Electrode Potentials and Fuel Cells

Question 1

Explain the term redox

Answer 1

reduction and oxidation together as complementary processes - the transfer of electrons
reduction - gain of electrons, loss of oxygen, gain of hydrogen or reduction in oxidation number.
oxidation - loss of electrons, gain of oxygen, loss of hydrogen or an increase in oxidation number.

Question 2

Explain the term oxidation number

Answer 2

the charge an atom would have if it were present in an ionic compound - e.g. if water were ionic H would have 1+ charge and O2-

Question 3

What is an oxidising agent? reducing agent?

Answer 3

oxidising agent - a substance that accept electrons and is reduced reducing agent - a substance that donates electrons and is oxidised.

Question 4

what is disproportionation?

Answer 4

the simultaneous oxidation and reduction of one type of atom within a species

Question 5

What are the rules for assigning oxidation numbers?

Answer 5

• The oxidation number of an atom in an element, e.g. H2, is 0
• The oxidation number of an atom in a simple ion e.g. Mg2+ is the charge on that ion
• The sum of oxidation numbers in a neutral compound is 0
• The more electronegative element in a covalent compound is given the negative number.
• The sum of oxidation numbers in an ion with more than one element equals the charge on the ion
• group 1: always 1+
  group 2: always 2+
  hydrogen: 1+ except in metal hydrides(-1)
  fluorine: -1
  oxygen: -2 except with fluorine(+2), peroxides(-1) and superoxides(-0.5)

Question 6

What rules to follow when combining/balancing half equations to predict redox equations?

Answer 6

• use oxidation numbers to see what has been oxidised and what has been reduced
• balance atoms using H2O to balance oxygen atoms and H+ to balance hydrogen atoms
• balance charge using electrons: on the right if oxidation has occurred, on the left if reduction has occurred
• when combining equations to deduce overall equation, multiply throughout so that electrons cancel when the 2 equations are added together

Question 7

define the term standard electrode potential

Answer 7

the potential difference between the standard hydrogen electrode and a half cell at 298K, 1atm and all concentrations having a concentration of 1moldm-3

Question 8

describe how to measure, using a hydrogen electrode, standard electrode potentials of metals or non-metals,

Answer 8

Metals: an electrode made of the metal is placed contact with their ions in aqueous solution, connected to a hydrogen half cell by a voltmeter and a salt bridge
Non-metals: if solid, see above. If gas, pass gas over solution of its aqueous ions with a platinum electrode in.

Question 9

describe how to measure, using a hydrogen electrode, standard electrode potentials of ions of the same element in different oxidation states;

Answer 9

An inert platinum electrode is placed in a solution containing the two aqueous ions

Question 10

What does a more negative electrode potential say about a metal?

Answer 10

The more reactive the metal, the greater the tendency to lose electrons and so the more negative the charge on the metal surface.
More negative electrode potential = more reactive metal = better reducing agent - it is the negative electrode, electrons flow from it to ‘positive’ electrode.

Question 11

Given the standard electrode potential data for elements in a half cell how can the cell reaction be deduced?

Answer 11

Half equations are combined with the more positive electrode potential going forwards (reduction) and the more negative backwards (oxidation). Electrons on either side cancel out and the combined reactions give the cell reaction.

Question 12

What are electrode potentials a measure of?

Answer 12

Oxidising and reducing strength:
more negative = better reducing agent
more positive = better oxidising agent

Question 13

How is cell potential calculated?

What does the cell potential tell you about the feasibility of the reaction?

Answer 13

By finding the difference in potential between the reduction half cell and the oxidation half cell. Ecell = Ered - Eox
e.g. the more positive value - the more negative value
If Ecell is positive, then the reaction is feasible, if negative it is not.

Question 14

Why might a reaction with a positive Ecell not appear to be feasible?
- limitations of predictions made using standard cell potentials

Answer 14

May have a high activation energy
May have a low rate
Non-standard conditions also have a significant effect on electrode potential values and can alter the sign of Ecell and therefore the value.

Question 15

How can principles of electrode potentials be applied to modern storage cells?

Answer 15

A storage cell (or battery) is an electrochemical cell that can generate an electric current because of the difference in potential between two half cells.
A rechargeable battery has a reversible cell reaction that can be reused after recharging with an external power source

Question 16

How do fuel cells create a voltage?

Answer 16

uses the energy from the reaction of a fuel e.g. hydrogen with oxygen
the two are separated by a polymer electrolyte membrane

Question 17

explain the changes that take place at each electrode in a hydrogen–oxygen fuel cell

Answer 17

In the hydrogen half-cell, hydrogen gas is oxidised to hydrogen ions in the presence of platinum
H2 –> 2H+ + 2e-
The hydrogen ions cross the electrolyte (e.g. polymer membrane) where they combine with oxygen to produce water
1/2O2 + 2H+ + 2e- –> H2O
Overall cell reaction: H2 + 1/2O2 –> H2O

Question 18

What is the effect of diluting the concentration on the electrode potential of a half cell?

Answer 18

in a metal/metal ion equilibriumL
Mn+ + ne- –> M
Diluting the solution reduced the concentration of the Mn+ ion and thus disturbs the equilibrium, which shifts to the left to compensate, forming more electrons and therefore making the half cell electrode potential more negative

Question 19

What are fuel-cell vehicles?

Answer 19

due to the finite nature of fossil fuels and global warming, vehicles are being developped that are powered by fuel cells, fuelled by:

• hydrogen gas
• hydrogen-rich fuels e.g. methanol, natural gas or petrol, which are converted into H gas by an onboard reformer

Question 20

what are some advantages of FCVs over conventional petrol or diesel-powered vehicles?

Answer 20

• less pollution and less CO2 as the only product is water(if using pure hydrogen - hydrogen-rich fuels produce only small amounts of air pollutants and CO2)
• greater efficiency: 45% compared with 22% for a diesel vehicule

Question 21

what are some disadvantages of hydrogen fuel cells?

Answer 21

• the fuel cells themselves are made with toxic chemicals and disposal is a problem
• have a limited lifetime, requiring regular replacement and disposal, and high production costs
• difficulties with the storage and transport of hydrogen: leaks easily, is very flammable, efficiency drops if hydrogen is stored as a high-pressure gas, , feasibility of a pressurised liquid and a limited life cycle of a solid ‘adsorber’ or ‘absorber’

Question 22

A ‘hydrogen economy’ may contribute largely to future energy needs, but what are some limitations of using hydrogen as a fuel?

Answer 22

• problems with the safety, storage and handling of hydrogen
• hydrogen is an energy carrier and not a source of energy - energy has to be used to extract it from water (by electrolysis) or other sources
• is has not gained public and political acceptance as a fuel
• currently no infrastructure designed to supply hydrogen as a fuel