Dry Cell: a galvanic cell with the electrolyte contained in a paste thickened by starch. anode and an inert graphite cathode.

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Joan Hicks
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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

1 1 BATTERIES Text Pages: , 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!

) 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.

2 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 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)

4 4 For the spontaneous galvanic reaction: LEOA: Pb (s) + SO 4 PbSO 4(s) + 2 e - GERC: PbO 2(s) + 4 H + + SO e - PbSO 4(s) Overall cell reaction: Pb (s) + PbO 2(s) + 4 H 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 e - the PbO 2 anode!) Overall cell reaction: 2 PbSO 4(s) Pb (s) + PbO 2(s) + 4 H 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 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.

QUESTION. Electrochemistry. Dry Cell Battery. Electrochemical Processes in Batteries. Chemistry of Batteries QUESTION

QUESTION. Electrochemistry. Dry Cell Battery. Electrochemical Processes in Batteries. Chemistry of Batteries QUESTION Meals Ready to Eat (MREs) were developed during the Vietnam War. They need hot water to be reconstituted. The military includes a small packet of metal to heat the water through an oxidation-reduction