Redox and electrode potentials Flashcards

Question

Why is a high resistance voltmeter added to the circuit?

Answer 1

So that no current flows, acting like an open circuit, allowing the maximum potential difference is calculated.

Answer 2

The E.M.F is the maximum potential difference (voltage) of a cell when no current flows (in an open circuit)

Answer 3

Because the standard hydrogen electrode is used as a reference electrode as it gives a value of 0.00 V and therefore all other electrode potentials are compared to this standard.

Answer 4

It consists of hydrogen gas in equilibrium with H+ ions of concentration 1.00 mol dm-3 (at 100KPa) An inert platinum electrode is in contact with the hydrogen gas and H+ ions. 2H+ (aq) + 2e- ⇌ H2 (g)

Answer 5

Metal / metal ion Non metal / non metal ion Ion / ion (the ions are in different oxidation states)

Answer 6

Standard cell potential = standard potential of positive electrode (reduction) - standard potential of negative electrode (oxidation).

Answer 7

On the surface of the platinum catalyst.

Answer 8

If the standard cell potential is positive. If its negative it is not feasible

Answer 9

The feasibility of a reaction can be determined from the electrode potential however it gives no information about the rate of reaction. Another limitation is many redox reactions arent aqueous. Another limitation is the standard electrode potential only applied under standard conditions therefore if conditions change, le chatelier's principle occurs.

Answer 10

Primary cells are non-rechargeable (irreversible) Secondary cells are rechargeable.

Answer 11

The Daniell cell which consisted of a simple metal-metal ion system. There was a zinc rod immersed in a solution of zinc sulfate, a copper cylinder filled with copper sulfate solution and a porous pot that separated the copper sulfate from the zinc sulfate. Zinc acted as the negative electrode and the copper as the positive. However, the cell is impractical to use as a portable device because of the hazardous liquids in the cell.

Answer 12

Zinc - carbon cells has a zinc casting which acts as the negative electrode, a paste of ammonium chloride which acts as an electrolyte as well as the positive electrode, a carbon rod which acts as an electron carrier in the cell. As the cell discharges, the zinc casting eventually wears away and the corrosive contents of the electrolyte paste can lead out - disadvantage relatively cheap.

Answer 13

Use of an alkaline paste in the electrolyte as they have a longer operating life but are more expensive than regular zinc-carbon cells.

Answer 14

A rechargeable cell which included chemical reactions that can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction.

Answer 15

Lead - acid batteries NiCad cells Lithium cells.

Answer 16

Consist of 6 cells joined together in a series. The cells use lead metal as the negative electrode and lead (IV) oxide as the positive electrode. The electrolyte is sulfuric acid. Used in a commercial car battery. When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead (IV) oxide. They are designed to produce a high current for a short period of time therefore used in powering a starter motor in car engines. The disadvantage of lead-acid batteries are: they are very heavy contain toxic materials: lead and lead (IV) oxide the sulfuric acid electrolyte is very corrosive.

Answer 17

Nickel-cadmium cells. Used in many standard sizes and voltages so that can replace almost any application of traditional zinc-carbon cells. Although they are more expensive, because they can be recharged hundreds of times, they are more commercially viable. The negative electrode = cadmium The positive electrode = nickel (II) hydroxide-oxide system Cadmium is a toxic metal so the disposal of old NiCad cells is an environmental issue. NiCad cells gradually lose their charge after repeated charge cycles when the cell is not fully discharged. Lithium - ion cells dont have this issue.

Answer 18

Lithium ion cells power phones and laptops. Lithium is used because it has a very low density and relatively high electrode potential. The cell consists of a positive lithium cobalt oxide electrode. a negative carbon electrode a porous polymer membrane electrolyte the polyer electrolyte cant leak since its not a liquid or paste, which presents advantages over other types of cells. The cells consist of a sandwich of different layers of lithium cobalt oxide and carbon. When the cell is charged and discharged, the lithium ions flow between the negative and positive through the solid electrolyte. NiCad cells gradually lose their charge after repeated charge cycles when the cell is not fully discharged. Lithium-ion cells dont have this issue. Some disadvantages: A global shortage of lithium is lieky to make lithium ion cells unsustainable. If the cells arent recycled but thrown away in landfills, then a huge amount of lithium becomes lost to future generations Reports of lithium ion cell fires have raised concerns about their safety. Lithium is a very reactive element in group 1, which is why it has a high electrode potential.

Answer 19

A fuel cell is an electrochemical cell in which fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.

Answer 20

As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell. Different electrolytes and fuels can be used to set up different types of fuel cells.

Answer 21

The hydrogen - oxygen fuel cell

Answer 22

A fuel cell which combined both elements to release energy and water. The fuel cell consists of a reaction chamber with separate inlets for H2(g) and O2(g), an outlet for the product - water, an electrolyte of aq NaOH and a semi-permable membrane that separates the H2(g) and O2(g)

Answer 23

Water is the only reaction product so the fuel cells have environmental advantages over other types of cells. The reaction is the same as hydrogen combusting in oxygen but since the reaction takes place at room temp without combustion, all the bond energy is converted in electrical energy instead of heat and light. No harmful oxides of nitrogen produced - which normally happens in high temp combustions where air is present Fuel cells have been used on space craft, where the product can be used as drinking water for astronauts.

Answer 24

Hydrogen is a highly flammable gas and the production and storage of hydrogen carries safety hazards. Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts. The production of hydrogen is a by-product of the crude oil industry therefore its a non-renewable finite source Until a cheap way is found to make hydrogen, the widespread use in fuel cells is limited Hydrogen has high energy density (the amount of energy contained in 1g of the fuel is high compared to other fuels) but because its a gas, its energy density per unit volume is low which means larger container are needed compared to liquid fuels

Answer 25

The cell operates continuously as long as there is a supply of hydrogen and oxygen - the energy is not stored in the cell.

Answer 26

Zn2+ (aq) + 2e- -> Zn (s) E0 = -0.76V 2NH4+ (aq) + 2e- -> 2NH3 (g) + H2 (g) E0 = +0.74 2NH4+ (aq) + Zn (s) -> 2NH3 (g) + H2(g) + Zn2+ (aq) Eθcell = +1.50V

Answer 27

Pb (s) + SO4 2- (aq) -> PbSO4 (s) + 2e- E0 = -0.36V PbO2 (s) + 4H+ (aq) + SO4 2- (aq) + 2e- -> PbSO4 (s) + 2H2O (l) Eθ = +1.70 V bO2 (s) + 4H+ (aq) + 2SO42- (aq) + Pb (s) -> 2PbSO4 (s) + 2H2O (l) Eθcell = +2.06 V

Answer 28

Nickel - cadmium cell Cd (s) + 2OH- (aq) -> Cd(OH)2 (s) + 2e- Eθ = -0.82V NiO(OH) (s) + H2O (l) + e- -> Ni(OH)2 (s) + OH- (aq) Eθ = +0.38V 2NiO(OH) (s) + 2H2O (l) + Cd (s) -> 2Ni(OH)2 (s) + Cd(OH)2 (s) Eθ = +1.2 V

Answer 29

Li (s) → Li+ (s) + e– Eθ = -3 V Li+ (s) + CoO2 (s) + e– → Li + (CoO2) – (s) Eθ = +1 V Li (s) + CoO2 (s) → Li + (CoO2) – (s) Eθcell = +3.5

Redox and electrode potentials Flashcards

(53 cards)