Electrolysis

Question 1

What is electrolysis?

Answer 1

• passing an electric current through an ionic substance that’s molten or in solution, breaking it down into a new substance

Question 2

What is the liquid used to conduct electricity called and how is it made?

Answer 2

• the electrolyte
• melting or dissolving ionic compounds
• the free ions are conducting the electricity

Question 3

How does electrolysis work?

Answer 3

• the ionic solid is dissolved in water. ions can now freely move within the solution
• live electrodes are placed in the solution. one is positively charged (the anode) and one is negatively charged (the cathode)
• the negative anions are attracted to the positively- charged anode. the positive cations are attracted to the negatively charged cathode. they move towards it
• at the electrodes, the ions become neutral by gaining or losing electrons. these electrons are either taken from or donated to the electrode. e.g. 2Cl- to Cl2 + 2e-
• e.g. the cations gain electrons form the cathode and become neutral

Question 4

What happens when an electrolyte is in aqueous solution?

Answer 4

• at the cathode hydrogen gas is produced (2H (aq)+ + 2e- = H2(g)) unless the cation is derived from an element that is less reactive than hydrogen
• at the anode, a halogen is produced if possible. if not possible, oxygen is produced: 4OH- = 2H20 + O2 + 4e-

Question 5

What four elements are less reactive than hydrogen?

Answer 5

• copper
• silver
• gold
• platinum

Question 6

What happens if you increase the number of electrons?

Answer 6

• you increase the amount of substance produced

- achieved by: electrolysing for a longer time, increasing the current

Question 7

What is the equation for coulombs (charge)?

Answer 7

• Q = I x T

- charge = current x time

Question 8

What is a faraday?

Answer 8

• one faraday is 96,000 coulombs

- one faraday cations one mole of electrons

Question 9

Some molten lead(ii) chloride is electrolysed for 20 mins. the current flowing is 5 amps. find the mass of lead produced

Answer 9

1. write out a balanced half equation for the cathode
2. calculate the number of faradays
3. calculate the number of moles of lead produced (divide the number of faradays by the number of electrons in the half equation)
4. write in the Mr values from the periodic table to work out the mass of the solid lead produced.

Question 10

What happens in the electrolysis of sulphuric acid?

Answer 10

• a solution of sulphuric acid (H2SO4) contains 3 different ions: SO4 2, H+ and OH-
• hydrogen ions (from the water or sulphuric acid) accept electrons
• so at the cathode, hydrogen gas is produced
• 2H+ + 2e- = H2
• hydroxide ions lose electrons more easily than sulphate ions
• so at the anode, oxygen and water are produced
• 4OH- = O2 + 2H2O + 4e-

Question 11

What happens in the electrolysis of sodium chloride?

Answer 11

• a solution of sodium chloride (NaCl) contains four different ions: Na+, Cl-, OH- and H+
• hydrogen ions accept electrons more easily than sodium ions
• so at the cathode, hydrogen gas is produced
• 2H+ + 2e- = H2
• chloride ions lose electrons more easily than hydroxide ions
• so at the anode, chlorine gas is produced
• 2Cl- = Cl2 + 2e-

Question 12

What happens in the electrolysis of copper(ii) sulphate?

Answer 12

• a solution of copper (ii) sulphate (CuSO4) contains four different ions: Cu2+, SO4 2-, H+ and OH-
• copper ions accept electrons more easily than hydrogen ions
• so at the cathode, copper metal is produced
• Cu2+ + 2e- = Cu
• hydroxide ions lose electrons more easily than sulphate ions
• so at the anode, oxygen and water are produced
• 4OH- = O2 + 2H2o + 4e-