Question 17.2: INTERPRETING THE SHORTHAND NOTATION FOR A GALVANIC CELL Give...
INTERPRETING THE SHORTHAND NOTATION FOR A GALVANIC CELL
Given the following shorthand notation
Pt(s)|Sn^{2+}(aq), Sn^{4+}(aq)||Ag^{+}(aq)|Ag(s)
write a balanced equation for the cell reaction, and give a brief description of the cell.
STRATEGY
We can obtain the cell half-reactions simply by reading the shorthand notation. To find the balanced equation for the cell reaction, add the two half-reactions after multiplying each by an appropriate factor so that the electrons will cancel. The shorthand notation specifies the anode on the extreme left, the cathode on the extreme right, and the reactants in the half-cell compartments.
Because the anode always appears at the left in the shorthand notation, the anode (oxidation) half-reaction is
Sn^{2+}(aq) → Sn^{4+}(aq) + 2 e^{-}
The platinum electrode is inert and serves only to conduct electrons. The cathode (reduction) half-reaction is
2 × [Ag^{+}(aq) + e^{-} → Ag(s)]
We multiply the cathode half-reaction by a factor of 2 so that the electrons will cancel when we sum the two half-reactions to give the cell reaction:
Sn^{2+}(aq) + 2 Ag^{2+}(aq) → Sn^{4+}(aq) + 2 Ag(s)
The cell consists of a platinum wire anode dipping into an Sn^{2+} solution—say, Sn(NO_{3})_{2}(aq)—and a silver cathode dipping into an Ag^{+} solution—say, AgNO_{3}(aq). As usual, the anode and cathode half-cells must be connected by a wire and a salt bridge containing inert ions.
Although the anode half-cell always appears on the left in the shorthand notation, its location in a cell drawing is arbitrary. This means that you can’t infer which electrode is the anode and which is the cathode from the location of the electrodes in a cell drawing. You must identify the electrodes based on whether each electrode half-reaction is an oxidation or a reduction.