Electrode Potentials and Fuel Cells

Question 1

What is the oxidation state of uncombined elements?

Answer 1

Zero.

Question 2

For a monoatomic ion, what is the oxidation state of the element?

Answer 2

Its the same charge of the ion.

Question 3

What are the oxidation states of oxygen?

Answer 3

The oxygen state in all compounds is -2 but in hydrogen peroxide (H2O2), when it is -1, or in OF2 when it’s +2.

Question 4

What is the sum of all oxidation states in a neutral compound?

Answer 4

Zero.

Question 5

What must the sum of all the oxidation states in a compound equal?

Answer 5

The overall charge.

Question 6

What is oxidation?

Answer 6

The loss of electrons.

Question 7

What is reduction?

Answer 7

The gain of electrons.

Question 8

What is a reducing agent?

Answer 8

A reactant that causes another reactant to be reduced in a reaction (itself, usually, being oxidised).

Question 9

What is an oxidising agent?

Answer 9

A reactant that causes another reactant to be oxidised (itself usually being reduced).

Question 10

What do fuel cells do?

Answer 10

They generate electricity from an electrochemical reaction in which oxygen (from air) and a fuel (e.g. hydrogen) combine to form water. The electricity produced can be use to power cars, buses etc. The by-product, heat, can also be used.

Question 11

Describe the structures of fuel cells.

Answer 11

Fuel cells consist of 2 electrodes, a negative anode and a positive cathode.
The electrodes are separated by a by a solid or liquid electrolyte.
Catalysts (e.g. Pt) are often used to speed up reaction.
Electricity is generated when oxygen and hydrogen combine to form water.

Question 12

In a Proton Exchange Membrane Fuel Cell (PEMFC), what is the fuel and what is the catalyst?

Answer 12

Fuel is Hydrogen, catalyst is Platinum.

Question 13

In a Proton Exchange Membrane Fuel Cell (PEMFC), describe the operation.

Answer 13

• Hydrogen is oxidised to protons at the anode
• Protons move through the electrolyte
• Electrons pass through the external circuit
• Oxygen is produced at the cathode
• Water is produced
• A catalyst accelerates the reaction at the electrodes

Question 14

In a Proton Exchange Membrane FUEL CELL (PEMFC), What happens at the anode?

Answer 14

2H2(g) -> 4H+(aq) + 4e-

Eo = 0.00V

Question 15

In a Proton Exchange Membrane FUEL CELL (PEMFC), what happens at the cathode?

Answer 15

O2(g) + 4H+(aq) + 4e- -> 2H2O(l)

E0 = +1.23V

Question 16

In a Fuel Cell (PEMFC), what is the overall reaction?

Answer 16

2H2(g) + O2(g) -> 2H2O(l)

Eo = +1.23V

Question 17

What happens if other substances travel though an electrolyte?

Answer 17

They can disrupt the chemical reaction.

Question 18

Why use fuel cells?

Answer 18

Need alternative to fossil fuels which are non-renewnable and prices and resources are dwindling. The atmosphere is becoming more polluted.

Question 19

What are the limitations of using hydrogen-oxygen fuel cells?

Answer 19

• storage of hydrogen and transportation
• feasibility of liquefied hydrogen under pressure
• limited life cycle of ad/bsorber and cell
• high production cost and use toxic chem. in cells.

Question 20

How is hydrogen manufactured?

Answer 20

Ideally from non-polluting and renewable resources; (solar, wind or hydropower), from hydrocarbon fuels by reforming, from methane of ethanol, electrolysis of water.

Question 21

How is hydrogen made from methane?

Answer 21

Reforming

CH4 + H2O –> CO + 3H2

Question 22

How do you store hydrogen?

Answer 22

• liquid stored under pressure or
• adsorbed on the surface of a solid or
• absorbed within a solid

Question 23

What are the advantages of having fuelled cell vehicles (FCV’s)?

Answer 23

• Produce less pollution from exhaust gases (no NOx, CO, unburnt hydrocarbons)
• produce less CO2
• are more efficient

Question 24

What are the pros for using fuel cells?

Answer 24

• Water is the waste product (no poluuting waste products)

- It is more efficient that conventional cells

Question 25

What are the cons of using fuel cells?

Answer 25

• They’re expensive to make (Pt and membrane is $$).
• Production of fuel cells involve the use of toxic chemicals, which need to be disposed of afterwards ($$$).
• Have a limited life span so need to be replaced which means new ones need to be made and old ones need to be disposed of (which is $$$).

Question 26

Describe the standard hydrogen electrode.

Answer 26

Reaction: 2H+(aq) + 2e- H2(g)

Pt electrode, 1M of H+ solution, H2 gas at 1atm, potential is 0.00V.

Question 27

What is standard electrode potential?

Answer 27

The potential difference of a cell when the the electrode is connected to the standard hydrogen electrode under standard conditions.

Question 28

What is the standard elecrode potential value affected by?

Answer 28

• temperature
• pressure of any gas
• solution concentration

Question 29

The ultimate reference for SEP is the standard hydrogen electrode. What are its conditions?

Answer 29

298K, 1m= mol dm-3 with respect to H+ and hydrogen at 1atm, Pt catalyst.

Question 30

Why use platinum as a catalyst?

Answer 30

You need an inert metal to get the electrons in/out. They’re inert and aren’t involved in the reaction.

Question 31

What needs to be overcome before a hydrogen economy can be established?

Answer 31

• Ppl concerned over safety and reliability
• Infrastructure for HF supplies is $$ to set up
• If H is going to be the future, it needs to be cheap or cheaper than existing fuels
• Hydrogen is energy carrier so if you want to make clean fuel you need to make it using a clean energy source (e.g. wind) which is v $$.

Question 32

What is meant by the term ‘hydrogen is an energy carrier’?

Answer 32

It needs an energy source to make it.