Transcript Physical Properties - Winthrop University

Special Note: Endothermic Reactions
• As long as the total
entropy is slightly
positive, endothermic
reactions will go
forward
• Spontaneity is
determined by the
increase in the
entropy of the
system, NOT a
decrease in the
ENERGY (heat) of
the system!

Equilibrium
• A system at equilibrium has no net
change in products or reactants
• This does not mean that nothing is
happening!!!!
– The Forward rate equals the Reverse rate
 STot for systems at equilibrium equals
zero
STot = 0 (at equilibrium)

Gibbs Free Energy
• The Gibbs Free Energy, G, is a value that
allows us to predict spontaneity by taking
both Enthalpy AND Entropy into account
1) We know that:
STot = S + SSurr
STot = S -H/T
(but SSurr=- H/T)
(at constant T and P)
2) We’ll define the Gibbs free energy as:
G = H - TS
Gibbs Free Energy
But we want to know the change in free energy
when a reaction occurs, so we change it to:
G = H -TS
(at constant T)
But if pressure is constant, we know that:
STotSH/T
Which we can combine and rearrange into:
G = -TSTot
(at constant T and P)
Gibbs Free Energy
G = H - TS
G = -TS
(at constant T)
(at constant T and P)
• A negative value of G indicates that a
reaction will spontaneously occur
• Large negative H values (like we’d have in a
combustion reaction) would probably give you
a large negative G
• If TS is large and positive, the value of G
may be large and negative
Free Energy and Spontaneity

Free Energy and Temperature
•Free energy decreases
(becomes more negative) as
temperature increases
•At low T, Gm for solid phase is
lower than that of liquid or
vapour, so the solid phase is
prevalent
•As we increase T to Tfus and
higher, the liquid state has a
lower Gm, so it is the phase
that prevails
•As we increase T further to Tb,
the gas phase has the lowest
value of Gm
7.13: Gibbs Free Energy or Reaction
• To determine the spontaneity of a
reaction, we use the change in the
Gibbs Free Energy, G, or the Gibbs
Free Energy of Reaction
G  nGm Products 

nG
m Reactants
We’ve seen something like
this before somewhere…
Standard Gibbs Free Energy of Formation,
Gf°
 Gf° = The standard Gibbs Free Energy of
reaction per mole for the formation of a
compound from its elements in their most
stable form.
• Most stable form?
–
–
–
–
Hydrogen = ?
Oxygen = ?
Iodine = ?
Sodium = ?
 Gf°is for the formation of 1 mole of product
– Different amounts of reactants may be used…Be vigilant!

Gf°: What Does it Mean?
• Compounds with Gf° > 0 are
Thermodynamically Unstable
• Compounds with Gf° < 0 are
Thermodynamically Stable
Gibbs Free Energy and Nonexpansion Work
• we = ‘Extra work’
– Nonexpansion work is any kind of work other than
that done against an opposing pressure
• Stretching a spring, moving a rope, importing
a sugar molecule into a cell are all examples
of nonexpansion work
• All cellular processes are examples of
nonexpansion work
• How are the Gibbs Free Energy and we
related?
Gibbs Free Energy and we
 G = we
• If we know the change in free energy,
we know how much nonexpansion work
can be done
• What does this mean?
– Let’s look at the combustion of glucose.
G° and the Combustion of Glucose
C6H12O6 (s) + 6 O2 (g) --> 6 CO2 (g) + 6 H2O (l)
• The G° of the reaction is -2879 kJ
For 1 mole of glucose, we get 2879 kJ of energy
Or
For 180 g of glucose, we get 2879 kJ of energy
• To make one mole of peptide bonds, 17 kJ of
work must be done.
– If we get 2879 kJ of energy from one mole of
glucose, we should be able to make 170 moles of
peptide bonds
One molecule of glucose will provide
enough energy to add 170 amino acids to
a growing protein
(in actuality, you can only add 10 amino acids)
The Effect of Temperature on G°
• Remember that H° or S° is the sum of
the individual enthalpies or entropies of the
products minus those of the reactants
• If we change the temperature, both are
affected to the same extent, so the H° and
S° values don’t significantly change
• This is not the case with G°. Why?
G° = H° - TS°
The Effect of Temperature on G°
The Effect of Temperature on G°
The Effect of Temperature on G°
The Effect of Temperature on G°
