Video Transcript
The following figure describes a
galvanic cell. The cell contains a copper anode
and a silver cathode. What symbols should replace boxes
one through four? (A) NO3−, K+, electron,
electron. (B) K+, NO3−, electron,
electron. (C) K+, NO3−, K+, NO3−. (D) Electron, NO3−, K+, NO3−. (E) Electron, electron, K+,
NO3−.
In this question, we want to
determine which symbols should replace the boxes in the pictured galvanic cell.
A galvanic cell is a type of
electrochemical cell where electrons are generated spontaneously through a redox
reaction. In a galvanic cell, chemical energy
is converted to electrical energy. These electrons pass through an
external circuit, which is the wire. Since electrons move in the wire,
we can immediately label boxes three and four as electrons. And so we can see that only answer
choices (A) and (B) are feasible answers, since they have electrons as labels for
boxes three and four. Therefore, we can eliminate answers
(C), (D), and (E).
To complete the circuit, ions move
in the beakers and in the salt bridge. The copper metal and the silver
metal rods are called electrodes. An electrode is a conductor used to
transfer electrical current to and from a nonmetallic part of a circuit, in this
case solutions of ions. We are told that the copper
electrode is the anode and the silver electrode is the cathode. The anode is where oxidation
occurs, and it is the negative electrode. The cathode is where reduction
occurs, and it is the positive electrode.
The positive and negative charges
of the electrodes in a galvanic cell depend on the ease with which a metal substance
can gain electrons and be reduced, giving us values for the standard reduction
potential of various chemicals. In this case, we do not have the
values for copper and silver. So we are unable to compare their
respective values. However, we do have arrows on the
diagram indicating the direction of the electron flow.
Due to electrostatic repulsion,
electrons will move away from the more negative electrode. And oxidation will occur as
electrons are being lost. As electrons are lost, so Cu2+ ions
will leave the anode and go into solution, giving this oxidation half equation. Due to electrostatic attraction,
negatively charged electrons traveling in the wire are attracted to the positively
charged cathode. Ag+ ions in solution can combine
with the incoming electrons to form silver metal atoms according to this reduction
half reaction.
The electrolyte solution in the
salt bridge is aqueous potassium nitrate, which contains potassium ions and nitrate
ions. As the electrons move around the
circuit, the concentration of copper two plus ions in the anode half-cell beaker
draws or attracts the negatively charged nitrate ions into the left-hand beaker. So label one must be NO3−. And as the concentration of silver
plus ions in the cathode half-cell beaker decreases over time, so the right-hand
beaker will draw the positively charged potassium ions from the salt bridge into the
right-hand beaker. So label two must be K+.
The correct label symbols match
answer choice (A), not (B). Finally, what symbols should
replace boxes one through four? The answer is (A) NO3−, K+,
electron, electron.
