1 BATTERIES Text Pages: 764-766, 787,788 Battery: a set of galvanic cells connected in series - The negative electrode of one cell is connected to the positive electrode of the next cell - The total voltage for a set of cells is the sum of the individual voltages. There are two general types of batteries: primary batteries and secondary batteries. A.Primary battery: a disposable battery Three types of primary batteries are dry cell, alkaline and button batteries. 1. Dry Cell Batteries Dry Cell: a galvanic cell with the electrolyte contained in a paste thickened by starch The cheapest AAA-, AA-, C-, and D-size batteries are dry cells. Dry cells have a maximum potential of 1.55 V. Dry cells stop producing electricity when the reactants are used up. Dry cells consist of a Zn (s) anode and an inert graphite cathode. The electrolyte is a paste of MnO 2, ZnCl 2, NH 4 Cl and carbon black, C (s) (soot) The half-cell reactions are: Zn 2+ (aq) + 2 e- GERC: Overall cell reaction: 2 MnO 2(s) + 2 e - Mn 2 O 3(s) + 2 OH - (aq) 2 MnO 2(s) + Zn (s) Mn 2 O 3(s) + Zn 2+ (aq) + 2 OH- (aq) Note that the cathode reaction is complicated, hence only an approximation has been given!
2 2. Alkaline Cell Battery These are slightly more expensive than dry cell batteries. They are also improved and longer lasting. The NH 4 Cl and ZnCl 2 of dry cells are replaced by KOH (hence the name alkaline!) The half-cell reactions are: + 2 OH - (aq) ZnO (s) + 2 e- GERC: MnO 2(s) + 2 e - Mn(OH) 2(s) + 2 OH - (aq) Overall cell reaction: Zn (s) + MnO 2(s) ZnO (s) + Mn(OH) 2(s) 3. Button Cell Battery These are much smaller than alkaline batteries and are commonly used in watches, hearing aids and pacemakers. Two common types of button cell batteries both have a zinc container that acts as the anode and an inert stainless steel cathode. One type has an alkaline electrolyte paste that contains HgO, and has a voltage of about 1.3 V.
3 The half-cell reactions for this cell are: + 2 OH - (aq) ZnO (s) + 2 e- GERC: HgO (s) + 2 e - Hg (l) + 2 OH (aq) Overall cell reaction: Zn (s) + HgO (s) ZnO (s) + Hg (l) Can you think of one disadvantage to this type of battery? The other type of button cell battery has an alkaline paste that contains Ag 2 O and has a voltage of about 1.6 V. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ B. Secondary battery: a rechargeable battery Two types of secondary batteries are lead storage and Ni-Cd batteries. 1. Lead Storage Batteries (Lead-Acid Batteries) These batteries are typically found in your car and consist of 6 cells with a total potential of ~ 12 V. When in use the cells in the battery operate like a galvanic cell they partially discharge and release electricity. At the same time a generator supplies electricity to the lead storage battery. This supplied electricity (external voltage) forces the reverse, non-spontaneous, cell reaction. The cell now behaves electrolytically and is recharged. Lead storage batteries have anodes consisting of powdered lead packed into a grid and cathodes consisting of powdered PbO 2 packed into another grid. The electrolyte is a 4.5 mol/l solution of H 2 SO 4 (aq).
4 For the spontaneous galvanic reaction: LEOA: Pb (s) + SO 4 PbSO 4(s) + 2 e - GERC: PbO 2(s) + 4 H + + SO 4 + 2 e - PbSO 4(s) Overall cell reaction: Pb (s) + PbO 2(s) + 4 H + + 2 SO 4 2 PbSO 4(s) For the nonspontaneous galvanic reaction: GERC: PbSO 4(s) + 2 e - Pb (s) + SO 4 ( @ the Pb cathode!) LEOA: PbSO 4(s) PbO 2(s) + 4 H + + SO 4 + 2 e - ( @ the PbO 2 anode!) Overall cell reaction: 2 PbSO 4(s) Pb (s) + PbO 2(s) + 4 H + + 2 SO 4 2. Ni-Cd (Nicad) Battery These usually contain three cells in series with a total potential of ~ 1.4 V. The anode is pressed cadmium, the cathode is NiO(OH) and the electrolyte is KOH or NaOH. For the spontaneous galvanic reaction: LEOA: Cd (s) + 2 OH (aq) Cd(OH) 2(s) + 2 e- GERC: 2 NiO(OH) (s) + e - 2 Ni(OH) 2(s) + 2 OH (aq) Overall cell reaction: Cd (s) + 2 NiO(OH) (s) Cd(OH) 2(s) + 2 Ni(OH) 2(s) Rechargeable batteries cannot be recharged infinitely, and eventually they do wear out.
5 ELECTROCHEMISTRY WORKSHEET #8 1. The reaction products in a silver button battery are solid zinc oxide and solid silver. Write the two half-reactions and the equation for the overall reaction in the battery. 2. Write the two half-reactions and the overall cell reaction for the process that occurs when a Ni- Cd battery is being recharged. 3. What external voltage is required to recharge a lead storage battery? 4. The iron-chromium redox battery makes use of the reaction Cr 2+ (aq) + Fe 3+ (aq) 6 Cr 3+ (aq) + Fe 2+ (aq) occurring at a chromium anode and an iron cathode. (a) Sketch and label a cell diagram for this battery. Note: When all reactants and products are aqueous, use inert electrodes. (b) Calculate the theoretical voltage of the battery.