Question

The concentration of a pure solid or liquid phase is not included in the expression of equilibrium constant because (a) solid and liquid concentrations are independent of their quantities. (b) solid and liquids react slowly. (c) solid and liquids at equilibrium do not interact with gaseous phase. (d) the molecules of solids and liquids cannot migrate to the gaseous phase.

Short answer

The correct answer is (a) because the concentration of a pure solid or liquid is constant and does not change with the amount of substance present.

Step-by-step solution

Understanding the Concept of Equilibrium Constants

The equilibrium constant expression for a chemical reaction is dependent on the concentrations of the reactants and products that are in the gaseous or dissolved state. For pure solids and liquids, their concentrations do not appear in the equilibrium expression.

Recognizing the Properties of Pure Solids and Liquids

Pure solids and liquids have fixed concentrations (represented as density or molarity) that are independent of the amount present. This is because their densities are constant at a given temperature and pressure, so their concentration essentially remains constant during the reaction, making their inclusion in the expression unnecessary.

Eliminating Incorrect Options

Option (b) suggesting that solids and liquids react slowly is not relevant to their being excluded from equilibrium expressions. Option (c) and (d) are incorrect because they refer to interactions and migrations to the gaseous phase, which is not linked to the reason for their exclusion from the equilibrium constant expression.

Key concepts

Chemical Equilibrium

Understanding chemical equilibrium is fundamental to mastering concepts in physical chemistry. At the heart of this concept is the state where the concentrations of reactants and products remain constant over time. It's akin to a tug-of-war with equally matched teams; the action is ongoing, but there's no discernible movement to either side. In a chemical reaction, this state is achieved when the rate of the forward reaction, where reactants convert to products, equals the rate of the reverse reaction, where products convert back to reactants.

What is vital to grasp is that equilibrium does not mean the reactants and products are equal in concentration, but that their rates of conversion are stable. In a closed system, once equilibrium is reached, the composition of the reaction mixture doesn't change unless disturbed by external factors like temperature or pressure changes. The fact that pure solids and liquids have constant concentrations under constant temperature and pressure makes their inclusion in the equilibrium constant expression redundant, as it would not affect the balance of the reaction.

Reaction Quotient

The reaction quotient, denoted as Q, acts like a snapshot of a reaction's progression at any given moment, before reaching equilibrium. It has the same form as the equilibrium constant expression but is applicable to any point in time. Calculated by plugging in the current concentrations of the reactants and products into the equilibrium expression, Q can tell you where the reaction stands relative to equilibrium.

If Q is less than the equilibrium constant (K), the forward reaction is favored, and the system will shift to produce more products. Conversely, if Q is greater than K, there will be a shift toward the reactants to reach equilibrium. This concept is a powerful tool since it can predict the direction of the shift in reaction conditions, providing insights on how a reaction must be manipulated to achieve the desired equilibrium state.

Physical Chemistry

Physical chemistry is the branch of science that deals with the principles governing the behavior and interactions of chemical substances. It encompasses a wide array of studies, including thermodynamics, reaction kinetics, quantum chemistry, and equilibrium states. A foundational aspect of physical chemistry is understanding how and why chemical reactions occur and how to control them.

The exercise related to the equilibrium constant offers an excellent example of the practical application of physical chemistry principles. By understanding the conditions of constant concentration in pure solids and liquids, and why they are omitted from the equilibrium constant expression, students gain insights into the meticulous and interconnected nature of chemical systems. This knowledge empowers future chemists to manipulate reactions deliberately to obtain desired outcomes in laboratory and industrial processes.