Chapter 19: Free Energy and Thermodynamics (Exam 3)

Question 1

Spontanteous Process

Answer 1

• One that occurs without ongoing outside intervention

Question 2

Spontaneity

Answer 2

• Direction in which and to what extent a chemical reaction proceeds (in thermodynamics)

Question 3

Speed

Answer 3

• How fast a reaction takes to complete (kinetics)

Question 4

Entropy + Equation

Answer 4

• (S)
• Thermodynamic function that increases with number of energetically equivalent way to arrange components of a system to achieve a particular state
• S = klnW
• k = Boltzmann’s constant
• W = number energetically equivalent ways to arrange components of the system

Question 5

Macrostate

Answer 5

• Given set of conditions (P, V, T)

Question 6

Microstate

Answer 6

• Exact internal energy distribution among particles at any one instant
• State in which given amount of energy is more highly dispersed has more entropy

Question 7

Second Law of Thermodynamics

Answer 7

• For any spontaneous process, the entropy of the universe increases
• Entropy is a state function and therefore its value is dependent upon the state of the system

Question 8

Units of Enropy

Answer 8

• Joules / Kelvin
• Measure of energy dispersal (joules) per unit temperature (Kelvin)

Question 9

State Changes and Energy (in terms of what causes an increase in entropy)

Answer 9

Increase in Entropy:

• Solid –> Gas
• Solid –> Liquid
• Liquid –> Gas
• Increase in number of moles of a gas during a chemical reaction (gas has more microstates)

Question 10

Translational Energy

Answer 10

• Straight-line motions of molecules

Question 11

Rotational Energy

Answer 11

• Rotations of the molecules

Question 12

Entropy (∆S˚rxn) Standards

Answer 12

• Gas: Standard is at 1atm
• Liquid or Solid: Most stable format 1 atm and temperature (usually 25˚C)
• Substance in solution: 1M

Question 13

Standard Entropy Change For Reaction

Answer 13

∆S˚rxn = S˚(products) - S˚(reactants)

• Standard molar entropies are used to calculate S˚rxn

Question 14

Third Law of Thermodynamics

Answer 14

• Entropy of a perfect crystal at absolute zero is zero (only one way to arrange components)

Question 15

Effect of:

• Allotropes
• Molar mass
• Molecular complexity
• Dissolution

on entropy

Answer 15

• Allotrope: Some elements that exist in two or more forms with different structures; more stable have less entropy
• Entropy increases with molar mass
• Entropy increases with increasing molecular complexity
• Dissolution of crystalline solid into solution results in increase in entropy

Question 16

Standard Entropy Change for Reaction (equation)

Answer 16

∆S˚rxn = ∑n_pS˚(products) - ∑n_rS˚(reactants)

Question 17

Spontaneity and

• Exothermic process
• Endothermic process

Answer 17

• Increases entropy of surroundings (Q_sys = - | ∆S_surr = +)
• Decreases entropy of surroundings

Question 18

Entropy Changes in Surroundings

Answer 18

∆S_surr = -H_sys/T

(assuming constant P and T)

Question 19

Gibbs Free Energy

Answer 19

∆G = ∆H - T∆S

∆G < 0 : Spontaneous

∆G > 0 : Nonspontaneous

Question 20

+ H˚ and + S˚

Answer 20

• Low temperatures: Spontaneous
• High temperatures: Spontaneous

Question 21

+ H˚ and - S˚

Answer 21

• Low temperatures: Nonspontaneous
• High temperatures: Nonspontaneous

Question 22

• H˚ and - S˚

Answer 22

• Low temperatures: Spontaneous
• High temperatures: Nonspontaneous

Question 23

+ H˚ and + S˚

Answer 23

• Low temperatures: Nonspontaneous
• High temperatures: Spontaneous

Question 24

Free Energy of Formation + Equation

Answer 24

• ∆G˚f
• Change in free energy when 1 mol of compound forms from its constituent elements in their standard states
• ∆G˚rxn = ∑n_p∆G˚f(products) - ∑n_r∆G˚f(reactants)

Question 25

Free Energy in Stepwise Reactions (similar to enthalpy)

Answer 25

1) If chemical equation is multiplied by some factor, ∆Grxn is multiplied by same factor.
2) If chemical equation is reversed, ∆Grxn changes sign.
3) If chemical equation can be expressed as sum of series of steps, ∆Grxn for overall equation is sum of free energies of reactions for each step.

Question 26

Reversible Reactions

Answer 26

• Reaction that achieves the theoretical limit of available free energy
• Occurs infinitesimally slowly; free energy can only be drawn out in same infinitesimally small increments

Question 27

Irreversible Reactions

Change in free energy = positive signifies what?

Answer 27

• All real reactions
• Do not achieve theoretical limit of available free energy
• ∆G˚rxn represents minimum amount of energy required to make reaction occur

Question 28

Free Energy Change of Reaction under Nonstandard Conditions

Answer 28

∆Grxn = ∆G˚rxn + RTlnQ

• Q: Reaction quotient
• T: Temperature in Kelvins
• R: Gas constant (8.314 J/mol•K)

Question 29

Relationship between ∆G˚rxn and K

Answer 29

∆G˚rxn = -RTlnk

1) K < 1 : lnK is negative; ∆G˚rxn is positive
2) K > 1 : lnK is positive; ∆G˚rxn is negative
3) K = 1 : lnK = 0; ∆G˚rxn = 0

Question 30

Temperature Dependence + Equilibrium Constant

Answer 30

lnK = -∆H˚rxn/R x (1/T) + ∆S˚rxn/R

• Form of a straight line; use when two different temperatures are known and equilibrium constant is given at those temperatures