3B Scientific Hoffmann Electrolysis Apparatus User Manual
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PHYSICSPHYSICS
U14332 Hofmann water-decomposition apparatus
Instruction sheet
11/03 ALF
®
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The purpose of the water-decomposition is for the electrolysis of water (converting electrical energy into
chemical energy), quantitative determination of the
resulting gases and confirmation of Faraday’s laws.
1. Safety instructions
• Since the conductivity of distilled water is too low,
electrolysis is carried out using dilute sulfuric acid
(c =1 mol/l approx.).
• Carefully add the sulfuric acid to the water while
stirring. Never do this the other way round.
• Wear protective goggles when mixing the solution
and when releasing the gases.
• Students should always be informed of the dan-
gers of the chemicals needed for the experiment.
• Caution. Any acid that escapes can cause irrepa-
rable stains and holes in clothing.
• Be careful when taking the glass tubing off its se-
curing plate.
• Do not subject the glass components of the water-
decomposition apparatus to mechanical stress.
2. Description, technical data
The water-decomposition apparatus consists of an Hshaped section of glass tubing attached to a securing
plate fixed to a stand rod that rests on a base-plate.
The glass section involves two gas collection tubes each
1 Base-plate with stand rod
2 Platinum electrodes
3 GL-18 screw fitting
4 GL-14 screw fitting
5 Gas collection tubes
6 Securing plate
7 Ground stopcock
8 Plastic hose
9 Stand ring
bl Leveling bulb
with a measuring scale. At the top of each tube there
is a ground stopcock. Two platinum electrodes are secured at the lower ends via GL-18 screw fittings. A flexible plastic hose leads to a leveling bulb for equalising
the pressure in the collection tubes.
Dimensions:
Water-decomposition apparatus:
Height: 800 mm approx.
Width: 150 mm
Base-plate: 250 mm x 160 mm
Rod: 750 mm x 12 mm Ø
Securing plate: 120 mm x 110 mm
Gas collection tubes:
Height: 510 mm
Width: 150 mm
Tube diameter: 19 mm
Scale: 50 ml each with 0.2 ml divisions
Leveling bulb:
Volume: 250 ml
2.1 Scope of delivery:
1 Glass section with gas collection tubes
1 Base-plate with stand rod and securing plate
1 Pair of platinum electrodes with 4-mm sockets
1 Leveling bulb with plastic hose
1 Stand ring to hold the leveling bulb
1 Universal sleeve
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2.2 Spares
n
Q
eNpVRT
mol
L
=
⋅
=
⋅
⋅2
FeN
QRT
pV
Cmol
L
=⋅ =
⋅⋅
⋅⋅
=296500 / .
U14333 Gas collection tubes
U14334 Pair of platinum electrodes
U14335 Leveling bulb, 250 ml
3. Theory
Unlike metallic conductors, where current is carried
by electrons, current in electrolytes is transported via
ions.
In water to which sulfuric acid has been added the fol-
–
lowing ions are present: HSO
4
2–
, SO
and H3O+. When
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a voltage is applied, ions begin to move and the water
is electrolyzed. This leads to the liberation of hydrogen and oxygen gas. At the cathode (the negative pole)
two 2 H
O+ ions combine to form an H2 molecule. At
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the anode (positive pole) O2 is formed. The sulfuric acid
remains unchanged and acts solely as a catalyst for
the electrolysis of water.
The charge Q transported between the electrodes during electrolysis can be calculated from the current Ι
and the duration of the electrolysis t by means of the
following equation:
Q = Ι · t.
If an ion has a charge of z times the charge on an electron e, then Q/ze ions are released.
For H3O+ z = 1 so that Q/2e H2 molecules are produced.
2 ions are needed to produce one molecule. To release
n moles of H
therefore requires a charge
2
Q = 2e · N
L
· n
where NL is the Loschmidt or Avogadro number that
represents the number of molecules per mole
(N
= 6.0 · 1023/mol).
L
If n and Q are known, the equation can be used to find
the Faraday constant F, which is the product of the
two fundamental constants, the charge on an electron
and the Avogadro number:
F = e · NL ~ 105 C/mol
The number n of moles released can simply be determined from the volume.
The gas law
p · V = n · R · T,
summarizes the relationship between pressure p, volume V , temperature T and the number of moles n.
The temperature T in Kelvin can easily be determined
from the temperature in Celcius tc (T = tc + 273 K). R is
the universal gas constant and takes the value
R = 8.3 J mol–1K
–1
(joules per mole per Kelvin).
A charge Q produces Q/2e H2 molecules at the cathode. If the Avogadro number NL = 6 · 1023/mol, we
then obtain from
a value for the Faraday constant of
3. Example experiments
3.1 Investigation of the conductivity and
composition of water
Required equipment:
Water-decomposition apparatus
Voltage supply (e.g. U11760 AC/DC power supply)
Connecting leads
Distilled water
Dilute sulfuric acid
Experiment procedure:
• Set up the experiment according to Figure 1.
• Pour distilled water into the leveling bulb with both
stopcocks open.
Fill the gas collection tubes completely by altering
the height of the leveling bulb.
• Close the glass stopcocks. The water level in the
leveling bulb should be higher than that in the collection tubes.
• Check the apparatus for leaks and tighten connec-
tions where necessary.
• Turn on the power supply and observe the elec-
trodes.
• Since there is no perceptible reaction, turn the
power supply off again.
• Add a few drops of dilute sulfuric acid (c = 1 mol/l
approx.).
• After waiting for about 5 minutes, switch on the
power supply again.
• Gas bubbles should rise from both electrodes.
• When the gas collection tube at the negative pole
(cathode) is half filled with gas, turn off the power
supply.
• To achieve a precise reading of the gas volumes,
lower the leveling bulb until the water in the bulb
is level with that in the tube to be measured.
• Release the gases through the stopcocks and col-
lect them in upturned test tubes.
• Demonstrate the presence of hydrogen by the pop
test and the presence of oxygen using a glowing
splint.
Result:
• Electrolysis does not take place when distilled wa-
ter is used on its own.
• Addition of dilute sulfuric acid has a catalytic ef-
fect so that the distilled water is electrolyzed into
its two components, hydrogen and oxygen.
• The volume of hydrogen gas formed at the cath-
ode is twice the volume of the oxygen gas formed
at the anode.
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