Principles of Chemistry - Electrolysis (Paper 2)

Question 1

do covalent compounds conduct electricity and why?

Answer 1

covalent compounds do not conduct electricity because there are no charged particles that are free to move

Question 2

do ionic compounds conduct electricity and why?

Answer 2

• when solid, ions in an ionic compound cant move
• when molten or in solution ions in ionic compounds are free to move so can conduct

Question 3

what is a negative ion?

Answer 3

an anion

Question 4

what is a positive ion?

Answer 4

a cation

Question 5

what is electrolysis?

Answer 5

the breaking down of a substance caused by passing an electric current through an ionic compound which is molten or in solution
new substances are formed

Question 6

what happens during the electrolysis of molten compounds? (example compound: lead II bromide)

Answer 6

• solid lead bromide is heated and becomes molten
• electrodes attached to a power source are placed in the molten lead bromide
• the left-hand electrode becomes positively charged, this is called the anode. the right hand becomes negatively charged, this is called the cathode
• at the anode, a brown gas is given off. this is bromine gas (g)
• at the cathode, a shiny substance is formed. this is molten lead (l)

Question 7

what is the negatively charged electrode called?

Answer 7

cathode

Question 8

what is the positively charged electrode called?

Answer 8

anode

Question 9

what is the rule for working out elements formed from the electrolysis of solutions at the cathode?

Answer 9

• the metal will be produced if it is less reactive than hydrogen
• hydrogen gas will be produced if the metal is more reactive than hydrogen

Question 10

what is the rule for working out elements formed from the electrolysis of solutions at the anode?

Answer 10

the product of electrolysis is always oxygen gas unless the solution contains Cl–, Br­- or I– ions.

Question 11

what happens during the electrolysis of ionic solutions? (example solution: sodium chloride solution)

Answer 11

• solid sodium chloride is dissolved in water
• the solution also contains hydrogen ions (H+) and hydroxide ions (OH–)
• chloride ions (Cl–) and hydroxide ions (OH–) are attracted to the anode
• sodium ions (Na+) and hydrogen ions (H+) are attracted to the cathode
• at the anode, a green gas is given off, chlorine gas
• at the cathode, a colourless gas is given off, hydrogen gas

Question 12

what happens during the electrolysis of copper sulfate solution?

Answer 12

• copper sulfate solution is composed of copper ions (Cu²⁺), sulfate ions (SO₄²⁻), hydrogen ions (H⁺) and hydroxide ions (OH⁻)
• at the cathode, a brown layer is formed, copper
• at the anode, bubbles of gas are given off, oxygen

Question 13

what happens during the electrolysis of sulfuric acid?

Answer 13

• sulfuric acid is composed of sulfate ions (SO₄²⁻), hydrogen ions (H⁺) and hydroxide ions (OH⁻)
• at the cathode, bubbles of gas are formed, this is hydrogen gas
• at the anode, bubbles of gas are given off, this is oxygen gas

Question 14

what are the half equations for the electrolysis of lead (II) bromide?

Answer 14

• at the cathode (negative electrode): Pb²⁺(l) + 2e⁻ → Pb(l)
• at the anode (positive electrode): 2Br⁻(l) → Br₂(g) + 2e⁻

Question 15

what are the half equations for the electrolysis of aluminium oxide?

Answer 15

• at the cathode: Al³⁺ + 3e⁻ → Al
• at the anode: 2O²⁻ → O₂ + 4e⁻

Question 16

what are the half equations for the electrolysis of sodium chloride solution?

Answer 16

• at the cathode: 2H⁺(aq) + 2e⁻ → H₂(g)
• at the anode: 2Cl⁻(aq) → Cl₂(g) + 2e⁻

Question 17

what are the half equations for the electrolysis of copper sulfate solution?

Answer 17

• at the cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
• at the anode: 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻

Question 18

how would you investigate the electrolysis of aqueous solutions?

Answer 18

• the electrolyte is an aqueous solution
• add the solution to a beaker and connect two electrodes to a power supply
• fill two small test tubes with the solution and position a tube over each electrode
• the test tubes over the electrodes must not completely cover them so the ions are free to move throughout the solution