Chemical Thermodynamics

Introduction to Thermodynamics

Chemical thermodynamics is the study of energy changes and the direction of processes in chemical reactions. It helps predict whether a reaction will occur spontaneously and how energy is transferred between a system and its surroundings.

• System: The part of the universe under study (e.g., a chemical reaction in a flask).

• Surroundings: Everything outside the system.

• State Function: A property whose value depends only on the state of the system, not on how it got there (e.g., energy, enthalpy, entropy).

First Law of Thermodynamics

The first law states that energy cannot be created or destroyed, only transferred or converted from one form to another. The total energy of the universe remains constant.

• Energy Conservation:

• Energy can be transferred as heat or work between the system and surroundings.

Enthalpy and Entropy

Two key thermodynamic quantities are enthalpy and entropy, both of which influence the spontaneity of processes.

• Enthalpy (H): The heat absorbed or released by a system at constant pressure.

• Entropy (S): A measure of the randomness or disorder of a system.

Spontaneous Processes

A spontaneous process occurs without outside intervention. Spontaneity does not imply speed; some spontaneous reactions are slow. The reverse of a spontaneous process is nonspontaneous, but not impossible—it may require energy input.

• Spontaneous: Occurs naturally (e.g., ice melting at room temperature).

• Nonspontaneous: Requires energy input to occur (e.g., water freezing above 0°C).

Factors Affecting Spontaneity

Temperature and pressure can influence whether a process is spontaneous. For example, ice melts spontaneously above 0°C but freezes below 0°C.

Reversible and Irreversible Processes

A reversible process can be reversed by infinitesimal changes, restoring both system and surroundings to their original states. Irreversible processes cannot be exactly reversed; all spontaneous processes are irreversible.

• Reversible: Maximizes work done by the system.

• Irreversible: Cannot be undone by simply reversing the process.

Entropy and the Second Law of Thermodynamics

Entropy: Definition and Calculation

Entropy (S) quantifies the disorder or randomness in a system. It is a state function, and its change is given by:

• For a reversible process at constant temperature:

Second Law of Thermodynamics

The second law states that the entropy of the universe increases in any spontaneous process:

• For a reversible process:

• For an irreversible (spontaneous) process:

Entropy on the Molecular Scale

On the molecular level, entropy is related to the number of possible arrangements (microstates) of molecules. The more microstates, the higher the entropy.

• Microstate (W): A specific arrangement of molecules' positions and energies.

• Boltzmann's Equation:

Statistical Thermodynamics

Statistical thermodynamics connects the molecular view (microstates) with macroscopic thermodynamic properties using probability and statistics.

• More microstates correspond to higher entropy.

• Changes in volume or temperature increase the number of microstates and thus entropy.

Molecular Motions and Entropy

Molecules can move in different ways, contributing to the number of microstates:

• Translational: Movement from one place to another.

• Vibrational: Atoms within a molecule move periodically.

• Rotational: Molecule rotates about an axis.

• More atoms = more possible motions = higher entropy.

Entropy and Physical States

Entropy increases with the freedom of motion of molecules:

• Processes that increase entropy:

  • Formation of gases from solids or liquids

  • Formation of liquids or solutions from solids

  • Increase in the number of gas molecules in a reaction

Third Law of Thermodynamics

The third law states that the entropy of a perfect crystalline substance at absolute zero (0 K) is zero. At this temperature, there is only one microstate.

Standard Entropies

Standard molar entropy () is the entropy of one mole of a substance at 1 atm and 298 K. Standard entropies increase with molar mass and the number of atoms in the formula.

Calculating Entropy Changes

Entropy change for a reaction is calculated similarly to enthalpy change:

• n and m are stoichiometric coefficients.

Entropy Changes in the Surroundings

Heat flow into or out of the system changes the entropy of the surroundings. For an isothermal process:

• At constant pressure, , so

Entropy Change in the Universe

The total entropy change is the sum of the system and surroundings:

• For a spontaneous process:

Gibbs Free Energy and Spontaneity

Gibbs Free Energy (G)

Gibbs free energy combines enthalpy and entropy to predict spontaneity at constant temperature and pressure:

• If , the process is spontaneous.

• If , the system is at equilibrium.

• If , the process is nonspontaneous (spontaneous in reverse).

Standard Free Energy Changes

Standard free energy of formation () is the free energy change when 1 mole of a compound forms from its elements in their standard states.

• Standard states: solid (pure), liquid (pure), gas (1 atm), solution (1 M), element in standard state ()

Effect of Enthalpy and Entropy on Spontaneity

The signs of and determine the temperature dependence of spontaneity:

Free Energy and Equilibrium

Free energy change under nonstandard conditions relates to the reaction quotient (Q):

• At equilibrium (), :

Coupling Reactions

Many biological and chemical processes that are nonspontaneous alone can occur when coupled with a spontaneous process. For example, ATP hydrolysis is coupled to drive nonspontaneous cellular reactions.

Additional info: These notes provide a comprehensive overview of the key concepts in chemical thermodynamics, including the laws of thermodynamics, entropy, Gibbs free energy, and their applications to chemical and biological systems.