Video Transcript
Which of the following diagrams depicts the equilibrium that exists between a piece
of zinc metal in contact with an aqueous solution of zinc ions?
The diagrams in this question depict one of the fundamental principles that underpin
galvanic cells. A galvanic cell is a type of electrochemical cell that consists of two
half-cells. Half-cells can be thought of as an electrode submerged in a solution of ions. In the diagrams, we see four potential options for a zinc half-cell with a piece of
zinc metal submerged in a solution of zinc atoms or ions. This experimental apparatus could be connected to another half-cell via a salt
bridge. The circuit could then be completed either by an external device, such as a lightbulb
or a high-resistance voltmeter that can measure the cell potential.
At first glance, we may be surprised by what these diagrams are suggesting. We have no external power supply attached. And so we may be surprised to see any sort of movement of atoms or ions between the
piece of metal and the solution. However, if we look more closely at what is happening in either half-cell, we will
see that there will be movement of atoms into ions in solution, and the reverse
process of ions collecting electrons and becoming reduced atoms of metal deposited
on the electrode.
This process is an equilibrium reaction and can be represented by the reaction shown,
where the letter 𝑀 is the metal in the redox reaction and the letter 𝑛 is the
number of electrons lost or gained in the process. However, we can imagine that this equilibrium is very much in favor of the metal
electrode. And so it is not something we can see on the microscopic level.
Now if we exchange the general example with zinc, as in the question, we should be
able to revisit and answer the question. Which of the following diagrams depicts the equilibrium that exists between a piece
of zinc metal in contact with an aqueous solution of zinc ions? If we now combine our knowledge with the answer choices, we can see in answer choice
(A) we have positive ions in solution with electrons left behind on the
electrode. This matches what we would expect to see when writing a half-cell equation for this
half-cell. Zn2+ aqueous plus two electrons react in equilibrium with Zn solid. And so we can correctly choose answer choice (A).
