Question

A liquid solvent is in equilibrium with its vapour. When a non-volatile solute is added to this liquid, the instant effect is the rate at which the solvent molecules leaves the (a) vapour phase, decreases. (b) vapour phase, increases. (c) solution, increases. (d) solution, decreases.

Short answer

The rate at which the solvent molecules leaves the vapour phase decreases.

Step-by-step solution

Understanding Raoult's Law

Raoult's Law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature multiplied by the mole fraction of the solvent in the solution. Adding a non-volatile solute to the solvent decreases the mole fraction of the solvent, hence, the vapor pressure of the solvent decreases.

Analyzing the Instant Effect

The immediate effect of adding a non-volatile solute is a reduction in the solvent's vapor pressure. This causes fewer solvent molecules to escape into the vapour phase. Thus, the rate at which the solvent molecules leaves the vapor phase decreases.

Choosing the Correct Option

Based on Raoult's Law and the immediate effect described, the correct statement is that the rate at which the solvent molecules leaves the vapour phase decreases. The answer is (a) vapour phase, decreases.

Key concepts

Vapor Pressure

We often hear about boiling points, but another crucial concept in chemistry is vapor pressure. It's the force exerted by a vapor when it is in thermodynamic equilibrium with its liquid or solid form. In everyday terms, it's how much of a liquid has evaporated into a gas at a given temperature.

When a liquid is heated, molecules start to move more vigorously and some are able to escape the liquid surface and become vapor. This occurs even at temperatures below the boiling point. The equilibrium is reached when the same number of molecules escape from the liquid to the gas as return from the gas to the liquid.

This dynamic is directly influenced by Raoult's Law. It's important to note that the vapor pressure is a signature property of any pure substance. However, when we mix substances, especially when adding a non-volatile solute, we effectively lower the vapor pressure because the presence of the solute molecules makes it harder for the solvent molecules to escape into the vapor phase.

Non-Volatile Solute

We often associate the term non-volatile with computer memory, but in chemistry, a non-volatile solute refers to a substance that does not vaporize easily. This means the solute has a very low vapor pressure compared to the solvent at a given temperature. Non-volatile solutes include things like table salt or sugar when added to water – they dissolve, but don't contribute to the air above the solution.

When such a solute is mixed into a solvent, it disrupts the solvent's ability to vaporize. This happens because the solute particles take up space at the surface and throughout the solution, making it less likely that a solvent molecule will find its way to the surface and escape. An analogy would be trying to exit a crowded room – the more crowded it is, the harder it is to reach the door. This is why, according to Raoult's Law, adding a non-volatile solute reduces the vapor pressure of the solution.

Chemical Equilibrium

When we talk about a balanced state in chemistry, we are referring to chemical equilibrium. This term describes a condition where the concentrations of reactants and products are constant over time because the rates of the forward and reverse reactions are equal. It's like a stand-off in a spy movie, where both sides are evenly matched, and no one is winning.

In the case of vapor pressure equilibrium, it means that the number of solvent molecules evaporating equals the number of vapor molecules condensing back into liquid. This equilibrium is disturbed when we add a non-volatile solute because it reduces the rate at which the solvent molecules are able to transition into the vapor phase – they are held back by the added solute particles.

This equilibrium shift, according to Raoult's Law, results in a lower vapor pressure for the solvent. Understanding this concept allows us to predict the behavior of solutions under various conditions, such as boiling point elevation and freezing point depression, which are both related to the changes in solvent vapor pressure when a solute is added.