Electrolysis Post Lab (Answer Key):
Check your answers with those that follow.
Create a note page titled “Balancing Equations.” Write the notes that follow the lab
answers below. (Last page in this file).
Electrolysis -- Decomposition of Water
Question:
Safety:
What can be learned about water’s structure using an electric current?
1. Wear goggles.
2. Read all directions carefully. 3. Do not plug in power
until teacher approves your set up. 4. Never touch the metal electrodes when
the power is on. 5. Turn off the power before handling the test tubes. 6. Do
not point test tubes at partner when doing the flame tests.
Materials & Setup:
Procedure: Read All Directions Carefully
1. Set up your equipment as instructed. Follow the diagram above. Have your teacher
check your set up.
2. Turn on your power supply until a reaction begins (max). Observe what is happening.
On your lab paper record your observations.
(1) your observations: You should have mentioned that bubbles are coming
off of both electrodes. You may note that electrode A seems to
have a greater quantity of bubbles.
3. While you are waiting for your experiment to run to completion answer the following
questions on your paper.
(2) What is the chemical formula for water? H2O
(3) Is water an element, a compound or a mixture? Compound
(4) Write this sentence & fill in the correct blanks using these terms (each only once):
compound, element, molecule
Atom is to Element, as Molecule is to Compound
(5) Write this sentence & fill in the correct blanks using these terms (you may use terms
more than once.): atom, compound, element, molecule
Each molecule of the compound water contains two atoms of the
element hydrogen and one atom of the element oxygen.
(6) Draw a sketch of a water molecule.
(7) Make a table labeled “Properties” and fill in the following info:
Hydrogen
Oxygen
Water
Solid, liquid or gas
at room temp.?
Flammability
(8) Do compounds have the same properties as the elements that they are made of?
Explain using water as an example. (see last question).
Compounds do not have the same properties as the elements of which they
are made. For example, water is made of hydrogen and oxygen, two
gasses, yet water is a liquid at room temperature. Also, water will
smother fires whereas hydrogen is flammable and oxygen supports
combustion (burning).
4. As soon as one tube has been emptied of water, turn off the power supply.
(9) How full is tube A? FULL Tube B? Half Full
(10) Which tube do you think is hydrogen? Why? Tube A. Because for every
molecule of water that is split we get two hydrogen atoms but only
one oxygen atom. Thus I would expect the tube that fills faster to
be hydrogen.
5. Stopper the test tube which is full of gas before removing it from the water. Keeping it
upside down, rinse the outside of the tube and your hands in fresh water. Perform the
flame test on this tube using a lighted match or glowing splint. Be sure to leave the
stopper in place until the flame is lit and near the tube opening.
(11) What is the gas in tube A? H In tube B? O Where you right? YES!
6. SYNTHESIS OF WATER: : (MUST WEAR GOGGLES!)
Step 1: Gather a mixture of hydrogen and oxygen gases. You will need a large test
tube, stopper and wire electrode from your teacher. Set up as shown below.
WARNING: bottom of test tube must be above the top of the electrodes.
Wear Goggles!
Step 2: Create H2O! When tube is full of gas, disconnect power, stopper the tube and
get permission to perform the flame test.
(12) From a mixture of H & O gasses you created H2O. Write & balance this equation.
H2 + O2
H20 (SEE NOTES BELOW)
(13) Explain why this flame test resulted in a significantly larger explosion.
This tube contained a mixture of hydrogen and oxygen gas. Thus
the hydrogen atoms had oxygen atoms to combine with creating
H2O. This reaction realeases energy. In the tube of pure
hydrogen no explosion took place because there was no oxygen for
the hydrogen to combine with, therefore there was no reaction. We
did get a little “poof” in the hydrogen tube because some hydrogen
combined with oxygen in the air near the mouth of the test tube.
(14) What’s most stable: water, hydrogen or oxygen? _____________
Explain why. Discuss valence electrons, energy levels and bonding (or electron
sharing).
Water is much more stable then pure hydrogen or oxygen because
in their pure forms hydrogen and oxygen do not have full valence
energy levels. When the hydrogen and oxygen atoms share their
valence electrons in the compound water it makes the substance
more stable (less reactive).
NOTES: BALANCING EQUATIONS