Equilibrium Involving Dissolution Of Solid Or Gas In Liquid


Everything in universe tries to attain stability by lowering their energy. This can be achieved by various physical and chemical processes. All 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 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 reaction.

Equilibrium involving Dissolution of Solid or Gases in Liquids:

Solids in liquids:

Have you ever tried dissolving sugar in water? Can you dissolve any amount sugar? After certain quantity, the added sugar gets deposited. We call a solution saturated when the solution cannot dissolve any more solute in it, at a given temperature. In a saturated solution, a dynamic equilibrium exists between solute molecules in the solid state and in the solution at given temperature. The increase in temperature increases the solubility of the solution i.e. solubility depends on temperature.

\(~~~~~~~~~~~~~~~\) Sugar (solution)  ⇌ sugar (solid)
\(~~~~~~~~~~~~~~~\)Rate of dissolution of sugar = Rate of crystallisation of sugar

Gases in Liquids:

Why do you think the carbonated drinks (cold drinks) are refrigerated? What will happen when the carbonated cold drink is at room temperature? Do you observe any difference? Let’s try to understand the reason behind this.

We notice that the gas fizzing out from the cold drink at room temperature is more than the drink that is refrigerated. This happens because the solubility of carbon dioxide at different pressures. The relation between the quantity of gas dissolved can be given by Henry’s Law. This law states that the mass of a gas dissolved in the given amount of solvent at any temperature is proportional to the pressure of the gas above the solvent.The solubility of gases decreases  with increase in temperature.

\(~~~~~~~~~~~~~~~\)(Mass of the gas) α (pressure of the gas)
\(~~~~~~~~~~~~~~~\)m α p
\(~~~~~~~~~~~~~~~\)mm = kHp

Where kH is the proportionality constant, called Henry’s law constant and depends on the temperature.The cold drink is sealed under a pressure of the gas (generally higher than atmospheric pressure) where its solubility is high in the solvent (water). When the drink is opened, it is exposed to the atmosphere (lower pressure than the drink) due to which some CO2 gas escapes the bottle to establish the new dynamic equilibrium in accordance with low pressure, namely its partial pressure in the atmosphere. If the drink is exposed to surrounding most of the dissolved gas escapes because the solubility is low under low pressure and high temperature (than in refrigerator). Hence, more gas pops out of a drink kept at room temperature than a drink which is just taken out from a refrigerator.

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Practise This Question

For the reaction PCl5 (g)  PCl3 (g) + Cl2 (g) if we start with only a moles of PCl5
PCl5PCl3Cl2(g)Moles t=0a00Moles At Eq.a(1α)aαaα
α is called the degree of dissociation - True or false?