Video Transcript
The following tables use the M and M 𝑛+ symbols to represent atoms and ions. The M symbol represents a metal atom. The M 𝑛+ symbol represents its associated cation. The 𝑛 symbol denotes the number of electrons that are needed to change the M 𝑛+
cation into the M-type atom. Which table correctly describes reactions at the cathode and anode of a galvanic
cell? (A) Reaction at the anode, M reacts to form M 𝑛+ plus 𝑛e‒; reaction at the cathode,
M 𝑛+ plus 𝑛e‒ reacts to form M. (B) Reaction at the anode, M 𝑛+ plus 𝑛e‒ reacts to form M; reaction at the cathode,
M reacts to form M 𝑛+ plus 𝑛e‒. (C) Reaction at the anode, M 𝑛+ plus 𝑛e‒ reacts to form M; reaction at the cathode,
M 𝑛+ plus 𝑛e‒ reacts to form M. Or (D) reaction at the anode, M reacts to form M 𝑛+ plus 𝑛e‒; reaction at the
cathode, M reacts to form M 𝑛+ plus 𝑛e‒.
To answer this question, we need to know about half-cells, which can be described as
electrodes in solutions of ions. Two half-cells make up an electrochemical cell, which is a system that converts
chemical energy to electrical energy or vice versa. Here, we are interested in galvanic cells, which specifically converts chemical
energy to electrical energy through redox reactions.
Let’s look at a cell diagram to help visualize a galvanic cell. Here, we see pieces of zinc and copper placed in solutions. Each beaker acts as a half-cell. The materials act as electrodes to transfer ions to and from the solution. The salt bridge contains a nonreactive ion species, such as KNO3, to help complete
the circuit. The current generated by the difference in potentials between the two half-cells can
perform work, such as powering a lightbulb.
In this example, it is favorable for zinc to undergo oxidation and copper to undergo
reduction. Therefore, 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 is oxidized and its ions enter
into a solution, while electrons travel across the wire and combine with copper ions
to form the copper solid. Here, we can see the half reactions written at each terminal.
And now if we replace the elements from this specific example with the general
letters M and 𝑛, we can revisit and answer the question. Which table correctly describes reactions at the cathode and anode of a galvanic
cell? And we can see that the correct answer is answer choice (A). Reaction at the anode, M reacts to form M 𝑛+ plus 𝑛e‒; reaction at the cathode, M 𝑛+
plus 𝑛e‒ reacts to form M.
