Chapter 19Chemical

Thermodynamics

John D. BookstaverSt. Charles Community CollegeSt. Peters, MO2006, Prentice Hall, Inc.Modified by S.A. Green, 2006Modified by D. Amuso 2011

Thermodynamics

The study of the relationships between heat, work, and the energy of a system.

First Law of Thermodynamics

• You will recall from Chapter 5 that energy cannot be created nor destroyed.

• Therefore, the total energy of the universe is a constant.

• Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

Second Law of Thermodynamics

The total Entropy of the universe increases in any spontaneous, irreversible process.

Entropy• Entropy can be thought of as a

measure of the randomness or disorder of a system.

• It is related to the various modes of motion in molecules (microstates).

Vibrational

Rotational

Translational

Entropy

Molecules have more entropy (disorder) when:

1)Phase Changes from: S L G

Example: Sublimation

CO2(s) CO2(g)

2)Dissolving occurs (solution forms):

Example:

NaCl(s) Na+(aq) + Cl-(aq)

Entropy3) Temperature increases

Example:

Fe(s) at 0 oC Fe(s) at 25 oC

4) For Gases ONLY, when

Volume increases or Pressure decreases

Examples:

2 Liters He(g) 4 Liters He(g)

3 atm He(g) 1 atm He(g)

Entropy5) Rx results in more molecules/moles of gas

Examples:

2NH3(g) N2(g) + 3H2(g)

CaCO3(s) CaO(s) + CO2(g)

N2O4(g) 2 NO2(g)

This one is difficult to predict:

N2(g) + O2(g) 2 NO(g)

Entropy6) When there are more moles

Example:

1 mole H2O(g) 2 moles H2O(g)

7) When there are more atoms per molecule

Examples:

1 mole Ar(g) 1 mole HCl(g)

1 mole NO2(g) 1 mole N2O4(g)

Entropy8) When an atom has a bigger atomic

number

1 mole He(g) 1 mole Ne (g)

Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously move to just one vessel.

Spontaneous Processes

• Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures.

• Above 0C it is spontaneous for ice to melt.• Below 0C the reverse process is spontaneous.

Spontaneous Processes

Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

Spontaneous processes are irreversible.

Second Law of Thermodynamics

A reversible process results in no overall change in Entropy while an irreversible, spontaneous process results in an overall increase in Entropy.

Reversible:

Irreversible (Spontaneous):

Third Law of Thermodynamics

The Entropy of a pure crystalline substance at absolute zero is zero. why?

Third Law of Thermodynamics

Standard Entropies

• Standard Conditions:

298 K, 1 atm, 1 Molar

• The values for Standard Entropies (So) are expressed in J/mol-K.

• Note: Increase with increasing molar mass.

Standard Entropies

Larger and more complex molecules have greater entropies.

Entropy Changes

So = n Soproducts - mSo

reactants

Be careful: S°units are in J/mol-K

Note for pure elements:

Gibbs Free Energy

Use G to decide if a process is spontaneous

G = negative value = spontaneous G = zero = at equilibrium

G = positive value = not spontaneous

Note: equation can be used w/o the o too.

Go = Ho – TSo

Gibbs Free Energy

1. If G is negative

G = maximum amt of energy ‘free’ to do

work by the reaction

2. If G is positive

G = minimum amt of work needed

to make the reaction happen Go = Ho – TSo

Gibbs Free Energy

In our tables, units are:

Go = kJ/mol

Ho = kJ/mol

So = J/mol-K

Go = Ho – TSo

Free Energy and Temperature

• There are two parts to the free energy equation: H— the enthalpy term TS — the entropy term

• The temperature dependence of free energy comes from the entropy term.

What causes a reaction to be spontaneous?

• Think Humpty Dumpty• System tend to seek:

Minimum Enthalpy

Exothermic Rx, H = negative

Maximum Entropy

More disorder, S = positive• Because: Go = Ho – TSo

- = (-) - (+)

Free Energy and Temperature

By knowing the sign (+ or -) of S and H, we can get the sign of G and determine if a reaction is spontaneous.

At Equilibrium

Go = zero Therefore: Ho = TSo

Or: SoHo

T

Use this equation when asked to calculate enthalpy of vaporization or enthalpy of fusion.

Go = Ho – TSo

Go = n Gof products - mGo

f reactants

Standard Free Energy Changes

Be careful: Values for Gf are in kJ/mol

G can be looked up in tables or calculated from S° and H.

Go = Ho – TSo

Free Energy and Equilibrium

Remember from above:If G is 0, the system is at equilibrium.

So G must be related to the equilibrium constant, K (chapter 15). The standard free energy, G°, is directly linked to Keq by:

Go = RT ln K Where R = 8.314 J/mol-K

Go = RT ln K

If the free energy change is a negative value, the reaction is spontaneous, ln K must be a

positive value, and K will be a large number meaning the equilibrium mixture is mainly products.

If the free energy change is zero, ln K = zero and K = one.

Relationships

Free Energy and Equilibrium

Under non-standard conditions, we need to use G instead of G°.

Q is the reaction quotient from chapter 15.