Question

Balance the following redox equation.

HNO3 + P -> H3PO4 + NO
I've already tried adding a coefficient of 3 to HNO3, but then an extra O2 would be needed on the right side, and so far I haven't been able to balance it.

Answer 1

i think it's impossible to balance this equation, i think the way you have written is incorrect.

Let's try to write the correct equation first. this is in acidic condition because we have two strong acids.

reduction: HNO3 + 3e + 3H+ -> NO + 2H2O
oxidation: P + 4H2O -> PO4^3- + 8H+ +5e

multiply the reduction half-equation by 5 and the oxidation half by 3 to balance the number of electrons, then add them together.

5 HNO3 + 3 P + 2H2O -> 5NO+ 3 PO4^3- + 9H^+

three H pluses combine with one phosphate to give H3PO4, so:

5 HNO3 + 3 P + 2H2O -> 5NO+ 3 H3PO4

checking if it's balanced:

H: 9 on LHS, 9 on RHS
N: 5, 5
O: 17, 17
P: 3, 3

so it's balanced!