Chapter 19 - Thermodynamics and Free Energy

What does thermodynamics predict?

Whether or not a process will occur under the given conditions, tells us which direction a reaction might proceed but does not tell us how fast

First Law of Thermodynamics

Energy cannot be created nor destroyed, the internal energy of the universe remains unchanged

ΔE_univ equation

ΔE_univ = 0 = ΔE_sys + ΔE_surroundings; ΔE = q + w; ΔE_sys = -ΔE_surroundings

Second Law of Thermodynamics

For any spontaneous process, the entropy of the universe increases (ΔS_univ > 0)

Third Law of Thermodynamics

The entropy for a perfect crystal at 0 K is zero

Spontaneous process

Occurs without ongoing outside intervention, reaction stops once all reactants are completely consumed (combustion, rusting, dissolving)

Nonspontaneous process

Require a constant energy input to occur

Spontaneity is determined by:

Comparing the chemical potential energy of the system before the reaction with the free energy of the system after the reaction.

A thermodynamically favorable reaction has:

less potential energy after the reaction than before the reaction.

Spontaneous process from higher to lower potential energy

Exothermic

Spontaneous process from lower to higher potential energy

Endothermic

Entropy (S)

The degree of randomness/disorder of a system, amount of energy UNAVAILABLE to do work, # of energetically equivalent ways a system can exist (state function)

Entropy equation and units

ΔS = ΔS_final - ΔS_initial, J/mol K

Macrostate

State defined by a given set of conditions (P, V, T)

Microstate

Exact internal energy distribution among the particles at any one instant (W is # of possible microstates of a system)

Entropy Change in System and Surroundings

ΔS_univ = ΔS_sys + ΔS_surr

Exothermic system process

Add heat to the surroundings, therefore increasing entropy of surroundings

Endothermic system process

Take heat from the surroundings, therefore decreasing the entropy of the surroundings

Temperature Dependence of ΔS_surroundings

There is greater impact on entropy when heat is added to surroundings at a lower temperature than a higher temperature

Gibbs Free Energy, G

Maximum amount of work energy that can be released to the surroundings by a system for a constant temperature and pressure system, determines if a reaction is spontaneous or nonspontaneous

ΔG equation

ΔG_sys = ΔH_sys - TΔS, ΔG = -ΔS_univ

-ΔG

Spontaneous process

+ΔG

Nonspontaneous process

ΔG = 0

Reaction is at equilibrium

-ΔH, +ΔS

Spontaneous at all temperatures (-ΔG, favorable)

+ΔH, -ΔS

Nonspontaneous at all temperatures (+ΔG, unfavorable)

-ΔH, -ΔS

Spontaneous at low temperatures (-ΔG), nonspontaneous at high temperatures (+ΔG), ΔH favorable, ΔS unfavorable

+ΔH, +ΔS

Nonspontaneous at low temperatures (+ΔG), spontaneous at high temperatures (-ΔG), ΔH unfavorable, ΔS favorable

Standard conditions

The state of a compound at a defined set of conditions, gas: pure gas at exactly 1 atm; liquid or solid: pure substance in its most stable form at exactly 1 atm pressure and usually 25 °C; solutions: substance in a solution with a concentration of 1 M

Absolute entropy

The amount of energy it has due to dispersion of energy through its particles (0 J/mol K, always positive value)

The larger the molar mass…

The larger the entropy.

Allotropes

Two or more different physical forms of which an element can exist (graphite, charcoal, diamond)

The less constrained the structure of an allotrope…

The larger the entropy

The larger and more complex the molecule…

The larger the entropy

Standard entropy change

Difference in absolute entropy between the reactants and products under standard conditions

Standard free energy of formation

The change in free energy when 1 mol of a compound forms from its constituent elements in their standard states (0 for formation of pure elements)

Freezing

-ΔH, -ΔS

Melting

+ΔH, +ΔS

Condensation

-ΔH, -ΔS

Vaporization

+ΔH, +ΔS

Sublimation

+ΔH, +ΔS

Deposition

-ΔH, -ΔS

Heat (q)

Type of energy caused by difference in temperature, flow of thermal energy, Joules (J) and Calories (cal)

Temperature

Average kinetic energy of particles, degree of hotness or coldness of an object/matter, increase in temperature = increase in average kinetic energy

Energy

Capacity to do work,

System

Anything under the study

Surroundings

Anything beyond the system

ΔE/ΔU

Internal energy of a system/universe/object

Heat equation

q = mC_sΔT

Work equation

w = -pΔV

Enthalpy (ΔH)

Heat content of a system, amount of heat absorbed or released during a chemical reaction

-ΔH

Potential energy of products will be less than potential energy of reactants (favorable process, exothermic)

+ΔH

Potential energy of products will be more than potential energy of reactants (unfavorable process, endothermic)

Two ways to write exo reactions:

1) A + B = C + D, ΔH = -120 kJ/mol

2) A + B = C + D + 120 kJ/mol

Two ways to write endo reactions:

1) A + B = C + D, ΔH = +120 kJ/mol

2) A + B + 120 kJ/mol = C + D

ΔH and ΔE relationship

ΔH = ΔE + pΔV, when volume change is negligible then ΔH = ΔE, enthalpy and internal energy typically have the same numerical value, except when a large quantity of gas is produced to have a significant ΔV

ΔH is measured by

Coffee cup calorimetry

ΔE is measured by

Bomb calorimetry

ΔS_surroundings equation

ΔS_surr = -ΔH_rxn/T

Standard enthalpy of formation for pure elements

0

Standard entropy/enthalpy/free energy of reaction equations:

ΔS°_rxn = ΣnS°_products - ΣnS°_reactants

ΔH°_rxn = ΣnH°_products - ΣnH°_reactants

ΔG°_rxn = ΣnG°_products - ΣnG°_reactants

Entropy standard condition rules

S increases as molar mass increases; S increases as molecule becomes more complex; S increases from solid to liquid to gas

Free energy overall reaction rules

1) if a reaction is reversed, sign of ΔG is reversed

2) if a reaction is multiplied by a factor, ΔG is multiplied by the same factor

3) if a reaction is divided by a factor, ΔG is divided by the same factor

4) add up the revised ΔG values to get overall ΔG

(literally just the same steps as Hess’ Law)

Finding ΔG under nonstandard conditions:

ΔG_rxn = ΔG°_rxn + RTlnQ; ΔG°_rxn = -RTlnK_eq (R = 8.314 J/mol K)

If -ΔG (spontaneous)

K_eq > 1 (product favored)

If ΔG = 0

K_eq = 1 (neither direction is favored)

If +ΔG (nonspontaneous)

K_eq < 1 (reactant favored)

Slope relationship

slope = -ΔH_rxn/R; ΔH_rxn = -slopeR

y-intercept relationship

ΔS°_rxn = y-intR