Ch.15 - Chemical Equilibrium

Problem 151b

Chapter 15, Problem 151b

The F-F bond in F₂ is relatively weak because the lone pairs of electrons on one F atom repel the lone pairs on the other F atom; K_p = 7.83 at 1500 K for the reaction F₂(g) ⇌ 2 F(g). (b) What fraction of the F₂ molecules dissociate at 1500 K?

Verified step by step guidance

Step 1: The given reaction is F2(g) ⇌ 2F(g). The equilibrium constant expression for this reaction is Kp = [F]^2 / [F2].

Step 2: Let's assume that 'x' is the fraction of F2 molecules that dissociate. Therefore, the initial concentration of F2 is (1-x) and the concentration of F at equilibrium is 2x.

Step 3: Substitute these values into the equilibrium constant expression. So, Kp = (2x)^2 / (1-x).

Step 4: You are given that Kp = 7.83 at 1500 K. Substitute this value into the equation from step 3.

Step 5: Solve the resulting equation for 'x'. This will give you the fraction of F2 molecules that dissociate at 1500 K.

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Key Concepts

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

Bond Strength and Lone Pair Repulsion

The strength of a chemical bond is influenced by the presence of lone pairs of electrons. In the case of the F2 molecule, the lone pairs on each fluorine atom repel each other, leading to a weaker bond. This repulsion reduces the overall bond strength, making it easier for the F2 molecules to dissociate into individual fluorine atoms.

Equilibrium Constant (Kp)

The equilibrium constant (Kp) quantifies the ratio of the partial pressures of products to reactants at equilibrium for a gaseous reaction. In this case, Kp = 7.83 at 1500 K indicates that at this temperature, the products (dissociated F atoms) are favored over the reactants (F2 molecules), allowing us to calculate the extent of dissociation.

Dissociation Fraction

The dissociation fraction represents the proportion of a substance that has dissociated into its constituent parts at equilibrium. To find this fraction for the F2 molecules at 1500 K, one can use the equilibrium expression derived from Kp, which relates the concentrations of the dissociated species to the original concentration of F2, allowing for the calculation of how many molecules have broken apart.

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Textbook Question

Halogen lamps are ordinary tungsten filament lamps in which the lamp bulb contains a small amount of a halogen (often bromine). At the high temperatures of the lamp, the halogens dissociate and exist as single atoms.(c) When the WBr41g2 diffuses back toward the filament, it decomposes, depositing tungsten back onto the fila- ment. Show quantitatively that the pressure of WBr4 from part (a) will cause the reaction in part (a) to go in reverse direction at 2800 K. [The pressure of Br1g2 is still 0.010 atm.] Thus, tungsten is continually recycled from the walls of the bulb back to the filament, allow-ing the bulb to last longer and burn brighter.

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Textbook Question

The F-F bond in F₂ is relatively weak because the lone pairs of electrons on one F atom repel the lone pairs on the other F atom; K_p = 7.83 at 1500 K for the reaction F₂(g) ⇌ 2 F(g). (a) If the equilibrium partial pressure of F₂ molecules at 1500 K is 0.200 atm, what is the equilibrium partial pressure of F atoms in atm?

Textbook Question

The F-F bond in F₂ is relatively weak because the lone pairs of electrons on one F atom repel the lone pairs on the other F atom; K_p = 7.83 at 1500 K for the reaction F₂(g) ⇌ 2 F(g). (c) Why is the F-F bond in F₂ weaker than the Cl-Cl bond in Cl₂?

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Textbook Question

The equilibrium constant K_c for the gas-phase thermal decomposition of cyclopropane to propene is 1.0 ⨉10⁵ at 500 K:

(a) What is the value of K_p at 500 K?

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The F-F bond in F2 is relatively weak because the lone pairs of electrons on one F atom repel the lone pairs on the other F atom; Kp = 7.83 at 1500 K for the reaction F2(g) ⇌ 2 F(g). (b) What fraction of the F2 molecules dissociate at 1500 K?

Verified video answer for a similar problem:

Key Concepts

Bond Strength and Lone Pair Repulsion

Equilibrium Constant (Kp)

Dissociation Fraction

The F-F bond in F₂ is relatively weak because the lone pairs of electrons on one F atom repel the lone pairs on the other F atom; K_p = 7.83 at 1500 K for the reaction F₂(g) ⇌ 2 F(g). (b) What fraction of the F₂ molecules dissociate at 1500 K?