First Law of Thermodynamics

• You will recall from previous chapter 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.

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

Spontaneous Processes

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

Spontaneous Processes

• Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures.
• Above 0°C it is spontaneous for ice to melt.
• Below 0°C the reverse process is spontaneous.

Reversible Processes

In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

​

Changes are infinitesimally small in a reversible process.

Irreversible Processes

• Irreversible processes cannot be undone by exactly reversing the change to the system.
• All Spontaneous processes are irreversible.
• All Real processes are irreversible.

Entropy

• Entropy (S) is a term coined by Rudolph Clausius in the 19th century.
• Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered,

Entropy

• Entropy can be thought of as a measure of the randomness of a system.
• It is related to the various modes of motion in molecules.

Entropy

• Like total energy, E, and enthalpy, H, entropy is a state function.
• Therefore,

ΔS = S_final − S_initial

Entropy

• For a process occurring at constant temperature (an isothermal process):

q_rev = the heat that is transferred when the process is carried out reversibly at a constant temperature.

T = temperature in Kelvin.

Second Law of Thermodynamics

The second law of thermodynamics: The entropy of the universe does not change for reversible processes

and

increases for spontaneous processes.

Reversible (ideal):

Irreversible (real, spontaneous):

Second Law of Thermodynamics

Reversible (ideal):

Irreversible (real, spontaneous):

“You can’t break even”

Second Law of Thermodynamics

The entropy of the universe increases (real, spontaneous processes).

But, entropy can decrease for individual systems.

Reversible (ideal):

Irreversible (real, spontaneous):

Entropy on the Molecular Scale

• Ludwig Boltzmann described the concept of entropy on the molecular level.
• Temperature is a measure of the average kinetic energy of the molecules in a sample.

Entropy on the Molecular Scale

• Molecules exhibit several types of motion:
• Translational: Movement of the entire molecule from one place to another.
• Vibrational: Periodic motion of atoms within a molecule.
• Rotational: Rotation of the molecule on about an axis or rotation about σ bonds.

Entropy on the Molecular Scale

• Boltzmann envisioned the motions of a sample of molecules at a particular instant in time.
• This would be akin to taking a snapshot of all the molecules.
• He referred to this sampling as a microstate of the thermodynamic system.

Entropy on the Molecular Scale

• Each thermodynamic state has a specific number of microstates, W, associated with it.
• Entropy is

S = k lnW

where k is the Boltzmann constant, 1.38 × 10⁻²³ J/K.

Entropy on the Molecular Scale

Implications:

​

• more particles

-> more states -> more entropy

• higher T

-> more energy states -> more entropy

• less structure (gas vs solid)

-> more states -> more entropy

​

Entropy on the Molecular Scale

• The number of microstates and, therefore, the entropy tends to increase with increases in
• Temperature.
• Volume (gases).
• The number of independently moving molecules.

Entropy and Physical States

• Entropy increases with the freedom of motion of molecules.
• Therefore,

S(g) > S(l) > S(s)