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Give the thermodynamic criteria for equilibrium(7 MARKS)(At least 2 pages)

Give the thermodynamic criteria for equilibrium(7 MARKS)(At least 2 pages)

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~For a system to be in thermodynamic equilibrium, there should be no differences in temperature, pressure, or chemical potential of any of its components. (Chemical potential is the free energy/mole of substance.)

If these conditions are met, the system will not appear to change at the macroscopic level. Nothing will flow. No heat transfers. No material transfers (e.g., evaporation, precipitation, mixing), and no chemical reaction.

A system is said to be in Thermodynamic Equilibrium if it is in phase, thermal, mechanical and chemical equilibrium at same time. If any one of the equilibrium condition disturb then the system cannot comes under Thermodynamic Equilibrium. Let us understand these four equilibrium.


The system is said to be in thermal equilibrium if it has equality of temperature with surrounding. For example a dead body is the perfect example of thermal equilibrium with surrounding.

Normal human body temperature is approximately 37 degree Celsius whereas temperature of surrounding keeps on changing, let us consider surrounding temperature is 25 degree Celsius. Therefore a living human body is a not under thermal equilibrium with surrounding because metabolism of the body is continuously working or producing heat. The metabolism of a dead body shuts down therefore after sometime dead body comes under same temperature with surrounding.


When mass of each phase remains constant with time then the system is in phase Equilibrium. For example -Let us consider a closed container that is partially filled with liquid. Now what happened is evaporation takes place inside the container. The air above the liquid can absorb the water vapour molecules upto a certain limit and this limit is known as saturation limit. This closed container is our system and remember I am considering atmospheric pressure and temperature conditions which means pressure and temperature are not changing. When saturation limit occurs then the mass of liquid present in container will remain constant and similarly mass of vapour present in container will also remain constant. At saturation limit condition there is no more tendency of phase change, so at this point our system is in phase equilibrium.


Chemical equilibrium is the state when both reactants and products are present in concentrations and have no further tendency to change with time, so that there is no observable change in properties of system. Generally it occurs when rate of forward reaction is equal to rate of backward reaction. In simple language we can say chemical reaction potential should not be present.

Mechanical Equilibrium -

A system is in mechanical equilibrium when net forces acting on that system will be zero. It is basically equality of forces. It can also be defined as, the the pressure acting at any point of the system does not change with time.

In Thermodynamic equilibrium is death. Energy can be extracted from a system until it reaches thermodynamic equilibrium whereupon no more energy can be extracted. A battery that is in thermodynamic equilibrium is a dead battery. If you want to extract energy from a system, that system must be far from equilibrium.

There is an equation that we use to calculate the amount of energy that can be extracted from a system and put to useful work. It is called the Gibbs Equation:


The ΔG∘ΔG∘ term fixes the equilibrium position of the reaction. The RTlnQRTlnQ term tells you how far away from equilibrium you are and the ΔGΔG term tells you how much energy can be extracted from the system. If the system is at equilibrium (Q = Keq) then ΔG = 0.

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