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Given the following reactants:

$\ce{P2O5}$ and $\ce{H2O}$

How does one determine what product the two will form in a reaction?

I instantly tried to treat the problem as a double displacement reaction. However, the compounds that I got by doing so were absurd. Do double replacement reactions only occur with ionic compounds (e.g., the cations of two ions switch)?

The two elements listed above form $\ce{H3PO4}$. How do I even go about determining that? What indicates that such a product forms from those two reactants?

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    $\begingroup$ Hint: try an addition reaction instead of double replacement. $\endgroup$
    – A.K.
    Commented Apr 13, 2016 at 1:42

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The reaction between the two substance you have mentioned above in synthesis it is an acid-base reaction . and about the other question you asked above i can say that we have reactions where two ionic substances take part in oxidation reduction reactions instead of double replacement.

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Let me outline the answer previously given by eyesun:

In order to figure out how something might react with water, it is helpful to describe and categorize the starting material and to find analogies.

  1. Apparently, $\ce{P2O5}$ is a group 15 oxide, the oxide of a non-metal.
  2. Other non-metal oxides that come to our mind are from the same element ($\ce{P2O3}$), from the same group ($\ce{N2O5}$), or from the neighboring group 14 ($\ce{CO2}$) and group 16 ($\ce{SO2}$, $\ce{SO3}$, $\ce{SeO2}$).

What other compounds do we know that are similar to these oxides?

  1. We do know that a number of oxo acids exist. Mostly, they do contain one atom of a group 14-16 element with a couple of oxygen atoms and some protons attached.

  2. Can we match these?

    $$\ce{H2SO3 <=> SO2 + H2O}$$ $$\ce{H2SO4 <=> SO3 + H2O}$$ $$\ce{H2CO3 <=> CO2 + H2O}$$ $$\ce{2HNO3 <=> N2O5 + 2H2O}$$

We conclude that these non-metal oxides are like oxo acid minus water and we therefore decide to call them anhydrides.

  1. Having worked our way through all of these, we decide to apply our knowledge to $\ce{P2O5}$:

    $$\ce{2 H3PO4 <=> P2O5 + 2 H2O}$$

to realize that phosphorous pentoxide is the anhydride of orthophosphoric acid.

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  • $\begingroup$ I would add that the complete name of this acid is the orthophosphoric acid. \ce{H3PO4}. $\endgroup$
    – Another.Chemist
    Commented Apr 16, 2016 at 17:23
  • $\begingroup$ @Another.Chemist Good point. Fixed. $\endgroup$
    – Klaus-Dieter Warzecha
    Commented Apr 16, 2016 at 17:39

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