Electrolysis Processes - Topic 3

Question 1

What is the movement of ions like in electrolysis?

Answer 1

• during electrolysis positively charged ions (cations) move to the negative electrode (cathode) and negatively charged ions move to the positively charged electrode (anode)
• ions are discharged at the electrodes producing elements

Question 2

What is oxidation in terms of electrons?

Answer 2

oxidation is the loss of electrons

Question 3

What is reduction in terms of electrons?

Answer 3

gain of electrons

Question 4

Electrolytes:

Answer 4

ionic compounds that conduct electricity in the molten state or dissolved in water e.g. lead bromide

Question 5

Non-electrolytes:

Answer 5

compounds that does not conduct electricity in the molten state or dissolved in water e.g. lead bromide

Question 6

Why and how do electrons move through a circuit?

Answer 6

electrons repelled by negative terminal and attracted to the positive terminal → flow from negative to positive

Question 7

Electrolysis:

Answer 7

a process in which electrical energy, from a direct current supply, decomposes electrolytes

Question 8

Electricity:

Answer 8

movement/flow of electric charge/electrons

Question 9

Conductors:

Answer 9

material that allows charge to easily move/flow through it e.g. metals like iron

Question 10

Why do metals like iron and a non-metal like graphite conduct electricity?

Answer 10

as they both contain electrons (can carry charge) and move through materials

Question 11

Insulators:

Answer 11

material that does allow charge to easily move/flow through it e.g. plastic

Question 12

Why do non-metals like diamond and sulphur not conduct electricity?

Answer 12

contain no charged particles that can move through substance - charge cannot move through the substance

Question 13

Important half-equation to learn:

Answer 13

4OH⁻ —> 2H₂O + O₂ + 4e ⁻

Question 14

Why is graphite often used as an electrode?

Answer 14

• conducts electricity - contains free electrons
• unreactive - so won’t react with solution and form other products

Question 15

Diagram of electrolysis:

Answer 15

Question 16

What takes place at the cathode in electrolysis?

Answer 16

• reduction occurs at the cathode
• positive cations attracted to the cathode
• cathode is negative as it has a surplus of electrons
• cations gain electrons to form an atom (generally metal atoms)

Question 17

What takes place at the anode in electrolysis?

Answer 17

• oxidation occurs at the anode
• negative anions attracted to the anode
• anode is positive as it has a shortage of electrons

Question 18

What is said to flow in electrolysis?

Answer 18

• electrons said to be taken from ___ (non-metal) and given up to the ___ (metal)
• there is an overall flow of electrons and so an electrical current is said to flow

Question 19

What is formed at the cathode in the electrolysis of solutions?

Answer 19

• if metal is more reactive than hydrogen then hydrogen will form at cathode as it is easier to discharge H+ ions than metal ions
• 2H⁺ + 2e⁻ ——> H₂
• in a mixture of ions it is the ion which is lower in the reactivity series that will be discharged and gain electrons (i.e. is produced)

Question 20

What is formed at the anode in the electrolysis of solutions?

Answer 20

• if the solution contains halide like Cl-, Br- and I- then the halide ions will become discharged and elements like chlorine, bromine and iodine are released from the anode
• the negative ions lose electrons and are oxidised
• if the solution contains other negative ions which are not halides like SO₄²⁻ then oxygen gas is released from the anode
• so oxygen gas is formed
• 4OH⁻ ——> 2H₂O + O₂ + 4e⁻

Question 21

Apparatus needed to carry out electrolysis of copper sulphate solution using inert electrodes:

Answer 21

• eye protection e.g. goggles
• emery paper
• low voltage d.c. supply
• ammeter
• variable resistor
• connecting leads
• crocodile clips
• 100cm3 beaker
• stop clock
• 2 copper electrodes
• 2 pieces of copper foil
• access to propanone (in a fume cupboard)
• mass balance
• graph paper

Question 22

Safety measures to take when carrying out electrolysis of copper sulphate solution using copper electrodes:

Answer 22

• wear eye protection e.g. goggles
• propanone is an irritant + highly flammable
  
  • must not be any naked flames in laboratory

Question 23

Diagram of electrolysis of copper sulphate solution using copper electrodes:

Answer 23

Question 24

Method to carry out electrolysis of copper sulphate solution using copper electrodes:

Answer 24

1. Select 2 pieces of copper foil to use as electrodes and clean them with emery paper - label one of the electrodes as the ‘anode’ and the other as ‘cathode’
2. Measure and record the mass of each electrode using a mass balance
3. Half fill the beaker with copper sulphate solution
4. Set up circuit as showing in diagram
5. Turn the power on and adjust the variable resistor to give a current of 0.3A - record the current and leave the power on for 15mins adjusting the variable resistor to keep the current constant if necessary
6. Turn off the power and remove the electrodes from the beaker
7. Gently rinse the electrodes with distilled water then dip them into propanone - remove the electrodes from the propanone and gently shake them until the propanone evaporates
8. Measure and record the masses of each of the dry electrodes
9. Repeat the experiments except using currents of 0.4A, 0.5A and 0.6A
10. Reaction is quite slow so cell should be left for around 30mins in order to achieve a measurable change in mass
11. Measure the change in mass of electrodes by finding the difference between the masses of the electrodes before and after the experiment

Question 25

Graph to show change in mass of electrodes during electrolysis of copper sulphate solution using inert electrodes as current increases:

Answer 25

Question 26

Half-equation at anode for electrolysis of copper sulphate solution using copper electrodes:

Answer 26

Cu(s) ——> Cu²⁺(aq) + 2e⁻

Question 27

How does the current affect the change in mass at the electrodes during the electrolysis of copper sulphate solution using copper electrodes?

Answer 27

as current increases change in mass at electrodes increases

Question 28

What happens to the masses at the electrodes during the electrolysis of copper sulphate solution using copper electrodes and why?

Answer 28

- anode lost copper and cathode gains copper - at anode copper atoms oxidised forming Cu2+ ions and lose 2 electron - copper ions leave anode which causes mass loss - copper ions in solution move to cathode - at cathode copper ions are reduced and gain 2 electrons forming copper atoms at the cathode which causes its mass to increase

Question 29

Why is the change in mass at the cathode not the same as the change in mass at the anode?

Answer 29

change in mass greater at cathode as Cu²⁺ from copper sulphate solution is also used to form copper on cathode no just Cu²⁺ from the copper anode

Question 30

Diagram of apparatus used to carry out electrolysis of copper sulphate solution using inert electrodes:

Answer 30

Question 31

Explain the formation of the product at each electrode for the electrolysis of copper sulphate solution using inert electrodes:

Answer 31

- copper formed at cathode as less reactive than H⁺ ions so is discharged more easily - oxygen formed at anode as copper sulphate solution does not contain any halides but contains SO₄²⁻ so O²⁻ is discharged instead

Question 32

Half-equation at cathode for electrolysis of copper sulphate solution using inert electrodes:

Answer 32

Cu²⁺(aq) + 2e⁻ ——> Cu(s)

Question 33

Half-equation at anode for electrolysis of copper sulphate solution using inert electrodes:

Answer 33

4OH⁻(aq) ——> 2H₂O(l) + O₂(g) + 4e⁻

Question 34

Why does the copper sulphate solution in the electrolysis of copper sulphate remain same colour?

Answer 34

- copper sulphate solution is blue colour - as same number of ions enter and leave the solution - for every copper ion entering the solution at the anode, one is removed from the solution at the cathode - concentration of copper sulphate solution remains the same

Question 35

In the electrolysis of copper sulphate solution what is the solid that forms under the anode?

Answer 35

Solid is the insoluble impurities falling from the anode

Question 36

What is observed at the electrodes in copper sulphate electrolysis?

Answer 36

- pink/brown colour on surface if cathode - solid copper deposited on the cathode - cathode increases in size/gains mass - anode decreases in size/loses mass