Question

Does a substance in the liquid state appear in the equilibrium expression for a gaseous state reaction?

Short answer

No, liquids do not appear in the equilibrium expression for a gaseous reaction.

Step-by-step solution

Understanding Equilibrium Expression

In a chemical equilibrium expression, only the concentrations of gases and aqueous solutions are included. Solids and pure liquids do not appear in the expression because their concentrations do not change during the reaction.

Analyze the Role of Liquids

Since the equilibrium expression only involves substances whose concentrations change, liquids (like water in many reactions) are not included. This is because they are considered as having a constant concentration, much like solids.

Apply to a Gaseous State Reaction

For a gaseous state reaction, only the gases are included in the equilibrium expression. If there is a liquid involved in the reaction, it remains excluded from the expression regardless of its participation in the reaction.

Conclusion

Conclude that the concentration of a pure liquid is not included in the equilibrium constant expression for a gaseous state reaction, as its concentration remains constant.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry that describes the state at which the forward and reverse reactions of a chemical process occur at the same rate. This means that the concentrations of reactants and products remain constant over time. In any equilibrium system, it is important to understand that it does not imply that reactants and products are in equal concentrations, but rather that their rates of formation are equal.

Mathematically, this is represented by an equilibrium expression, which is a ratio of the concentrations of products to reactants, each raised to the power of their respective coefficients from the balanced chemical equation. The equilibrium constant, denoted as \( K \), is the value obtained from the equilibrium expression and provides insight into the extent of a reaction:- If \( K \) is large, the equilibrium lies towards the products, indicating a more complete reaction.- If \( K \) is small, the equilibrium favors the reactants, which means there is less conversion to products.

Understanding chemical equilibrium is crucial for predicting how a system will respond to changes in concentration, pressure, or temperature, as well as understanding the dynamic nature of reactions.

Gaseous Reactions

When dealing with gaseous reactions, it's essential to recognize that the equilibrium expression only includes the concentrations of gases. This is because gases are compressible and their concentrations can change during the reaction. Hence, they contribute significantly to the equilibrium position.

In a gaseous state reaction, pressures are often used in place of concentrations, due to the ideal gas law which relates pressure to concentration. The equilibrium expression thus often involves partial pressures of the gaseous reactants and products rather than their concentrations:- The equilibrium constant expressed in terms of partial pressures is denoted by \( K_p \).- It is particularly useful when all reacting substances are gases.

A key point to remember is that although gases and solutions are included in equilibrium expressions, substances that remain in their solid or liquid state do not affect the gaseous equilibrium, as their concentration or amounts do not change.

Pure Liquids Exclusion

In chemical reactions, especially those involving gases, pure liquids are excluded from the equilibrium expression. This exclusion is based on the premise that the activity (effective concentration) of a pure liquid is constant.

Pure liquids, like pure solids, possess a fixed density and volume; therefore, their concentration does not change during the course of the reaction. This implies that they do not influence the system's equilibrium position.

For instance, in the reaction of gaseous reactants that involves water as a solvent or reactant, water's participation as a liquid does not alter the equilibrium expression. Here are a few guiding principles:

• Pure liquids' and solids' concentrations are regarded constant and thus omitted from the expression.
• Liquids only contribute to the reaction through solvent effects or participating as reactants/products but do not change the equilibrium constant.

Pure liquids' exclusion simplifies the equilibrium expression, focusing only on the components that genuinely fluctuate and affect the state of equilibrium.