Electrolysis and Extracting Metals

Question 1

What is oxidisation?
What is reduction?
What is the pneumonic to rememer?

Answer 1

Process of losing electrons in a reaction

Process of gaining electrons in reaction

(OIL RIG)

Question 2

Why do molten compounds conduct electricity?

How do aqueous solutions conduct electricity?

Answer 2

• Ions are free to move (breaks ionic lattice)
• allows flow of charge to move = electrical current
• when substance dissolves in water, ionic bonds are broken
• splits compound into its ions which are free to move

Question 3

Describe the (typical) electrodes used in electrolysis
What process happens at each electrode?

Answer 3

Usually made of carbon (graphite) as it is good electrical conductor

Anodes are positive electrodes as they attract anions- OXIDATION

Cathodes are negative as they attract cations- REDUCTION

Question 4

What is an electrolyte?

Answer 4

Substance that has electricity passed through it

Question 5

What are the half equations for electrolysis of molten compounds?

Answer 5

At cathode:
Positive cation reduced (gains electrons) to form element

At anode:
Negative anion oxidised (loses electrons) to form element

Question 6

What are the rules for the electrolysis of aqueous solutions?

Answer 6

At cathode:
If metal MORE reactive than hydrogen, HYDROGEN is produced.
If metal is Cu, Ag, Au or Platinum, metal is produced.

At anode:
If non-metal is halide, halogen is produced.
Else 4OH- minus 4e- ——-> 2H2O and O2.

(The metal/hydrogen and non-metal/hydroxide that hasn’t reacted will react together eg. Sodium Hydroxide)

Question 7

What are the uses of:
Chlorine? Hydrogen? Sodium hydroxide (brine)?

What apparatus is used in industry for electrolysis of brine?

Answer 7

Killing bacteria in water, bleach | Making margarine | Making soap and paper

Diaphragm cell (diaphragm in middle to separate products as toxic)

Question 8

What is the standard method for electrolysis calculations?

Answer 8

1) balanced half equations
2) Q = IT
3) moles of electrons = Q/F
4) molar ratio
5) moles to mass/volume

Question 9

What is the definition of electrolysis?

Answer 9

The use of electricity to separate compounds into elements

Question 10

How is aluminium extracted from its main ore?

Answer 10

Bauxite (main ore) is treated to produce pure aluminium oxide, which has high melting point so would be too expensive.
Al2O3 is dissolved in molten cryolite to reduce melting point.
learn diagram
Aluminium produced at cathode, oxygen at anode

Question 11

What machine is used for extraction of iron?

Answer 11

Blast furnace

Question 12

What are the inputs into extraction of iron?

Answer 12

Iron ore (hematite) , limestone and coke (carbon)

Question 13

Write the 4 equations that occur in extraction of iron

Answer 13

C + O2 —> CO2
CO2 + C —> 2CO

Fe2O3 + 3CO —> 2Fe + 3CO2

SiO2 + CaCO3 –> CaSiO3 + CO2

Question 14

Why do covalent compounds not conduct electricity?

Answer 14

They have no free electrons and no ions

No flow of charge

Question 15

What does one Faraday represent and what is its value?

Answer 15

Represents one mole of electrons

96 500 C

Question 16

What are the half equations in aluminium extraction?

Answer 16

Al3+ plus 3e- —> Al

O2+ minus 2e- —> O2

Question 17

What are the properties and uses of cast iron?

Answer 17

Very impure and brittle, doesn’t shrink when solidified

Used for castings eg. Manhole covers, guttering

Question 18

Properties and uses of mild steel?

Answer 18

Iron with 0.25% carbon
Iron increases hardness and strength

Used for wire, nails, car bodies etc.

Question 19

Properties and uses of wrought iron?

Answer 19

Pure iron
Soft and lacks strength

Used for decorative work eg. gates and railings

Question 20

Properties and uses of high-carbon steel?

Answer 20

Iron with 1.5% carbon
Hard but brittle

Used for cutting tools and masonry nails

Question 21

Properties and uses for stainless steel?

Answer 21

Alloy of iron, chromium and nickel
Strong oxide layers protect it from corrosion

Used for cutlery, cooking utensils, kitchen sinks etc.

Question 22

How does iron rust? (What is formula of rust)

Answer 22

Iron corrodes in presence of O2 and H2O
It loses electrons to form Fe2+ ions, then oxidised by air to Fe3+

FeO3.xH2O

Question 23

How can you prevent rust?

Answer 23

Barriers
Alloying iron
Sacrificial metals