Electrolysis

Question 1

What is electrolysis?

Answer 1

• passing electric current through electrolyte
• causes ions to move to electrodes
• positively charged ions -> cathode
• negatively charged ions -> anode
• ions are discharged at electrodes
• produces elements

Question 2

What is an electrolyte?

Answer 2

• liquid or solution that can conduct electricity
• contains ions that can move

Question 3

What is the charge of the non metal ion in electrolysis?

Answer 3

• negative

Question 4

What is the charge of a metal ion in electrolysis?

Answer 4

positive

Question 5

Give a half equation for the formation of Br2 at the positive electrode

Answer 5

2Br- -> Br2 + 2e-

Question 6

Give a half equation of the formation of Mg at the negative electrode

Answer 6

Mg2+ + 2e- -> Mg

Question 7

Why does aluminium need to be extracted from electrolysis?

Answer 7

• it is too reactive to be extracted by reduction with carbon

Question 8

Why is cryolite added to aluminium oxide?

Answer 8

• to lower the melting point
• reduces energy costs

Question 9

Why does the carbon anode need to be replaced frequently?

Answer 9

• carbon reacts with oxygen produced to form carbon dioxide

Question 10

What is an aqueous compound?

Answer 10

• dissolved in water

Question 11

What is the rule for the product of the negative electrode in the electrolysis of aqueous solutions?

Answer 11

• hydrogen is produced if metal is more reactive than hydrogen

Question 12

What is the rule for the product of the positive electrode in the electrolysis of aqueous solutions?

Answer 12

• oxygen is produced unless solution contains halide ions (group 7)

Question 13

Describe the test and expected observations for the hydrogen

Answer 13

• put lit splint near the gas
• products squeaky “pop” sound = hydrogen is present

Question 14

Describe the test and expected observations for oxygen

Answer 14

• put glowing splint near gas
• if relights = oxygen is present

Question 15

Describe the test and expected observations for chlorine

Answer 15

• bleaches litmus paper

Question 16

What is the difference between a molten substance and an aqueous solution?

Answer 16

• molten substance only contains 2 types of ions
• aqueous solutions contains hydrogen and oxygen ions

Question 17

What does discharged mean?

Answer 17

ion loses its charge and becomes neutral

Question 18

Describe what happens at the electrodes in terms of reduction and oxidation

Answer 18

• cathode - positively charged ions gain electrons, reduced
• anode - negatively charged ions lose electrons, oxidised

Question 19

What uses the most energy during the electrolysis of aluminium?

Answer 19

• melting the compounds
• producing electrical current

Question 20

What is the half equation at the cathode during electrolysis of aluminium oxide?

Answer 20

Al 3+ + 3e- –> Al

Question 21

What is the half equation at the anode during electrolysis of aluminium oxide?

Answer 21

2O2- –> O2 + 4e-

Question 22

Reactivity series for electrolysis of aqueous solutions

Answer 22

potassium
sodium
lithium
calcium
magnesium
zinc
iron
hydrogen
copper
silver

hydrogen and up = hydrogen at cathode
hydrogen down = metal at cathode

Question 23

What do water molecules break down into?

Answer 23

• hydrogen ions H+
• hydroxide ions OH-

Question 24

What is the half equation for copper sulfate solution?

Answer 24

Cu2+ + 2e- –> H2
4OH- –> O2 + H2O + 4e-

metal less reactive than hydrogen
no halide ion in compound = oxygen

Question 25

What is the half equation for sodium chloride solution?

Answer 25

2H+ + 2e- –> H2
2Cl- –> Cl2 + 2e-

sodium more reactive than hydrogen
halogen present

Question 26

Describe the test and expected observations for the carbon dioxide

Answer 26

• turns limewater cloudy when bubbled into it