What are the products formed when nitrogen dioxide reacts with water?

Question

I've seen two different equations for the reaction of nitrogen dioxide with water:

$$\ce{2NO2 + H2O -> H+ + NO3- + HNO2}$$

and

$$\ce{3NO2 + H2O -> 2H+ + 2NO3- + NO}$$

Are both of these correct to some degree, and is one more prominent over the other?

Answer 1

Both of the reactions are correct. However, the second reaction is written by 'adding' two reaction equations.

When nitrogen dioxide($\ce{NO2}$) is dissolved in water, it produces a 1:1 mixture of nitric acid($\ce{HNO3)}$ and nitrous acid(HNO2). $$\ce{2NO2(g) +H2O(l)->HNO3(aq) +HNO2(aq)}$$ i.e.$$\ce{2NO2(g) +2H2O(l)->H3O+(aq) +NO3-(aq) + HNO2(aq)}$$

However, since nitrous acid is unstable in any environment except very cold solution, it decomposes slowly into $\ce{NO}$ and $\ce{HNO3}$: $$\ce{3HNO2(aq)->2NO(g) +H3O+(aq) + NO3-(aq)}$$

Your second reaction equation is obtained by adding these two reaction together.

Reference:

1. Housecroft, C. E.; Constable, E. C. Chemistry, 4th ed.; Pearson, 2010, p 777.

Note:

I wrote the $\ce{H+}$ ions as $\ce{H3O+}$, so there is an extra water molecule on the left side of the equation, in each case.

Answer 2

Both can take place, but nitrous acid is unstable. In warm or concentrated solutions, the nitrous acid will disproportionate, forming nitric oxide and more nitric acid. So the nitrous acid is essentially only a temporary intermediate.