Electrolysis - ES

Question 1

What is electrolysis

Answer 1

breakdown of an ionic compound using electricity

Question 2

When can you use electrolysis on ionic compounds and why

Answer 2

ionic compounds conduct electricity when molten or in a solution as the ions are free to move. They move to the electrode of the opposite charge where they either gain or lose electrons

Question 3

Apparatus for electrolysis

Answer 3

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Question 4

Molten lead (II) bromide what happens at the cathode and anode

Answer 4

cathode = Pb²⁺ ions move here and gain electrons (reduced)
Pb²⁺ + 2e⁻ -> Pb

anode = Br⁻ ions move here and they lose electrons (oxidised)
2Br⁻ -> Br₂ +2e⁻

Question 5

Why is there a difference in electrolysis between molten and aqueous ionic compounds

Answer 5

molten ionic compounds only contain one type of cation and one type of anion but when in a solution water is also present so different cations/anions can react

Question 6

General rule for aqueous ionic compound electrolysis for cathode and anode

Answer 6

cathode = cations of more reactive metals (group 1 and 2 and Al³⁺) will form hydrogen gas, any other metal ion with form the metal which is plated onto the electrode
Hydrogen is also given if acid solutions are electrolysed

anode = halide ions react if present in preference to water (Cl₂, Br₂, I₂) but OH⁻ will react in preference to other anions (sulphate/nitrate) and give oxygen gas
Oxygen is also given if solutions of alkalis are electrolysed

Question 7

Half equations of hydrogen gas at cathode and hydroxide at anode

Answer 7

2H⁺ + 2e⁻ -> H₂

4OH⁻ -> O₂ + 2H₂O + 4e⁻

Question 8

What is formed at the cathode and anode in electrolysis of KCl solution

Answer 8

cathode = H⁺ reacts and electrons are gained - 2H⁺ + 2e⁻ -> H₂ (reduction)
bubbles of hydrogen gas
anode = Cl⁻ reacts and electrons are lost - 2Cl⁻ -> Cl₂ + 2e⁻ (oxidation)
bubbles of chlorine gas

Question 9

What is formed at the cathode and anode in electrolysis of H₂SO₄

Answer 9

cathode = H⁺ reacts and electrons are gained - 2H⁺ + 2e⁻ -> H₂ (reduction)
bubbles of hydrogen gas
anode = OH⁻ reacts and electrons are lost 4OH⁻ -> O₂ + 2H₂O + 4e⁻ (oxidation)
bubbles of oxygen gas

Question 10

Unreactive electrode types

Answer 10

ones made from graphite and platinum

Question 11

How can some metals be purified by electrolysis

Answer 11

when electrolysing a solutions of their ions with electrodes made of that metal. The anode is the impure metal and it gradually goes into the solution. Pure metal gradually builds up on the cathode

Question 12

Purifying copper (II) sulphate using copper electrodes

Answer 12

cathode (pure copper) = copper metal coats the electrode and it gets bigger
Cu²⁺ + 2e⁻ -> Cu
anode (impure copper) = copper ions pas into the solution and the anode gets smaller
Cu -> Cu²⁺ + 2e⁻

Question 13

What would happen in the concentration of Cu²⁺ during purification of copper (II) sulphate using copper electrodes

Answer 13

it stays the sae as the rate of Cu²⁺ made at the anode is the same as the rate it is used up at the cathode

Question 14

Electrolysis of brine - What is brine and how is it used

Answer 14

concentrated solution of NaCl used in industry to manufacture chlorine

Question 15

Electrolysis of brine - What happens at the anode and cathode

Answer 15

when electrolysed it produces hydrogen gas at the cathode and chlorine at the anode and leaves NaOH in solution. Products have to be kept separate otherwise they would react together

Question 16

Half equation of electrolysis of brine

Answer 16

2NaCl + 2H₂0 -> Cl₂ + H₂ + NaOH

Question 17

How is chlorine used to extract bromine

Answer 17

some of the chlorine produced is used to extract Br⁻ which is how bromine is produced in industry