Chapter 19- Chemical Thermodynamics

Entropy, S

measure of the disorder (possible arrangements of particles) of a system

ΔS > 0

randomness increases

ΔS

order increases

What dominates when there is an increase in entropy in one process that is associated with a decrease in entropy in another?

the increase in entropy

Where do spontaneous reactions proceed?

lower energy or higher entropy

Microstates

single possible arrangment of position and kinetic energy of molecules; entropy is measure of how many of this are associated with a particular macroscopic state

What do thermodynamic quantities tell us about reactions?

whether it’s thermodynamically favorable (spontaneous) under given conditions

What do thermodynamic quantities not tell us about reactions?

the rate of a reaction

State Functions

values that depend on the state of the substance and not on how the state was reached

What are state functions?

entropy, density, Gibbs free energy, temperature, pressure, volume, enthalpy, internal energy

1st Law of Thermodynamics

energy is conserved

2nd Law of Thermodynamics

spontaneous processes have a direction; entropy of the universe must increase in a spontaneous process

ΔS_univ = 0

reversible process

ΔS_univ > 0

spontaneous process; irreversible process

In an isolated system, ΔS_sys = 0

reversible process

In an isolated system, ΔS_sys > 0

spontaneous process

3rd Law of Thermodynamics

the entropy of a perfect crystal at 0K is zero; as heat from 0K, entropy increases

Entropy increases when

• liquids or solutions are formed from solids

• gases are formed from solids or liquids

• the number of gas molecules increase

• the temperature is increased

Thermodynamics

study of relations between heat, work, temperature, and energy

Temperature Change

can affect spontaneity

Spontaneous

any process that occurs without outside intervention; has a direction which can depend on temperature

Heat Transfer

movement of thermal energy; exothermic & endothermic

What does a zero for ΔS?

perfect order with no molecular motion and one possible microstate

What does a zero for ΔG mean?

reaction is at equilibrium

What are free states for elements?

elements existing in uncombined, pure form (not bonded); noble gases, certain metals (Au, Pt, Ag), carbon (graphite, diamond), sulfur (S₈)

How do free states affect thermodynamic constants?

if in standard state, constants are zero

Reversible Process

one that can go back and forth between states along the same path; heat can be added and removed but temperature cannot change; in equilibrium

Can spontaneous processes be reversible?

no, they are irreversible

Can thermodynamics predict the speed the process will occur at?

no

Any process that increases the number of gas molecules leads to ___________ in entropy

increase

Three atomic modes of motion (Molecular Motion)

translation, vibration, rotation

Translation

the movement of a molecule from one point to another

Vibration

shortening and lengthening of bonds or change in bond angles

Rotation

spinning of a molecule about some axis

In a perfect crystal at 0K, what happens to translation, rotation, and vibration of molecules?

there is none; state of perfect order

The more energy stored in translation, vibration and rotation, the greater the

degrees of freedom and the higher the entropy

What is required to get a molecule to translate, vibrate, or rotate?

energy

Absolute Entropy

determined experimentally

Standard molar entropy, S°

entropy of a substance in its standard state; similar to ΔH°; units = J/mol-K

S° increases with ________ molar mass and ___________ number of atoms in the formula

increasing; increasing

Reactions with large negative ΔH are

spontaneous

Gibbs Free Energy, G

H - TS; at constant temperature: ΔH - TΔS

ΔG < 0

forward reaction is spontaneous; favor products; spontaneous and exergonic

ΔG = 0

reaction is at equilibrium and no net reaction will occur

ΔG > 0

forward reaction is not spontaneous; favor reactants; work must be supplied from the surroundings to drive the reaction (magnitude will show minimum amount of work)

What does the free energy of the reactants do in a reaction?

decreases to minimum (equilibrium) and then increases to the free energy of the products

ΔH units

kJ/mol

ΔS units

J/mol-K

ΔG units

kJ/mol

ΔG° < 0

K > 1; thermodynamically favored (spontaneous)

ΔG° = 0

K = 1; at equilibrium

ΔG° > 0

K < 1; not thermodynamically favored in direction it is written in

ΔG° and K refer to

standard contitions

ΔG and Q apply to

any conditions