Chemical changes - electrolysis

Question 1

what is electrolysis

Answer 1

splits ionic compounds into the different elements that they are made of using electricity

Question 2

what happens during electrolysis

Answer 2

an electric current is passed through an electrolyte

- the ions move towards the electrodes where they react and the compound decomposes

Question 3

what ions in the electrolyte will go to the negative electrode

Answer 3

the positive ion

Question 4

what ions in the electrolyte will go to the positive electrode

Answer 4

the negative ion

Question 5

what happens when the ions gain or lose electrons in electrolysis

Answer 5

they form the uncharged element and are discharged from the electrolyte

Question 6

why cant an ionic solid be electrolysed

Answer 6

because the ions are in fixed positions and can’t move around the structure and carry charge

Question 7

why can molten / aqueous ionic compounds be electrolysed

Answer 7

because the ions can move freely and carry charge, as the forces of attraction are broken

Question 8

what ions are reduced and what ions are oxidised

Answer 8

• the positive metal ions are reduced to their element at the cathode as they gain electrons
• the negative non - metal ions are oxidised to their element at the anode

Question 9

what is the positive electrode called

Answer 9

anode

Question 10

what is the negative electrode called

Answer 10

cathode

Question 11

what gas is produced at the anode

Answer 11

oxygen

Question 12

what gas is produced at the cathode when the metal ion is more reactive than hydrogen

Answer 12

hydrogen gas

Question 13

how is aluminium electrolysed

Answer 13

• high melting point so mixed with cryolite to lower the melting point

Question 14

what ore is aluminium extracted from

Answer 14

bauxite

Question 15

what happens during electrolysis of aluminium

Answer 15

• the positive Al3 + ions are attracted to the cathode, each pick up 3 electrons and turn into neutral aluminium atoms, these then sink to the bottom of the electrolysis tank
• negative O2- ions are attracted to the anode where they each lose 2 electrons, neutral atoms will then combine to form O2 molecules

Question 16

using half equations, what happens at the anode and cathode during extraction of aluminium

Answer 16

at the cathode - reduction
Al3+ + 3e- —- Al

at the anode - oxidation
2O2- —– o2+ 4e

Question 17

why do the carbon electrodes need to be replaced regularly during the extraction of aluminium

Answer 17

• anode made of carbon, needs to be replaced regularly as aluminium oxide will react to produce carbon dioxide
• will corrode

Question 18

what are negatively charged ions called

Answer 18

anions

Question 19

what are positively charged ions called

Answer 19

cations

Question 20

why is carbon used in electrodes

Answer 20

cheap and good conductor

Question 21

disadvantages of electrolysis

Answer 21

• lots of energy needed to melt the solid ionic compound and produce the electrical current
• energy costs money

Question 22

what is an electrolyte

Answer 22

a form of an ion in melted or dissolved in water

Question 23

what is produced at the cathode if the metal ion is less reactive than hydrogen

Answer 23

a solid layer of the pure metal will be produced

Question 24

what is produced at the anode if halides ( Cl-, I - Br - ) are present

Answer 24

gases of bromine, chlorine, iodine etc will be formed

Question 25

what is produced at the anode if there are no halides present

Answer 25

OH - ions are discharged and oxygen will be formed

Question 26

what happens during the electrolysis of copper sulfate

Answer 26

contains cu2+ , SO4 2+ , H+ and OH- ions

copper is less reactive than hydrogen so at the cathode, copper metal is produced and coats the cathode

cu 2+ + 2e- — Cu

no halide ions present, so at the anode there will be oxygen gas

4OH- + O2 —- 2H2O + 4e-

Question 27

what happens during the electrolysis of sodium chloride

Answer 27

contains Na + , Cl - , OH - , H+

• sodium more reactive than hydrogen, so hydrogen gas is produced at the cathode

2H+ + 2e —- H2

• chloride ions are present in the solution, so chlorine gas is formed at the anode

2Cl —- Cl2 + 2e-

Question 28

how can we test for chlorine

Answer 28

use litmus paper, chlorine will bleach it, turning it white

Question 29

how can we test for hydrogen

Answer 29

light a splint, hydrogen is present if you hear a “squeaky pop” sound

Question 30

how to test for oxygen

Answer 30

oxygen will light a glowing splint

Question 31

What type of reaction happens at the anode (positive electrode) during electrolysis

Answer 31

oxidation

Question 32

What type of reaction happens at the cathode (negative electrode) during electrolysis

Answer 32

reduction

Question 33

half equation for the electrolysis of Aluminium oxide

Answer 33

Al3+ + 3e- —> Al (negative cathode)

2O2 —> O2 + 4e- (positive electrode)

Question 34

why must the anode be replaced regularly

Answer 34

because the oxygen molecules produced at the anode react with graphite (carbon) forming co2 gas

Question 35

why is electrolysis expensive

Answer 35

• melting the compound requires a great deal of energy
• a lot of energy is required to produce the electric current