Chemistry Chapter 23
Thermodynamics
Teacher Lecture Notes
23-1 Spontaneous Process
Spontaneous chemical reactions proceed on their own without
outside intervention.
Thermodynamics answers the questions
 The spontaneous direction of a given reaction
 How far the reaction will proceed until it reaches
equilibrium (Keq = 1) Keq >>1 products are favored
Section Review 23-1
1. If energy must be put into a reaction to make it go
in the opposite direction of the spontaneous
direction what does that tell you about energy and
the spontaneous rxn?
23-2 Enthalpy
Enthalpy of a substance represents its energy. Points to remember
 Delta H = Hp - Hr
 Negative enthalpies indicate that energy has been released and
therefore products have less energy than the reactants. The
lower the energy the more stable the substance
 Standard enthalpy (Delta H prime) is the heat transferred
between reactants and products at 1atm 25C
 Enthalpy change is proportional to the amount of substance
 Hess’s Law – net enthalpy can be derived from the sum of two
or more reactions
Most exothermic reactions are spontaneous, but some endothermic
reactions are also spontaneous.
Section Review 23-2
1. Is the enthalpy of a rxn a good indicator of its
spontaneity? Explain why or why not
2. How are enthalpy changes of rxns determined?
3. What happens to the delta H sign if the rxn is
reversed?
23-3 Entropy
Entropy (S) is a quantative measure of the disorder, or randomness,
in the substance involved in a rxn. The greater the disorder the
greater the S value. Where is disorder greater in gases or solids?
Entropy changes (delta S) = Sp - Sr Sound familiar? Unlike
delta H, you cannot directly measure delta S with heat
However if delta S is positive (products are more disordered than
the reactants) Delta Srxn = Sp > Sr
In any spontaneous process, the overall entropy of the universe
always increases.
Increase entropy occurs when
 Solids or liquids become gases
 Solutions form from liquids or gases
 More gas molecules are present as
products than reactants
 Temperature of a substance increases
Student Notes
C2.2e Compare the
entropy of solids,
liquids, and gases.
C3.4e
Predict if a chemical
reaction is spontaneous
given the enthalpy (ΔH)
and entropy (ΔS)
changes for the reaction
using Gibb’s Free
Energy, ΔG = ΔH - TΔS
(Note: mathematical
computation of ΔG is not
required.)
C3.4f Explain why some
endothermic reactions
are spontaneous at room
temperature.
Delta Universe = delta Srxn – delta Ssurr
When Delta Universe is positive the reaction is spontaneous
Neg delta H are exothermic, heating up the surroundings, therefore
increasing the entropy of the surroundings (+ delta S)! visa versa
Sooo
- H with a + S are spontaneous
+ H with a - _S are not spontaneous
Section Review 23-3
1. Explain the concept of entropy
2. An endothermic rxn has a positive delta S. Is the rxn
spontaneous? (hint look carefully at the table on pg 760)
3. Explain how entropy changes of the surroundings are related to
the enthalpy change of the rxn.
4. Why Did Ms. B name her boat entropy?
H2O(l) --> H20(g)
23-4 Gibbs Free Energy
Gibbs Free Energy incorporates the concepts of entropy and
NaCl(s) --> Na+ (aq) + Cl- (aq)
enthalpy
G= H -T S
2NH3(g)--> N2(g) + 3H2(g)
In other words, the change in Gibbs Free Energy is equal to the
enthalpy change minus the product of absolute temperature and the
H2O(l, 25C) --> H2O(l, 50C)
rxns entropy change
When delta G is a large negative number (G<<<0) products are
heavily favored (Keq >>1) Conversely if (G>>>0) reactants are
heavily favored (Keq <<1)
Energy is defined as the ability to do __________. Therefore
spontaneous rxns can do __________! So Gibbs Free Energy
change (negative delta G) represents the maximum work that a
spontaneous rxn can perform! Hence how it got the name “Free
Energy” this really represents the amount of energy that is “free” to
do work, the remainder is lost to the environment to meet the
criterion of increasing entropy of the universe! So cool Gibbs! A
positive delta G represents the minimum amount of work that must
be put into the rxn to make it run!
Section Review 23-4
1. Write the sign for delta G for a spontaneous rxn
2. How is the equilibrium constant for a reaction related to its
Free Energy and
free energy?
Spontaneity
3. What must be the sign of delta G if a process is
endothermic and has decreased entropy? Was work done
G
Rxn
Keq
by the rxn or work put into the rxn?
+
Not spon <<1
4. Why must society seek alternative energy sources if
spont
>>1
“Energy is neither created nor destroyed”?
0
At eq
=1
Special Notes
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