pBlock ElementsII 1 Question 16
16. The reaction of white phosphorus with aqueous $\mathrm{NaOH}$ gives phosphine alongwith another phosphorus containing compound. The reaction type, the oxidation states of phosphorus in phosphine and the other product respectively are
(2012)
(a) redox reaction, $-3$ and $-5$
(b) redox reaction, $3$ and $+5$
(c) disproportionation reaction, $-3$ and $+5$
(d) disproportionation reaction, $-3$ and $+3$
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Answer:
Correct Answer: 16. (c)
Solution:
- The reaction of white phosphorus with aqueous alkali is
$\mathrm{P} _{4}+3 \mathrm{NaOH}+3 \mathrm{H} _{2} \mathrm{O} \longrightarrow \mathrm{PH} _{3}+\mathrm{NaH} _{2} \mathrm{PO} _{2}$
In the above reaction, phosphorus is simultaneously oxidised $\left[\mathrm{P} _{4}(0) \longrightarrow \mathrm{NaH} _{2} \stackrel{+1}{\mathrm{P}} \mathrm{O} _{2}\right]$ as well as reduced $\left[\mathrm{P} _{4}(0) \longrightarrow \stackrel{-3}{\mathrm{P}} \mathrm{H} _{3}\right]$ .Therefore, this is an example of disproportionation reaction. Oxidation number of phosphorus in $\mathrm{PH} _{3}$ is $-3$ and in $\mathrm{NaH} _{2} \mathrm{PO} _{2}$ is $+1$ . However, $+1$ oxidation number is not given in any option, one might think that $\mathrm{NaH} _{2} \mathrm{PO} _{2}$ has gone to further decomposition on heating.
$2 \mathrm{NaH} _{2} \mathrm{PO} _{2} \stackrel{\Delta}{\longrightarrow} \mathrm{Na} _{2} \mathrm{H} \stackrel{+5}{\mathrm{P}} \mathrm{O} _{4}+\mathrm{PH} _{3}$
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