Question

Explain why the position of a heterogeneous equilibrium does not depend on the amounts of pure solid or pure liquid reactants or products present.

Short answer

In a heterogeneous equilibrium involving gaseous and solid species, the position of the equilibrium does not depend on the amounts of pure solid or pure liquid reactants or products present, because their activities are always defined as 1. The equilibrium constant $K$ depends only on the activities of the gaseous species in the reaction. Since the activities of pure solid and pure liquid reactants or products are constant and do not change during the course of the reaction, their presence or absence, as well as their amounts, do not affect the equilibrium position. The equilibrium position depends only on the concentrations or partial pressures of the gaseous and dissolved species, and not on the amounts of pure solids or pure liquids present.

Step-by-step solution

Understand the concept of heterogeneous equilibrium

Heterogeneous equilibrium refers to a situation where the reactants and products are in more than one phase. A common example is a reaction involving solid reactants and gaseous products. In a heterogeneous equilibrium, only the concentrations (or partial pressures) of the gaseous and dissolved species are considered, while the amounts of pure solid or pure liquid reactants or products present are not taken into account.

Define activity and its role in the equilibrium constant

In thermodynamics, the concept of activity is used to describe the effective concentration of a chemical substance in a mixture. Activity is a dimensionless quantity that measures how "active" a species is chemically relative to its standard state. For pure solids and pure liquids, the activity is defined to be 1, because their concentrations remain constant and do not change during the course of a reaction.

Establish the relationship between the equilibrium constant and activities

The equilibrium constant $K$ for a reaction is defined as the product of the activities of the products raised to their stoichiometric coefficients divided by the product of the activities of the reactants raised to their stoichiometric coefficients. In heterogeneous equilibria involving pure solids and/or pure liquids, their activities are equal to 1, as mentioned earlier.

Derive the equilibrium constant expression for a heterogeneous equilibrium

Let's consider a general equilibrium reaction involving gaseous and solid species: $a{A}_{(g)}+b{B}_{(s)}⟷c{C}_{(g)}+d{D}_{(s)}$ The equilibrium constant expression, considering activities, would be $$K=\frac{(a_C{)}^c(a_D{)}^d}{(a_A{)}^a(a_B{)}^b}$$ However, since the activities of the pure solid species B and D are both 1, $$K=\frac{(a_C{)}^c}{(a_A{)}^a}$$ Hence, the equilibrium constant depends only on the activities of the gaseous species in the reaction.

Explain the independence of the equilibrium position on pure solids and liquids

Since the activities of pure solid and pure liquid reactants or products are always defined as 1, their presence or absence, as well as their amounts, do not affect the equilibrium constant and thus the position of the heterogeneous equilibrium. The equilibrium position depends only on the concentrations or partial pressures of the gaseous and dissolved species, and not on the amounts of pure solids or pure liquids present.

Key concepts

Equilibrium Constant

The equilibrium constant, denoted as $K$, is a fundamental concept in chemistry when evaluating the state of equilibrium in a reaction. It quantifies the ratio of activities, which is essentially the "effective concentrations," of the products to the reactants at equilibrium.

For any given chemical reaction at equilibrium, the equilibrium constant is expressed in a form derived from the balanced chemical equation. The value of $K$ gives insight into the position of equilibrium, indicating the relative amounts of products and reactants when the system reaches equilibrium. A large $K$ value suggests that the products are favored at equilibrium, while a small $K$ means the reactants are favored.

In heterogeneous equilibria, where different phases exist, only the gaseous and dissolved species appear in the expression for $K$.

Activity in Chemistry

In thermodynamics, the concept of "activity" is a way to represent the effective concentration of specific chemical species in a mixture. This notion helps predict how a substance behaves in a reaction.

Activity is represented as a dimensionless quantity that is standardized to reference the standard state of the species in question. For pure solids and liquids, their activity is consistently assigned a value of 1. This is because their concentration does not vary over the course of the reaction, maintaining a constant presence.

• Solids and pure liquids remain at consistent phases.
• Their activities do not fluctuate.

The determination of activity is particularly crucial in correctly establishing the equilibrium constant for any reaction.

Phases in Reactions

When discussing reactions, the word "phases" refers to the different states of matter involved: gaseous, liquid, solid, and sometimes aqueous phases.

In heterogeneous reactions, components in different phases interact. This means that the reaction involves matter in solid, liquid, or gaseous states, like solid reactants with gaseous products.

• Heterogeneous reactions are widespread in both natural processes and industrial applications.
• They often include systems where reactants or products are immobilized, such as catalysts.

Understanding how different phases interact is vital because the characteristics of each phase can significantly impact the reaction dynamics and equilibrium.

Gaseous and Dissolved Species

In chemical equilibria, gaseous and dissolved species play a central role because they are often directly involved in the reaction's equilibrium expression. This is particularly the case in heterogeneous equilibria.

These species appear in the balanced chemical equation and affect the calculation of the equilibrium constant $K$.

• Gaseous species are included as they have partial pressures that fluctuate and are integral to the system's behavior.
• Dissolved species, often in the aqueous phase, contribute to concentration-based equilibrium expressions.

Only the gas-phase concentration or the dissolved concentration matters in the equilibrium constant expression because their amounts significantly impact the reaction's direction and extent.

Pure Solids and Liquids

In the evaluation of equilibrium, pure solids and liquids have a unique role. Unlike gaseous or dissolved species, their concentrations remain constant throughout a reaction, meaning their activities are always 1. This constancy arises from their physical nature—their quantities, masses, do not change significantly.

Because of this stability, pure solids and liquids are not included in the equilibrium constant expression. They do not impact the equilibrium ratio, as their activity does not contribute.

• This simplification helps focus only on reactive components that affect the equilibrium position.
• It allows the equilibrium constant to reflect the true dynamical relationships between reactive gaseous and dissolved species.

The presence and amount of these pure phases might impact the speed of reaching equilibrium, but not the position of "where" equilibrium lies.