Thermodynamic equilibrium is something that we know about intuitively. It is related to heat exchange or any kind of dynamical change that occurs in a system. That is because, every change in nature is a consequence of the change in a system, to reach this equilibrium. Thermodynamics is a major branch of physics, that deals with all kinds of energy changes that occur in nature. It particularly involves the study of conversion of heat into mechanical work and energetics of any kind of physical or chemical change in nature. It deals with the changes in macroscopic parameters like temperature, pressure, and volume of any system. Equilibrium is a state of balance, characterized by exact nullification of opposing forces and no change in state whatsoever. There can be two states of equilibrium actually. One is static equilibrium and the other is dynamic equilibrium. Static equilibrium may be reached due to an absence of active dynamic forces and dynamic equilibrium may be attained due to active forces that exactly nullify each other. Every dynamic system in nature is in a state of flux to achieve equilibrium. Definition When you keep a hot cup of coffee on a table for a while, slowly it begins to cool due to exchange of heat with the surroundings. The temperature of the coffee being higher than that of the surroundings, more heat is transferred from the coffee to the surroundings, than vice versa. The coffee (unless it's drunk), keeps cooling off until its temperature becomes equal to that of the surroundings. That is, it keeps cooling off to reach thermal equilibrium with the surroundings. Thermal equilibrium is just one condition that needs to be satisfied to achieve this type of equilibrium. It is defined as follows: 'A system exists is in a state of thermodynamic equilibrium, when it has achieved mechanical, thermal, chemical, and radiative equilibrium'. In thermodynamics, the type of equilibrium reached, is always dynamic by nature. When two systems come in contact with each other, their intensive parameters change due to exchange of matter and/or energy until they reach a state of equilibrium. Properties of System in Equilibrium Let me explain the necessary conditions for achievement of this equilibrium, between two systems in contact with each other:

• They have to be in thermal equilibrium, which means their temperatures should be the same.
• They must be in mechanical equilibrium, which means their pressures should be the same.
• The systems should be in diffusive equilibrium, which means they should have the same chemical potential.

The process which leads to the achievement of thermal equilibrium is called thermalization. A system in such an equilibrium, if left isolated from the surroundings, doesn't change its state with time. Helmholtz free energy is a type of thermodynamic potential which quantifies the useful amount of work that can be obtained from any thermodynamic system, which is at constant volume and constant pressure. Gibbs free energy is the thermodynamic potential that quantifies the maximum amount of work of the 'non-expansion' type, which can be obtained from an isobaric and isothermal system. A state of equilibrium has minimum Helmholtz free energy and minimum Gibbs free energy. Necessary conditions are:

• When a system is completely isolated in time, its entropy (S) doesn't change with time, i.e. ΔS = 0.
• A system at constant temperature and volume, which is in equilibrium, will show no change in its Helmholtz free energy (F) i.e. ΔF = 0.
• A system in equilibrium, which is at constant pressure and temperature, shows no change in Gibbs free energy (G), i.e. ΔG = 0.

In chemistry, reactions in equilibrium are characterized by constants that are used to calculate the values of various thermodynamic potentials. Although, a system may not be globally in a state of thermodynamic equilibrium, it can have a local equilibrium which occurs in parts, where the macroscopic parameters are very slowly varying in space and time.