Reverse engineering of a dry cell
Andy Cherkas and Brian Dalziel <[email protected]>
Stouffville District Secondary School
183 Bramble Crescent, Stouffville, ON L4A 7Z1
(Take note of an article related to this one on page 10. It goes
into more detail about the chemistry of the dry cell.)
Introduction
carbonic acid.)
Carbon + oxygen Æ carbon dioxide
C(s) + O2(g) Æ CO2(g)
Carbon dioxide + water Æ carbonic acid
It is often better to catch students’ attention by having them work
on materials that they are familiar with. Most students use dry
cells, so they are familiar with them. How do they work? By
taking one apart and analyzing the contents, one can study
conductors, insulators, types of reactions, tests for materials,
catalysts, acids and bases, complex ions, activity series and
redox. Here is an outline of what can be done.
To begin
Take one regular (non-alkaline) dry cell. Place the dry cell in a
vice and cut through the top with a hacksaw. Cut just through
the hard outer shell only, turning the dry cell as you cut through
to make a cut around the top edge. Separate the components
as follows.
•
Rotate the cut end around and pull out the centre post. This
electrode post is a hard, brittle, black solid.
•
Empty the black powder that is inside the cell into a 250-mL
beaker.
•
The inside case is a shiny, silver coloured solid.
What are these materials?
As the dry cell is taken apart you will also find an inner paper
liner, a plastic liner, an outer case, and a metallic top and bottom
piece. Ask students for the purpose of these parts.
The black centre post (electrode)
[Teacher’s note: The electrode is made up of an epoxy carbon
composite. When ignited it will decompose the epoxy, which
liquefies. The liquid burns with an orange flame. This is not
indicative of carbon. What to do? I suggest that you take a
piece of charcoal or a charcoal bricket; break it up into small
pieces, and use these to simulate the post. Tell the students
that you have previously broken up the post material.]
Take a piece of the “electrode”, heat it in a Bunsen burner flame.
Burn the material in a gas bottle filled with oxygen. Add
limewater to the combustion product in the gas bottle. The
limewater turns cloudy indicating that the gas produced is
carbon dioxide. Therefore, the electrode must be carbon. (A
second piece could be burned in a bottle containing oxygen that
has neutral [green] bromthymol blue solution in it. The solution
turns yellow showing the presence of an acid, in this case
8 CHEM 13 NEWS/January 2006
CO2(g) + H2O(l) Æ H2CO3(aq) Æ H+(aq) + HCO3-(aq)
Test for carbon dioxide using the standard test with limewater.
Carbon dioxide + calcium hydroxide Æ calcium carbonate + water
CO2(g) + Ca(OH)2(aq) Æ CaCO3(s) + H2O(l)
(A second verification to show the presence of carbon dioxide
gas is to have the carbon dioxide further react with the
suspension of calcium carbonate in water to form the soluble
calcium hydrogen carbonate. But the only other common gas to
form a white precipitate with limewater is sulfur dioxide. This
gas would be immediately noted by its odour, so the second part
of the test is not really necessary.)
The metallic case
Cut a piece of the metallic case and place it into 3 M HCl(aq).
Then touch a piece of copper metal to it. What happens? What
gas is produced? Collect some of the gas and insert a burning
splint. The gas explodes with a pale blue flame, indicating that
hydrogen gas is produced.
The pure zinc of the metal case reacts slowly with the acid.
Copper will catalyse the reaction. A bit of copper(II) sulfate
added to the acid and zinc will speed the reaction to obtain the
hydrogen. When mossy zinc is used in the lab, impurities in the
mossy zinc (including copper) result in the faster reaction of zinc
with acid that is normally observed. Ask the students why pure
zinc is desirable in the dry cell.
copper
Zinc + hydrochloric acid ⎯⎯ ⎯⎯→ hydrogen + zinc chloride
Cu
Zn(s) + 2 HCl(aq) ⎯⎯
⎯→ H2(g) + ZnCl2(aq)
Hydrogen + oxygen Æ water
2 H2(g) + O2(g) Æ 2 H2O(l)
After the metal has dissolved in the acid, add 3 M NaOH(aq).
A white gelatinous precipitate forms. This precipitate dissolves
in aqueous ammonia solution indicating the metal case is zinc.
The sodium hydroxide neutralizes the acid.
Sodium hydroxide + hydrochloric acid Æ sodium chloride + water
NaOH(aq) + HCl(aq) Æ NaCl(aq) + H2O(l)
Excess sodium hydroxide reacts with the zinc chloride to give
zinc hydroxide.
Sodium hydroxide + zinc chloride Æ sodium chloride + zinc hydroxide
2 NaOH(aq) + ZnCl2(aq) Æ 2 NaCl + Zn(OH)2(s)
The zinc hydroxide precipitate then forms the
tetraamminezinc(II) ion with ammonia and dissolves in the
ammonia solution.
Zinc hydroxide + ammonia Æ tetraamminezinc(II) hydroxide
Zn(OH)2(s) + 4 NH3(aq) Æ [Zn(NH3)4](OH)2(aq)
The black powder
Stir 20 mL of distilled water into the black powder. Stir, decant
the liquid into a filter paper in a funnel and collect the filtrate in a
125-mL flask. Repeat two more times with 20-mL aliquots of
water. Dry the remaining black solid, which is insoluble in water.
Check the clear, colourless filtrate with litmus paper to show it is
acidic. Add a few drops of the liquid to silver nitrate solution to
obtain a white precipitate, indicating the presence of chloride
ions. Evaporate the liquid to obtain a white solid. Mix some of
this white solid with solid calcium hydroxide and heat the mixture
gently. Hold moist red litmus in the gas produced to show it is
basic. Note the odour of ammonia. The white solid is
ammonium chloride.
Silver ions + chloride ions Æ silver chloride solid
Ag+(aq) + Cl-(aq) Æ AgCl(s)
Calcium hydroxide + ammonium chloride
Æ ammonia + calcium chloride + water
Take a carbon electrode and a zinc electrode and place them in
a thick paste of ammonium chloride and manganese dioxide.
What voltage is given? (1.5 V, the voltage of a dry cell.)
The half cell reactions are:
Zn(s) Æ Zn2+ (aq) + 2 e2 NH4+(aq)
0.76 V (oxidation)
+ 2 e-
2 MnO2(s) +
Æ Mn2O3(s) + 2 NH3(aq) + H2O(l)
0.75 V (reduction)
The carbon electrode is an inert electrode for the reduction and
conduction of electricity.
How do we get 6 volt and 9 volt batteries?
Cut one open and you will find four or six cells connected in
series.
Caution!
Alkaline dry cells are very caustic when opened. The centre
post is a brass pin surrounded by cotton soaked in a very
alkaline solution. Instead of ammonium chloride mixed with the
manganese(IV) oxide, potassium hydroxide is used. These dry
cells can damage skin — and the powder, if inhaled, can
damage mucous membranes. These alkaline dry cells should
not be opened.
g
Jarape’s lab
Ca(OH)2(s) + 2 NH4Cl(s)
Æ 2 NH3(g) + CaCl2(s) + 2 H2O(g)
The black solid looks like carbon powder. Heat some to see if it
burns. It does not. Add some to hydrogen peroxide solution.
Test the gas produced with a glowing splint. Note the
production of oxygen gas. What other black solid acts as a
catalyst for the decomposition of hydrogen peroxide?
Manganese dioxide does, which is the identity of the black solid.
manganese dioxide
Hydrogen peroxide ⎯⎯ ⎯ ⎯ ⎯ ⎯ ⎯ ⎯
⎯→ oxygen + water
MnO
2 → O (g) + 2 H O(l)
2 H2O2(aq) ⎯⎯ ⎯
⎯
2
2
If you obtain the mass of the original mixture, and the mass of
the manganese(IV) oxide recovered, a quantitative
determination of the proportions of ammonium chloride to
manganese dioxide can be made. Dry the powdered mixture
before measuring the mass of the powder, since some water is
added to make a thick paste in the dry cell.
Putting it all back together
At one time dry cell kits were available so students could build
one. The source of these for me has disappeared. The building
of a dry cell and its use is very instructive. If anyone finds a
source for the kits, please get them for your students.
(May be copied for non-profit, classroom use.
You can contact JARAPE at: [email protected].)
January 2006/CHEM 13 NEWS 9