Question

The vapour pressure of a liquid in a closed container depends on : (1) temperature of liquid (2) quantity of liquid (3) surface area of the liquid (a) 1 only (b) 2 only (c) 1 and 3 only (d) 1,2 and 3

Short answer

The vapour pressure of a liquid in a closed container depends on the temperature of the liquid (1) only. So the correct answer is (a) 1 only.

Step-by-step solution

Understand the concept of vapour pressure

Vapour pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.

Analyze the effect of temperature on vapour pressure

As the temperature of a liquid increases, its molecules have more kinetic energy and are more likely to enter the vapor phase. This causes the vapour pressure to increase. Therefore, temperature does affect vapour pressure.

Analyze the effect of quantity of liquid on vapour pressure

The quantity of liquid does not affect the vapour pressure, as the vapour pressure depends on the substance's temperature and nature, not on the amount of liquid.

Analyze the effect of surface area on vapour pressure

The surface area of the liquid doesn't change the vapour pressure itself; it affects the rate at which the liquid evaporates. However, in a closed system, once equilibrium is reached, vapour pressure will be the same regardless of the surface area.

Key concepts

Vapour Pressure and Temperature

Exploring the direct relationship between vapour pressure and temperature reveals fundamental aspects of how substances behave under different conditions. Vapour pressure is essentially the pressure at which a liquid's vapor phase is in equilibrium with its liquid phase, in a closed container. As we increase the temperature, we are essentially pumping more energy into the liquid. This added energy is transferred to the molecules, which begin to move faster.

When these molecules gain sufficient kinetic energy, they can break free from the liquid’s surface and enter the vapor phase, which contributes to increased vapour pressure. This is because there are more molecules in the gas phase colliding with the walls of the container, which in technical terms, exert greater pressure. An integral point to remember is that vapour pressure does not depend on the total volume of the liquid or vapor but is determined by the substance's nature and the temperature at which it is measured.

Thermodynamic Equilibrium

Thermodynamic equilibrium is a state in which all the macroscopic properties of a system are unchanging in time. This implies that the system is perfectly balanced in terms of energy distribution. In the context of vapour pressure, thermodynamic equilibrium occurs when the rate at which the liquid molecules evaporate (escape into the vapor phase) is equal to the rate at which vapor molecules condense (return to the liquid phase).

At this point of dynamic balance, the vapour pressure remains constant. However, if the temperature changes, the system will move out of equilibrium until a new balance is established at a different vapour pressure. It is crucial to underscore that reaching thermodynamic equilibrium in a closed container means that the observed vapour pressure is not influenced by the liquid's surface area or quantity but solely by the temperature and intrinsic properties of the liquid.

Kinetic Energy of Molecules

The kinetic energy of molecules plays a pivotal role in determining the state of matter of a substance. In a liquid, molecules are closely packed but they still vibrate and move around. The kinetic energy is the energy that these molecules possess due to their movement and is directly proportional to temperature. As we raise the temperature of the liquid, its molecules move more vigorously due to the increase in kinetic energy.

At a certain threshold, some molecules will have enough energy to overcome the intermolecular forces that keep them within the liquid (such as hydrogen bonds or van der Waals forces), allowing them to evaporate and contribute to the vapour phase. This is the microscopic view of how vapour pressure is generated and why it increases with temperature. Nevertheless, the kinetic energy distribution among the molecules also means there are always some molecules with higher or lower energy, which allows for the ongoing exchange between the vapor and liquid phases, essential to the concept of thermodynamic equilibrium.