Diphosphorus pentoxide reacts with water to produce phosphoric acid (#H_3PO_4#). How do you write the balanced equation for this reaction?

1 Answer
Jan 15, 2016

Answer:

#"P"_4"O"_text(10(s]) + 6"H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])#

Explanation:

The interesting thing about diphosphorus pentoxide, #"P"_2"O"_5#, is that it usually exists as a dimer.

This implies that #"P"_2"O"_5# is actually the compound's empirical formula and that you should use #"P"_4"O"_10# as its molecular formula.

Now, diphosphorus pentoxide reacts violently with water to form phosphoric acid, #"H"_3"PO"_4#. The reaction is highly exothermic and leads to the formation of toxic fumes.

The unbalanced chemical equation for this reaction looks like this

#"P"_4"O"_text(10(s]) + "H"_2"O"_text((l]) -> "H"_3"PO"_text(4(aq])#

To balance this equation, start by multiplying the phosphoric acid by #4# to get equal numbers of atoms of phosphorus on both sides of the equation

#"P"_4"O"_text(10(s]) + "H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])#

Notice that you have #2# atoms of hydrogen on the reactants' side and #12# on the products' side. Multiply the water molecule by #6# to balance the hydrogen atoms out.

Incidentally, this will also balance out the atoms of oxygen, since you'd now have #16# on the reactants' side and #16# on the products' side.

The balanced chemical equation for this reaction will thus be

#"P"_4"O"_text(10(s]) + 6"H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])#

It's worth mentioning that diphosphorus pentoxide is a very powerful dehydrating agent.

SIDE NOTE You'll sometimes see this reaction written using the empirical formula of diphosphorus pentoxide, #"P"_2"O"_5#. In that case, the balanced chemical equation will be

#"P"_2"O"_text(5(s]) + 3"H"_2"O"_text((l]) -> 2"H"_3"PO"_text(4(aq])#