Transcript Potential Energy Diagrams Power Point

Potential Energy
Diagrams Made Easy
Exothermic followed
by Endothermic
In any exothermic reaction, energy would be a product
Potential energy
in
kJ/mol
time
In any exothermic reaction, energy would be a product
The potential energy of the reactants, labeled R, is steady, because
it is based upon the actual chemicals. That line is flat to start.
Potential energy
in
kJ/mol
R
time
In any exothermic reaction, energy would be a product
As you add energy, the energy of the reactants increases
Potential energy
in
kJ/mol
R
time
In any exothermic reaction, energy would be a product
As you add energy, the energy of the reactants increases
Until you reach the top of the arch, the minimum amount of energy
needed to make this reaction go forward, the ACTIVATION ENERGY.
Potential energy
in
kJ/mol
R
time
In any exothermic reaction, energy would be a product
As you add energy, the energy of the reactants increases
The reaction “pays back the activation energy first.
Potential energy
in
kJ/mol
R
time
The green arrow
represents the activation
energy required for this
reaction to start.
In any exothermic reaction, energy would be a product
The reaction continues to give off more energy (that’s what
exothermic reactions do). The reaction moves to completion.
Potential energy
in
kJ/mol
R
time
In any exothermic reaction, energy would be a product
The reaction finishes, the products end with less potential energy
than the reactants started with (labeled P).
This Potential energy remains a constant as well.
Potential energy
in
kJ/mol
R
P
time
In any exothermic reaction, energy would be a product
The net change in potential energy between the starting & the
ending potential energy is called the ∆H. In this case, since the
difference is LOWER, the ∆H is negative. A ―∆H is exothermic.
Potential energy
in
kJ/mol
R
― ∆H
time
P
The activated complex is a very short lived melding of the
reactants as the become products. For about one ten trillionth of
a second they are not reactants or products, but an intermediate
substance. The bonds are breaking and reforming as the products
form. This is labeled with the star.
Potential energy
in
kJ/mol
This is the “point of no
return” for the reaction.
R
― ∆H
time
P
In any exothermic reaction, energy would be a product
∆H is the NET difference between starting and ending energies.
The activation energy is required to start the reaction, but it is
“paid back”. Only the NET difference is the energy released to the
environment. When a reaction releases energy, that’s exothermic.
Potential energy
in
kJ/mol
R
― ∆H
time
P
In any exothermic reaction, energy would be a product
If a catalyst is used to speed up a chemical reaction, the potential
energy of the reactants and products does not change. The ∆H
doesn’t either. What does happen is that the ACTIVATION ENERGY
is lowered, making the reaction easier to go forward.
Potential energy
in
kJ/mol
R
― ∆H
time
P
In any exothermic reaction, energy would be a product
Catalysts lower activation energy by providing an easier, alternative
pathway for the reaction to proceed. This makes the reaction
faster, but not
Potential energy
in
kJ/mol
The catalyst can
only changes the
activation energy,
it cannot change
potential energies,
and therefore,
cannot change the
∆H either.
Blue arrow is the lower activation
energy required due to the catalyst.
R
― ∆H
time
P
Endothermic Potential Energy Diagrams
ΔH is positive
Energy is absorbed to make the reaction go forward.
In any endothermic reaction, energy would be a reactant.
The reactant Potential Energy is a constant because of
what the reactants are. It’s labeled “R”.
Potential energy
in
kJ/mol
R
time
In any endothermic reaction, energy would be a reactant.
As you add energy, the energy levels rise,
towards the minimum needed to cause a reaction,
the ACTIVATION ENERGY.
Potential energy
in
kJ/mol
R
time
In any endothermic reaction, energy would be a reactant.
If you reach the top of the arch, if you provide enough
energy, the reaction goes forward and “pays back” some
energy of activation.
Potential energy
in
kJ/mol
R
time
In any endothermic reaction, energy would be a reactant.
In an endothermic reaction, the products “P”, have more
energy than the reactants they started with.
Potential energy
in
kJ/mol
P
R
time
In any endothermic reaction, energy would be a reactant.
The ACTIVATION ENERGY is from the starting potential
energy to the top of the curve, the minimum energy
needed to make this reaction start.
Potential energy
in
kJ/mol
P
R
time
A.E.
In any endothermic reaction, energy would be a reactant.
The difference between the potential energy at the start
of the reaction, and the end of the reaction, between
reactants and products, is the ΔH.
Potential energy
in
kJ/mol
P
R
time
A.E.
∆H
Potential energy
in
kJ/mol
In any endothermic reaction, energy would be a reactant.
A catalyst for this reaction would require less activation
energy, providing an easier, alternate path for the
reaction. It does not change potential energies for
reactants or products, or the ΔH.
P
R
Lower A.E.
time
Potential energy
in
kJ/mol
In any endothermic reaction, energy would be a reactant.
The star represents that moment in time where the
reactants are reacting, no longer exactly what they were,
but not quite fully formed into the products. The
ACTIVATED COMPLEX is that in between state.
time
Potential energy diagrams show the energy flow of a chemical reaction, from the
starting point with the reactants, through the energy “spark” that will start a
reaction, through the formation of the products at the end.
A chemical reaction will result in a net gain of energy (endothermic, +∆H) or a net
loss of energy (exothermic, ─ ∆H). The energy absorbed comes from the
immediate environment of the reaction, which makes that feel colder, since it’s a
loss of kinetic energy.
In an exothermic reaction, since the products have less energy than they started,
the energy is not “lost”, rather it is let go into the environment, making that feel hot.
Without enough energy, the activation energy, you will not get the reaction to move
forward to completion. You might get warmer reactants, but not products.