5.2.9: Standard electrode potentials

Question 1

Define the standard electrode potential of a half cell.

Answer 1

The e.m.f of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 moldm-3 and gas pressure of 100kPa.

Question 2

What is a half cell comprised of?

Answer 2

• A half cell comprises an element in two oxidation states.

- The simplest half cell has a metal placed in an aqueous solution of its ions.

Question 3

For example, how could you set up a copper half cell?

Answer 3

A copper half cell comprises…
-A solution containing Cu2+(aq) ions (oxidation state +2) into which a strip or rod of copper metal (oxidation state 0) is placed.

Question 4

In the example of a copper half cell, an equilibrium exists at the surface of the copper between the two oxidation states of copper. Write the symbol equation for this equilibrium and state which direction is reduction and oxidation.

Answer 4

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

• The forward reaction involves electron gain (reduction)
• The reverse reaction involves electron loss (oxidation)

Question 5

What does the electrode potential of a ha;f cell indicate?

Answer 5

-Its tendency to lose or gain electrons in the equilibrium which exists between the element in its two oxidation states (the metal and its ions)

Question 6

Half cells can also be made from non-metals in equilibrium with non-metal ions.
Give an example.

Answer 6

-A half cell of bromine and its ions.
Br2(g) +2e- 2Br-(aq)

-Hydrogen gas (H2) in contact with hydrogen ions (H+)
2H+(aq) + 2e- H2(g)

Question 7

If a hydrogen half cell were to be connected to another half cell, to form a cell, there would be no piece of metal that could act as an electrode. How is this overcome?

Answer 7

• By the use of a platinum electrode.
• The platinum is inert and does not react at all.
• Its sole purpose is to be in contact with both the H2(g) and the H+ ions to allow the transfer of electrons into and out of the half cell via a connecting wire.

Question 8

What is the platinum electrode coated by in a hydrogen half cell?

Answer 8

-The surface of the platinum electrode is coated with platinum black, a spongy coating in which electrons can be transferred between the non-metal and its ions.

Question 9

What does a standard hydrogen half cell comprise?

Answer 9

• Hydrochloric acid, HCl(aq), of concentration 1 mol dm-3, as the source of H+(aq).
• Hydrogen gas, H2(g), at 100 kPa pressure
• An inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire.

Question 10

Give an example of a half cell containing ions of the same element in different oxidation states (metal ion /metal ion half cell)

Answer 10

-A half cell containing Fe3+ (aq) and Fe2+(aq)

Fe3+(aq) + e- Fe2+(aq)

Question 11

What is a standard Fe3+(aq)/Fe2+(aq) half cell made up of?

Answer 11

• A solution containing Fe2+(aq) and Fe3+(aq) ions with the same concentrations (‘equimolar’)
• An inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire.

Question 12

Different half cells have different electrode potentials. When two half cells are connected to form a cell, they will have an overall cell potential. What is this a measure of and what does it depend on?

Answer 12

• How well electrons can be ‘pushed around’ the cell.
• The larger the overall cell potential, the more electrons are ‘pushed around’.
• The overall cell potential depends on the half cells involved.

Question 13

How can we determine the standard electrode potential of a half cell?

Answer 13

• By connecting it to a hydrogen half cell.
• The hydrogen half cell has an e.m.f value of 0 V, so it can be used as a reference to measure other half cells against.

Question 14

what is the tendency for different half cells to accept or release electrons measured as?

Answer 14

An electromotive force (e.m.f), or voltage, measured in volts, V.

Question 15

What are are two half cells in a cell joined by?

Answer 15

• A wire, this allows electrons carrying charge to flow through it.
• A salt bridge, this connects the two solutions and allows ions carrying a charge to be transferred between the half cells.

Question 16

What is the salt bridge in a cell made of?

Answer 16

-Salt bridges are usually made from a piece of filter paper soaked in an aqueous solution of an ionic substance, usually KNO3(aq) or NH4NO3(aq).

Question 17

What is an electrochemical series?

Answer 17

Half cells in order of their standard electrode potentials.

Question 18

Each reaction in an electrochemical series is shown as a reduction reaction as electrons are gained. What does an negative electrode potential tell us and what does a positive electrode potential tell us?

Answer 18

• If the electrode potential is a negative value, the backward reaction will occur when compared with the standard hydrogen electrode.
• If the electrode potential is a positive value, the forward reaction will occur when compared with the standard hydrogen electrode.

Question 19

How can you work out which half cell undergoes oxidation and which one undergoes reduction?

Answer 19

• The more negative the electrode potential, the greater the tendency towards the half cell undergoing oxidation.
• The more positive the electrode potential, the greater the tendency towards the half cell undergoing reduction, when connected in a cell.

Question 20

What can E(theta) values for standard electrode potentials be used to predict?

Answer 20

-What will happen when half cells are connected to form a cell.