Transcript Document

16.2 Driving Force of
Reactions
POINT > Recall enthalpy of reactions
POINT > Define entropy
POINT > Describe enthalpy and entropy
interactions
POINT > Define Gibbs Free Energy
POINT > Use Gibbs Free Energy to predict
reaction spontaneity
POINT > Recall enthalpy of reactions
We defined enthalpy ∆H = Hproducts - Hreactants
If ∆H is positive, energy is required from the
surroundings and the reaction is endothermic
If ∆H is negative, energy is released to the
surroundings and the reaction is exothermic
WB CHECK:
If ∆H is positive is the reaction is exothermic or
endothermic?
POINT > Recall enthalpy of reactions
The products of exothermic reactions, having lost
energy, tend to be more stable
Most naturally occurring reactions are exothermic,
leading to a lower energy state
Some endothermic reactions, however, do occur
spontaneously at certain temperatures
H2 + CO2  H2O + CO ∆H = 11kJ
POINT > Define entropy
If an endothermic reaction happens spontaneously, it
is due to an increase in entropy
Entropy (S) is a measure of the disorder of a system
(how random it is)
For example, liquid water has higher entropy than
ice
Water vapor has higher entropy than liquid water
POINT > Define entropy
In general, reactions proceed toward creating
higher entropy (S)
Increasing disorder (entropy) is a characteristic of
the universe
POINT > Define entropy
Entropy has unit kJ/mol•K (similar to enthalpy)
POINT > Define entropy
In general:
Entropy for gas phase is greater than that of liquid
or solid of same substance
i.e. I2 (g) has greater entropy than I2 (s)
Increasing temperature increases entropy
POINT > Define entropy
In general:
An increase in moles = increase in entropy
POINT > Describe enthalpy and entropy interactions
Reactions are favored when change in enthalpy ∆H
is negative and change in entropy ∆S is positive
If ∆H is negative and ∆S is positive, a reaction will
always be spontaneous
If ∆H is positive and ∆S is negative a reaction will
never be spontaneous
If the two are in conflict, one will predominate
WB CHECK:
Spontaneous reactions are favored when
a) ∆H is negative and ∆S is negative
b) ∆H is positive and ∆S is positive
c) ∆H is negative and ∆S is positive
d) ∆ H is positive and ∆S is negative
POINT > Define Gibbs Free Energy
Gibbs Free Energy (G) combines enthalpy and
entropy to quantify the overall energy state of a
reaction
G = H – T S
(T = ºK)
A G that is negative will be a spontaneous
reaction
A G that is positive will not occur spontaneously
POINT > Define Gibbs Free Energy
G = H – T S
A G that is negative will be a spontaneous
reaction
A G that is positive will not occur spontaneously
So, a negative H (exothermic) and positive S
(increased entropy) both lead to a negative G and
a spontaneous reaction
WB CHECK:
G = H – T S
H for a reaction is 1196 kJ/mol and S for the
reaction is 4.36 kJ/mol •ºK
Find G and predict whether the reaction is
spontaneous at 25 C
G = -103 kJ yes, the reaction is spontaneous
POINT > Use Gibbs Free Energy to predict reaction
spontaneity
G = H – T S
If asked to find the temperature at which a reaction
will occur spontaneously, set G = 0 and solve for T
Ex Practice problem 1 page 520
Thermodynamics in Living Systems
Example
C6H12O6 oxidation: G° = 2880 kJ/mol
ADP  ATP
G° = 31 kJ/mol
Protein synthesis: forming peptide bonds between
amino acids
alanine + glycine  alanylglycine
G° = 29 kJ/mol
Homework: Read the book!! Pages 516-520
F.A. page 520
#21, 23-25 page 524