Disappearing Water: The Electrolysis of H2O
Background:
In the wonderful world of chemistry there are many different types of chemical reactions (synthesis, decomposition, replacement reactions, etc). Today you will take H2O and pass a 6 volt electrical current (the activation energy needed to cause the reaction to occur) through it.
This will cause the water in the test tubes to seemingly disappear into thin “air”.
The chemical equation for this chemical reaction is:
The following information will be helpful for the setup and understanding of the experiment:
· Electrode: a metal used to carry electrons (conduct electricity). There are two in this lab.
· Anode Connection: Positive electrode
· Cathode Connection: Negative electrode (the black cat is negative)
· Electrolysis: a method used to break the chemical bond between the hydrogen and oxygen in water through the use of an electrical current.
· Electrolyte: a chemical substance (metal ion usually) that allows an electric current to better flow though water.
· Combustible: ability of a substance to burn in the presence of oxygen and heat.
Purpose:
To collect the products that form when an electrical current is passed through water (H2O) mixed with a salt (sodium sulfate (Na2SO4) or sodium chloride (NaCl)) to form an electrolytic solution. The salt is not part of the reaction (acts king of like a catalyst). It is only used to help water better conduct electricity and speed up the formation of the gases in the test tubes.
Hypothesis:
Develop a hypothesis for which gas will collect in which of the two test tubes and which test tube will have the largest volume of gas at 16 minutes. Include your reason for your hypothesis.
Gas in Tube #1: ______(Anode) Gas in Tube #2: ______(Cathode)
Data:
Quantitative Data:
Data Table 1 – Volume of Gases Produced
Time(minutes) / Volume of Tube #1
(mL) / Volume of Tube #2
(mL)
2
4
6
8
10
12
14
16
Qualitative Data:
Physical Properties – Color of reactants and products and state (solid, liquid, gas).
· Water: ______
· Hydrogen: ______
· Oxygen: ______
Chemical Properties (use burning/glowing wooden splint to conduct this test)-
· Hydrogen: ______
· Oxygen: ______
Graph:
· Construct a graph that shows the volume of hydrogen and oxygen gas vs. time. Remember to include the following: a color-coded key, title, units, labels the correct scale. Remember to use the SULTAN method to check your graph.
Graph Analysis:
1. Were the results shown in the graph what you expected? Explain.
2. Error analysis: What could be some reasons why the ratio of Hydrogen to Oxygen atoms was not exactly 2 to 1?
3. Use your graph to predict how long it would take for all the test tube containing the oxygen to completely fill.
Scientific Drawing:
· Create a colored drawing of the experimental design, correctly labeled, with a short description of what is shown. Diagram should also show the flow of energy from anode to cathode using arrows. Please include the following concepts in the drawing: (Anode (+), cathode (-), electrodes, electrolyte solution, test tube #1 & #2, wires (red & black), and container (plastic bath that holds water).
Conclusion Questions: Answer the following questions and explain each answer.
1. Which gas is attracted to the positive electrode? Why?
2. Which gas is attracted to the negative electrode? Why?
3. How much hydrogen should you have theoretically collected compared to oxygen? Why was it not perfect? Do some research and explain possible sources of error.
4. How did you chemically test to see which test tube had hydrogen which had oxygen gas?
5. If some students decided one day to connect a lot of batteries to all the water in the ocean would we be in danger of turning all ocean water into oxygen and hydrogen? Explain.
6. I performed this experiment last year. I attached the batteries and came back about 30 minutes later. To my surprise one test tube was completely filled with one of the gases, but the other test tube still had some water in it, but the bubbles had stopped forming in both. I checked to see that all the wires were still connected and they were. What caused the reaction to stop? Come up with a logical conclusion or a way to test what stopped the rxn.
CLASS COPY: DO NOT SYNTHESIS, DECOMPOSE OR INVOLVE IN A REPLACEMENT RXN.