The Foundations of Chemistry

Aluminum is oxidized; it is the reducing agent. H 2 O is reduced; it is the oxidizing agent.

OH
H 2 O
Al88n[Al(OH) 4 ]
H 2


 3

 1

We make the totalincrease and decrease in oxidation numbers equal.

Oxidation Numbers Change/Atom Equalizing Changes Gives

Al 0 88nAl
 3 
31(
3)
 3

H
 1 88nH 0 13(1) 3

Each change must be multiplied by two because there are 2 H in each H 2.

2(
3)
6 (total increase) 2(3)6 (total decrease)

Now we balance the redox part of the equation. We need 2 Al on each side. Because only one
H in each H 2 O molecule is reduced (the other H is in OH), we show 6 H 2 O on the left and
3 H 2 on the right.

OH
 6H 2 O 
 2Al 88n 2[Al(OH) 4 ] 
 3H 2

The net charge on the right is 2, and so we need 2 OHon the left to balance the net ionic
equation.

2OH(aq)
6H 2 O(
)
2Al(s)88n2[Al(OH) 4 ](aq)
3H 2 (g)

This reaction occurs in excess NaOH solution. We need 2 Na
(aq) on each side to balance
the negative charges.

2NaOH(aq)
6H 2 O(
)
2Al(s)88n2Na[Al(OH) 4 ](aq)
3H 2 (g)

You should now work Exercise 58.

EXAMPLE 11-21 Balancing Redox Equations (CON method)

The breathalyzer detects the presence of ethanol (ethyl alcohol) in the breath of persons
suspected of drunken driving. It utilizes the oxidation of ethanol to acetaldehyde by dichro-
mate ions in acidic solution. The Cr 2 O 72 (aq) ion is orange (see Example 11-23). The Cr^3
(aq)
ion is green. The appearance of a green color signals alcohol in the breath that exceeds the
legal limit. Balance the net ionic equation for this reaction.

H
(aq)
Cr 2 O 72 (aq)
C 2 H 5 OH(
)88nCr^3 
(aq)
C 2 H 4 O(
)
H 2 O(
)

Plan

We are given the unbalanced equation, which includes H
. This tells us that the reaction occurs
in acidic solution. We apply our five-step procedure.

Solution

We first assign oxidation numbers to the elements that change.

H

C 2 H 5 OH
Cr 2 O 72 88nCr^3 

C 2 H 4 O
H 2 O(
)

Many breathalyzers have been

developed over the years. This

model has proved to be effective.

11-7 Change-in-Oxidation-Number Method 423

8 n

888888 888n

(^888888)
88888888888888888888888888888
8888888888

1 0
(^30)
 2
6
3  1