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P Block Elements question

2012 · Shift 2 · Q7
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P Block Elements question

2012 · Shift 2 · Q7

JEE AdvancedChemistryP Block ElementsMCQ+3 / −1
The reaction of white phosphorous with aqueous NaOH gives phosphine along with another phosphorus containing compound. The reaction type; the oxidation states of phosphorus in phosphine and the other product are, respectively,
  1. A
    redox reaction; −-− 3 and −-− 5
  2. B
    redox reaction; +3 and +5
  3. C
    disproportionation reaction; −-− 3 and +1
  4. D
    disproportionation reaction; −-− 3 and +3
View written solutionFree

Correct answer: C

Step-by-step Solution:

  1. Identify the Reactants and Products: The question states that white phosphorus (P4P_4P4​) reacts with aqueous NaOH. One product is given as phosphine (PH3PH_3PH3​). We need to identify the other phosphorus-containing compound and the reaction type. The reaction is a well-known disproportionation reaction of white phosphorus in a hot, concentrated alkali solution. The unbalanced chemical equation is: P4+NaOH+H2O→PH3+NaH2PO2P_4 + NaOH + H_2O \rightarrow PH_3 + NaH_2PO_2P4​+NaOH+H2​O→PH3​+NaH2​PO2​ The other product is sodium hypophosphite (NaH2PO2NaH_2PO_2NaH2​PO2​).

  2. Determine the Oxidation States: To classify the reaction and find the required oxidation states, we calculate the oxidation state (O.S.) of phosphorus in the reactant and products.

    • Reactant (White Phosphorus, P4P_4P4​): In its elemental form, the oxidation state of phosphorus is 0.
    • Product (Phosphine, PH3PH_3PH3​): Let the oxidation state of P be xxx. Hydrogen has an oxidation state of +1 when bonded to a more electronegative non-metal like phosphorus. x+3(+1)=0  ⟹  x=−3x + 3(+1) = 0 \implies x = -3x+3(+1)=0⟹x=−3 So, the O.S. of P in PH3PH_3PH3​ is -3.
    • Other Product (Sodium Hypophosphite, NaH2PO2NaH_2PO_2NaH2​PO2​): Let the oxidation state of P be yyy. Sodium (Na) is an alkali metal, so its O.S. is +1. Oxygen (O) generally has an O.S. of -2. Hydrogen (H) has an O.S. of +1. (+1)+2(+1)+y+2(−2)=0(+1) + 2(+1) + y + 2(-2) = 0(+1)+2(+1)+y+2(−2)=0 1+2+y−4=01 + 2 + y - 4 = 01+2+y−4=0 y−1=0  ⟹  y=+1y - 1 = 0 \implies y = +1y−1=0⟹y=+1 So, the O.S. of P in NaH2PO2NaH_2PO_2NaH2​PO2​ is +1.
  3. Determine the Reaction Type: In this reaction, the oxidation state of phosphorus changes from 0 in the reactant (P4P_4P4​) to -3 in one product (PH3PH_3PH3​) and +1 in another product (NaH2PO2NaH_2PO_2NaH2​PO2​).

    • P(0)→P(−3)P(0) \rightarrow P(-3)P(0)→P(−3) is a reduction (gain of electrons).
    • P(0)→P(+1)P(0) \rightarrow P(+1)P(0)→P(+1) is an oxidation (loss of electrons). Since the same element (phosphorus) from a single reactant is simultaneously oxidized and reduced, the reaction is a disproportionation reaction. A disproportionation reaction is a specific type of redox reaction.
  4. Match with the Options: We are looking for the option that correctly identifies:

    • Reaction type: disproportionation reaction
    • O.S. of P in phosphine: -3
    • O.S. of P in the other product: +1

    Let's evaluate the given options:

    • A: redox reaction; −-− 3 and −-− 5. (Incorrect type and O.S.)
    • B: redox reaction; +3 and +5. (Incorrect type and O.S.)
    • C: disproportionation reaction; −-− 3 and +1. (This matches our findings.)
    • D: disproportionation reaction; −-− 3 and +3. (Incorrect O.S. for the other product.)
  5. Conclusion: The reaction is a disproportionation reaction, and the oxidation states of phosphorus in phosphine and sodium hypophosphite are -3 and +1, respectively. Therefore, option C is the correct answer.

    The balanced chemical equation for the reaction is: P4(s)+3NaOH(aq)+3H2O(l)→PH3(g)+3NaH2PO2(aq)P_4(s) + 3 NaOH(aq) + 3 H_2O(l) \rightarrow PH_3(g) + 3 NaH_2PO_2(aq)P4​(s)+3NaOH(aq)+3H2​O(l)→PH3​(g)+3NaH2​PO2​(aq)

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