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Ch.15 - Chemical Equilibrium
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; 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?

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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.
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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.
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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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