Illustrated Guide to Home Chemistry Experiments

Chapter 16 Laboratory: Electrochemistry 291

In the preceding lab session, we used an

external electrical current to force a non-

spontaneous redox reaction to occur. But

many redox reactions occur spontaneously,

without requiring an external energy source

to initiate or sustain them. In this lab, we’ll

examine a very familiar spontaneous redox

reaction, the oxidation of iron-to-iron oxide,

otherwise known as rusting.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ test tubes (6)

£ beaker, 150 mL

£ stoppers or corks for test tubes (2)

£ test tube rack

£ ring stand

£ support ring

£ wire gauze

£ alcohol lamp or burner

£ beaker tongs

£ spatula

£ steel wool or sandpaper

£ marking pen

£ common iron or steel nails (ungalvanized), 6d to 12d (6)

£ sodium chloride, solid (~5 to 10 g)

£ sodium chloride, 1 m (~100 mL)

£ distilled or deionized water (~100 mL)

Although rusting occurs spontaneously, it happens only when
specific conditions exist; namely, the presence of oxygen and
water. The balanced equation for this electrochemical reaction
shows why:

4 fe(s) + 6 H 2 o(l) + o 2 (g) → 2 fe 2 o 3 · 3H 2 o(s)

In the absence of oxygen or water, the reaction cannot proceed.
And, although these conditions are both necessary and sufficient
for the reaction to occur, the reaction rate varies with the
presence or absence of electrolytes. Because the current flow
inherent to an electrochemical reaction involves the migration
of ions, the presence of an electrolyte, such as sodium chloride
(common salt) or another ionic salt, increases the reaction rate.
That’s why, for example, iron and steel rust much faster near
the ocean than they do in drier areas far from salt air, and why
automotive rust is a major problem in areas where roads are
salted during winter weather.

In this lab session, we’ll examine the rusting of iron. We’ll
expose clean iron surfaces to oxygen, water, and salt, alone or
in any combination, and observe the effects of each of these
environments on the reaction rate.

LABORATORY 1 6.2:

oBSERvE THE ELECTRoCHEmICAL oxIdATIoN of IRoN

CUTIOA nS

This experiment uses flame. Be careful with the flame, have

a fire extinguisher readily available, and use care in handling

hot objects (a hot beaker looks exactly like a cold beaker).

Wear splash goggles, gloves, and protective clothing.

z

POCEDURER

1.ou have not already done so, put on your splash If y

goggles, gloves, and protective clothing.

2. Label six clean, dry test tubes A through F and place them
   in the test tube rack.


3. Use the steel wool or sandpaper to polish six iron or steel
   nails until they are bright and shiny. Make sure to clean
   the entire nail, including the head, shank, and tip.


4. Place one of the nails in test tube A and return the test
   tube to the rack.


5. Fill test tube B with tap water to just below the rim and
   insert the stopper. The goal is to determine the water
   level needed to allow the stopper to seat, leaving only
   water in the test tube, with no air bubble. When you have
   determined how much water is needed, mark the level on
   the outside of the tube and empty the tube.