Chem 2 Ch 19 Chemical Thermodynamics In-Depth Notes

First Law of Thermodynamics

• Energy cannot be created or destroyed, total energy of the universe is constant.

• Energy can be transformed between forms or exchanged between a system and its surroundings.

Enthalpy and Entropy

• Enthalpy: Heat absorbed by a system at constant pressure.

• Entropy: Measure of disorder or randomness in a system.

• Both properties influence the spontaneity of processes.

Spontaneous Processes

• Spontaneous processes occur without external assistance.

• Spontaneity is direction-dependent: spontaneous in one direction, nonspontaneous in reverse.

• Spontaneous does not equate to fast; slow reactions can also be spontaneous.

• Nonspontaneous processes can be made spontaneous with energy input.

Factors Affecting Spontaneity

• Temperature and pressure play significant roles in spontaneity.

• Example: Melting of ice vs. freezing; temperature influences state changes.

Reversible and Irreversible Processes

Reversible Processes

• A process where the system can return to its original state by reversing the direction of the process.

• Maximizes work done by the system on its surroundings.

Irreversible Processes

• Cannot be reversed by simply reversing the change in state.

• All spontaneous processes are irreversible.

Understanding Entropy

• Entropy measures disorder and is defined as a state function.

• It can be determined through heat transfer from surroundings at a specific temperature.

Second Law of Thermodynamics

• Entropy of the universe increases in spontaneous processes.

Entropy at the Molecular Level

• Entropy can be analyzed using microstates (arrangements of molecular positions and energies).

• Spontaneous expansion of gases leads to increased molecular arrangements.

Statistical Thermodynamics

• Relates macroscopic properties to molecular behavior using statistical mechanics.

• A microstate represents a distinct arrangement of molecules.

Boltzmann’s Principle

• Relationship between the number of microstates (W) and entropy (S):
  S = k imes ext{ln}(W)
  where k is the Boltzmann constant.

• More microstates lead to higher entropy.

Factors Increasing Entropy

• Increasing volume permits more positions for molecules, increasing microstates.

• Increased temperature raises average kinetic energy, broadening speed distribution, and increasing entropy.

Molecular Motions

• Molecules exhibit:

  • Translational: Entire molecule moves.

  • Vibrational: Atoms in a molecule move periodically.

  • Rotational: Molecule rotates about an axis.

• More atoms correlate with greater microstates.

Entropy and Physical States

• Entropy increases with freedom of motion:

  • S(g) > S(l) > S(s)

• Entropy rises when:

  • Gases form from solids or liquids.

  • Liquids/solutions form from solids.

  • Number of gas molecules increases in a reaction.

Third Law of Thermodynamics

• Entropy of a perfect crystalline substance at absolute zero ( 0 ext{K} ) is zero (only one microstate).

Standard Entropies

• Standard entropies vary according to:

  • Temperature (reference at 0 K).

  • Molar mass (larger molar mass generally means higher entropy).

  • Number of atoms in the molecule (greater number increases disorder).

Calculating Entropy Changes

• Similar to riangle H calculations, changes in entropy ( riangle S ) can be derived from a balanced equation.

Entropy Changes in Surroundings

• Heat transfer alters surrounding entropy.

• For isothermal processes, at constant pressure, it is equated to riangle H° of the system.

Entropy Change in the Universe

• Comprises the system and surroundings; for spontaneous processes:
  riangle S{ ext{universe}} = riangle S{ ext{system}} + riangle S_{ ext{surroundings}} > 0

Total Entropy and Spontaneity

• Substitute for entropy of surroundings and rearrange to express conditions for spontaneity.

Gibbs Free Energy (G)

1. If riangle G < 0 : the forward reaction is spontaneous.

2. If riangle G = 0 : the system is at equilibrium.

3. If riangle G > 0 : the reaction is nonspontaneous in the forward direction but spontaneous in the reverse.

Standard Free Energy Changes

• Similar to standard enthalpy changes; based on stoichiometric coefficients in equations.

Free Energy and Reaction Spontaneity

• Effects of heat and entropy on riangle G vary with temperature. Spontaneity depends on the signs and magnitudes of both.

Equilibrium & Free Energy Relation

• At equilibrium, Q = K and riangle G = 0 ; expressions can be manipulated for standard conditions.

Coupling Reactions

• Many natural processes are nonspontaneous on their own but can be coupled with spontaneous reactions, enabling favorable conditions.

• Example: Free energy from glucose oxidation is used to convert ADP to ATP.