Example #1

Example #1

EXAMPLE #1 Balancing Redox Reactions Using the Oxidation Number Method

Balance the following redox equation using either the inspection technique or the oxidation number method. Be sure to check that the atoms and the charge are balanced.

      HNO3(aq) + H3AsO3(aq)  -->  NO(g) + H3AsO4(aq) + H2O(l)


Step #1:  Try to balance the atoms by inspection.

The H and O atoms are difficult to balance in this equation. You might arrive at the correct balanced equation using a trial and error technique, but if you do not discover the correct coefficients fairly quickly, proceed to Step #3.

Step #3:  Is the reaction redox?

The N atoms change from +5 to +2, so they are reduced. This information is enough to tell us that the reaction is redox. (The As atoms, which change from +3 to +5, are oxidized.)

Step #4:  Determine the net increase in oxidation number for the element that is oxidized and the net decrease in oxidation number for the element that is reduced.

As  +3 to +5     Net Change = +2

N  +5 to +2      Net Change = -3

Step #5:  Determine a ratio of oxidized to reduced atoms that would yield a net increase in oxidation number equal to the net decrease in oxidation number.

As atoms would yield a net increase in oxidation number of +6. (Six electrons would be lost by three arsenic atoms.) 2 N atoms would yield a net decrease of -6. (Two nitrogen atoms would gain six electrons.) Thus the ratio of As atoms to N atoms is 3:2.

Step #6: To get the ratio identified in Step 5, add coefficients to the formulas which contain the elements whose oxidation number is changing.

2HNO3(aq) + 3H3AsO3(aq)  
                 -->  NO(g) + H3AsO4(aq) + H2O(l)

Step #7:  Balance the rest of the equation by inspection.

2HNO3(aq) + 3H3AsO3(aq)   
                   -->  2NO(g) + 3H3AsO4(aq) + H2O(l)

Example 2

Example 3

Click here to see an exercise that will allow you to try this task yourself. 

Return to the Balancing Redox Equations Page.


Home ] Up ] Study Sheet ] [ Example #1 ] Example #2 ] Example #3 ] Exercise ]