Electrolysis

Question 1

Electrolysis

Answer 1

Is the process which uses electricity to split up molten or aqueous ionic compounds.

Question 2

Electrolyte

Answer 2

Is a substance which conducts electricity and can be split up by a current when molten or in a solution. It will contain ions which are able to move.

Question 3

Ions

Answer 3

Are charged particles formed when atoms lose or gain electrons.

Question 4

Cation

Answer 4

Positively charged ion (goes to cathode).

Question 5

Anion

Answer 5

A negatively charged ion (goes to anode).

Question 6

Oxidation

Answer 6

Is a type of reaction involving the gain of oxygen or the loss of electrons.

Question 7

Reduction

Answer 7

A type of reaction involving the loss of oxygen or the gain of electrons.

Question 8

Redox

Answer 8

A type of reaction in which oxidation and reduction take place at the same time.

Question 9

Electrode

Answer 9

Is a solid conductor of electricity, which is used to make electrical contact with an electrolyte.

Question 10

When does hydroxide (OH-) go to the anode?

Answer 10

Unless the other negatively charged ion is a halide, hydroxide will go to the anode and form oxygen gas.
E.G CL- or OH-, CL- will go.

Question 11

What happens when the hydroxide goes to anode?

Answer 11

Hydroxide goes to the anode but forms oxygen gas.

Question 12

When does hydrogen go to the cathode?

Answer 12

If the metal is more reactive than hydrogen, hydrogen will go to the cathode and form H2.
If the metal is less reactive than hydrogen, that metal ion will go to the cathode (copper, silver and gold).

Question 13

What is an electric current?

Answer 13

A flow of electrons or ions

Question 14

What is the anode and what is the cathode?

Answer 14

PANiC
The anode is positive - Oxidation always happens at the anode
The cathode is negative - Reduction always happens at the cathode

Question 15

What are the electrodes usually made out of?

Answer 15

Graphite as it is cheap and doesn’t decay quickly

Question 16

When molten lead bromide is electrolysed what goes to the anode?

Answer 16

Bromide ions as the negative bromide ions are attracted to the positive anode
-When the bromIDE ions get to the anode, the extra electron which makes the bromide ion negatively charged moves onto the electrode because this electrode is short of electrons
-The loss of the extra electron turns each bromIDE ions into a bromINE atom. These join in pair to make bromine molecules
2Br- –>Br2 + 2e-

Question 17

When molten lead bromide is electrolysed what goes to the cathode?

Answer 17

Lead ions as the negatively charged lead ions are attracted to the negative cathode
-When the positive lead ions reach the cathode, they pick up two electrons from the electrode to form neutral lead atoms. Since they have lost their charge they fall to the bottom of the container as molten lead
Pb2+ + 2e- –> Pb

Question 18

What happens when the power is switched on but the lead bromide is still solid?

Answer 18

• As soon as you connect the power source, it pumps mobile electrons away from the left-hand electrode to the right-hand electrode
• The excess of electrons on the right-had electrode makes it negatively charged. The left-hand electrode is positively charged because it is short of electrons. There is a limit to how many electrons the ‘pump’ can squeeze into the negative electrode because of the repulsion by the electrons already there

Question 19

What happens when lead bromide is electrolysed?

Answer 19

When the lead bromide melts:

• The bulb lights up showing that electrons are flowing through it
• There is bubbling around the anode as bromine gas is given off
• Nothing seems to happen at the cathode but afterwards metallic lead is found underneath it
• When you stop heating lead (II) bromide solidifies again, everything stops - there is no more bubbling and the bulb goes out

Question 20

Can all ionic compounds be electrolysed?

Answer 20

Some can’t even when they are molten as their melting point is too high e.g. sodium chloride (801℃) but it ca be done industrially

Question 21

What happens during the electrolysis of sodium chloride solution (Brine)?

Answer 21

1. Cl- goes to the anode (oxidised) (colour of litmus is white (bleached))
   - Two chloride ions lose their electrons and become one chlorine molecule
   - Chlorine gas is given off
2. H+ goes to the cathode (reduced) (colour of litmus is blue)
   - Two hydrogen ions accept two electrons to become one hydrogen molecule
   - Hydrogen gas is given off.
3. NaOH is left in solution

Question 22

What is the equation for the electrolysis of sodium chloride solution (Brine)?

Answer 22

2NaCl + 2H2O –> H2 + Cl2 + 2NaOH

Question 23

Describe a bit about the chlor-alkali industry

Answer 23

• In the modern method salt is pumped out of the ground as brine
• Beginning of 19th century salt was used to make soap, bleach and soda (domestic) and salt products to make textile, paper and glass (industry)
• Electrolysis is used to decompose sodium chloride solution

Question 24

What are some uses for the products of the electrolysis of sodium chloride solution (Brine)?

Answer 24

1. Chlorine: manufacture of bleach and HCL, PVC, disinfectant (so kills bacteria), sterilising water supplies (chlorination)
2. Hydrogen: fuel cells, used in Haber process, to make ammonia (fertiliser), to change oils into fats for making margarine
3. NaOH: manufacture of bleach, paper and soap
   - Sodium hydroxide is a very strong base and is used widely in the chemical industry

Question 25

In a diaphragm cell which particles are prevented from passing through the porous diaphragm?

Answer 25

- Chlorine (chloride ions and not chlorine gas) (and hydrogen??) - If chlorine comes into contact with the sodium hydroxide solution, it reacts to make bleach - If chlorine comes into contact with hydrogen, it produces a mixture which would explode violently on exposure to sunlight or heat to give hydrogen chloride

Question 26

How has the NaOH become alkaline?

Answer 26

As the NaCl contains a halide, that goes to the cathode to form chlorine, leaving OH- in the solution which is alkaline, the H+ ions have gone to the cathode and been removed The OH- ions join with the Na+ ions left, to form NaOH which is alkaline

Question 27

In general aqueous solutions what goes to the cathode? (m+ or H+)

Answer 27

1. If the metal is more reactive than hydrogen (the only ones which are less are copper, gold and silver), the H+ ions will go to the cathode to form H2: 2H+ + 2e- --> H2 2. If the metal is less reactive than hydrogen the metal ion goes to the cathode, as since it is below hydrogen int he reactivity series its ion is easier to discharge: Cu2+ (aq) + 2e- --> Cu (s)

Question 28

In general aqueous solutions what goes to the anode? (OH- or x-)

Answer 28

- If x- is a halide (F-, Cl-, Br-, I-) than the halide forms at the anode: 2F- --> 2e- + Cl2 - In all other cases (since they take more time to get to the anode as they are bigger and bulkier than OH-), OH- ions go to the anode and from oxygen gas (O2): 4OH- (aq) --> O2 (g) + 2H2O (l) + 4e-

Question 29

What are the typical equation in general aqueous solutions?

Answer 29

H2O --> H+ + OH - | mx (aq) --> m+ + x-

Question 30

Describe what happens in the electrolysis of Dilute Sulphuric acid?

Answer 30

1. Twice as much hydrogen is produced than oxygen 2. Every time one O2 is formed, you can get two H2. 3. For every four electrons that flow around the circuit you can get one molecule of oxygen, but you can also can instead two molecules of hydrogen 4. There is twice as much hydrogen produced as oxygen because each oxygen releases four electrons and each hydrogen only needs two electrons. therefore twice as much hydrogen is produced 5. You get twice the number of molecules of hydrogen as of oxygen, and twice the number of molecules occupy twice the volume 6. The concentration of sulphuric acid is increasing as the water is electrolysed and oxygen and hydrogen are lost from the solution 7. You would not be able to use HCL as it has a halogen so chlorine could get mixed

Question 31

Describe what happens in the electrolysis of Dilute Sulphuric acid?

Answer 31

- Product at the cathode: the carbon electrode is coated in copper - Product at the anode: oxygen - Left in the solution: hydrogen and sulphate ions, which will turn into dilute sulphuric acid, the electrolysis will then continue as for dilute sulphuric acid - Eventually after a very long time, all the copper (II) ions will be used up and so the colour of the solution will fade from blue to colourless

Question 32

What happens at the anode during the purification of copper?

Answer 32

At the anode the Cu atoms are pulled from the anode to become positively charged Cu2+ ions. The Cu2+ ions are repelled from the anode (as it is positively charged) and flows through the solution and is attracted to the cathode (as it is negatively charged). The Cu2+ ions are then deposited as copper atoms and stick to the cathode due to a metallic bond

Question 33

What happens at the cathode during the purification of copper?

Answer 33

At the cathode the Cu2+ ions gain electrons to become copper atoms. The copper atoms 'stick' to the copper cathode forming a new layer

Question 34

What happens to the size of the electrodes during the purification of copper?

Answer 34

The pure copper cathode, increases in size, whilst the anode gets smaller. The impurities left behind at the anode form a silt layer/mud (usually gold Au/silver Ag) beneath it

Question 35

What actually happened experimentally?

Answer 35

(The mass lost by the anode should equal the mass gained by the negative electrode?) However the mass gained by the cathode was less that the mass lost by the anode why: -The anode may have lost impurities -Some of the copper may have feel into the solution (weak attachment) and even more when handling -Cathode was still wet (we didn't use propane) (wouldn't the results be the other way round?) but more could have been lost in the drying process (like above second statement) -Some copper ions could have come from the copper sulphate solution (wouldn't the results be the other way round?) ALWAYS assume that there are some impurities

Question 36

What does electrolysis involve?

Answer 36

- The formation of new substances when ionic compounds conduct electricity 1. If you pass an electric current through an ionic substance that is molten or in solution it breaks down into new substances, this is called electrolysis 2. It requires a liquid to conduct the electricity called the electrolyte 3. Electrolytes are made by meting or dissolving ionic compounds 4. In either case it is the free ions which conduct the electricity 5. For the circuit to be complete, there has got to be a flow of electrons. Electrons are taken away from the ions at the anode and given to other ions at the cathode 6. As ions gain or lose electrons they become atoms or molecules

Question 37

How do you distinguish between electrolytes and non-electrolytes?

Answer 37

- Electrolytes are liquids that conduct electricity 1. When you place a conductivity probe in an electrolyte, current flows through the circuit, so you can measure its conductivity 2. When you place a conductivity probe in a non-electrolyte, no current flows, so you will get a reading of zero conductivity 3. Another way of determining whether a substance is an electrolyte or not is to set up an electrolytic cell and if the substance undergoes electrolysis the it is an electrolyte

Question 38

Describe the electrolysis of copper (II) sulphate

Answer 38

- A solution of copper (II) sulphate (CuSO4) contained four different ions, Cu 2+, SO4 2-, H+ and OH- - Cooper ions accept electrons more easily than hydrogen ions, so at the cathode, copper metal is produced - Hydroxide ions lose electrons more easily than sulphate ions, so at the anode oxygen and water are produced

Question 39

How is sodium hydroxide and chlorine manufactured?

Answer 39

By the electrolysis of concentrated sodium chloride solution (brine) in a diaphragm cell