Question

Given below are two statements: 
Statement I : The N-N single bond is weaker and longer than that of P-P single bond 
Statement II : Compounds of group 15 elements in +3 oxidation states readily undergo disproportionation reactions. 
In the light of above statements, choose the correct answer from the options given below

• A. Statement I is true but Statement II is false
• B. Both Statement I and Statement II are false
• C. Statement I is false but Statement II is true
• D. Both Statement I and Statement II are true

Hint:

Consider the bond strengths and lengths in nitrogen and phosphorus. Recall the trend of disproportionation reactions in group 15 elements.

Answer 1

The Correct Option is B

This question presents two statements related to the properties of group 15 elements and asks to determine their truthfulness.

Concept Used:

Statement I involves comparing the bond properties (strength and length) of N-N and P-P single bonds. This requires knowledge of chemical bonding trends, including atomic size and inter-electron repulsion.

• Bond Length: Generally, bond length increases with atomic size.
• Bond Energy: Bond energy is influenced by factors like bond length and electron-electron repulsions. For small atoms with lone pairs, lone pair-lone pair repulsion can significantly weaken the single bond.

Statement II concerns the redox chemistry of group 15 elements, specifically the stability of the +3 oxidation state and its tendency to undergo disproportionation.

• Disproportionation: A redox reaction in which a substance is simultaneously oxidized and reduced. For an element in an intermediate oxidation state, it can be converted to species with higher and lower oxidation states.
• Inert Pair Effect: For heavier p-block elements, the reluctance of the outermost s-electrons to participate in bonding leads to the stabilization of a lower oxidation state (which is 2 less than the group oxidation state).

Step-by-Step Solution:

Step 1: Analyze Statement I.

"The N-N single bond is weaker and longer than that of P-P single bond"

• Bond Length: Nitrogen (N) is in Period 2, while Phosphorus (P) is in Period 3. Atomic radius increases down a group, so P is larger than N. Consequently, the P-P single bond length (approx. 222 pm) is significantly longer than the N-N single bond length (approx. 147 pm). The statement claims the N-N bond is longer, which is incorrect.
• Bond Strength: The N-N single bond has a bond dissociation energy of about 160 kJ/mol, while the P-P single bond has a bond dissociation energy of about 200 kJ/mol. The N-N bond is weaker due to the high inter-electronic repulsion between the lone pairs on the small, adjacent nitrogen atoms. The statement claims the N-N bond is weaker, which is correct.

Since one part of the statement (about bond length) is incorrect, Statement I is false.

Step 2: Analyze Statement II.

"Compounds of group 15 elements in +3 oxidation states readily undergo disproportionation reactions."

Let's examine the elements of group 15: N, P, As, Sb, Bi.

• For Nitrogen and Phosphorus, the +3 oxidation state is indeed unstable with respect to disproportionation. For example: 
  \( 3\text{HNO}_2 \rightarrow \text{HNO}_3 (+5) + 2\text{NO} (+2) + \text{H}_2\text{O} \) 
  \( 4\text{H}_3\text{PO}_3 \rightarrow 3\text{H}_3\text{PO}_4 (+5) + \text{PH}_3 (-3) \)
• For Arsenic (As) and Antimony (Sb), the stability of the +3 oxidation state increases due to the inert pair effect. Disproportionation is less common.
• For Bismuth (Bi), the inert pair effect is very strong, making the +3 oxidation state the most stable one. Bi(V) compounds are extremely powerful oxidizing agents, and it is very difficult to oxidize Bi(III) to Bi(V). Therefore, Bi(III) compounds do not readily undergo disproportionation.

The statement claims this behavior for "group 15 elements" in general and that they "readily" do so. Since this is not true for the heavier elements, especially Bismuth, the statement is an incorrect generalization. Therefore, Statement II is false.

Step 3: Conclude based on the analysis.

Both Statement I and Statement II are false.

Therefore, the correct option is (2) Both statement I and statement II are false.

Answer 2

N-N single bond weaker than P-P due to more lp-lp repulsion. Bond length \( d_{N-N}>d_{P-P} \) (size↑, B.L.↑) 
In group 15 elements only N and P show disproportionation in +3 oxidation state. 
As, Sb and Bi have almost inert for disproportionation in +3 oxidation state. So both statements are false.