Balance the equation : NO2 + H2O ---> N2O + HNO3

Posted by Kyo on June 25, 2008 at 14:51:27:

In Reply to: Spot the Chemistry Errors in this YouTube video! posted by Kyo on May 22, 2008 at 09:32:51:

I find the following equation to be good practice for my students (both 'O' levels and 'A' levels). Note that this is a simple, non-ionic, non-redox equation.

You know how you learnt at 'O' levels how nitrogen dioxide contributes to acid rain? Well, try writing a balanced equation for the reaction in which nitrogen dioxide dissolves in water to produce the nitric acid. You might find it helpful to be told that one of the two products is dinitrogen monoxide (for 'A' level students, also draw the Kekule structure or displayed structural formula of dinitrogen monoxide, showing all lone pairs and formal charges).

Note : you can easily google the balanced equation, but the point of this question is to teach students the correct (ie. systematic) way to balance this and other equations, instead of the haphazard trial and error method.

I suggest fellow educators (teachers and/or tutors) use this equation to help your students learn the correct (ie. systematic) way of balancing (simple, non-ionic, non-redox) equations.

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Solution :

NO2 + H2O ---> N2O + HNO3

Let coefficient of H2O be x.

NO2 + xH2O ---> N2O + HNO3

Then coefficient of HNO3 becomes 2x.

NO2 + xH2O ---> N2O + 2xHNO3

Let coefficient of N2O be y.

NO2 + xH2O ---> yN2O + 2xHNO3

Then coefficient of NO2 becomes 2x+2y.

(2x+2y)NO2 + xH2O ---> yN2O + 2xHNO3

Looking at oxygen, we have :

2(2x+2y) + x = y + 6x

Simplifying, we get :

x = 3y

Notice that x is the larger value here, and y the smaller value.

Hence, let y be the smallest possible integer, ie. 1.

Consequently, y = 1, x = 3.

Substituting these values, we obtain the balaced equation :

(8)NO2 + (3)H2O ---> (1)N2O + (6)HNO3

This is the systematic way of balancing equations that I teach to my students. I always remind my students, "Algebra is your friend who is here to make your life easier, so use it whenever you can in Chemistry calculations!"

Follow Ups:

• NH3 vs PH3 ; H2O vs H2S - Kyo 06/26/08 (1)
  • Br2 (aq) vs Br2 (CCl4) ; and Methylbenzene vs Phenol - Kyo 07/06/08 (0)

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