❓3. Required Practical: Predicting The Products of Electrolysis Flashcards
Aim Of This Practical
The aim of this practical is to make accurate predictions and observations about the products formed at the electrodes during the electrolysis of ionic compounds dissolved in aqueous solution.
Hypothesis for What will happen at each electrode
At the Negative Electrode:
At the cathode, bubbles of hydrogen gas is produced unless the metal is LESS reactive than hydrogen i.e. copper and silver.
At the Positive Electrode:
At the anode, oxygen is produced unless the solution contains halide ions when the halogen is produced, e.g Cl^-, Br^-, I^- which will be discharged.
Predictions For Product At Positive and Negative Electrode:
- Sodium Chloride
- Copper (II) Chloride
- Sodium Sulphate
- Copper (II) Sulphate
Negative Electrode (Cathode):
Sodium Chloride - Hydrogen Gas (bubbles of a colourless gas)
Copper (II) Chloride - Copper (brown coating on the electrode)
Sodium Sulphate - Hydrogen Gas (bubbles of a colourless gas)
Copper (II) Sulphate - Copper (brown coating on the electrode)
Positive Electrode (Anode):
Sodium Chloride - Chlorine Gas (bubbles of a gas that bleach damp litmus paper)
Copper (II) Chloride - Chlorine Gas (bubbles of a gas that bleach damp litmus paper)
Sodium Sulphate - Oxygen Gas (bubbles of a gas that do not bleach damp litmus paper)
Copper (II) Sulphate - Oxygen Gas (bubbles of a gas that do not bleach damp litmus paper)
Health and Safety Measures
Do not leave the cells running for more than 5 minutes each.
- Small amounts of chlorine may be produced which is toxic – do this in a well ventilated room.
- Copper compounds are harmful.
- Use low voltage (no higher than 6 V)
The practical should be carried out in a well-ventilated laboratory as significant amounts of toxic chlorine can be produced, as well as highly flammable hydrogen. Take care if you are asthmatic.
Equipment for this Practical
- 1 beaker with electrodes
- retort stand and clamp to hold electrodes
- DC power supply (6 V)
- 2 x leads
- 2 x crocodile clips
- 1 mol/dm3 copper(II) chloride
- 1 mol/dm3 copper (II) sulfate
- 1 mol/dm3 sodium sulfate
- blue litmus paper
Method for this Practical
- Set up apparatus as shown in picture. Clean the electrodes lightly using emery paper.
- Half fill a 100 cm3 beaker with one of the solutions.
- Connect the circuit as shown in the picture. The light bulb is present to check that the circuit is working correctly.
- Look for bubbling at each electrode. If there is bubbling at the positive electrode, test the gas by holding a piece of litmus paper in the solution at that electrode using tweezers.
- If there is bubbling at both electrodes and you have identified both gases, turn off the cell and start with the next solution.
- If there is not bubbling at both electrodes, then leave the cell connected for 5 minutes and then disconnect and study the electrodes for evidence of a metal coating.
Sodium Chloride Observations and Element formed at each Electrode
Negative Electrode (Cathode)
Observations - Bubbles of gas that bleach litmus paper
Element Formed - Chlorine
Positive Electrode (Anode):
Observations - Colourless bubbles of hydrogen gas
Element Formed - Hydrogen
Copper (II) Chloride Observations and Element formed at each Electrode
Negative Electrode (Cathode)
Observations - Bubbles of gas that bleach litmus paper
Element Formed - Chlorine
Positive Electrode (Anode):
Observations - Copper (brown coating on the electrode)
Element Formed - Copper
Sodium Sulphate Observations and Element formed at each Electrode
Negative Electrode (Cathode)
Observations - Oxygen Gas (bubbles of a gas that do not bleach damp litmus paper)
Element Formed - Oxygen
Positive Electrode (Anode):
Observations - Colourless bubbles of hydrogen gas
Element Formed - Hydrogen
Copper (II) Sulphate Observations and Element formed at each Electrode
Negative Electrode (Cathode)
Observations - Oxygen Gas (bubbles of a gas that do not bleach damp litmus paper)
Element Formed - Oxygen
Positive Electrode (Anode):
Observations - Copper (brown coating on the electrode)
Element Formed - Copper
Why should this experiment be done in a ventilated room?
The experiment should be done in a well ventilated room because the chlorine gas is poisonous, so having windows open and e.c.t stops the build up of too much chlorine.
Why should the electrolysis cells be connected for no more than 5 minutes?
The electrolysis cells should not be connected for more than 5 minutes, becuase it will start to not work, as the concentration of the solution would’ve changed. Also if it’s left on for too long, then more gases like chlorine can be produced.
Why is the light bulb in the circuit?
The light bulb is in the circuit because it allows you to see if the circuit is on, or if the circuit is even working, so you know when electrolysis is taking place.
How to Test for Chlorine (Cl2) and What Shows The Result
Test:
You use damp litmus paper (usually blue), and the water dissolves some of the chlorine so it can react with the indictator on the litmus paper.
Result:
If chlorine is present, then the litmus paper will be bleached by the chlorine.
How to Test for Hydrogen (H2) and What Shows The Result
Test:
You place a lighted spint in a test tube containing the gas, and wait to see if there is a squeaky pop sound.
Result:
If the gas is hydrogen, there will be a squeaky pop sound.
