Question

I know that addition of a non-volatile solute in a pure solvent effects the rate of evaporation as the number of molecules leaving the liquid bulk per unit time decreases. However when the rate of evaporation decreases immediately , rate of condensation should automatically increase , this is because, if we consider a liquid 'X' in equilibrium as:

X(l)↽−−⇀X(g)

Now when some amount of solute is added in the pure X
, this dynamic equilibrium is obviously disturbed. (Rate)evap↓ so to attain equilibrium (Rate)cond↑ as reaction shifts backward. also when equilibrium is attained the number of molecules entering a liquid has decreased , so at equilibrium rate of condensation has decreased. So, overall it has been affected.

But when it browsed various sites , all agreed that it is unchanged by solute. So, where's the problem? Isn't my logic correct?

Answer 1

The rate of condensation remains unchanged when a nonvolatile solute is added to a volatile solvent, while the rate of evaporation decreases. This adjustment maintains the dynamic equilibrium at a lower vapor pressure. Raoult's Law explains how the presence of solute particles affects the colligative properties of the solution.

Step-by-step explanation:

When a nonvolatile solute is added to a volatile solvent, it results in the lowering of the vapor pressure of the solvent. At equilibrium, the rate of evaporation decreases because there are fewer solvent molecules at the surface due to the presence of solute molecules. However, the rate of condensation does not increase; it remains unchanged.

Over time, because the rate of evaporation is reduced and condensation is constant, fewer molecules of the solvent will be in the vapor phase. Thus, the dynamic equilibrium is maintained at a lower vapor pressure than that of the pure solvent.

This can be understood using Raoult's Law, which states that the decrease in vapor pressure, increase in boiling point, and decrease in freezing point depend on the total number of dissolved nonvolatile solute particles.

The overall shift in dynamic equilibrium due to the addition of a solute highlights the colligative properties of solutions, meaning they depend on the quantity of solute particles rather than their identity.