Application of Le Chatelier's Principle to Physical and chemical Change

Application of Le Chatelier's Principle to physical change

    Le Chatelier's principle says that when a reversible process in equilibrium is subjected to any change in pressure temperature or concentration then the equilibrium shifts in such direction that the effect of change is compensated.

    This principle can be applied bot to the physical or chemical change. At first we will discuss about physical equilibrium and then the chemical equilibrium. 

Application to Physical Equilibrium:

    Let us apply the principle to a solid liquid equilibrium of water. 

                        Solid Ice  ⇌  Liquid Water

    As we know when a definite volume of water is converted into ice, the volume of ice becomes greater than that of liquid water. This is because of the cage like structure in ice due to inter molecular hydrogen bonds between water molecules.

 For your particular interest, we will discuss why the ice in Himalayan mountains becomes harder and harder as we go higher and higher in the mountain.

    According to Le Chatelier's principle, if we change (increase/decreases) the pressure or temperature or concentration of reactants or products of the system at equilibrium then the equilibrium tries to relieve the stress caused due to the change and shifts either in forward or backward direction accordingly. 

    Now, let us focus on the main concept, the ice water equilibrium. As we go higher and higher, the presuure decreases. The eqilibrium shifts in a direction in which the effect of decresing pressure is rilieved. Hence the shift takes place in the direction of increased volume. Clearly the equilibrium prefers ice side caise the volume of ice is greater than water. This is why ice on the mountain becomes harder and harder as  we go higher and higher.

    The reverse case also holds good. That means if presuure is increased the ice water equilibrium shifts towards the liquid water side. 

Application of Le Chatelier's Principle to Chemical Change:

    To understand the shifting of equilibrium in a chmical change, let us take a particular exampleof preparation of ammonia in Haber's process. 

    The reversible reaction is: N₂ + 3H₂ ⇌ 2NH₃

1.   Effect of changing the pressure:

a.      When pressures is increased: If we increase the pressure inside a vessel by decreasing the volume, then the equilibrium tries cancel out the effect of change, i.e., the equilibrium tries to decrease the pressure. This is only possible if the total number of molecules inside the vessel decreases. Hence the equilibrium shifts in the forward direction as less number of moles of products are formed compared the numbers of moles of reactants. Accordingly more ammonia is formed. When the temperature is set to the critical temperatue of ammonia it is liquefied which further supports the preparation of ammonia due to concentration effect. Because if we will decrease the concentration of product by withdrawing the liquid ammonia, the equilibrium will try to increase the concentraton of it and more ammonia is formed.

b.      When pressures is decreased: If we decrease the pressure inside a vessel by increasing the volume, then the equilibrium tries cancel out the effect of change, i.e., the equilibrium tries to increase the pressure. This is only possible if the total number of molecules inside the vessel increases. Hence the equilibrium shifts in the backward direction as more number of moles of reactants are formed compared the numbers of moles of products.

        As a conclusion, in increasing the pressure, the equilibrium shifts in the direction in which lesser number of moles is prepared.

    We woluld like to listen from you, other forms of physical equilibrium and how Le Chatelier's priciple works there. Comment bellow.

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