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Ch.15 - Chemical Equilibrium
Chapter 15, Problem 151c

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). (c) Why is the F-F bond in F2 weaker than the Cl-Cl bond in Cl2?

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The F-F bond in F_2 is weaker than the Cl-Cl bond in Cl_2 due to the smaller size of the fluorine atom compared to the chlorine atom.
Fluorine atoms have a high electron density and small atomic radius, which leads to significant electron-electron repulsion between the lone pairs on adjacent fluorine atoms.
This repulsion weakens the F-F bond, making it easier to break compared to the Cl-Cl bond.
Chlorine atoms are larger, so the electron pairs are further apart, reducing the repulsion between them and resulting in a stronger Cl-Cl bond.
Additionally, the bond length in F_2 is shorter than in Cl_2, which also contributes to the increased repulsion and weaker bond strength in F_2.

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Key Concepts

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

Lone Pair Repulsion

Lone pair repulsion refers to the repulsive interactions between non-bonding electron pairs on atoms. In the case of F<sub>2</sub>, each fluorine atom has three lone pairs, which repel each other, leading to increased electron-electron repulsion. This repulsion weakens the bond between the two fluorine atoms, making the F-F bond less stable compared to bonds where such repulsion is minimized.
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Bond Strength and Atomic Size

Bond strength is influenced by the size of the atoms involved in the bond. Chlorine atoms are larger than fluorine atoms, which allows for better overlap of the bonding orbitals in Cl<sub>2</sub>. This greater orbital overlap results in a stronger Cl-Cl bond compared to the F-F bond, where the smaller size of fluorine leads to increased lone pair repulsion and less effective bonding.
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Thermodynamics and Equilibrium

The equilibrium constant (K<sub>p</sub>) provides insight into the stability of a chemical species at a given temperature. A K<sub>p</sub> value of 7.83 at 1500 K for the dissociation of F<sub>2</sub> indicates that at this temperature, the formation of free fluorine atoms is favored. This suggests that the F-F bond is relatively weak, as a significant amount of F<sub>2</sub> can dissociate into individual F atoms, contrasting with the stronger Cl-Cl bond that would have a lower tendency to dissociate.
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Related Practice
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; Kp = 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; 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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Textbook Question

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

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

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Textbook Question

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

(c) Can you alter the ratio of the two concentrations at equilibrium by adding cyclopropane or by decreasing the volume of the container? Explain.

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