The Foundations of Chemistry

The oxidation half-reaction involves three electrons, and the reduction half-reaction involves

two electrons. We balance the electron transfer and add the half-reactions term by term.

2(CrO 2 
4OH88nCrO 42 
2H 2 O
 3 e)

3(ClO
H 2 O
 2 e88nCl
2OH)

2CrO 2 
8OH
3ClO
3H 2 O88n2CrO 42 
4H 2 O
3Cl
6OH

We see 6 OHand 3 H 2 O that can be eliminated from both sides to give the balanced net

ionic equation.

2CrO 2 (aq)
2OH(aq)
3ClO(aq)88n2CrO 42 (aq)
H 2 O(
)
3Cl(aq)

You should now work Exercise 58.

For brevity, we will refer to this as the
CON method.

420 CHAPTER 11: Reactions in Aqueous Solutions II: Calculations

Problem-Solving Tip:Converting Ionic to Formula Unit Equations

We learned in Section 4-3 how to convert the formula unit equation to the net ionic

equation. To do this, we convert the formulas for all strong electrolytesinto their ions, and

then cancel spectator ionsfrom both sides of the equation. In Example 11-16 we reverse

this procedure. To balance this excess charge, we must add negatively charged spectator

ions to combine with the positively charged reactants, and we add positively charged

spectator ions to combine with the negatively charged reactants. Any spectator ions added

to the reactant side of the equation must also be added to the product side. Then we

combine species to give complete formula units. Now we can write total ionic and formula

unit equations for Exercise 11-17 if we know what spectator ions are present. Just for

practice, consider the spectator ions to be Na
.

CHANGE-IN-OXIDATION-NUMBER METHOD

The next few examples illustrate this method, which is based on equal total increases and

decreases in oxidation numbers.The general procedure follows.

1.Write as much of the overall unbalancedequation as possible.

2.Assign oxidation numbers (Section 4-4) to find the elements that undergo changes

in oxidation numbers.

3. a.Draw a bracket to connect atoms of the element that is oxidized. Show the
   increase in oxidation number per atom.Draw a bracket to connect atoms of the
   element that is reduced. Show the decrease in oxidation number per atom.
   b.Determine the factors that will make the totalincrease and decrease in oxidation
   numbers equal.

11-7

These common household
chemicals, vinegar and bleach,
should never be mixed because they
react to form chlorine, a very
poisonous gas.

2H
(aq)
ClO(aq)
Cl(aq)88n
Cl 2 (g)
H 2 O(
)