B.Sc. 2nd Year Chemistry — Physical Chemistry
Thermodynamics is the branch of physical chemistry that deals with energy changes accompanying physical and chemical processes. It describes the relationships among heat, work, temperature and other forms of energy.
Thermodynamics helps determine whether a process is possible, the amount of energy involved and the equilibrium condition of a system. It is applicable to chemical reactions, phase changes, biological processes, engines, refrigeration and many industrial operations.
A system is the part of the universe selected for thermodynamic study.
Everything outside the system that can interact with it is called the surroundings.
The real or imaginary surface separating the system from its surroundings is called the boundary.
The system and its surroundings together constitute the universe.
| System | Definition | Example |
|---|---|---|
| Open system | Exchanges both matter and energy with surroundings. | Open beaker |
| Closed system | Exchanges energy but not matter. | Sealed container |
| Isolated system | Exchanges neither matter nor energy. | Ideal thermos flask |
The measurable characteristics of a system are called thermodynamic properties.
Properties that depend on the amount of substance are called extensive properties.
Properties that do not depend on the amount of substance are called intensive properties.
The condition of a system described by properties such as pressure, temperature, volume and composition is called its thermodynamic state.
A state function depends only on the initial and final states and not on the path followed during the process.
Examples include internal energy, enthalpy, entropy and Gibbs free energy.
| State Function | Path Function |
|---|---|
| Depends only on initial and final states. | Depends on the path followed. |
| Independent of path. | Dependent on path. |
| Examples: U, H, S, G. | Examples: heat and work. |
A process carried out at constant temperature is called an isothermal process.
A process in which no heat is exchanged with the surroundings is called an adiabatic process.
A process occurring at constant pressure is called an isobaric process.
A process occurring at constant volume is called an isochoric process.
A process in which the system returns to its original state is called a cyclic process.
Heat is energy transferred between a system and its surroundings because of a temperature difference.
Work is energy transferred when a force causes displacement or when a system expands against an external pressure.
Heat and work are path functions because their values depend on the path followed during a process.
Internal energy is the total energy contained within a system due to the motion and interactions of its particles.
It is represented by U. The absolute value of internal energy cannot usually be measured, but changes in internal energy can be determined.
The first law of thermodynamics is the law of conservation of energy. It states that energy can neither be created nor destroyed, although it can be converted from one form into another.
Using the chemistry sign convention:
Where ΔU is change in internal energy, q is heat supplied to the system and w is work done on the system.
| Quantity | Sign | Meaning |
|---|---|---|
| q | + | Heat absorbed by system |
| q | − | Heat released by system |
| w | + | Work done on system |
| w | − | Work done by system |
When a gas expands or contracts against an external pressure, pressure-volume work is performed.
For expansion, ΔV is positive and work done by the system is negative under the chemistry sign convention.
A reversible process is an ideal process that can be reversed through infinitesimal changes so that both the system and surroundings return to their original states.
An irreversible process occurs with finite driving forces and cannot be completely reversed without producing changes in the surroundings.
| Reversible | Irreversible |
|---|---|
| Very slow process. | Usually rapid or occurs with finite gradients. |
| System remains nearly at equilibrium. | System passes through non-equilibrium states. |
| Maximum work obtained. | Less work obtained. |
Enthalpy is a thermodynamic state function defined as the sum of internal energy and pressure-volume term.
The change in enthalpy is:
For a reaction involving ideal gases at constant temperature:
A reaction that releases heat to the surroundings is called an exothermic reaction.
A reaction that absorbs heat from the surroundings is called an endothermic reaction.
Heat capacity is the amount of heat required to raise the temperature of a system by one degree.
Molar heat capacity is the heat required to raise the temperature of one mole of a substance by one degree.
Specific heat capacity is the heat required to raise the temperature of unit mass of a substance by one degree.
At constant volume, no pressure-volume expansion work occurs for a simple closed system.
At constant pressure:
For an ideal gas:
Hess's law states that the total enthalpy change of a reaction is the same whether the reaction occurs in one step or through a series of steps, provided the initial and final states are the same.
This law is a consequence of the fact that enthalpy is a state function.
The enthalpy change accompanying the formation of one mole of a compound from its constituent elements in their standard states is called standard enthalpy of formation.
It is represented by ΔH°f.
The enthalpy change when one mole of a substance is completely burned in oxygen under specified conditions is called enthalpy of combustion.
Combustion reactions are generally exothermic.
The enthalpy change when an acid and base react to form one mole of water under specified conditions is called enthalpy of neutralization.
The enthalpy change when one mole of a substance dissolves in a specified amount of solvent to form a solution of specified concentration is called enthalpy of solution.
The first law tells us about conservation of energy but does not determine the direction in which a process occurs. The second law provides the criterion for spontaneity and introduces the concept of entropy.
A spontaneous process proceeds naturally in a particular direction under given conditions.
Entropy is a thermodynamic state function that measures the degree of disorder or randomness of a system. It is represented by S.
For a spontaneous process, the entropy of the universe increases.
Entropy generally increases when:
Entropy generally decreases when a system becomes more ordered.
The third law of thermodynamics states that the entropy of a perfectly crystalline pure substance approaches zero as the temperature approaches absolute zero.
Gibbs free energy is a thermodynamic function used to determine the spontaneity of a process at constant temperature and pressure.
Therefore:
| ΔG | Meaning |
|---|---|
| ΔG < 0 | Process is spontaneous under the specified conditions. |
| ΔG > 0 | Process is non-spontaneous in the forward direction. |
| ΔG = 0 | System is at equilibrium. |
The spontaneity of a process depends on the relative contributions of enthalpy and entropy.
| ΔH | ΔS | Spontaneity |
|---|---|---|
| Negative | Positive | Spontaneous at all temperatures |
| Positive | Negative | Non-spontaneous at all temperatures |
| Negative | Negative | Depends on temperature |
| Positive | Positive | Depends on temperature |
Helmholtz free energy is useful for processes occurring at constant temperature and volume.
Its change is:
At equilibrium, there is no net tendency for the forward or reverse reaction to proceed. The Gibbs free energy change for the reaction is zero under equilibrium conditions.
The relationship between standard Gibbs free energy change and equilibrium constant is:
Therefore, a large equilibrium constant corresponds to a negative standard Gibbs free energy change.
Thermodynamics provides the relationship between electrical work and Gibbs free energy in electrochemical cells.
Under standard conditions:
Combining this with:
gives:
Thermodynamics deals with energy changes associated with physical and chemical processes. Important concepts include system, surroundings, thermodynamic properties, heat, work, internal energy and enthalpy. The first law establishes conservation of energy, while Hess's law allows enthalpy changes to be calculated through different reaction pathways. The second law introduces entropy and explains the direction of spontaneous processes. Gibbs free energy provides an important criterion for spontaneity and equilibrium. The third law describes the behavior of entropy near absolute zero. Thermodynamic principles have wide applications in chemical reactions, equilibrium, electrochemistry, biological systems and industrial processes.