Electrochemistry & Electrochemical Cells (Batteries) · Electrochemistry & Electrochemical Cells (Batteries) Zn Cu 2+ Cu 2+ Cu Cu Cu Zn Cu2+ Zn ... Daniell Cell with Salt Bridge Na+
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Line Notation(Shorthand Notation for Electrochemical Cells)
electrode sp.1(aq), sp.2(aq) … sol. liq. gas sp.1(aq), … sol. liq. gas electrode
Anode half-cell Cathode half-cell
Notes:1. Only the species appearing in each net-ionic half rxn. are listed.2. Species in same solution are separated by a comma.3. Species in different phases are separated by a vertical line.4. The first half-cell listed is always for the Anode.5. The electrodes are listed at the extremes.6. A double line is used if there is a physical separation between the two half-
cell compartments.
Example for the Daniell cell (with and without the salt bridge cells before):
Zn Zn2+ Cu2+ Cu
Question: How would you design an electrochemical cell for which the net-ionic rxn. is:
If there are competing half-rxns., the one with the higher will be the most favored.
Electrolysis of Aqueous NaCl
What are the possible reduction rxns.?
Na+(aq) + e– Na(s), εo = –2.71 V
2H2O(l) + 2e– H2(g) + 2OH–, εo = –0.83 V
NaCl(aq) Na+(aq) + Cl–(aq)
What are the possible oxidation rxns.?
2Cl–(aq) Cl2(g) + 2e–, εo = –1.36 V
2H2O(l) O2(g) + 4H+ + 4e–, εo = –1.23 V (So, is it really a bit easier to oxidize H2O)
But , note the following:1. Non-Standard State effects favor Cl– oxidation: The cell is never allowed to reach
standard-state conditions. The solution is typically 25% NaCl by mass, which significantly decreases the potential required to oxidize the Cl – ion.
2. pH Effects do not favor oxidation of H2O: The pH of the cell is also kept very high (see H2O reduction above), which decreases the oxidation potential for water.
3. Overvoltage: This is the deciding factor. It is the extra voltage that must be applied to a reaction to get it to occur at the rate at which it would occur in an ideal system.
Compare with the electrolysis of molten NaCl: 2Na+(l) + 2Cl–(l) 2Na(l) + Cl2(g)
Because the demand for chlorine is much larger than the demand for sodium, electrolysis of aqueous sodium chloride is a more important process commercially. Electrolysis of an aqueous NaCl solution has two other advantages. It produces H2
gas at the cathode, which can be collected and sold. It also produces NaOH, which can be drained from the bottom of the electrolytic cell and sold.
If the porous barrier is removed, the products of the electrolysis of aqueous sodium chloride react to form sodium hypochlorite, which is the first step in the preparation of hypochlorite bleaches, such as Chlorox.
If 96,485 C of charge is passed through a cell,equivalent amounts react and are produced
The charge on 1 mol of e’s is 96,485 C and an “equivalent” amount of any substance is that quantity that reacts with or is produced from 1 mol of e’s in a redox rxn.:
96,485 C ↔ 1 mol e’s ↔ 1 eq. of substanceIn short, use stoichiometry on the half-rxns. or the overall rxn. (example to follow)
Some Relevant Electrical PropertiesEquationUnitSymbolProperty
Question:If 1.50 A of current flows through an aqueous solution containing CuCl2 for 15.0 min, how many grams of Cu(s) are deposited at the _____ode?
Answer:Write the relevant net-ionic half-rxn.: Cu2+ + 2e– Cu(s)Use the Unit-Factor Method for the Electrical Stoichiometry:
Cu g _________ Cu mol 1
Cu g 546.63
e mol 2
Cu mol 1
C 96,485
e mol 1s .900
s
C 5.1
Q=It
Question:In the electrolysis of CuCl2(aq) above, the _____tion of Cl– to Cl2(g) occurs at the _____ode. ? Assume that standard state conditions apply with the current and time from the problem above, how much electrical energy (in J and in kWh) and power (in W) were needed?
If the electrolytic cell for the electrolysis of water draws a current of 0.775 A for 45.0 minutes, calculate the volumes of H2 and O2 produced if each gas is collected at 25 oCand P = 1.00 atm.
Problem:
An electroplating apparatus is used to coat jewelry with gold. What mass of gold can be deposited from a solution that contains Au(CN4)– ions if a current of 5.0 A flows for 30.0 min?
Galvanized Iron (Plated with Zn):Zn is the “sacrificial” anode; easier to oxidize Zn than Fe (check it!).Fe is now “forced” to be the cathode, but of course an inert one:Anode: Zn Zn 2+ + 2e–
Cathode: 2H2O + 4e– 4OH–
Aluminum:Al undergoes “passivation”: 2Al + 1½O2(g) Al2O3(s)Once the oxide is formed, it serves as a thin film that prevents any more Al to come into contact with the environment and react.
Automobile headlights typically draw 5.9 A of current. The galvanic cell of a lead storage battery consumes Pb and PbO2 as it operates. A typical electrode contains about 250 g of PbO2. Assuming that the battery can supply 5.9 A of current until all the PbO2 has been consumed, how long will it take for a battery to run down if the lights are left on after the engine is turned off?
Fuel Cell is an electrochemical cell that produces electricity from a fuel tank. The electricity is generated through the reaction, triggered in the presence of an electrolyte, between the fuel (on the anode side) and an oxidant (on the cathode side). The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.Fuel cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished – a thermodynamically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system.