Question
Mon January 09, 2012 By:

Balance the following redox reaction by oxidation number method: Zn + HNO3 --> Zn(NO3)2 + NO + H20

Expert Reply
Tue January 10, 2012

1.    The zinc atoms are changing their oxidation number from 0 to +2, and some of the nitrogen atoms are changing from +5 to +2.

2.    Determine the changes in oxidation number for the atoms oxidized and reduced.

            Zn = 0 to +2;   Net Change = +2

            Some N = +5 to +2;   Net Change = -3

3.    We need three Zn atoms (net change of +6) for every 2 nitrogen atoms that change (net change of -6).

4.     Because some of the nitrogen atoms are changing and some are not, place the 2 as coefficient in front of the NO (g) on the product side.

5.    The 3 for the zinc atoms is placed in front of the Zn(s).

     3Zn(s) + HNO3(aq)  -->  Zn(NO3)2(aq) + 2NO(g) + H2O(l)

6.    Balance the rest of the atoms but keep the ratio of Zn to NO as 3:2.

3Zn(s) + 8HNO3(aq) -->  3Zn(NO3)2(aq) + 2NO(g) + 4H2O(l)

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