Spring 2014 S6 – Electrolysis (Fresh vs Salt water)
Background
A water molecule is made of two hydrogen (H) atoms and one oxygen atom (O).
In order to break up a water molecule into the separate elements, we must
introduce energy. We can do that with electricity.
In the electrolysis setup, the two pencils are electrodes that conduct electricity
from the batteries. The negative electrode is called the cathode, and the
positive electrode is called the anode. As negatively-charged electrons flow out
of the cathode, they connect with positively-charged hydrogen ions in the water
to form hydrogen gas. This is called a reduction reaction because the atoms
gain negatively-charged electrons. The equation for this reduction reaction at the
cathode is: 2e- + 2 H+  H2
The opposite of a reduction reaction is an oxidation reaction, where electrons
are taken away from water molecules to form oxygen gas. The excess electrons
travel into the anode and back to the voltage source (in this case, a battery).
The equation for the oxidation reaction occurring at the anode is: 2 H2O  O2 +
4H+ + 4eCombining these two half-reactions and balancing them so that there are the
same number of atoms and electrons on each side of the arrow, the overall
reaction is: 4e- + 2 H2O  2 H2 + O2 + 4e- or 2 H20  2H2 + O2
You can see in this reaction that, under ideal circumstances, for every molecule
of oxygen, you should also have two molecules of hydrogen. You should see a
rough approximation of this in your experiment, where you will produce about
twice as much hydrogen gas from the cathode as you will oxygen gas from the
anode.
Pure water is a fairly good insulator, preventing a reaction without a lot of
electricity. By adding an electrolyte to the water, we make it much more
conductive, and able to produce more gas with less energy. We use baking soda
because it is a simple and safe electrolyte, and the nature of the baking soda
molecules do not interfere with the reaction.
Using a different electrolyte such as table salt (NaCl), introduces chlorine ions
(Cl-) that can outcompete the oxygen ions (O-) for the oxidation reaction. If this
happens, chlorine gas will be released instead of oxygen!
From: http://www.eastbaystem.net/wp/wpcontent/uploads/2013/04/Electrolysis-Of-Water.pdf
Experiments
I) Water model
Teaching goals:
-Water is made from different compounds (hydrogen and oxygen) that can be
broken apart.
Materials:
-Marshmallows big and small
-Toothpicks
Instructions:
Big marshmallow will represent oxygen, smaller marshmallows will represent
hydrogen. Give the kids a bunch of marshmallows and toothpicks and make
them put together a rigid ball-and-stick model of a water molecule: H2O.
Questions:
-Why can’t the two hydrogens go to the same side?
-How can two water molecules bind to form water? (through hydrogen bond)
-Talk about strength of hydrogen bonds. Compare them with stronger bonds
such as ionic bonds (metal). What properties do they confer?
-Make them break their model. Talk about when you break something you need
energy. Connect that with water  We will use electricity to break water
molecules!
II) Electrolytic cells
Teaching goals:
-Forming water releases energy.
-Breaking water/bonds needs energy.
Materials:
Alligator clips
Clear plastic cups
Pencils
Tape
Cardboard
9-volt battery
Wires
Solution mixes:
Salt
Sugar
Soap
Baking soda
Lemon juice
Baking soda, known by chemists as sodium bicarbonate (NaHCO3), isn’t an
electrolyte on its own. Remember—it needs to be in a solution! When dissolved
in water, it lets current flow through the water between the terminals of the
battery. In the water-baking soda solution, the gases that are produced are
hydrogen (H2), oxygen (O2) and carbon dioxide (CO2).
Table salt, or sodium chloride (NaCl), is also a good additive to form
electrolytes. In water, salt actually splits into Na+ and Cl- ions, which are very
good at carrying current, or the flow of electric charges. In the water-salt
solution, the gases that are produced are hydrogen, oxygen and chlorine gas
(Cl2)—you don’t want to inhale this stuff!
Lemon juice is a decent electrolyte because it is an acid. The acid dissociates in
water and the current will be carried by the ions. Distilled water on its own will
not carry any current. However, tap water is often able to conduct current
because of minerals and impurities (Don’t worry! These things are not bad for
you!).
Different amounts of gas are produced from each reaction because each reaction
will carry a different amount of current. The amount of gas produced is directly
proportional to the current that flows in the system.
Instructions:
1.
2.
3.
4.
5.
6.
7.
8.
Sharpen two pencils at both ends.
Put pencils through the cardboard.
Tape end of wires to battery.
Put the other end of the wires into alligator clips.
Clip alligator clips onto pencils.
Put pencils in cup filled with water + other things.
You should see bubbles!
Add different compounds. Do you see more or less bubbles?
In a perfectly efficient system, we should see more Hydrogen because we have
2. But that’s difficult.
Questions:
Why did speed of bubble formation changed when we added different
components?