Question

The correct sequence of bond enthalpy of ‘C–X’ bond is:

• A. CH₃–F>CH₃–Cl>CH₃–Br>CH₃–I
• B. CH₃–F<CH₃–Cl>CH₃–Br>CH₃–I
• C. CH₃–Cl>CH₃–F>CH₃–Br>CH₃–I
• D. CH₃–F<CH₃–Cl<CH₃–Br<CH₃–I

Hint:

Small atoms form strong bonds because they can get closer to each other, resulting in better orbital overlap. Fluorine is the smallest halogen, so C–F is the strongest bond.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Bond enthalpy, also known as bond dissociation energy, is a measure of bond strength. It is the energy required to break one mole of a specific type of chemical bond in the gas phase. For carbon-halogen (C-X) bonds, this strength is intimately related to the atomic size of the halogen involved.
Step 2: Key Formula or Approach:
There is a fundamental inverse relationship between bond length and bond enthalpy. As the size of the halogen atom increases, the carbon-halogen bond length increases. A longer bond implies less effective orbital overlap between the carbon and halogen atoms, which results in a weaker bond and consequently, a lower bond enthalpy.
Step 3: Detailed Explanation:
Let's consider the halogens belonging to Group 17 of the periodic table: Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I).
As you move down the group, the number of electron shells increases, leading to an increase in atomic radius. Thus, the order of atomic size is:
\[ \text{F}<\text{Cl}<\text{Br}<\text{I} \]
Because of this increasing size, the distance between the carbon nucleus and the halogen nucleus increases when they form a bond. This gives the following order for C-X bond lengths:
\[ \text{C-F}<\text{C-Cl}<\text{C-Br}<\text{C-I} \]
Since shorter bonds have more effective orbital overlap and are thus stronger, the energy required to break them is higher. Therefore, the bond enthalpy decreases as the halogen becomes larger:
\[ \text{C-F bond enthalpy}>\text{C-Cl bond enthalpy}>\text{C-Br bond enthalpy}>\text{C-I bond enthalpy} \]
This expected trend perfectly matches the sequence provided in option (a). {Note that 'CH-X' in the options is a general representation intended to mean the carbon-halogen bond.}
Step 4: Final Answer:
The correct decreasing sequence of bond enthalpy is CH-F > CH-Cl > CH-Br > CH-I.