Thermodynamics and Chemical Kinetics: A Concise Review

Posted on Feb 17, 2025 in Chemistry

Thermodynamics and Chemical Kinetics

Power: The ability of a body to produce work or heat, generating chemical reactions. Energy is exchanged between atoms, ions, and molecules. Each substance transfers energy through heat or work mechanisms. Work is defined as force applied over a distance (W = -PΔV). Heat and thermal energy are transferred naturally between bodies within thermodynamic systems.

  • Adiabatic Wall: Prevents heat exchange (thermally insulated) between a system and its surroundings.
  • Diathermic Wall: Conductive, allowing heat exchange until thermal equilibrium is reached.
  • Rigid Walls: Maintain constant volume, preventing expansion or compression, resulting in zero work.

Classification of Systems:

  • Open: Exchanges both matter and energy.
  • Closed: Exchanges energy but not matter.
  • Isolated: Exchanges neither matter nor energy (ΔE = 0).

Function of State:

A thermodynamic variable whose value depends only on the initial and final states of the system, not the path taken (e.g., U). Q and W are not state functions.

First Principle of Thermodynamics:

Every thermodynamic system has a property called internal energy (U), which increases when the system absorbs heat or work (U = Q + W).

Thermodynamic Processes:

  • Cyclic: ΔU = 0
  • Adiabatic: U = W (since Q = 0)
  • Isothermal: T = constant
  • Isochoric (Isometric): V = constant, U = Qv (if a state function, W = 0)
  • Isobaric: P = constant, H = U + PV, ΔH = Qp (if a state function), ΔH = ΔU + ΔnRT

Thermochemistry:

The study of heat changes in chemical reactions.

Thermochemical Equation:

A chemical equation that includes the thermodynamic parameters, especially enthalpy.

Energy of Reaction:

The total energy absorbed or released when a chemical reaction occurs under constant volume conditions (U = Qv).

Heat of Reaction:

The energy absorbed or released in a chemical reaction. At constant volume, it matches the change in internal energy. At constant pressure, it is the enthalpy change (ΔH = Qp).

Hess’s Law:

The enthalpy change of a chemical reaction depends only on the initial and final states, not the path taken. If a reaction can be expressed as the algebraic sum of other reactions, its enthalpy change is the sum of the enthalpy changes of those reactions.

Enthalpy of Formation:

The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states.

Bond Energy:

The energy required to break one mole of bonds in the gas phase.

Entropy:

A measure of the disorder of a system. Higher molecular disorder corresponds to higher entropy (S = Q/T).

Second Principle of Thermodynamics:

Any spontaneous process increases the total entropy of the universe.

Gibbs Free Energy:

A thermodynamic potential that determines the spontaneity of a process at constant temperature and pressure. A decrease in Gibbs free energy (G < 0) indicates spontaneity (G = H – TS).

Spontaneity Criteria:

  • Exothermic (H < 0) and increasing disorder (S > 0): Always spontaneous.
  • Endothermic (H > 0) and decreasing disorder (S < 0): Never spontaneous.
  • Exothermic and decreasing disorder: Spontaneous if T is low.
  • Endothermic and increasing disorder: Spontaneous if T is high.

Chemical Kinetics

The study of the factors influencing the rates of chemical reactions. It aims to find mathematical expressions that relate reaction rates to these factors.

Reaction Mechanism:

A sequence of simple steps that describe the molecular-level pathway of a reaction.

Elementary Step:

An individual step in a reaction mechanism.

Molecularity:

The number of molecules interacting in an elementary step.