Page 220 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 14] OXIDATION AND REDUCTION 209

EXAMPLE 14.21. Complete and balance the following equation in basic solution:
2−
Fe(OH) 2 + CrO 4 −→ Fe(OH) 3 + Cr(OH) 3
Ans. Step 1: Fe(OH) 2 −→ Fe(OH) 3 CrO 4 2− −→ Cr(OH) 3
Step 2: Already done. Already done
−
Step 3: Fe(OH) 2 −→ Fe(OH) 3 + e − 3 e + CrO 4 2− −→ Cr(OH) 3
−
−
+
Step 4: Fe(OH) 2 −→ Fe(OH) 3 + e + H + 5H 3 e + CrO 4 2− −→ Cr(OH) 3
Step 5: H 2 O + Fe(OH) 2 −→ Fe(OH) 3 + e + H + 5H + 3 e + CrO 4 2− −→ Cr(OH) 3 + H 2 O
+
−
−
Step 6: 3 H 2 O + 3Fe(OH) 2 −→ 3Fe(OH) 3 + 3 e + 3H +
−
+ − 2− − +
3H 2 O + 3Fe(OH) 2 + 5H + 3 e + CrO 4 −→ 3Fe(OH) 3 + 3 e + 3H + Cr(OH) 3 + H 2 O
+ 2−
2H 2 O + 3Fe(OH) 2 + 2H + CrO 4 −→ 3Fe(OH) 3 + Cr(OH) 3
To eliminate the H , which cannot exist in basic solution, add 2 OH to each side, forming 2 H 2 O on the left:
−
+
2− −
2H 2 O + 3Fe(OH) 2 + 2H 2 O + CrO 4 −→ 3Fe(OH) 3 + Cr(OH) 3 + 2OH
Finally,
2− −→ 3Fe(OH) 3 + Cr(OH) 3 + 2OH −
4H 2 O + 3Fe(OH) 2 + CrO 4
14.6. ELECTROCHEMISTRY
The interaction of electricity with matter was introduced in Chap. 8, where the electrical decomposition of
a melted salt was used to prepare active elements from their compounds. An illustration of an electrolysis was
shown in Fig. 5-2. Chemical reactions occur at the two electrodes. The electrode at which oxidation occurs is
called the anode; the one at which reduction takes place is called the cathode. Electricity passes through a circuit
under the influence of a potential or voltage, the driving force of the movement of charge. There are two different
types of interaction of electricity and matter, as follows:
Electrolysis: Electric current causes chemical reaction.
Galvanic cell action: Chemical reaction causes electric current, as in the use of a battery.

Electrolysis
The following are the requirements for electrolysis:

1. Ions (There must be charged particles to carry current. It might not be the ions that react, however.)
2. Liquid, either a pure liquid or a solution, so that the ions can migrate
3. Source of potential (In a galvanic cell, the chemical reaction is the source of potential, but not in an electrolysis
cell.)
4. Mobile ions, complete circuit (including wires to carry electrons), and electrodes (at which the current
changes from the flow of electrons to the movement of ions or vice versa)

If you electrolyze a solution containing a compound of a very active metal and/or a very active nonmetal, the
water (or other solvent) might be electrolyzed instead of the ion. For example, if you electrolyze molten sodium
chloride, you get the free elements:

electricity
2 NaCl(l) −−−→ 2Na(l) + Cl 2 (g)
However, if you electrolyze a dilute aqueous solution of NaCl, the water is decomposed. (The NaCl is necessary
+
to conduct the current, but neither Na nor Cl reacts at the electrodes.)
−
electricity
2H 2 O −−−−−→ 2H 2 + O 2
NaCl(dilute)