Electrochem

Question 1

What increases emf in a galvanic cell

Answer 1

Increase in concentration of reactants
Decrease in concentration of products
LCP: Forward - Increases Voltage
Reverse - Decreases Voltage

Question 2

How to increase current without affecting emf

Answer 2

Larger surface area of the electrode (Increases reaction rate)
A wider, shorter and more conductive salt bridge(Lowers internal resistance in cell)

Question 3

Redox reaction

Answer 3

A reaction involving the transfer of electrons.

Question 4

Oxidation

Answer 4

The loss of electrons.

Question 5

Reduction

Answer 5

The gain of electrons.

Question 6

Oxidising agents

Answer 6

A substance that accepts electrons.

Question 7

Reducing agent

Answer 7

A substance that donates electrons.

Question 8

Anode

Answer 8

The electrode where oxidation takes place.

Question 9

Cathode

Answer 9

The electrode where reduction takes place.

Question 10

Electrolyte

Answer 10

A substance that can conduct electricity by forming free ions when molten or dissolved in solution.

Question 11

Brine

Answer 11

A highly concentrated solution of sodium chloride.

Question 12

Galvanic cell

Answer 12

self-sustaining electrode reactions
conversion of chemical energy to electrical energy
Current is produced to the occurrence of spontaneous redox reactions taking place.
Has the capacity to produce current unless it reaches equilibrium or reaction runs to completion (Voltage = 0V when this happens)

Question 13

Function of Salt bridge

Answer 13

To complete the circuit

- maintaining electrical neutrality in the half-cells

Question 14

Why is a platinum (Inert metal used)

Answer 14

Metal is completely unreactive thus it provides a metal surface for redox reactions to take place

Question 15

Standard Hydrogen Electrode

Answer 15

The standard hydrogen electrode consists of hydrogen gas (H2) bubbled through an electrolyte containing hydrogen ions (H+). Thus it is a redox electrode which forms the basis of the scale of oxidation-reduction potentials.

Question 16

Standard electrode potential conditions

Answer 16

pressure = 101,3 kPa (1 atm)
temperature = 298 K (25 ℃)
concentration = 1 mol.dm−3

Question 17

Ecell = Ecat - Eano

Answer 17

E>0 - Spontneous

E < 0 - Non-spontaneous

Question 18

Electrolytic cells

Answer 18

electrode reactions that are sustained by a supply of electrical energy
conversion of electrical energy into chemical energy

Question 19

Mercury cell

Answer 19

Adv: A relatively high percentage purity of sodium hydroxide is obtained (50%) because the NaOH is separated from the brine.
It also allows Cl2 and H2 to be formed in separate areas. Cl2 and H2 can combine explosively if they met.

Disadv: The mercury cell is more expensive to build, run and maintain than the other cells.
• Mercury is highly toxic causing the NaOH and H2 (High contamination) to be purified before they can be used (which is expensive).
• The toxicity of mercury causes environmental concerns as the disposal of waste mercury is extremely hazardous.

Question 20

Diaphragm cell

Answer 20

Ad: Large quantities of sodium hydroxide and chlorine can be produced at reasonable costs and at quite acceptable levels of purity.

Dis: There is always a small amount of chloride in the NaOH produced.
• There are health and environmental problems with small losses of asbestos in making and using diaphragms. Asbestos is carcinogenic.
• Diaphragm is non-ion selective, so the control of ion movements is limited.

Question 21

Membrane cell

Answer 21

Ad: There is no by-product of sodium chloride due to the membrane preventing the chloride and hydroxide ions to pass through.
• The use of this process results in virtually pure sodium hydroxide being produced as there is no contamination by chloride ions.
• There is very little harmful impact on the environment as the membrane is environmentally friendly.
• It is cheaper to run the membrane cell compared to the other cells.

Question 22

How does a salt bridge maintain neutrality

Answer 22

Anions of the salt bridge move the anode to balance the increase in positive ions, cations of the salt bridge move the cathode to balance the decrease in the positive ions there

Question 23

Observations at electrolysis of salt

Answer 23

Coating at the cathode due reduction of cations into metal

Bubbles at the anode due to oxidation of anions into a gas

Question 24

Electroplating

Answer 24

If thing being coated with a metal like silver use an electrolyte with that cation (AgNO3)

Question 25

Hall Heroult process

Answer 25

Aqueous Al(OH)3 formed by treating the bauxite/impure Al2O3 with NaOH. It is then heated to 1000C in order to convert it back to pure Al2O3 (Alumina)
Alumina then is melted to release Al3+ ions for extraction however it is dissolved in cryolite in order to lower its melting point
It is then put in a reaction vessel with carbon electrodes forming the anode while the graphite lining is the cathode
Cathode - Al3+ is reduced into Al
Anode - Oxide ions are oxidised to form Oxygen gas, react with carbon electrodes to form CO2

Question 26

Potential risks of Hall Heroult process

Answer 26

Requires large amounts of electrical energy = Expensive
Green house emissions from CO2s
Fluoride wastes and emissions are dangerous to environment

Question 27

Why is Na+ reduced in a mercury cell instead of water

Answer 27

Know that in the mercury cell, the nature of the mercury cathode adjusts the electrode potentials of the Na+ ions and water, so that the reduction of Na+ is now more likely than that of water