Question

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

Short answer

No, solids do not appear in the equilibrium expression for gaseous state reactions.

Step-by-step solution

Understanding the Equilibrium Expression

The equilibrium expression for a reaction is derived from the concentrations of the reactants and products. In a general chemical reaction, only the substances in the gaseous and aqueous states are included because their concentrations can change. Solids and pure liquids have defined densities, so their concentrations remain constant.

Analyzing the Role of Solids

In a chemical equilibrium, the concentration of a solid is considered constant because it's based on its density and does not vary with the system's state. Thus, solids are not included in the equilibrium expression, as their 'concentration' does not influence the position or calculation of equilibrium.

Concluding the Expression

Therefore, when writing an equilibrium expression for a reaction involving solids and gases, you include only the gaseous reactants and products. The expression uses the partial pressures or concentrations of these gases, excluding any terms that represent solid substances.

Key concepts

Equilibrium Expression

The equilibrium expression is a vital concept in chemistry. It allows us to understand how chemical reactions behave under equilibrium conditions. When a reaction reaches equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. To capture this behavior mathematically, we use the equilibrium expression, a formula derived from the concentrations of reactants and products at equilibrium.

The equilibrium expression is represented as:

• For a general reaction: \( aA + bB \rightleftharpoons cC + dD \)
• The equilibrium constant \( K \) is given by: \( K = \frac{[C]^c[D]^d}{[A]^a[B]^b} \)

Importantly, only the reactants and products in the gaseous and aqueous states are included in this expression. This is because their concentrations can change, affecting the equilibrium position. On the other hand, solids and pure liquids have fixed concentrations, so they do not appear in the equilibrium expression.

Gaseous State

The gaseous state of matter plays a prominent role in chemical equilibria, especially because gases have variable concentrations. The molecules in a gas move freely, allowing their concentrations to change significantly compared to solids or liquids.

In the context of equilibrium:

• Gases are expressed in terms of concentration or partial pressure.
• The equilibrium constant might use partial pressures (denoted as \( K_p \)) when referring to gaseous reactions.
• The gaseous state makes it easier to reach dynamic equilibrium due to the mobility and diffuse nature of gas particles.

The inclusion of gaseous substances in equilibrium expressions underscores their significance. Unlike solids, they freely exchange with the surroundings, thus directly impacting the equilibrium condition.

Concentration

Concentration is a key measurement in chemistry that impacts numerous processes, including chemical equilibrium. It refers to the amount of a substance in a given volume and is crucial for determining the equilibrium state of a reaction.

Key points about concentration include:

• Primarily influences how we express equilibrium conditions—in terms of molarity (\( M \)) or partial pressures for gases.
• Determines the terms included in the equilibrium expression, as only varying concentrations (gases and aqueous solutions) are factored in.
• The concentration of reactants and products at equilibrium remains constant, albeit not necessarily equal.

Since solids retain constant concentration due to their density, their absence in equilibrium expressions emphasizes the dynamic nature of gases and solutions. Understanding concentration helps us manipulate reactions to achieve desired equilibriums efficiently.