Electrochemistry

Question 1

Define the standard electrode potential.

Answer 1

The electromotive force between a half cell and a standard hydrogen electrode under standard conditions: 1 moldm-3, 1 atm and 298K

Question 2

What is the change if the concentration of a half cell is non standard?

For example, what happens when concentration of aqueous copper is increased in:

Cu2+(aq) + 2e- Cu(s)

Answer 2

According to Le Chat’s principle, the reaction moves to the other side to oppose the change, so the solid is favoured rather than its solution.

The equilibrium position moves to RHS to minimise change in concentration, therefore producing more Cu(s)

Question 3

What does the E value indicate about a particular half cell? (positive and negative values)

Answer 3

A negative E value means it is more likely to loose electrons and be oxidised. A positive E value means the species is more likely to be reduced.

All standard electrode potentials are given as reduction reactions by default, so a -ve E value means it opposes the reduction.

Question 4

What is the purpose of the salt bridge?

Answer 4

It completes the circuit. It is soaked in KNO3 which does not react, but will carry charge.

Question 5

What kind of E value would you expect from a strong oxidising agent?

Answer 5

Positive E value. It reduces, so will gain electrons, so will be positive, as E values are for reduction equations.

Question 6

By convention, what are all half cell E value reactions?

Answer 6

Reductions - they are gaining electrons

Question 7

What apparatus do we use for a hydrogen half cell?

Answer 7

A gas electrode which allows hydrogen to enter at the top at 1 atm, and oxidise to H+ in solution with the platinum electrode which is highly inert

Question 8

What E value suggests the reaction is feasible?

Answer 8

If the E value is greater than 0.3, it will most likely react.

Question 9

What are the limiting factors of using E values to determine feasibility?

Answer 9

Many reactions won’t occur under standard conditions, so will be shown not to occur, whereas in reality the conditions just need to change.

Some reactions do not occur spontaneously in aqueous solutions, so will appear not to react.

Question 10

What are the two half reactions for a hydrogen fuel cell under alkaline conditions?

Answer 10

H2 + 2OH- –> 2H2O + 2e-

H2O + 0.5O2 + 2e- –> 2OH-

Question 11

What are the two half reactions for a hydrogen fuel cell under acid conditions?

Answer 11

2H2 + 4e- –> 4H+

4H+ + O2 + 4e- —> 2H2O

Question 12

What is the overall reaction of any hydrogen fuel cell?

Answer 12

2H2 + O2 –> 2H2O

Question 13

What is the efficiency of an internal combustion engine compared to a hydrogen fuel cell vehicle

Answer 13

ICE - 20%

FC - 40-60%

Question 14

Name two disadvantages of hydrogen fuel cells?

Answer 14

The adsorbers deteriorate over time to hold less hydrogen.

The storage must be under high pressure to ensure good space efficiency. This has safety hazards, in case it blows up.

Question 15

What are the two most common methods of storing hydrogen?

Answer 15

Adsorbing onto a surface

Absorbing into a hydrogen sponge like lanthanum

Question 16

When setting up an equation for an overall half cell, what is the only other thing you can add other than the species in the half equations and the electrons?

Answer 16

The species relating to the conditions, e.g. H+ or OH- if acid or alkali. This can be useful to form water with irritating left over oxygens. Balance the other species first, work out how many oxygens you have left over, then add the H+ to form H2O. Remember to add the electrons too.

Question 17

Give two reasons why a reaction may not happen, even if predicted to be feasible.

Answer 17

It may not have sufficient energy to reach activation energy so will be a slow reaction. The conditions may need to be non standard.

Question 18

What is a possible way to product hydrogen without reforming from hydrocarbons?

Answer 18

Hydrolysis of H2O

Question 19

When you have to two E values with correct signs, how do you find the overall cell potential?

Answer 19

Add them up.

Question 20

What is the rule for working out the half equations for hydrogen fuel cells under different conditions?

Answer 20

Hydrogen always reacts with/forms conditions. This is always the oxidation reaction
2H2 + 4OH- —> 4H2O + 4e-

or

2H2 —> 4H+ + 4e-

The second reaction combines the oxygen with the products of the first reaction.