Video Transcript
What is the correct cell notation for a half-cell consisting of a piece of silver
metal placed in a solution of silver ions? (A) Ag+ aqueous, one molar bar Ag solid. (B) Ag solid bar Ag+ aqueous. (C) Ag solid bar Ag+ aqueous, one molar. (D) Ag solid forward slash Ag+ aqueous, one molar. Or (E) Ag solid backward slash Ag+ aqueous.
To answer this question, let’s discuss properties of half-cells.
In electrochemistry, a half-cell refers to an electrode placed into a solution of
ions. Two half-cells correctly connected make up an electrochemical cell that can convert
chemical energy to electrical energy or vice versa. We are interested in galvanic cells, which specifically convert chemical energy to
electrical energy. Let’s look at a diagram to visualize a galvanic cell.
Here, we see pieces of zinc and copper placed in a solution. Each beaker thus acts as a half-cell. The pieces of metal act as electrodes to transfer charge to and from the solutions in
the beakers. The salt bridge contains a nonreactive ionic species, such as KNO3, to help complete
the circuit. However, in this diagram, we have a high-resistance voltmeter attached to the
electrodes. This prevents electrons from traveling but allows us to measure the potential
difference between the two-half cells created by the difference in their tendencies
to be reduced.
Let’s now look at the diagram where we replaced the voltmeter with a wire. In this example, zinc undergoes oxidation, and copper undergoes reduction. By definition, the zinc half-cell is the anode, and the copper half-cell is the
cathode. If we track the electron flow, we can see that zinc ions enter the solution, while
electrons travel across the wire and combine with copper ions to form copper
solid.
Here you can see the half reactions written at each terminal. Looking closely at the chemical equations, by combining the two half reactions, we
can arrive at the full redox equation. We should note that because electrons cancel out, we don’t need to write them into
the equation. To simplify the drawing of cells, we can draw a cell notation diagram, which we need
to answer the question.
Since the only information we are given is that we have one half-cell of silver metal
and silver ions, we will write the half-cell showing the oxidation process. We start by writing the solid metal and follow it with a vertical line or bar
representing the phase boundary. We then write the corresponding ion, making sure you note the concentration and that
it is an aqueous solution.
At this point, we should be able to revisit and answer the question. What is the correct cell notation for a half-cell consisting of a piece of silver
metal placed in a solution of silver ions? And using the standard notation techniques we have outlined for half-cells, we can
correctly choose (C) as the correct answer: Ag solid bar Ag+ aqueous, one molar.
