Predictions in Half-Cells

A cell contains a silver anode and a copper cathode. Give the half-cell reactions occurring at the anode and cathode, as well as standard cell notation for this cell.

Step 1: What type of reaction occurs at the anode, and what type occurs at the cathode?

Oxidation is loss of electrons at the anode. So the silver anode will be oxidised.

Reduction is gain of electrons at the cathode. So the copper cathode will be reduced.

Step 2: Write down the half-reactions as they would occur in the cell

\(\text{Ag}(\text{s})\) \(\to\) \(\text{Ag}^{+}(\text{aq}) + \text{e}^{-}\) (oxidation half-reaction)

\(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^{-}\) \(\to\) \(\text{Cu}(\text{s})\) (reduction half-reaction)

Step 3: Give the standard cell notation for this reaction

The anode is always written first (on the left): \(\text{Ag}(\text{s})|\text{Ag}^{+}(\text{aq})\)

The cathode is always written second (on the right): \(\text{Cu}^{2+}(\text{aq})|\text{Cu}(\text{s})\)

Therefore the standard cell notation is:

\(\text{Ag}(\text{s})|\text{Ag}^{+}(\text{aq})||\text{Cu}^{2+}(\text{aq})|\text{Cu}(\text{s})\)

The following half-reactions take place in a cell:

\(\text{Fe}(\text{s})\) \(\to\) \(\text{Fe}^{2+}(\text{aq}) + 2\text{e}^{-}\)

\(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^{-}\) \(\to\) \(\text{Cu}(\text{s})\)

Determine the overall reaction that takes place as a balanced chemical equation and in standard cell notation.

Step 1: Identify the oxidation and reduction half-reactions

\(\text{Fe}(\text{s})\) \(\to\) \(\text{Fe}^{2+}(\text{aq}) + 2\text{e}^{-}\) (oxidation half-reaction)

\(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^{-}\) \(\to\) \(\text{Cu}(\text{s})\) (reduction half-reaction)

Step 2: Which metal is the anode and which is the cathode?

Oxidation is loss of electrons at the anode, therefore \(\text{Fe}\) is the anode. Reduction is gain of electrons at the cathode, therefore \(\text{Cu}\) is the cathode.

Step 3: Compare the number of electrons in each equation

There are 2 electrons in both equations, so the charges are balanced.

Step 4: Combine the equations into one equation

\(\text{Cu}^{2+}(\text{aq}) + 2\text{e}^{-} + \text{Fe}(\text{s})\) \(\to\) \(\text{Cu}(\text{s}) + \text{Fe}^{2+}(\text{aq}) + 2\text{e}^{-}\)

Step 5: Clean reaction up by combining appropriate ions and molecules

\(\text{Cu}^{2+}(\text{aq}) + \text{Fe}(\text{s})\) \(\to\) \(\text{Cu}(\text{s}) + \text{Fe}^{2+}(\text{aq})\)

Step 6: Give the standard cell notation for this reaction

The anode is always written first (on the left): \(\text{Fe}(\text{s})|\text{Fe}^{2+}(\text{aq})\)

The cathode is always written second (on the right): \(\text{Cu}^{2+}(\text{aq})|\text{Cu}(\text{s})\)

Therefore the standard cell notation is:

\(\text{Fe}(\text{s})|\text{Fe}^{2+}(\text{aq})||\text{Cu}^{2+}(\text{aq})|\text{Cu}(\text{s})\)