A cup of hot tea is made and is left on a table.
What do you expect to spontaneously happen
to the temperature of the tea?
A cup of hot tea is made and is left on a table.
What do you expect to spontaneously happen
to the temperature of the tea?
Is this endothermic or exothermic?
A cup of hot tea is made and is left on a table.
What do you expect to spontaneously happen
to the temperature of the tea?
Is this endothermic or exothermic?
What if the tea was left long enough to
evaporate. Is this endothermic or exothermic?
A cup of hot tea is made and is left on a table.
What do you expect to spontaneously happen
to the temperature of the tea?
Is this endothermic or exothermic?
What if the tea was left long enough to
evaporate. Is this endothermic or exothermic?
Does there seem to be a connection between
being endothermic or exothermic and
spontaneous?
Enthalpy does have a connection with
whether or not a change is spontaneous, but
obviously it is not the only factor.
Enthalpy does have a connection with
whether or not a change is spontaneous, but
obviously it is not the only factor.
While making a cup of tea, the box of tea
bags falls and opens on the floor. What do
you expect the tea bags to spontaneously do?
Enthalpy does have a connection with
whether or not a change is spontaneous, but
obviously it is not the only factor.
While making a cup of tea, the box of tea
bags falls and opens on the floor. What do
you expect the tea bags to spontaneously do?
Does it seem increasing disorder is always
spontaneous?
The second law of thermodynamics says the
disorder of the universe is always increasing.
What two things are included in the universe?
The second law of thermodynamics says the
disorder of the universe is always increasing.
What two things are included in the universe?
If the disorder of the universe is always
increasing, how is it possible for you make
your bedroom less disordered?
The thermodynamic factor we have been
calling disorder is actually called entropy (S).
So the second law shows:
Suniverse = Ssystem + Ssurroundings and is always
increasing (+)
-S is decreasing disorder, +S is increasing
disorder
Brainstorm changes that increase entropy:
*Factors of ΔS that are of major importance:
*Factors of ΔS that are of major importance:
1. Phase changes
Which phase changes are +ΔS and which are
–ΔS?
*Factors of ΔS that are of major importance:
1. Phase changes
Which phase changes are +ΔS and which are
–ΔS? Which ones make the biggest change?
*Factors of ΔS that are of major importance:
2. Changing the number of moles
What are the two possibilities? Let’s show an
example of each:
*Factors of ΔS that are of major importance:
2. Changing the number of moles
What are the two possibilities? Let’s show an
example of each:
Can this be combined with #1? Let’s show an
example:
*Factors of ΔS that are of major importance:
3. Molecular complexity
consider H2(g) + Cl2(g) → 2HCl(g)
Is there a phase change?
Is there a change in the number of moles?
What effect does the reaction have on
entropy?
Chemists of course want to measure entropy,
so the third law of thermodynamics says that
the absolute entropy of any substance at
absolute zero is zero. What is standard
temperature?
Chemists of course want to measure entropy,
so the third law of thermodynamics says that
the absolute entropy of any substance at
absolute zero is zero. What is standard
temperature? If the substances are never at
absolute zero, what does that mean for
elements in their natural state at standard
temperature and pressure?
Chemists of course want to measure entropy,
so the third law of thermodynamics says that
the absolute entropy of any substance at
absolute zero is zero. What is standard
temperature? If the substances are never at
absolute zero, what does that mean for
elements in their natural state at standard
temperature and pressure? Can these standard
entropy values be + or -?
Utilizing standard entropy data, we can
calculate the change in entropy for a reaction:
ΔS = ΣSproducts - ΣSreactants
Utilizing standard entropy data, we can
calculate the change in entropy for a reaction:
ΔS = ΣSproducts - ΣSreactants
Would Hess’s Law and bond entropy data also
work for finding ΔS? What does that tell you
about S?
Utilizing standard entropy data, we can
calculate the change in entropy for a reaction:
ΔS = ΣSproducts - ΣSreactants
Would Hess’s Law and bond entropy data also
work for finding ΔS? What does that tell you
about S?
Beware, S is usually measured in J mol-1, not
kJ mol-1 as we did with enthalpy.
Rockets can use the following reaction as
propulsion: N2H4(l) + 2H2O2(l) → N2(g) + 4H2O(g)
What is the ΔS° (appendix B.16)
What is spontaneous for liquid water at 100°C?
What is the ΔH?
What is the ΔS?
What is spontaneous for liquid water at 100°C?
What is the ΔH?
What is the ΔS?
What is spontaneous for liquid water at 0°C?
What is the ΔH?
What is the ΔS?
Both enthalpy and entropy have an effect on
the spontaneity of a reaction, but both need to
be considered at the same time for us to predict
if a reaction will be spontaneous or not. This
relationship is called Gibb’s (Free) Energy.
Gibbs energy is a measure of the maximum
amount of useful (available) energy in a
system.
Gibbs energy is calculated as:
G = H - TS
as both enthalpy and entropy are state
functions, so is Gibbs energy, therefore:
ΔG = ΔH - TΔS
ΔG = ΔH - TΔS
if ΔG is -, then spontaneous
if ΔG is +, then not spontaneous
T must be in Kelvin
H is usually in kJ/mol but
S is usually in J/mol
ΔG = ΔH - TΔS
if ΔG is -, then spontaneous
if ΔG is +, then not spontaneous
A spontaneous (thermodynamically favorable)
reaction is one where the reactants will
naturally form products. The bigger the value of ∆G, the greater the tendency to form
products. The bigger the + value of ∆G, the
greater the tendency to form reactants.
ΔG = ΔH - TΔS
The above equation gives us many options.
We could solve for ΔG. What information
would we need?
ΔG = ΔH - TΔS
The above equation gives us many options.
We could solve for ΔG. What information
would we need?
We could solve for ΔH. What information
would we need?
ΔG = ΔH - TΔS
The above equation gives us many options.
We could solve for ΔG. What information
would we need?
We could solve for ΔH. What information
would we need?
We could solve for ΔS. What information
would we need?
ΔG = ΔH - TΔS
The above equation gives us many options.
We could solve for T. What information
would we need?
ΔG = ΔH - TΔS
A very interesting thing happens when you
solve for T. This tells you the temperature at
which a reaction that was not spontaneous
would become spontaneous. Note that at this
point where ΔG goes from + to - the value of
∆G is zero.
What is this formula?
ΔG = ΔH - TΔS
A very interesting thing happens when you
solve for T. This tells you the temperature at
which a reaction that was not spontaneous
would become spontaneous. Note that at this
point where ΔG goes from + to - the value of
∆G is zero.
What is this formula?
Why do reptiles sunbathe?
Consider this reversible reaction: Br2(l) → Br2(g)
At what temperature does this change
spontaneity?
What is significant about this temperature?
One other alternative way to find ΔG is
ΔG = ΣΔGproducts - ΣΔGreactants
It is VERY important you know how to predict
when a reaction will be thermodynamically
favorable or not. To be spontaneous, G must be -.
How can you find the sign of G?
ΔH - T ΔS =
+
ΔG
spont.
+
-
T+
-
spont.
spont.
+
-
not spont.
+
T-
not spont.
not spont.
T
T
+
S
H
Other things that can be done with ∆G:
∆G° = -n F E°
What is each variable?
Other things that can be done with ∆G:
∆G° = -n F E°
What is each variable?
What would this allow us to do?
Other things that can be done with ∆G:
∆G° = -n F E°
What is each variable?
What would this allow us to do?
What is spontaneous E and what is
spontaneous for G? How does this work out?
Other things that can be done with ∆G:
[products]
∆G° = -R T ln
[reactants]
What is each variable?
Other things that can be done with ∆G:
[products]
∆G° = -R T ln
[reactants]
What is each variable?
What would this allow us to do?
Other things that can be done with ∆G:
[products]
∆G° = -R T ln
[reactants]
What is each variable?
What would this allow us to do?
What is spontaneous for a chemical reaction
and what is spontaneous for G? How does thi
work out?