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Why is hypophosphorous acid $(\ce{H3PO2})$ a strong reducing agent? the oxidation state of phosphorous in $\ce{H3PO2}$ is $+5$, as hydrogen and oxygen both are more electronegative than phosphorous, and phosphorous can never increase its oxidation state to more than $+5$, so how can it be a strong reducing agent?

hypophosphorous_acid

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If you render the oxidation state of hypophosphorous as +5, then the electrons in the phosphorous-hydrogen bonds are attributed to the hydrogen atoms in those bonds. You can do that, but then the hydrogen atoms involved have oxidation state -1 not +1. So the reducing action of hypophosphorous acid is attributed to the negative oxidation state hydrogens.

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  • $\begingroup$ so its all because of the negative partial charge on hydrogen that will be responsible for the reducing ability of the acid? $\endgroup$ – Geet Suri Feb 18 '18 at 21:56
  • $\begingroup$ Hyrogen in a negative oxidation state (-1) is reducing. $\endgroup$ – Oscar Lanzi Feb 18 '18 at 22:15
  • $\begingroup$ can u plss explain how negative oxidation state of hydrogen is reducing? everytime i encountered with oxidation problems i always used to see the oxidation state of the central atom and then judge the oxidizing and reducing nature of the compound in a reaction.. $\endgroup$ – Geet Suri Feb 18 '18 at 22:24
  • $\begingroup$ The central atom is often decisive, but not always. You really have to consider the oxidation state of all atoms. $\endgroup$ – Oscar Lanzi Feb 18 '18 at 22:32
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The oxidation state of phosphorus in H3PO4 is indeed +5, but in this case you have only 2 and not 4 oxygens. This means that your P is only +1. The reason it's a reducing is because that P really wants to "become" +5.

Oscar Lanzi in his answer suggests that you can assume the H is H-, but this introduces a complication of this being a hydride, and these are not usually stable in solution. Note that this distinction is artificial. Eventually, this is all covalently bonded and the use oxidation numbers suggests ionic bonding.

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  • $\begingroup$ If you assume phosphorous is +5, then hydrogen attached to it is -1. You can assume instead that all the H is +1 but must then accept P being +1. No matter how you slice it, you render something in a state that makes the molecule strongly reducing. $\endgroup$ – Oscar Lanzi Feb 19 '18 at 13:32
  • $\begingroup$ $\ce{H3PO2}$ not $\ce{H3PO4}$ $\endgroup$ – A.K. Oct 12 '18 at 3:15
  • $\begingroup$ @A.K. yea I know, that's what I said "only 2 and not 4 oxygens". $\endgroup$ – Gimelist Oct 12 '18 at 3:30

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