
Notice how the potential energy of our reactants
is higher than the potential energy of the product.
So if we were to find the change in energy,
that would be the final minus the initial.
So the energy of the products minus the energy
of the reactance for this energy profile,
the energy of the products is about 50 kilojoules per mole,
and the potential energy of our reactance is at 100.
So this would be 50 minus 100
which is equal to negative 50 kilojoules per mole.
So on our energy profile, we could show Delta E
which would be this difference right here.
So that represents Delta E
and the change in energy Delta E is also equal to the change
in the enthalpy Delta H for this reaction.
So we know that the change in enthalpy
is equal to negative 50 kilojoules per moles.
Let's go ahead and plug that in over here on the left.
So by feeling the outside of the beaker,
we knew that the reaction was exothermic,
but the energy profile allowed us to figure out
how much energy was transferred from the system
to the surroundings.
So for an energy profile, when the energy of the reactants
is higher than the energy of the products,
this is the energy profile for an exothermic reaction.
Let's say we ran a similar reaction
where A plus B turned into C,
but this time when we felt the beaker,
the beaker felt cool to the touch.
If that's the case, it's because energy
was being transferred from the surroundings to the system.
And since these surroundings was losing energy,
that's why the beaker felt cool.
So heat flows from the surroundings to the system,
and this occurs in an endothermic reaction
and the change in entropy, Delta H is positive
for an endothermic process.
When we look at the energy profile
for an endothermic reaction.
The energy of the reactants.
So go ahead and write reactants in here.
The energy of the reactants is lower
than the energy of the products.
So this time, if we find Delta E
that would be the energy of our products
minus the energy of our reactants.
And the energy of our products
is about 100 kilojoules per mole
and the energy of our reactants is about 50.
So let's say it's 100 minus 50,
which would be positive 50 kilojoules per mole.
So on our diagram if we represent Delta E,
that would be this difference here on the energy profile.
And once again, Delta E is equal to the change
in the enthalpy Delta H for the reaction.
So Delta H for this hypothetical reaction
is positive 50 kilojoules per mole.
Since Delta H is positive,
we know that energy was transferred from the surroundings
to the system.
And that's the reason why the products
have a higher potential energy than the reactants
in our energy diagram.