Thermodynamics & Thermochemistry: A Comprehensive Guide
Thermodynamics and Thermochemistry
Introduction
Thermodynamics is the study of energy transformations, particularly those involving heat and work. Thermochemistry, a branch of thermodynamics, focuses on energy changes during chemical reactions, which is the main subject of this unit.
Thermodynamic Systems
A thermodynamic system is the specific part of the universe being studied. Its immediate environment is called the surroundings. Systems are classified as:
- Open: Exchange both matter and energy with the surroundings.
- Closed: Exchange energy but not matter with the surroundings.
- Isolated: Exchange neither matter nor energy with the surroundings.
State Variables and Functions
State variables describe the state of a system, including pressure, volume, temperature, concentration, and density. State functions depend only on the initial and final states of a system, not the path taken. If the path forms a closed loop (a cycle), the change in any state function is zero.
Types of Changes
- Isobaric: Constant pressure.
- Isochoric: Constant volume.
- Isothermal: Constant temperature.
- Adiabatic: No heat exchange.
Concept of Temperature
Temperature is related to the internal energy of a system, which includes:
- Potential energy: Energy stored in the bonds between particles.
- Kinetic energy: Energy of particle motion.
Thermometric Scales
Thermometers measure temperature according to a specific thermometric scale, which is established through a defined procedure.
Heat
Heat is the transfer of energy between a system and its surroundings due to a temperature difference. It is measured in joules (J). The heat absorbed or released by a substance depends on its mass (m), specific heat (ce), and temperature change (ΔT):
Q = mceΔT
Specific Heat
Specific heat is the amount of heat required to raise the temperature of 1 kg of a substance by 1 degree Celsius (or 1 Kelvin). It is measured in J/kg°C. Calorimeters are used to measure heat transfer, treating the system and surroundings as an isolated unit.
Work
In a system with a movable wall, expansion or contraction can perform work against an external force. At constant pressure, the work (in J) is:
W = -P(Vf – Vi)
First Law of Thermodynamics
First Law of Thermodynamics: In an isolated system, the total energy remains constant. Any energy gained or lost by a system is balanced by an equal and opposite change in the surroundings.
Internal Energy
Internal energy (U) is the total energy of a system. The change in internal energy (ΔU) is equal to the heat (Q) and work (W) exchanged with the surroundings:
ΔU = Q + W
Enthalpy
Enthalpy (H) is defined as H = U + PV. The change in enthalpy (ΔH) at constant pressure represents the heat exchanged:
Endothermic and Exothermic Reactions
- Exothermic reactions: Release energy to the surroundings (ΔH < 0).
- Endothermic reactions: Absorb energy from the surroundings (ΔH > 0).
Heat of Reaction
The enthalpy of reaction (ΔHr) is the heat released or absorbed during a chemical reaction at constant pressure.
Enthalpy Change of Formation
The enthalpy of formation (ΔHf) is the enthalpy change when one mole of a substance is formed from its elements in their standard states. The enthalpy change of a reaction can be calculated using the following equation:
ΔHr = ΣnpΔHf(products) – ΣnrΔHf(reactants)
Bond Enthalpy
Bond enthalpy is the energy required to break one mole of a specific bond in the gaseous state at standard conditions (1 atm and 298 K).
Heat of Combustion
Heat of combustion is the energy released during the combustion of a substance. The standard enthalpy of combustion (ΔHc°) is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. The calorific value is the energy released per kilogram of fuel.
Hess’s Law
Hess’s Law: The total enthalpy change for a reaction is independent of the pathway taken. It depends only on the initial and final states.
Enthalpy Diagram
Hess’s Law can be illustrated using an enthalpy diagram.
Entropy
Spontaneous processes occur without external intervention.
Reversible and Irreversible Processes
Irreversible processes occur spontaneously and cannot be reversed without external energy input. Reversible processes are idealized processes that occur infinitely slowly, with the system remaining at equilibrium throughout. They can be reversed by infinitesimal changes in external conditions.
Second Law of Thermodynamics
Second Law of Thermodynamics: The entropy of an isolated system always increases in a spontaneous process.
Entropy (S)
Entropy (S) measures the degree of disorder in a system. In a reversible process, the total entropy of the universe remains constant. In an irreversible process, the total entropy of the universe increases.
Third Law of Thermodynamics
Third Law of Thermodynamics: The entropy of a perfect crystal at 0 K is zero. Entropy increases with changes in physical state (e.g., solid to liquid), temperature, and molecular complexity.