Voltaic Cell Diagrams — Overview & Notation

We've been learning about redox reactions. But it's been a pain to write all those half-reactions! Do you need to write a balanced equation? Write the half-reactions. Do you need to find the change in oxidation state? Write the half-reactions. Do you need to calculate the voltage? Write the half-reactions. Do you need to find the Gibbs free energy change for electrical work? Start by writing the half-reactions.

Thankfully, chemists are lazy. We don't like having to write all those half-reactions. So, we developed a special notation for voltaic cells. Can you draw a vertical line? If yes, you're well on your way to being an expert at voltaic cell notation!

Rules for Writing Cell Notation

1. Electrons flow from the anode to the cathode. So, we start with the anode and move right towards the cathode.

2. We separate species within a half-cell by a single vertical line (|). We always move from reactants to products. For example, the anode's half-reaction is anode|ion. But, we write cathode half-reaction as ion|cathode.

3. We use two vertical lines (||) to denote the salt bridge. So our total voltaic cell looks like anode|anode's ion||cathode's ion|cathode. We don't list spectator ions in the notation.

4. Denote the phase in parenthesis. For example, anode (s)|anode's ion (aq)||cathode's ion (aq)|cathode (s). Remember, sometimes we have gases. For example, the standard hydrogen electrode has hydrogen gas.

5. Are any species not at standard conditions? Write their concentration or pressure in the parenthesis. For example, we write a 0.15 molar solution as (aq, 0.15M). For a gas at 0.45 atmospheres, we write (g, 0.45atm).

Example Problems

1. We have a voltaic cell made of the transition metals zinc and copper. Assume we are at standard conditions. Write the cell notation.

• First, we need to determine which metal is the cathode and anode. Do you remember how? We look at our standard electrode potentials. Copper has a larger voltage (0.34V) than zinc (-0.76V). So, copper will be the cathode.

• Remember, the problem says we are at standard conditions, so we only need to write the phases. If we don't denote the ion concentrations, we assume they are one molar.

$Zn(s)|Zn^{2^+}(aq)||Cu^{2+}(aq)|Cu(s)$

1. Write the cell notation for, the alkali metal, sodium, and the standard hydrogen electrode.

• We have the standard hydrogen electrode. So, we assume standard conditions.

• The standard hydrogen electrode has a higher voltage. So, it's the cathode.

$Na(s)|Na^{+}(aq)||H^{+}(aq)|H_{2}(g)$

1. Let's make a voltaic cell from $0.23M$ sodium chloride and $0.33M$ copper (II) chloride. Write the cell notation.

• First, notice that we don't have standard conditions. We will have to include the ion concentrations.

• Next, determine the cathode and anode. Copper has a larger voltage, so it's the cathode.

• Also, notice that the chloride ion is present in both cells. So, it's a spectator ion. We don't include it in the notation.

$Na(s)|Na^{+}(aq, 0.23M)||Cu^{2+}(aq, 0.33M)|Cu(s)$

• Notice that sodium and copper have different charges. That's okay! The cell notation does not account for the stoichiometry.

(Video) 19.3b Cell Notation & Cathodic Protection | General Chemistry

by Chad's Prep

In this section of the video, Chad explains cell notation. He demonstrates two examples. The first is a normal voltaic cell. But, the second example is trickier. It has an inert electrode. After watching this clip, you'll understand cell notation!