Ch.15 - Chemical Equilibrium

Problem 10a

Chapter 15, Problem 10a

The diagram shown here represents the equilibrium state for the reaction A2(𝑔) + 2B(𝑔) ⇌ 2AB(𝑔). (a) Assuming the volume is 2 L, calculate the equilibrium constant 𝐾𝑐 for the reaction.

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Step 1: Identify the equilibrium concentrations of the reactants and products from the diagram. The number of molecules of each species in the diagram represents the number of moles of that species. Divide the number of moles by the volume (2 L) to get the molar concentration.

Step 2: Write the expression for the equilibrium constant, Kc, for the reaction. For the reaction A2(g) + 2B(g) ⇌ 2AB(g), the equilibrium constant expression is Kc = [AB]^2 / ([A2][B]^2).

Step 3: Substitute the equilibrium concentrations from Step 1 into the equilibrium constant expression from Step 2. Remember that the concentrations should be in moles per liter (M).

Step 4: Perform the calculations as indicated in the equilibrium constant expression. Remember to square the concentration of AB and B as indicated in the equilibrium constant expression.

Step 5: The result of these calculations is the equilibrium constant, Kc, for the reaction. This value indicates the extent to which the reaction proceeds to the right (towards the products) at equilibrium.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Equilibrium Constant (Kc)

The equilibrium constant, Kc, is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. It is calculated using the formula Kc = [products]^[coefficients] / [reactants]^[coefficients]. A Kc value greater than 1 indicates that products are favored at equilibrium, while a value less than 1 suggests that reactants are favored.

Concentration

Concentration refers to the amount of a substance (solute) present in a given volume of solution. It is typically expressed in moles per liter (Molarity, M). In equilibrium calculations, the concentrations of the reactants and products at equilibrium are essential for determining the value of Kc, as they directly influence the ratio used in the equilibrium expression.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It involves using the coefficients from the balanced chemical equation to relate the amounts of substances consumed and produced. In the context of equilibrium, stoichiometry is crucial for determining the concentrations of reactants and products based on their initial amounts and the changes that occur as the system reaches equilibrium.

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Textbook Question

When lead(IV) oxide is heated above 300°C, it decomposes according to the reaction, 2 PbO₂(𝑠) ⇌ 2PbO(𝑠) + O₂(𝑔). Consider the two sealed vessels of PbO₂ shown here. If both vessels are heated to 400°C and allowed to come to equilibrium, which of the following statements is or are true? (c) The amount of PbO₂ remaining in each vessel will be the same. [Find more in Section 15.4]

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Textbook Question

The reaction A₂ + B₂ ⇌ 2 AB has an equilibrium constant K_c = 1.5. The following diagrams represent reaction mixtures containing A₂ molecules (red), B₂ molecules (blue), and AB molecules. (a) Which reaction mixture is at equilibrium?

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Open Question

The reaction PCl₃(g) + Cl₂(g) ⇌ PCl₅(g) has Kp = 0.0870 at 300 _x001F_C. A flask is charged with 0.50 atm PCl₃, 0.50 atm Cl₂, and 0.20 atm PCl₅ at this temperature. (d) The reaction is exothermic. What effect will increasing the temperature of the system have on the mole fraction of Cl₂ in the equilibrium mixture?

Textbook Question

Suppose that the gas-phase reactions A → B and B → A are both elementary reactions with rate constants of 4.7×10⁻³ s⁻¹ and 5.8×10⁻¹ s⁻¹, respectively. (a) What is the value of the equilibrium constant for the equilibrium A(g) ⇌ B(g)? (b) Which is greater at equilibrium, the partial pressure of A or the partial pressure of B?

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Textbook Question

The equilibrium constant for the dissociation of molecular iodine, I₂(g) ⇌ 2 I(g), at 800 K is K_c = 3.1 × 10^–5. (a) Which species predominates at equilibrium I₂ or I?

Textbook Question

The equilibrium constant for the dissociation of molecular iodine, I₂(g) ⇌ 2 I(g), at 800 K is K_c= 3.1×10⁻⁵. (b) Assuming both forward and reverse reactions are elementary reactions, which reaction has the larger rate constant, the forward or the reverse reaction?

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