Topic 3b (Electrolysis)

Question 1

3.22
Define the term electrolyte

Answer 1

Ionic compounds w/ freely moving ions able to conduct electricity

Question 2

3.23
Define the term electrolysis

Answer 2

Breaking down of compounds (electrolytes) using electricity

Question 3

3.23
Explain why graphite is suitable for electrolysis

Answer 3

-Can conduct electricity: delocalised electrons (C atoms form 3 covalent bonds -> 1 free to move, carry charge)
-Stable bc inert (full outer shell -> not reactive)
-Low cost

Note: First 2 reasons are better

Question 4

3.24
Explain the movement of ions during electrolysis

Answer 4

-Positive cations migrate to negative cathode
-Negative anions migrate to positive anode

Question 5

5.25
Explain how you can predict the products that form at the cathode and anode during the electrolysis of different substances

Answer 5

Cathode
-If H₂ more reactive metal forms
-If metal more reactive H₂ forms
Anode
-Simple negative ion -> element forms
-Polyatomic “ -> O₂ forms

Question 6

5.25
What products will form at the cathode and anode in sodium chloride solution? What ions does it contain?

Answer 6

Cathode: H₂
Anode: Chlorine

Na^+ H^+ Cl^- OH^-
(Hydrogen & hydroxide bc aqueos solution)

Question 7

5.25
What products will form at the cathode and anode in copper chloride solution? What ions does it contain?

Answer 7

Cathode: Copper
Anode: Chlorine

Cu^2+ H^+ Cl^- OH^-

Question 8

5.25
What products will form at the cathode and anode in sodium sulfate solution? What ions does it contain?

Answer 8

Cathode: H₂
Anode: O₂
Na^+ H^+ SO₄^2- OH^-

Question 9

5.25
What products will form at the cathode and anode in acidified water? What ions does it contain?

Answer 9

Cathode: H₂
Anode: O₂

H^+ SO₄^2- OH^-

Question 10

5.25
CHECK What products will form at the cathode and anode in molten lead bromide? What ions does it contain?

Answer 10

Cathode: Lead
Anode: Bromine

Pb^2+ Br^- H and OH?

Question 11

5.25
What will happen during the electrolysis of all molten salts?

Answer 11

Cathode: Metal
Anode: Non-metal

Question 12

5.26
Describe the electrolysis of solid lead bromide and what would be seen at each electrode

Answer 12

-Heat solid lead bromide w/ Bb -> molten
-Pb^2+ cations migrate to cathode -> gain 2e^- -> Pb atoms
-Br^- anions migrate to anode -> lose e^- -> Br atoms
-Cathode: Molten lead
-Anode: Bromine atoms join -> Bromine molecules (bromine gas) so brown fumes are seen

Question 13

5.26
Describe the electrolysis of a solid ionic compound

Answer 13

-Ionic substance -> crucible (heat w/ Bb)
-2 clean inert electrodes -> electrolyte
-Connect electrodes to power supply (wires + clips)
-Current flows through cell when power on

Question 14

3.27
State what type of half equation (reduction or oxidation) the following are:
a) 2Cl^- -> Cl₂ + 2e^-
b)Mg^2+ + 2e^- -> Mg

Answer 14

a)Oxidation
b)Reduction

(Why? Bc first one loses electrons so will be on right as electrons are ‘formed’. In b, Mg is reduced so has gained electrons. Therefore will be on reactant side bc it reacts with Mg^2+ ion to form an Mg atoms - the product - which is by itself on the right)

Question 15

3.27
Write the half equation (with state symbols) for when a metal is formed at the cathode. Use copper as an example

Answer 15

Cu^2+ (aq) + 2e^- -> Cu (s)

Question 16

3.27
Write the half equation (with state symbols) for when hydrogen is formed at the cathode

Answer 16

2H^+ (aq) + 2e^- -> H₂ (g)

Question 17

3.27
Write the half equation (with state symbols) for when an element is formed at the anode. Use chlorine as an example

Answer 17

2Cl^- (aq) -> Cl₂ (g) + 2e^-

Question 18

3.27
Write the half equation for when oxygen is formed at the anode

Answer 18

4OH^- (aq) -> 2H₂O (l) + O₂ (g) + 4e^-

Question 19

3.28/9
Define the term oxidation. At which electrode does it occur?

Answer 19

-Loss of electrons
(OILRIG)
-Anode

Question 20

3.28/9
Define the term reduction. At which electrode does it occur?

Answer 20

-Gain of electrons
(OILRIG)
-Cathode

Question 21

3.30
Describe how you would purify copper

Answer 21

Cathode
-Cu^2+ ions attracted to cathode
-Gain 2 electrons (reduced) -> Cu atoms
-Cu^2+ (aq) + 2e^- -> Cu (s)
-Pure Cu cathode mass increases
Anode
-Cu atoms lose 2 electrons (oxidised) -> Cu^2+ ions
-Cu (s) -> Cu^2+ (aq) + 2e^-
-Sludge (impurities) form
-Impure Cu anode mass decreases

Question 22

3.30
In the electrolysis of copper sulfate, explain why the solution remains the same shade of blue

Answer 22

Same no. of ions enter & leave solution

Question 23

3.31
Describe how to set up an electrolysis experiment which would collect the gases produced

Answer 23

-Electrolyte & electrodes into beaker
-Inverted test tubes over electrodes (don’t cover completely bc conductivity will decrease -> slow electrolysis)
-Connect to D.C power supply
-Electrolysis occurs
-Gases produced are collected in test tubes

Question 24

3.31
Compare the volume of gases produced in the electrolysis of acidified water

Answer 24

H₂ volume is twice O₂ volume

Question 25

What will be seen at the cathode when hydrogen forms?

Answer 25

Bubbles/fizzing