5.2.3- Electrode Potentials and Fuel Cells

Question 0

What is an oxidising agent

Answer 0

A substance that brings about the oxidation of another substance e.g. by removing electrons (it is reduced itself)

Question 1

What is a redox reaction

Answer 1

A reaction that involves oxidation (loss of electrons/increase in oxidation number) and
reduction (gain of electrons/decrease in oxidation number)

Question 2

What is a reducing agent

Answer 2

A substance that brings about the reduction of another substance e.g. by providing electrons (it is oxidised itself)

Question 3

What coating is used on the platinum electrode so that electrons can be transferred

Answer 3

Platinum black (It provides a large surface area)

Question 4

What aqueous compound is used in the salt bridge

Answer 4

KNO3 or NH4NO3

Question 5

Define the standard electrode potential of a half cell

Answer 5

It is the e.m.f of a half cell compared with standard hydrogen half cell, measured at standard conditions (298K, concentrations of one of 1 moldm^-3, and gas pressure of 100 kPa (1 atmosphere)

Question 6

Define the standard cell potential

Answer 6

The EMF between the two half cells making up the cell under standard conditions’

Question 7

Why should EMF be measured as soon as you connect the half cells

Answer 7

Because the concentrations of ions will change as the reactions occur at each electrode.

Question 8

How do fuel cells work

Answer 8

They use the energy from the reaction of a fuel with oxygen to create a voltage. The reactants flow in and the products flow out, while the electrolyte remains in the cell. Fuel cells do not have to be recharged and can operate virtually continuously as long as there is a constant supply of fuel and oxygen into the cell (they are open systems).

Question 9

What are the three main types of electrochemical cell

Answer 9

Non-rechargeable cells, rechargeable cells, fuel cells

Question 10

What are the electrode reactions and the overall reaction in an alkaline membrane (eg . KOH) hydrogen fuel cell?

Answer 10

Negative electrode: H2(g) + 2OH-(aq) → 2H2O(l) + 2e-
Positive electrode: 1/2O2(g) + H2O(l) + 2e- → 2OH-(aq)
Overall reaction: H2(g) + 1/2O2(g) → H2O(l)

Question 11

What are the electrode reactions and the overall reaction in an acid membrane (eg. Phosphoric Acid) hydrogen fuel cell?

Answer 11

Negative electrode: H2(g) → 2H+(aq) + 2e-
Positive electrode: 1/2O2(g)+ 2H+(aq) + 2e→H2O(l)
Overall reaction: H2(g) + 1/2O2(g) → H2O(l)

Question 12

What are the two types of fuels for fuel cell vehicles that are being developed

Answer 12

Hydrogen gas and hydrogen rich fuel cells

Question 13

What three ways can hydrogen gas be stored for FCVs?

Answer 13

As a liquid under pressure, aDsorbed onto the surface of a solid material, aBsorbed within a solid material

Question 14

Name some examples of hydrogen rich fuels and explain how they are converted to H2(g)

Answer 14

Methanol, natural gas or petrol.

They are converted by and on-board reformer in the reaction: CH3OH +H2O → 3H2 + CO2

Question 15

What are the advantages of using hydrogen rich fuel cells rather than hydrogen gas fuel cells

Answer 15

These are liquids and so are easier to store and transport than hydrogen; they are also not explosive, unlike hydrogen. However, these fuel cells would produce carbon dioxide.

Question 16

What are the advantages of fuel cell vehicles over conventionally powered vehicles

Answer 16

1. Less pollution and less CO2: pule H2 only produces water and hydrogen rich fuels only produce small amounts of CO2.
2. Great to efficiency: more than twice the efficiency (40-60% vs 20% for petrol engines). Therefore fuel consumption halves. (Other advanced technologies in the future can further increase efficiency)

Question 17

State for logistical limitations of hydrogen fuel cells

Answer 17

1. Storing and transporting hydrogen has safety problems (it is explosive).
2. The feasibility/convenience of storing a pressurised liquid.
3. The limited life cycle of current and adsorbers and absorbers as they need replacement and are expensive to produce
4. The use of toxic chemicals in their production

Question 18

The Hydrogen economy will contribute largely to the future of our energy needs, but name three limitations that need to be solved.

Answer 18

1. Hydrogen is an ‘energy carrier’ not energy source and takes energy to produce. This energy should come from renewable sources to make it carbon neutral.
2. Public and political acceptance of hydrogen as a fuel
3. Logistical problems in handling and maintenance of hydrogen systems

Question 19

As a cell approaches full charge, the aqueous KOH electrolyte decomposes to form H2 at one electrode and O2 at the other, why are the half equations that cause this? (PPQ)

Answer 19

4OH- –> O2 + 2H2O + 4e-

2H2O + 2e- –> H2 + 2OH-