ES3: Electrolysis Flashcards
Define “electrolysis”
The breaking down of a compound into its elements by passing an electric current through an ionic substance molten or in solution
What is an electrolyte?
Liquid that conducts electricity in electrolysis
What does an electrolyte contain?
A molten/dissolved ionic substance
Why does the ionic substance need to be molten/dissolved?
So the ions are free to move and carry charge
Which electrode is negative?
Cathode
Which electrode is positive?
Anode
What happens at the cathode?(4)
- Cathode is negative
- Attracts cations because of the opposite charge
- Cations migrate to it
- There they gain electrons and become an atiom
What happens at the anode?(4)
- Anode is positive
- Attracts anions because of the opposite charge
- Anions migrate to it
- There they lose electrons and become an atiom
Describe a method for setting up electrolysis.(6)
- Use wires + clips to connect each electrode to the power supply
- Electrode at the positive pole is the anode
- Electrode at the negative pole is the cathode
- Use inert electrodes (e.g carbon/platinum) so they don’t react + interfere with electrolysis
- Place electrodes in the beaker containing the electrolyte, making sure they don’t touch each other
- Turn the power supply on
What do products forming as metals look like?
A thin layer on the surface of the cathode
What do products forming as gases look like?
Bubbles at the cathode/anode
What happens to metal ions in the electrolysis of molten compounds?
They are positive and so move to the cathode and are reduced there
Give a half equation for the reduction of zinc.
Zn2+(l) + 2e- ====> Zn(s)
What happens to non-metal ions in electrolysis of molten compounds?
They are negative and so move to the anode and are oxidised there
Why are the products of the electrolysis of ionic solutions hard to determine?
The H+ and OH- ions from the water are competing with the ions
What is produced in the electrolysis of ionic solutions at the cathode when the metal is more reactive than hydrogen?
Hydrogen gas
What metals are more reactive than hydrogen?(3)
- Group 1 metals
- Group 2 metals
- Aluminium
Give the equation for the production of hydrogen at the cathode from water?
2H20 (l) + 2e- ===> 2OH- (aq) + H2 (g)
What is the pH surrounding the cathode in the electrolysis of ionic solutions when the metal is more reactive than hydrogen?
Alkaline because of the OH- ions surrounding the cathode
What is produced in the electrolysis of ionic solutions at the cathode when the metal is less reactive than hydrogen?
The metal is formed
What is produced at the cathode if there is an acid in the solution?
Hydrogen gas
Give the equation for the production of hydrogen at the cathode from hydrogen ions?
2H+(aq) + 2e- ===> H2 (g)
What is produced in the electrolysis of ionic solutions at the anode when the non metal is a sulfate/nitrate?
Oxygen gas
Give the equation for the production of oxygen at the anode from water.
H2O (l) ====> O2 (g) + 4e- + 4H+ (aq)
What is the pH surrounding the anode in the electrolysis of ionic solutions when the non metal is a sulfate/nitrate like?
Acid because of the H+ ions surrounding the cathode
What is formed at the anode in the electrolysis of a concentrated ionic solution with a halide ?
The halogen gas
What is the pH of the solution left behind after the production of hydrogen at the cathode and why?
Alkaline because of the hydroxide ions left behind in the solution
What is formed at the anode in the electrolysis of a dilute ionic solution with a halide ?
Oxygen is formed
What is formed at the anode in the electrolysis of a solution with a hydroxide ?
Oxygen is formed
Give the half equation for the formation of oxygen at the anode.
4OH- (aq) ===> O2 (g) + 2H2O (l) + 2e-
What is the pH of the solution left behind after the production of oxygen at the anode and why?
Neutral because the OH- ions have been reduced
Describe what happens when the solution contains the same metal as the one both the electrodes are made of. Use copper as the example. (5)
- At the anode, oxidation occurs and electrons are lost:
(Cu (s) ====> Cu2+ (aq) + 2e-) - Copper ions enter the solution and become attached to the cathode
- At the cathode, reduction occurs and electrons are gained:
(Cu2+ (aq) + 2e- ===> Cu (s)) - Copper atoms plate the pure copper cathode
- The concentration of the solution remains constant
What substance is the anode made of when the solution contains the same metal as the one both the electrodes are made of (e.g copper) ?
The impure metal (e.g impure copper)
What substance is the anode made of when the solution contains the same metal as the one both the electrodes are made of (e.g copper) ?
Pure metal (e.g pure copper)
What is used as an electrolyte in copper electroplating?
CuSO4
What two uses are there for using the same metal for the electrodes and the solution in electrolysis?
- Electroplating objects with the metal coating
- Purifying impure metals
What happens to the pure cathode when using the same metal for the electrodes and the solution in electrolysis?
Increases its mass
What happens to the impure anode when using the same metal for the electrodes and the solution in electrolysis?
It shrinks
How can chlorine be produced as a gas?
From the electrolysis of brine
What is brine?
A solution of water with a high concentration of halide salts
What halide salts in brine is mainly made up of?
Sodium chloride, with some bromine and iodine salts
Name 3 places brine can be found
- Occurring naturally as seawater
- Occurring naturally in salt lake
- By dissolving rock salt in water
How can bromine be produced using displacement?(3)
- Bubble chlorine gas through brine
- Collect the bromine produced
- Condense it into a liquid and purify it
Give the ionic equation for the displacement of bromine with chlorine
2Br-(aq) + Cl2 (g) ===> 2Cl- (aq) + Br2 (aq)
How can iodine be produced using displacement?(3)
- Bubble chlorine gas through brine
- Collect the iodine produced
- Purify it and condense it into a grey solid
Give a half equation for the reduction of bromine.
2Br-(l) ====> Br2 (g) + 2e-
