Question

Define critical temperature and critical pressure. In terms of the kinetic molecular theory, why is it impossible for a substance to exist as a liquid above its critical temperature?

Short answer

The critical temperature (Tc) is the temperature above which a substance cannot be liquefied, regardless of pressure applied. Critical pressure (Pc) is the minimum pressure required to liquefy a substance at its critical temperature. According to the kinetic molecular theory, a substance cannot exist as a liquid above its critical temperature because the kinetic energy of its particles is too high and overcomes the attractive forces between them, thus preventing the formation of a liquid state even under high pressure.

Step-by-step solution

Define critical temperature and critical pressure

Critical temperature (Tc) is the temperature above which a substance cannot be liquefied, regardless of the pressure applied. It is the highest temperature at which a substance can exist as a liquid. Critical pressure (Pc) is the minimum pressure required to liquefy a substance at its critical temperature. In other words, it is the pressure at which a substance can exist as a liquid and a gas in equilibrium at the critical temperature.

Understand the kinetic molecular theory

The kinetic molecular theory is a model that describes the behavior of gases, based on the idea that they consist of small particles (atoms or molecules) in constant chaotic motion. The key postulates of this theory are: 1. Gases consist of small particles separated by large distances relative to their size. 2. These particles are in constant random motion, colliding with each other and the walls of their container. 3. The motion of the particles is governed by Newton's laws, and the average kinetic energy of the particles is directly proportional to the temperature of the gas. 4. There are no forces of attraction or repulsion between the particles, except during collisions. 5. The total energy of the particles remains constant, as long as no external forces act on the gas.

Relate kinetic molecular theory to critical temperature

When a substance is heated, the average kinetic energy of its particles increases. As a result, the particles move faster and collide more frequently. When the temperature reaches the critical temperature, the kinetic energy of the particles becomes so high that the attractive forces between them are no longer strong enough to hold them together in a liquid state. In other words, the substance transitions from a liquid to a gas. At temperatures above the critical temperature, even applying pressure cannot bring the particles close enough to form a liquid, as the kinetic energy of the particles will always overcome the attractive forces. In conclusion, according to the kinetic molecular theory, a substance cannot exist as a liquid above its critical temperature because the kinetic energy of the particles is too high for attractive forces to maintain a liquid state.