Question

Identify the correct order of bond dissociation enthalpy of halogens from following.

• A. \(\text{I-I}>\text{Br-Br}>\text{Cl-Cl}>\text{F-F}\)
• B. \(\text{Cl-Cl}>\text{Br-Br}>\text{F-F}>\text{I-I}\)
• C. \(\text{I-I}>\text{Br-Br}>\text{Cl-Cl}>\text{F-F}\)
• D. \(\text{Br-Br}>\text{F-F}>\text{I-I}>\text{Cl-Cl}\)

Hint:

Bond enthalpy falls down the group, but fluorine is an exception because of lone pair repulsion in a very small molecule.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
Bond dissociation enthalpy usually decreases down a group as the bond length increases.

Step 2: Order for Cl, Br and I
\(\text{Cl-Cl}>\text{Br-Br}>\text{I-I}\), because the bond gets longer and weaker from Cl to I.

Step 3: Exception of fluorine
The \(\text{F-F}\) bond is unusually weak. The fluorine atoms are very small, so the lone pairs on the two atoms repel each other strongly.

Step 4: Compare the options
The fluorine bond is stronger than the iodine bond but weaker than the chlorine and bromine bonds, so the order is \(\text{Cl-Cl}>\text{Br-Br}>\text{F-F}>\text{I-I}\).

Final Answer:
The order is Cl-Cl > Br-Br > F-F > I-I. \[ \boxed{\text{(B) }\text{Cl-Cl}>\text{Br-Br}>\text{F-F}>\text{I-I}} \]