Cells and half cells

Question 1

Explain what a copper half cell is and the equilibrium that occurs in it.

Answer 1

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

An equilibrium exists at the surface of the copper between these oxidation states of copper:

Cu²+(aq) + 2e- ⇔ Cu(s)

Question 2

Describe, explain and draw a cell.

Answer 2

A simple electrochemical cell can be made by connecting together two half cells with different electrode potentials:

• One half cell releases electrons and the other half cell gains electrons.
• The wire connects the two metal allowing electrons to be transferred between the two half cells. The voltmeter has a high resistance and is used to minimise the current that flows. If a small bulb were to replace the voltmeter, the bulb would light up.
• The salt bridge connects the two solutions, allowing ions to be transferred between the half cells. A simple salt bridge can be made out of a strip of filter paper soaked in an aqueous solution of an ionic compound that does not react with either of the half-cell solutions. Usually aqueous KNO₃ or NH₄NO₃(aq) is used.

Question 3

In a cell what does the reading of the voltmeter tell you?

Answer 3

The reading on the voltmeter measures the potential difference of the cell - this measures the difference between the electrode potentials of the half cells.

Question 4

Draw a hydrogen half cell.

• Explain all the standard conditions for the half cell setup.

Answer 4

A hydrogen half cell comprises hydrogen gas, H₂, in contact with H*(aq) ions.

• Hydrochloric acid, HCl(aq), of concentration 1 mol dm^-3, as the source of H*.
• hydrogen gas, H₂(g), at 100KPa ( 1 atmosphere) pressure.
• An inert paltinum electrode to allow electrons to pass into or out of the half cell via a conecting wire.

Question 5

Explain the use of the platinum electrode in a hydrogen half cell.

Answer 5

Platinum electrode is place in the solution so that it is in contact with both H2(aq) and H*(aq) ions. The platinum is inert and does not react at all - its sole purpose is to allow the transfer of electrons into and out of the half cell via a connecting wire. 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 6

Draw and describe a metal ion/metal ion half cell such as Fe³⁺ and Fe²⁺.

Answer 6

• Fe²+(aq) + e- ⇔ Fe³+

A standard Fe²+(aq)/Fe³+(aq) half cell is made up of:

• A solution containing Fe²+ and Fe³+ 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.