CATALYST
1. Would a +ΔH or a –ΔH favor a chemical reaction to
occur? Why?
2. Would a +ΔS or a –ΔS favor a chemical reaction to
occur? Why?
FORMATION OF SODIUM CHLORIDE
JUSTIFY – TPS
Why a –ΔH or +ΔH indicate a chemical reaction will
occur under some conditions, but not under other
conditions?
LECTURE 6.1 – PREDICTING
SPONTANEITY
TODAY’S LEARNING TARGETS
• LT 6.1 – I can interpret the entropy of a chemical
reaction and how standard entropies can be use to
calculate the overall entropy of the system.
• LT 6.2 – I can explain the parameter of free energy
(ΔG) and discuss the values that indicate
spontaneous and non-spontaneous reactions.
• LT 6.3 – I can hypothesize the conditions (if any)
whereby a reaction will be spontaneous based on
the values of ΔH and ΔS.
SPONTANEOUS REACTIONS
• Recall, in chemistry, a spontaneous
reaction/process is one that will occur without any
additional assistance (electricity, new chemicals,
etc.).
• Reactions are spontaneous based on their values of
of ΔH and ΔS.
+ΔH
+ΔS
-ΔS
-ΔH
GIBBS FREE ENERGY (ΔG)
• ΔG is our final thermodynamic parameter that
relates ΔH and ΔS
• Gibbs simply applied the 2nd law to chemical
reactions:
DSuniverse = DSsurroundings + DSsystem
æ -DH system ö
DSuniverse = ç
÷ + DSsystem
T
è
ø
-TDSuniverse = DH system - TDSsystem
DG = DH - TDS DG = DH -TDS
o
o
o
THE POWER OF ΔG
• We do not need to measure anything outside the
system in order to determine spontaneity
• Using Gibbs Free Energy alone we will be able to
predict the spontaneity of a reaction.
• The value of ΔG indicates one of three options:
1. If ΔG < 0, then reaction is spontaneous as
written
2. If ΔG = 0, then reaction is at equilibrium
3. If ΔG > 0, then reaction is not spontaneous as
written (spontaneous in reverse direction)
PREDICTING SPONTANEITY
• Recall, -ΔH and +ΔS are favorable.
• ΔG equation explains why!
DG = DH -TDS
+ΔH
+ΔS
-ΔS
-ΔH
Spontaneous at
High
Temperatures
Always
Spontaneous
Never
Spontaneous
Spontaneous at
Low
Temperatures
MISCONCEPTION ALERT
• Just because a reaction is
spontaneous does not mean that
it will be fast
• A –ΔG simply means the reaction
is spontaneous. It may take 3
seconds or 3 million years. The
value simply indicates that it
will occur
CLASS EXAMPLE
• Without calculating, predict if the following reaction
will be spontaneous. If it is spontaneous, specify
under what conditions.
• You run the following reaction:
N2 (g) + O2 (g)  2 NO (g)
• You have ΔH = 180.7 kJ and ΔS = 24.7 J/K
TABLE TALK
• Without calculating, predict if the following reaction
will be spontaneous. If it is spontaneous, specify
under what conditions.
• You run the following reaction:
P4 (g) + 6 Cl2 (g)  4 PCl3 (g)
• You have ΔH = -190.2 kJ and ΔS = 423.4 J/K
STOP AND JOT
• Without calculating, predict if the following reaction
will be spontaneous. If it is spontaneous, specify
under what conditions.
• You run the following reaction:
N2 (g) + 3 H2 (g)  2 NH3 (g)
• The reaction is exothermic.
WHITE BOARD RACES
WHITE BOARD QUESTIONS
Using only the signs of Ho and
temperature dependence.
So, predict the signs and
1. CH3OH (l) + 3/2 O2 (g)  CO2 (g) + 2 H2O (g)
Exothermic reaction
2. The vaporization of water
3. CO (g) + H2O (g)  CO2 (g) + H2 (g)
Ho = -41.2 kJ and
So = -135 J/K
4. The condensation of water
5. P4 (g) + 6 Cl2 (g)  4 PCl3 (g)
The reaction is endothermic
6. N2 (g) + O2 (g)  2 NO (g)
The reaction is endothermic
7. The freezing of carbon dioxide
WORK TIME
• Work on drills for ΔH, ΔS, and ΔG.
• Due Thursday/Friday
EXIT TICKET
Using only the signs of Ho and
temperature dependence.
So, predict the signs and
1. P4 (g) + 6 Cl2 (g)  4 PCl3 (g) + Energy
2. The dissolving of NaCl in water. (This process is endothermic)
RATE YOURSELF
• Using your learning target log, rate yourself 1 – 4 on 6.1
and 6.3
CLOSING TIME
• Read 19.5 and 19.6
• Complete 6.1 ΔH, ΔS, and ΔG Drills