Introduction
Le Chateliers Principle is a fundamental law of thermodynamics which states that when a system is subjected to an external stress, it attempts to regain its equilibrium state through an adaptation in order to reduce the stress. The principle is derived from the fact that the free energy of a system tends to decrease and reach its minimum with time through an equilibrium state. This principle can be applied to a physical change in a system and chemical changes for which the system can assume two or more possible equilibrium states. One of the important applications of Le Chateliers Principle is in determining the conditions for phase transition in a system.
Overview of Phase Transitions
Phase transitions are transformations between the solid, liquid and gaseous phases of matter which occur when external parameters such as temperature, pressure, and component concentrations are changed. They are typically characterized by an abrupt and discontinuous change in some properties of the system. For example, the specific heat of a solid may increase suddenly upon melting or the crystalline form of a solid may change upon heating.
Phase transitions can be classified into two types: first order and second order. First order transitions are discontinuous transitions in which the order of the system changes abruptly, such as fusion (melting) and vaporization (boiling). Second order transitions are continuous transitions in which only the properties of the system change abruptly without a change of its order, such as the transition from glassy to rubber forms of a polymer or the Ising transition in magnetic metals. All transitions are governed by a fundamental law of thermodynamics known as Le Chatelier’s Principle.
Le Chatelier’s Principle during Phase Transition
When an external variable is altered, a system tends to react against the change in order to reach its equilibrium state. Depending upon the type of the variable, such response can be in terms of an adjustment in the forces between its components to regain the equilibrium. In most cases, this response results in a continuous change in properties of the system, but at times it can bring about an abrupt change in the form of a phase transition.
In the case of a first-order phase transition, the system reacts against the imposed external change by abruptly shifting the equilibrium to its more stable state. For example, when the temperature of a liquid is raised, the system reacts by passing through a phase transition from liquid to vapor, as the vapor state is more stable than the liquid at the new temperature.
The situation is slightly different for second-order phase transitions. Since these transitions are characterized by continuous changes in properties of the system (such as increased density or increased viscosity), the system remains in equilibrium during the transition and therefore does not need to adjust its forces in order to maintain its stability. However, when the external variables reach a critical value, the system acquires a more stable form, resulting in an abrupt phase transition.
Conclusion
Le Chatelier’s Principle plays an important role in determining the conditions necessary for phase transition in a system. It states that when an external stress is applied to a system, it reacts against it in order to regain its equilibrium through an adequate adaptation. This principle can be applied to both first-order and second-order phase transitions, and helps explain the abrupt changes in the properties of a system that occur during the transition.
