Equilibrium and Types of Equilibrium
We observe processes taking place, what is the reason behind this? Does it have something to do with energy? Let’s try to unfold the mystery.
Everything in universe tries to attain stability by lowering their energy. This is done by various physical and chemical processes. The reactions proceed unit they reach certain minimum energy in accordance with its surroundings. They reach the state where there is no visible change in the system and they remain that way unless disturbed. This state is called Equilibrium. In this state, the rate of forward reaction is equal to the rate of backward reactions.
Equilibrium in physical processes
Physical equilibrium is defined as the equilibrium which develops between different phases or physical properties. In these processes, there is no change in chemical composition. The following are some examples of Physical Equilibria:
- Solid-Liquid Equilibria
- Liquid-Gas Equilibria
- Solid-Vapor Equilibria
Consider ice and water in a perfectly insulated thermos flask at 00C in an open atmosphere. There will be no change in the level of water and quantity of ice, which implies that the rate of transfer of molecules from water to ice is equal to the rate of transfer of molecules from ice to water. Hence, we can conclude that this system is in steady state. This can be represented by the following equation:
H2O (s)⇌ H2O (l)
Rate of melting = Rate of freezing
Take distilled water in a closed container and start heating, the water converts to vapour. After a certain time, we will observe that the level of water becomes constant implying that there is no more conversion of water to vapour and vice-versa. We can technically say that the rate of evaporation (liquid to vapour) is equal to the rate of condensation (vapour to liquid) thus achieving steady state. This equation can be represented by the following equation:
H2O (l) ⇌ H2O (g)
Rate of evaporation = Rate of condensation
This kind of equilibrium can exist only in case of sublimates (solid directly converts to vapour). Consider heating of solid iodine in a closed container, slowly the vessel is filled with violet coloured vapour and the intensity of color increases with time. After a certain time, the intensity of color doesn’t change with time. This implies steady state is attained where the rate of sublimation of solid iodine is equal to the rate of deposition of iodine vapour.
I2 (s) ⇌ I2 (vap)
Rate of sublimation = Rate of deposition
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