Redox Chemistry

Question 1

Oxidisation

Answer 1

Gain of o2
Loss of hydrogen
Loss of electrons

Question 2

Reduction defined as

Answer 2

Loss of oxygen
Gain of hydrogen
Gain of electrons

Question 3

Oxidisation number

Answer 3

The charge an atom would have if the bonds were purely ionic

Question 4

Oxygen oxidation number

Answer 4

In combined state: -2
Peroxides: (h2o2) and (na2o2)= -1
F2O= +2

Question 5

Exception to hydrogen oxidation number

Answer 5

If it binds to metallic its -1

If it binds to non metallic its +1

Question 6

Analysing oxidation numbers

Answer 6

An oxidisation can be defined as an increase in oxidation number and a reduction is a decrease

Question 7

Conjugate redox pairs

Answer 7

Conjugate oxidant on the left and conjugate reluctant on the right,

Mg2+/Mg

Question 8

What do standard half cells have?

Answer 8

🔹 Oxidant and conjugate reducta not present
🔹 Electrode is a metal reductant or inert solid if no metal is present in the half equation
🔹 electrolyte contains all relevant ions at 1M
🔹 any relevant gas is bubbled over inert electrode at 1 atm
🔹 temperate is 25°

Question 9

If E° half cell is positive…

Answer 9

If E° half cell is positive half cell was positive relative to SHE, oxidant was reduced and the potential difference = E° value

Question 10

If E° value is negative

Answer 10

If half cell is negative the reductant was oxidised and potential difference = -E° value

Question 11

Metal displacement reactions

Answer 11

More reactive metal (oxidised) lower E° will react with the cation of a less reactive metal (reduced) higher E°

Question 12

What happens if reactants in a half cell are allowed to mix?

Answer 12

If they come into direct contact chemical energy is transformed to heat energy, not electrical energy

Question 13

Half cell

Answer 13

Contains a conjugated redox pair, often as the electrode in contact with the solution

Question 14

Electrodes

Answer 14

They function as electron conductors

Question 15

Electrolyte

Answer 15

Solution that conducts electricity due to mobile ions

Question 16

Wire

Answer 16

The wire is an electron conductor

Question 17

Salt bridge

Answer 17

Completed the circuit (current flows via mobile ions)

Maintains electrical neutrality - prevents the build up of charge in the half cell

Question 18

Where do electrons flow?

Answer 18

External circuit

Anode, wires, galvonimitre and cathode

Question 19

Internal circuit

Answer 19

Is where free moving charged particles flow

Electrolytes and salt bridge

Question 20

Dry corrosion

Answer 20

Is the reaction with O2 to form a metal oxide

Question 21

Wet corrosion

Answer 21

Operates like a mini galvanic cell, where electrons are transferred through the iron and ions flow through the water droplets. Occurs in the presence of water and oxygen

Question 22

Surface of corroding iron

Answer 22

🔹 The cathodic region where oxygen is reduced and occurs at sites of high oxygen concentration
🔹 the anodic region where iron is oxidised occurs in areas of stress in the metal where the lattice has been deformed and/or in areas of low oxygen

Question 23

Corrosion is accelerated in the presence of

Answer 23

Water
Impurities that dissolve in water. Dissolved ions promote corrosion by increasing conductivity of the water, eg salt
Impurities in the iron
A less reactive metal, the less reactive metal will act as the cathode while Fe will be more reactive and lose e- and be oxidised

Question 24

Corrosion is inhibited when iron

Answer 24

Is alloyed with other materials
Has a protective coating (prevents 02 and H20 from reaching the Re) such as a more reactive metal than iron
Has its surface electrochemistry changed (impressed current, sacrificial anode)

Question 25

alloying

Answer 25

The metals oxidise slightly in air, oxide coating produced and is continuous, unreactive and forms an impervious barrier

Question 26

Electroplating

Answer 26

Coats a thin layer of other metals which form a protective coating.
If metal is less reactive than iron it will only protect if the coating is complete. Any holes will promote corrosion

Question 27

Impressed current

Answer 27

Electrical connection to the negative electrode of a power source. The iron is given a negative charge and becomes the site of reduction, not oxidisation

Question 28

Sacrificial anode

Answer 28

Connection to a more reactive metal, stronger reductant which oxidises in preference to the iron. Fe acts as the cathode and the more reactive metal is the sacrificial anode.