L10 - Cell Diagrams

objectives

Cell Notation of the Example Cell

Figure 2. In this standard galvanic cell, the half-cells are separated; electrons can flow through an external wire and become available to do electrical work.

There are many possible galvanic cells, so a shorthand notation is usually used to describe them.
The cell notation (sometimes called a cell diagram) provides information about the various species involved in the reaction.
This notation also works for other types of cells.
A vertical line, │, denotes a phase boundary and a double line, ‖, the salt bridge.
Information about the anode is written to the left, followed by the anode solution, then the salt bridge (when present), then the cathode solution, and, finally, information about the cathode to the right.
The cell notation for the galvanic cell in Figure 2 is then:

Note that spectator ions are not included and that the simplest form of each half-reaction was used.
When known, the initial concentrations of the various ions are usually included.
One of the simplest cells is the Daniell cell.
It is possible to construct this battery by placing a copper electrode at the bottom of a jar and covering the metal with a copper sulfate solution.
A zinc sulfate solution is floated on top of the copper sulfate solution; then a zinc electrode is placed in the zinc sulfate solution.
Connecting the copper electrode to the zinc electrode allows an electric current to flow.
This is an example of a cell without a salt bridge, and ions may flow across the interface between the two solutions.
Some oxidation-reduction reactions involve species that are poor conductors of electricity, and so an electrode is used that does not participate in the reactions.
Frequently, the electrode is platinum, gold, or graphite, all of which are inert to many chemical reactions. One such system is shown in Figure 3.
Magnesium undergoes oxidation at the anode on the left in the figure and hydrogen ions undergo reduction at the cathode on the right.
The reaction may be summarized as:

The cell used an inert platinum wire for the cathode, so the cell notation is:

Active and Inert Electrodes

The magnesium electrode is an active electrode because it participates in the oxidation-reduction reaction.
Inert electrodes, like the platinum electrode in Figure 3, do not participate in the oxidation-reduction reaction and are present so that current can flow through the cell.
Platinum or gold generally make good inert electrodes because they are chemically unreactive.

Figure 3. The oxidation of magnesium to magnesium ion occurs in the beaker on the left side in this apparatus; the reduction of hydrogen ions to hydrogen occurs in the beaker on the right. A nonreactive, or inert, platinum wire allows electrons from the left beaker to move into the right beaker. The overall reaction is: Mg + 2H+ ⟶ Mg2+ + H2, which is represented in cell notation as: Mg(s) │ Mg2+(aq) ║ H+(aq) │ H2(g) │ Pt(s).

activity 1 - using cell notation

Using Cell Notation

Consider a galvanic cell consisting of

2Cr(s)+3Cu²⁺(aq)⟶2Cr³⁺(aq)+3Cu(s)

Write the oxidation and reduction half-reactions and write the reaction using cell notation. Which reaction occurs at the anode? The cathode?

Solution

By inspection, Cr is oxidized when three electrons are lost to form Cr3+, and Cu2+ is reduced as it gains two electrons to form Cu. Balancing the charge gives the overall reaction found below this drop box

Cell notation uses the simplest form of each of the equations, and starts with the reaction at the anode. No concentrations were specified so:

Cr(s)∣Cr³⁺(aq)∥Cu²⁺(aq)∣Cu(s)

Oxidation occurs at the anode and reduction at the cathode.

activity 2 - using cell notation

Using Cell Notation

Consider a galvanic cell consisting of

5Fe²⁺(aq)+MnO₋₄(aq)+8H⁺(aq)⟶5Fe³⁺(aq)+Mn²⁺(aq)+4H₂O(l)

Write the oxidation and reduction half-reactions and write the reaction using cell notation. Which reaction occurs at the anode? The cathode?

By inspection, Fe2+ undergoes oxidation when one electron is lost to form Fe3+, and MnO4− is reduced as it gains five electrons to form Mn2+. Balancing the charge gives the reaction found below this dropbox.

Cell notation uses the simplest form of each of the equations, and starts with the reaction at the anode. It is necessary to use an inert electrode, such as platinum, because there is no metal present to conduct the electrons from the anode to the cathode. No concentrations were specified so:

Pt(s)∣Fe²⁺(aq),Fe³⁺(aq)∥MnO₋₄(aq),H⁺(aq),Mn²⁺(aq)∣Pt(s)

Oxidation occurs at the anode and reduction at the cathode.

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