Chemistry Reference
In-Depth Information
E5.10: Electrolysis of Sodium Hydroxide Solution
Problem: The electrolysis of melts has been interpreted with the formation of metal
in E5.9. In E5.10 it is to be demonstrated that during the electrolysis of sodium
hydroxide solution, hydrogen forms at the cathode, instead of sodium metal, and
oxygen forms at the anode. In this context you can speak of the decomposition of
water and of a special “decomposition apparatus,” because hydrogen and oxygen
are generated in a volume ratio of 2:1. This apparatus can also be used for all other
electrolyses that produce gases at both electrodes.
Material: Decomposition apparatus with platinum electrodes, direct current voltage
source (10 V), cable, test tubes, wooden splint; diluted sodiumhydroxide solution (C).
Procedure: The apparatus is to be filled with sodium hydroxide solution and
connected to the direct current voltage source. The electrolysis is to be run with
5-10 V. The electrolysis is finished, when the cathode cylinder (minus pole) is filled
with gas. The volume in the electrode cylinders is to be measured. A test tube is to
be put over the cylinder of the minus pole. The tap is to be opened so that gas can
escape from the cylinder and be collected in the test tube for a combustibility test.
The tap of the plus pole cylinder is to be opened and a glowing wooden splint is to
be held above the opening. Afterwards the taps are to be closed again.
Observation: Gases emerge in the volume ratio of 2:1. The first gas burns with a
bang: hydrogen. The second gas reignites a glowing wooden splint: oxygen.
E5.11: Sodium Hydroxide on the Balance
Problem: By observing an increasing mass of sodium hydroxide on the balance,
students are motivated to explain this unknown phenomenon. Because sodium
hydroxide reacts becomes moist on the watch glass, the assumption follows that
the reaction with water vapor causes the increasing weight. One has to introduce the
term “hygroscopic” for substances that extract water vapor from humid air.
Material: Digital balance with overhead display, watch glass; sodium hydroxide
(pellets).
Procedure: The digital balance is to be switched on. The watch glass is to be placed
on the balance; the displayed mass does not change. About 20 sodium hydroxide
pellets are to be placed on the watch glass, the new mass is to be measured and
observed for 5 min.
Observation: The mass of sodium hydroxide increases by a few hundred
milligrams. The longer you wait, the higher is the mass; the pellets become moist
on the watch glass.
E5.12: Reactions of Sodium Hydroxide with Air
Problem: The reaction of sodium hydroxide with water has been shown in E5.11. But
it still needs to be clarified, whether other components of air react as well. Students
know the components of air, such as oxygen, nitrogen and carbon dioxide. The ques-
tion arises, whether these gases as well as water vapor react with sodium hydroxide.
The students can create an apparatus independently that makes it possible to answer
this question. One way: fill the gases into gas syringes and connect them gas-tight to
test tubes containing some sodium hydroxide pellets. A high volume decrease can
only be observed in the case of carbon dioxide: sodium hydroxide reacts to form solid
sodium hydrogen carbonate in the form of small white crystals.
