CHAPTER 19. ELECTRON TRANSFER AND ELECTROCHEMISTRY
** Electrochemistry represents the interconversion of chemical energy and
electrical energy. **
Electrochemistry involves redox reactions because electrical energy (flow of electrons) results
from loss of electrons (oxidation) at one place and gain of electrons (reduction) at another place.
1. Can we use the energy released (ΔG) when a spontaneous reaction occurs to do work?
YES (common e.g. car, batteries, barbecue, etc)
2. Can we cause a nonspontaneous reaction to occur by supplying energy? YES
We have already seen one way to accomplish 2, by supplying thermal energy (raising the
temperature) if ΔS > 0. Remember:
ΔG = ΔH TΔS
∴ if ΔS > 0, then TΔS 0).
Thus, products have higher free energy than reactants, e.g., metal plating.
(Surroundings do work on the system.)
GALVANIC OR VOLTAIC CELLS
Question: The previous figure (see above) showed Cu metal in a Ag soln this is a
spontaneous rxn, but how can we use it to do work??
4 Answer: use this reaction in a voltaic cell, which separates the two half reactions and gives a
flow of e between them (Figure 20.5)
The same redox reaction occurs, but reagents are kept separate
Cu(s) + 2Ag → Cu + 2Ag (s)
We find that the concentration of the Cu solution increases with time, concentration
of Ag solution decreases over time.
We find mass of Cu rod decreases. Silver metal is deposited on the silver rod. Electrons
flow through the external circuit and can do work, e.g., can light a bulb, run a motor or
Left compartment (Anode) Cu(s) → Cu + 2 e 2+
Right compartment (Cathode) Ag + e → Ag (s)
Net reaction Cu(s) + 2Ag → Cu + 2Ag (s)
Salt bridge serves to keep the solution neutral; without the salt bridge the Cu solution
would become more and more positively charged, and the Ag solution would become more
and more negatively charged, and the reaction would very soon stop.
** reduction occurs at the cathode (= red cat) **
5 Notation for a Voltaic Cell = shorthand way of describing cell
Reaction: Cu (s) + 2 Ag (aq) → Cu (aq) + 2 Ag (s)
Cell notation: Cu (s) | Cu (aq) || Ag (aq) | Ag (s)
1) vertical bars (|) indicate a phase boundary
2) electrodes placed at far left and right. If electrode not involved in halfreaction, put it
at end and show reagents in order they appear in halfreaction.
e.g. 2 I (aq) → I (s)2+ 2 e (anode = source of e i.e. e on right of eq)
MnO (a4) + 8 H (aq) + 5 e → Mn (aq) + 4 H O (l) 2
(cathode i.e. where e go and are used in a reduction
i.e. e on left)
Electrodes are graphite, platinum, etc. ∴ “inert” (= not involved in rxns)
graphite | I (aq) | I (s) || MnO (aq), H (aq), Mn (aq) | graphite 2+
or Pt (s) | I (aq) | I (s)2|| MnO (aq), H 4(aq), Mn (aq) | Pt (s)
PROBLEM: Cr bar dipping in Cr(NO ) solution on one side of cell, Ag bar in AgNO
3 3 3
solution on the other side, and connected by salt bridge. The Cr electrode is the negative one.
Draw diagram, and give cell reaction and cell notation.
ANSWER: Since Cr (s) | Cr (aq) is the negative electrode, it is the anode
∴ Ag (s) | Ag (aq) is the positive electrode ∴ it is the cathode
∴ Cr (s) → Cr (aq) + 3 e (anode, source of e, put on left of cell)
Ag (aq) + e → Ag (s) (cathode, where e are used, put on right of cell)
6 + 3+
∴ Cr (s) + 3 Ag (aq) → Cr (aq) + 3 Ag (s)
and cell notation is Cr (s) | Cr (aq) || Ag (aq) | Ag (s)
Why and how long does a Voltaic Cell Work?
Why? Because the rxn is spontaneous (ΔG