Question

Compare homogeneous and heterogeneous equilibria.

Short answer

Homogeneous equilibria involve reactants and products in the same phase, with the equilibrium constant expression including all species concentrations raised to their respective stoichiometric coefficients: \(K = \frac{[C]^c [D]^d}{[A]^a [B]^b}\). In contrast, heterogeneous equilibria involve reactants and products in different phases. The equilibrium constant expression only includes gaseous species: \(K = \frac{[C]^c }{[A]^a }\), as concentrations of solids and pure liquids are constant.

Step-by-step solution

Defining Homogeneous Equilibria

Homogeneous equilibria refer to a chemical equilibrium in which all the reactants and products are in the same phase (e.g., all reactants and products are gases, liquids, or solids). For example, the reaction between two gases producing a third gas is considered a homogeneous equilibrium.

Defining Heterogeneous Equilibria

Heterogeneous equilibria refer to a chemical equilibrium in which reactants and products are in different phases. For example, the reaction between a solid and a gas producing a different gas is considered a heterogeneous equilibrium.

Equilibrium Constant

The equilibrium constant (K) is a numerical value that indicates the ratio between the concentrations of products and reactants at equilibrium. It is used to predict the direction in which the reaction will proceed. The equilibrium constant expression is formulated based on the stoichiometry of the balanced chemical equation.

Equilibrium Constant for Homogeneous Equilibria

For a general homogeneous reaction represented as \(aA + bB \rightleftharpoons cC + dD\), where a, b, c, and d are stoichiometric coefficients and A, B, C, and D are species in the same phase, the equilibrium constant expression can be written as: \[K = \frac{[C]^c [D]^d}{[A]^a [B]^b}\] In this expression, the concentration terms are raised to the power of their respective stoichiometric coefficients in the balanced chemical equation.

Equilibrium Constant for Heterogeneous Equilibria

For a general heterogeneous reaction represented as \(aA_{(g)} + bB_{(s)} \rightleftharpoons cC_{(g)} + dD_{(s)}\), where a, b, c, and d are stoichiometric coefficients and A, B, C, and D are species in different phases, the equilibrium constant expression can be written as: \[K = \frac{[C]^c }{[A]^a }\] It is important to note that in the equilibrium expression for heterogeneous equilibria, only the concentrations of the species present in the gaseous phase are included; the concentration terms of solids and pure liquids are not included in the equilibrium expression.

Comparing Homogeneous and Heterogeneous Equilibria

The key differences between homogeneous and heterogeneous equilibria can be summarized as follows: 1. In homogeneous equilibria, all reactants and products are in the same phase, while in heterogeneous equilibria, reactants and products are present in different phases. 2. The equilibrium constant expression for homogeneous equilibria includes the concentrations of all species present in the reaction, while the equilibrium expression for heterogeneous equilibria only includes the concentrations of gaseous species, as the concentrations of solids and pure liquids are constant.

Key concepts

Homogeneous Equilibria

In chemical reactions, when all the reactants and products are in the same phase, we talk about homogeneous equilibria. Imagine a situation where all involved substances are gaseous or all are in solution. That's homogeneous. For instance, when nitrogen and hydrogen gases react to form ammonia gas:

• \[ N_2 (g) + 3H_2 (g) ightleftharpoons 2NH_3 (g) \]
• This reaction involves only gases, making it homogeneous. Sometimes, homogeneous equilibria can also involve solutes in a solution, like when table salt dissolves in water and remains in aqueous form. The beauty of homogeneous equilibria is simplicity. Since everything is in the same phase, we can use concentrations like molarity directly in our calculations. By examining homogeneous equilibria, chemists can better predict how changes to conditions like concentration or pressure may affect the system. Understanding this concept provides a fundamental basis to more complex systems where different phases are involved.

Heterogeneous Equilibria

Heterogeneous equilibria occur when reactants and products are in different phases. This might seem more complicated, but it often simplifies equilibrium calculations since solids and pure liquids don't appear in the equilibrium expression. Let's consider the decomposition of calcium carbonate:

• \[ CaCO_3 (s) ightleftharpoons CaO (s) + CO_2 (g) \]
• In this case, calcium carbonate and calcium oxide are solids, while carbon dioxide is a gas. Thus, we have a heterogeneous scenario. Only the concentration of the gas is used in the equilibrium expression. This is because the activities of solids and pure liquids are taken as constants. They have a fixed concentration throughout the process. By focusing only on gaseous or solute components, it simplifies predicting how equilibrium will shift. In practical terms, understanding heterogeneous equilibria is pivotal for processes like distillation and extraction where phase changes are integral.

Equilibrium Constant

The equilibrium constant, represented as \(K\), is fundamental in determining the dynamics of a chemical reaction at equilibrium. It indicates the ratio of concentrations of products to reactants. This ratio remains constant as long as the temperature is fixed. Consider the general reaction:

• \[ aA + bB ightleftharpoons cC + dD \]
• The expression for the equilibrium constant \(K\) would be:
  • \[ K = \frac{[C]^c [D]^d}{[A]^a [B]^b} \]
  • For homogeneous reactions, calculations include all species. In heterogeneous reactions, concentrations for solids and pure liquids are omitted.Understanding \(K\) allows chemists to predict not only the composition of a reaction mixture at equilibrium but also how shifts in conditions may alter it. If \(K\) is large, the reaction favors products; if small, it favors reactants. This insight aids in industrial processes and varieties of chemical syntheses.