Topic 20: Entropy, Free Energy, and Reaction Direction

Question 1

Definition of system

Answer 1

Part of the universe chosen for study

Question 2

Types of systems

Answer 2

a) Open - Freely exchanging energy and matter
b) Closed - Exchanging energy, but not matter
c) Isolated - Not interacting with its surroundings

Question 3

Surroundings definition

Answer 3

Part of the universe outside the system which the system interacts with

Question 4

State function definition

Answer 4

Properties that depend on the initial and final state of the system

Question 5

Standard condition convention

Answer 5

a) Pressure of 1atm and 1M
b) Pure substance

Question 6

Spontaneous change definition

Answer 6

A change that occurs without a continuous input of energy from outside the system (Product favored)

Question 7

If a change is spontaneous in one direction, …

Answer 7

it is not in the other

Question 8

First law of thermodynamics

Answer 8

Energy is neither created nor destroyed, it is transformed
(∆Esys + ∆Esurr = ∆Euniv = 0)

Question 9

How is change of internal energy measured?

Answer 9

∆E=q+w

Question 10

Does the first law of thermodynamics predict the direction of a spontaneous change?

Answer 10

No

Question 11

Examples of endothermic reactions that are spontaneous

Answer 11

a) Melting / Vaporization
b) Dissolution of salts

Question 12

Examples of exothermic reactions that are spontaneous

Answer 12

a) Freezing / Condensation
b) Burning of methane
c) Oxidation of metals
d) Formation of ionic compounds

Question 13

3rd Law of Thermodynamics

Answer 13

A perfect crystal has zero entropy at absolute zero

Question 14

2nd Law of Thermodynamics

Answer 14

All real processes occur spontaneously in the direction that increases Suniv
∆Suniv = ∆Ssys + ∆Ssurr > 0

Question 15

General idea for predicting the change in entropy

Answer 15

When a system becomes more disordered, the more positive the value of ∆S

Question 16

Definition of a reversible isothermal process

Answer 16

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

Question 17

Formula for predicting the change in entropy for an isothermal process

Answer 17

∆Ssys = q/T

Question 18

Formula for predicting the change in entropy for phase change

Answer 18

a) ∆Sfus = (∆Hfus) / Tf
b) ∆Svap = (∆Hvap) / Tb

Question 19

Trouton’s rule

Answer 19

Most liquids have a ∆Svap value close to 88 J/mol K

Question 20

Factors that affect the change of entropy

Answer 20

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

Question 21

Effect of change of state in entropy

Answer 21

Increases from solid => liquid => gas

Question 22

Effect of dissolution of solid / liquid in entropy

Answer 22

Ionic solutes have more freedom of motion
a) Exp: Small charged ions that order water molecules

Question 23

Effect of atomic size in entropy

Answer 23

Within a periodic group, molar entropy increases with heavier atoms

Question 24

Effect of molecular complexity in compounds

Answer 24

Entropy increases with chemical complexity
a) Exception: Ring structure restricts freedom of motion

Question 25

Effect of number of particles

Answer 25

An increase in # of gaseous substances causes an increase in entropy

Question 26

Effect of temperature in entropy at constant volume and pressure

Answer 26

Directly proportional

Question 27

Effect of volume in entropy at constant pressure and temperature

Answer 27

Directly proportional

Question 28

Effect of pressure in entropy at constant volume and temperature

Answer 28

Inversely proportional

Question 29

Standard entropy change calculation (J K-1 mol -1)

Answer 29

∆S=∑mS(products) -∑nS(reactants)

Question 30

Units of entropy change

Answer 30

J K-1 mol-1

Question 31

Relationship between the entropy of the surroundings and…
a) Endothermic reaction
b) Exothermic reaction

Answer 31

∆Ssurr = -∆Hsys/T
a) Decrease
b) Increase

Question 32

When does a process reach equilibrium?

Answer 32

∆Suniv=0

Question 33

Definition of free energy

Answer 33

Measure of the spontaneity of a process and of the useful energy available from it

Question 34

Definition of entropy

Answer 34

Measure of the distribution of available energy among the particles in a system
S = k ln(W)

Question 35

Formula to calculate ∆G

Answer 35

∆G = ∆H - T∆S
-T∆S_Univ=-T∆S_Sys+∆H_Sys

Question 36

Spontaneity and Sign of ∆G
a) ∆G<0
b) ∆G=0
C) ∆G>0

Answer 36

a) Spontaneous
b) Equilibrium
c) Nonspontaneous

Question 37

Standard free energy of formation (∆Gθf)

Answer 37

When 1 mol of a compound is made from its elements in their standard states

Question 38

What does change in free energy represent? (+ / -)

Answer 38

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

Question 39

What is the difference between the theoretical and actual change in free energy?

Answer 39

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

Question 40

Effect of temperature on spontaneity
a) ∆H (-) | ∆S (+)
b) ∆H (+) | ∆S (-)
c) ∆H (+) | ∆S (+)
d) ∆H (-) | ∆S (-)

Answer 40

a) Spontaneous at all T
b) Nonspontaneous at all T
c) Spontaneous at high T
d) Spontaneous at low T

Question 41

How to drive nonspontaneous reactions

Answer 41

Coupling of reactions, in which one step supplies enough free energy for the other to occur
(e.g., ATP in metabolism)

Question 42

Relationship between Q/K and ∆G
a) Q/K < 1
b) Q/K > 1
c) Q/K = 1

Answer 42

a) ∆G < 0 (Spont. to the right)
b) ∆G > 0 (Spont. to the left)
c) ∆G = 0 (Equilibrium)

Question 43

Standard Free Energy Change and the Equilibrium Constant
(Small change in G causes large change in K)

Answer 43

∆Gθ = -RTln⁡(K)

Question 44

Free Energy Change under any conditions

Answer 44

∆G = ∆Gθ + RT ln⁡(Q)