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Chapter 15: Problem 29

Explain why we normally exclude pure solids and liquids from equilibrium- constant expressions.

Short Answer

Expert verified
We normally exclude pure solids and liquids from equilibrium constant expressions because their concentrations do not change during the reaction, as they are determined by their densities and molar masses which remain constant. Including them in the equilibrium constant expression would result in a misleading result, making it seem like the position of the equilibrium is dependent on these constant concentrations. Excluding them simplifies the expressions and allows us to focus on the substances that actually impact the position of the equilibrium.

Step by step solution

01

Understanding Equilibrium Constants

Equilibrium constants, denoted as K, are used to determine the extent to which a chemical reaction proceeds when it reaches equilibrium. The equilibrium constant expression of a reaction can be determined by using the concentrations, or partial pressures, of reactants and products participating in the reaction. For a given reaction, let's represent reactants as A and B and products as C and D: \[A + B \rightleftharpoons C + D\] The equilibrium constant expression for this reaction would be written as: \[K_c = \frac{[C][D]}{[A][B]}\] where [A], [B], [C], and [D] represent the molar concentrations of the reactants and products at equilibrium.
02

Understanding Concentrations of Pure Solids and Liquids

The concentration of a substance is defined as the amount of that substance present in a specific volume. However, for pure solids and liquids, their concentrations are determined by their densities and molar masses, which remain constant regardless of how much of the substance is present. This means that the concentrations of pure solids and liquids will not change during the reaction, unlike the concentrations of gases and solutions, which do change as the reaction proceeds.
03

Excluding Pure Solids and Liquids from Equilibrium Constants

Since the concentrations of pure solids and liquids do not change as the reaction proceeds, they do not influence the position of the equilibrium. Therefore, their concentrations do not need to be included in the equilibrium constant expression. Including them would result in a misleading result, making it seem like the position of the equilibrium is dependent on the constant concentrations of the solids and liquids. In essence, excluding pure solids and liquids from equilibrium constant expressions is because they do not impact the position of the equilibrium. Their constant concentrations help simplify the expressions, making it easier to understand and use them in various chemical equilibrium problems.

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Most popular questions from this chapter

If \(K_{c}=1\) for the equilibrium \(2 \mathrm{~A}(g) \rightleftharpoons \mathrm{B}(g)\), what is the relationship between \([\mathrm{A}]\) and \([\mathrm{B}]\) at equilibrium?

Consider the following equilibrium for which \(\Delta H<0\) $$2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{SO}_{3}(g)$$ How will each of the following changes affect an equilibrium mixture of the three gases: (a) \(\mathrm{O}_{2}(g)\) is added to the system; (b) the reaction mixture is heated; (c) the volume of the reaction vessel is doubled; (d) a catalyst is added to the mixture; (e) the total pressure of the system is increased by adding a noble gas; (f) \(\mathrm{SO}_{3}(g)\) is removed from the system?

(a) How is a reaction quotient used to determine whether a system is at equilibrium? (b) If \(Q_{c}>K_{c}\), how must the reaction proceed to reach equilibrium? (c) At the start of a certain reaction, only reactants are present; no products have been formed. What is the value of \(Q_{c}\) at this point in the reaction?

At \(1200 \mathrm{~K}\), the approximate temperature of automobile exhaust gases (Figure 15.15\(), K_{p}\) for the reaction $$2 \mathrm{CO}_{2}(g) \rightleftharpoons 2 \mathrm{CO}(g)+\mathrm{O}_{2}(g)$$ is about \(1 \times 10^{-13}\). Assuming that the exhaust gas (total pressure 1 atm \()\) contains \(0.2 \% \mathrm{CO}, 12 \% \mathrm{CO}_{2},\) and \(3 \% \mathrm{O}_{2}\) by volume, is the system at equilibrium with respect to the \(\mathrm{CO}_{2}\) reaction? Based on your conclusion, would the CO concentration in the exhaust be decreased or increased by a catalyst that speeds up the \(\mathrm{CO}_{2}\) reaction?

How do the following changes affect the value of the equilibrium constant for a gas-phase exothermic reaction: (a) removal of a reactant (b) removal of a product, (c) decrease in the volume, (d) decrease in the temperature, (e) addition of a catalyst?
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