Video Transcript
The cell diagram can be used to
understand reactions at the cathode and anode of a galvanic cell: Cu solid, Cu2+
aqueous, Ag+ aqueous, Ag solid. Which table shows the correct
cathode and anode equilibrium reactions for this galvanic cell system? (A) The anode reaction is Cu solid
reacting to form Cu2+ aqueous plus two electrons and the cathode Ag+ aqueous plus
electron reacting to form Ag solid. Reaction (B) anode, Ag+ aqueous
plus electron reacting to form Ag solid, and cathode, Cu solid reacting to form Cu2+
aqueous plus two electrons. Reaction (C) anode, Cu solid
reacting to form Cu+ aqueous plus electron, and cathode, Ag+ aqueous plus electron
reacting to form Ag solid. Reaction (D) anode, Cu solid
reacting to form Cu2+ aqueous plus two electrons, and cathode, Ag2+ aqueous plus two
electrons reacting to form Ag solid. Or reaction (E) anode, Ag+ aqueous
plus electron reacting to form Ag solid, and cathode, Cu solid reacting to form Cu+
aqueous plus electron.
In this question, we want to know
which of the answers, which all show different half equations, show reactions that
fit the cell notation provided. For a particular galvanic cell,
this is the cell notation, also referred to as a cell diagram. In order to convey the setup of a
galvanic cell, a cell diagram, such as this one, is used to show what substances or
materials are used to make up the cathode and the anode, as well as the electrolytes
in the two half-cells of a galvanic cell.
Let’s clear some space to look at
the cell notation to see which part is the anode information and which part is the
cathode information. And from this, we will determine
the half reactions at the anode and cathode.
A galvanic cell is an
electrochemical cell in which electrons are generated spontaneously by a redox
reaction and then passed through an external circuit. If we examine a galvanic cell, we
will find it is made of two half-cells: one is the anode and the other the
cathode. The anode is positive and is where
oxidation occurs, and the cathode negative and is where reduction occurs. We can write information about the
half-cells in the cell notation.
When we write this information in
the cell diagram, there are certain conventions that we should follow. On the left-hand side, information
regarding the anode is written. We start by writing the metal
electrode symbol, in this case copper, followed by a vertical line. A phase boundary is represented by
this vertical line. The symbol and charge of the
matching ion in solution are then written for the anode half-cell. So, in this question, we have a
solution containing copper two plus ions. Sometimes, the concentration of the
solution is written in parentheses, although not in this example.
The salt bridge linking the two
half-cells is represented by the double vertical line. On the right-hand side, information
regarding the cathode is written, including the metal ion in solution, which in this
case is Ag+ or silver one ions. There is a vertical line for a
phase boundary and the symbol for the cathode metal, which in this case is silver,
symbol Ag.
Since we know the left-hand side of
the cell notation shows the anode, and in this case copper being oxidized to copper
two plus, we can deduce that the half reaction for oxidation is Cu solid reacting
reversibly to give Cu2+ aqueous plus two electrons. And on the right-hand side of the
cell notation at the cathode, we know that reduction occurs with silver plus ions
being transformed into silver metal, with the reduction half reaction being Ag+
aqueous plus electron reacting reversibly to give Ag solid.
The question asked us to identify
the correct cathode and anode equilibrium reactions for this galvanic cell
system. We know the anode half reaction and
the cathode half reaction. All that is left now is to match
these two equations to the correct answer option in the table.
So let’s move these equations up
and bring back the table. The two half reactions in answer
option (A) match our equations. So which answer shows the correct
cathode and anode equilibrium reactions for this galvanic cell system? The answer is (A), for the anode,
Cu solid reacting reversibly to give Cu2+ aqueous plus two electrons, and for the
cathode, Ag+ aqueous plus electron reacting reversibly to give Ag solid.
