PAG 2: Electrolysis

Question 1

Safety

Answer 1

Copper(II) chloride solution– IRRITANT
Oxygen gas – OXIDISING
Hydrogen gas – EXREMELY FLAMMABLE
Chlorine gas – TOXIC
The electrolysis of brine produces a solution of sodium hydroxide, which is corrosive.
Wear chemical splash proof eye protection and if you are collecting a gas by displacement, wear nitrile gloves and only complete the practical in a well-ventilated room. Take extra care if you are asthmatic

Question 2

What PAG says - spec

Answer 2

• PAG C2 – Electrolysis of aqueous sodium chloride solution or aqueous copper sulfate solution testing for the gases produced

Question 3

Electrolysis of aqueous Copper(II) sulfate solution - method

• 6 points

Answer 3

1. Pour some copper(II) sulfate solution into a beaker.
2. Place two graphite rods into the copper sulfate solution. Attach one electrode to the negative terminal of a dc supply and the other electrode to the positive terminal.
3. Use a dropping pipette to completely fill two small test tubes with copper(II) sulfate solution. Quickly turn each test tube upside down, keeping its mouth under the surface of the copper(II) sulfate solution. Position a test tube over each electrode. Rinse any copper sulfate solution off your hands.
4. Make sure the electrodes do not touch each other, then turn on the power supply. If no bubbles are observed, check all the electrical connections and the dc supply.
5. Continue to collect any gases in the test tubes, then turn off the dc supply.
6. Oxygen should be produced at the positive electrode. Confirm this by holding a glowing splint just inside the open mouth of its test tube. Record whether the splint relights.

Question 4

Electrolysis of sodium chloride solution

Answer 4

1. Pour some sodium chloride solution into a beaker.
2. Place two graphite rods into the sodium chloride solution. Attach one electrode to the negative terminal of a dc supply and the other electrode to the positive terminal.
3. Use a dropping pipette to completely fill two small test tubes with sodium chloride solution. Quickly turn each test tube upside down, keeping its mouth under the surface of the sodium chloride solution. Position a test tube over each electrode. Rinse any sodium chloride solution off your hands.
4. Make sure the electrodes do not touch each other, then turn on the power supply. If no bubbles are observed, check all the electrical connections and the dc supply.
5. Continue to collect any gases in the test tubes, then turn off the dc supply.
6. Hydrogen should be produced at the negative electrode. Confirm this by holding a lighted splint near the open mouth of its test tube. Record whether a squeaky pop is obtained.
7. Chlorine should be produced at the positive electrode. Confirm this by holding a piece of damp blue litmus paper just inside the open mouth of its test tube. Record whether this turns red then white.

Question 5

Results + analysis

Answer 5

Record what happens at each electrode, including the results of the gas tests.

Analysis

Identify the gases produced during the two experiments.

Question 6

Name the substance produced at the negative electrode during the electrolysis of copper(II) sulfate.

Answer 6

The (brown) substance was copper.

Question 7

Suggest an explanation for why it may be difficult to obtain positive results in the tests for gases.

Answer 7

The volumes of gas collected are very small, making it difficult to carry out the tests.