Topic 20: Entropy, Free Energy, and Reaction Direction Flashcards
Definition of system
Part of the universe chosen for study
Types of systems
a) Open - Freely exchanging energy and matter
b) Closed - Exchanging energy, but not matter
c) Isolated - Not interacting with its surroundings
Surroundings definition
Part of the universe outside the system which the system interacts with
State function definition
Properties that depend on the initial and final state of the system
Standard condition convention
a) Pressure of 1atm and 1M
b) Pure substance
Spontaneous change definition
A change that occurs without a continuous input of energy from outside the system (Product favored)
If a change is spontaneous in one direction, …
it is not in the other
First law of thermodynamics
Energy is neither created nor destroyed, it is transformed
(∆Esys + ∆Esurr = ∆Euniv = 0)
How is change of internal energy measured?
∆E=q+w
Does the first law of thermodynamics predict the direction of a spontaneous change?
No
Examples of endothermic reactions that are spontaneous
a) Melting / Vaporization
b) Dissolution of salts
Examples of exothermic reactions that are spontaneous
a) Freezing / Condensation
b) Burning of methane
c) Oxidation of metals
d) Formation of ionic compounds
3rd Law of Thermodynamics
A perfect crystal has zero entropy at absolute zero
2nd Law of Thermodynamics
All real processes occur spontaneously in the direction that increases Suniv
∆Suniv = ∆Ssys + ∆Ssurr > 0
General idea for predicting the change in entropy
When a system becomes more disordered, the more positive the value of ∆S
Definition of a reversible isothermal process
One that occurs in such tiny increments that the system remains at equilibrium and the direction of the change can be reversed by an infinitesimal reversal of conditions
Formula for predicting the change in entropy for an isothermal process
∆Ssys = q/T
Formula for predicting the change in entropy for phase change
a) ∆Sfus = (∆Hfus) / Tf
b) ∆Svap = (∆Hvap) / Tb
Trouton’s rule
Most liquids have a ∆Svap value close to 88 J/mol K
Factors that affect the change of entropy
a) Change of state
b) Dissolution of solid / liquid
c) Dissolution of gas
d) Atomic size
e) Molecular complexity
f) Number of particles
g) Temperature
h) Volume
i) Pressure
Effect of change of state in entropy
Increases from solid => liquid => gas
Effect of dissolution of solid / liquid in entropy
Ionic solutes have more freedom of motion
a) Exp: Small charged ions that order water molecules
Effect of atomic size in entropy
Within a periodic group, molar entropy increases with heavier atoms
Effect of molecular complexity in compounds
Entropy increases with chemical complexity
a) Exception: Ring structure restricts freedom of motion
Effect of number of particles
An increase in # of gaseous substances causes an increase in entropy
Effect of temperature in entropy at constant volume and pressure
Directly proportional
Effect of volume in entropy at constant pressure and temperature
Directly proportional
Effect of pressure in entropy at constant volume and temperature
Inversely proportional
Standard entropy change calculation (J K-1 mol -1)
∆S=∑mS(products) -∑nS(reactants)
Units of entropy change
J K-1 mol-1
Relationship between the entropy of the surroundings and…
a) Endothermic reaction
b) Exothermic reaction
∆Ssurr = -∆Hsys/T
a) Decrease
b) Increase
When does a process reach equilibrium?
∆Suniv=0
Definition of free energy
Measure of the spontaneity of a process and of the useful energy available from it
Definition of entropy
Measure of the distribution of available energy among the particles in a system
S = k ln(W)
Formula to calculate ∆G
∆G = ∆H - T∆S
-T∆S_Univ=-T∆S_Sys+∆H_Sys
Spontaneity and Sign of ∆G
a) ∆G<0
b) ∆G=0
C) ∆G>0
a) Spontaneous
b) Equilibrium
c) Nonspontaneous
Standard free energy of formation (∆Gθf)
When 1 mol of a compound is made from its elements in their standard states
What does change in free energy represent? (+ / -)
a) Maximum useful work done by a system during a spontaneous process at constant T and P
b) Minimum work that must be done to a system to make a nonspontaneous process occur at constant T and P
What is the difference between the theoretical and actual change in free energy?
a) Maximum work is never done on the surroundings due to heat loss
b) For any real machine, the actual work done on the system is always more than the minimum
Effect of temperature on spontaneity
a) ∆H (-) | ∆S (+)
b) ∆H (+) | ∆S (-)
c) ∆H (+) | ∆S (+)
d) ∆H (-) | ∆S (-)
a) Spontaneous at all T
b) Nonspontaneous at all T
c) Spontaneous at high T
d) Spontaneous at low T
How to drive nonspontaneous reactions
Coupling of reactions, in which one step supplies enough free energy for the other to occur
(e.g., ATP in metabolism)
Relationship between Q/K and ∆G
a) Q/K < 1
b) Q/K > 1
c) Q/K = 1
a) ∆G < 0 (Spont. to the right)
b) ∆G > 0 (Spont. to the left)
c) ∆G = 0 (Equilibrium)
Standard Free Energy Change and the Equilibrium Constant
(Small change in G causes large change in K)
∆Gθ = -RTln(K)
Free Energy Change under any conditions
∆G = ∆Gθ + RT ln(Q)
