Galvanic_Cell

Dry cell compared to Button cell. The common dry cell battery is a galvanic cell that uses a zinc casing as an anode and a manganese dioxide (MnO2) and carbon (graphite) as a cathode. The carbon in the cathode does not involve the reaction, it only conducts the electricity. The electrolyte consists of a paste of ammonium chloride and zinc chloride rather than a solution. The paste prevents the cell contents from mixing and thus a salt bridge is not required. The diagram is shown as below: Oxidation reaction at the anode ( left ): Zn (s) → Zn2+ (aq) + 2e-. Reduction reaction at the cathode ( right ): 2MnO2 (s) + 2NH4+ (aq) + 2e- → Mn2O3 (s) + H2O (l) + 2NH3 (g). The overall equation is: 2MnO2 (s) + 2NH4+ (aq) + Zn (s) → Mn2O3 (s) + H2O (l) + Zn(NH3)22+ (aq). However, in the reduction reaction, NH4+ ions only provide H+ ions needed for cathode process. In fact, only MnO2 gains the electrons. So when we calculate the potential difference, the 2 half equations are: MnO2 + 4H+ + e- → Mn3+ + 2H2O. Eo = +0.95 V Zn (s) → Zn2+ (aq) + 2e-. Eo = +0.76 V So that the net ionic equation is MnO2 + 4H+ + Zn → Mn3+ + 2H2O + Zn2+ . Eo = 0.95 + 0.76 = 1.71 V It is little higher than the usual data given (1.5 V) because during the whole process of producing the galvanic cell, some of the energy is lost or the conditions of the cells do not meet the standards such as temperature errors. The standard dry cell is very cheap to produce, and can be made in a variety of sizes so it is very as flashlights, remote controls, portable radios, many toys, etc. It has a poor shelf life as reactions with its zinc casing slowly occur. Sometimes the battery 'leaks' when this casing has been perforated. It is also easy to transport, especially when compared to liquid cells such as lead-acid cell. In fact, the dry cell encouraged the development of all these devices. Small portable electrical devices would never have been invented if cells and batteries had not been available to power them. It was the first commercialised battery, so had a massive impact on society, allowing for electrical goods to be made portable. The environmental impact of this technology is minimal, since the chemicals involved do not cause any significant problems when disposed get into landfill rubbish dumps. Button cell is a small single cell battery shaped as a squat cylinder typically 5 to 12 mm in diameter and 1 to 6 mm high—like a button on a garment, hence the name. Zinc acts as the anode whilst silver oxide acts as the cathode. Zinc atoms are oxidised to zinc ions whilst silver ions are reduced to silver atoms in a KOH electrolyte. The carbon (graphite) and silver produced assist the conductivity of the cell. Constant OH- concentration helps to maintain stable voltage. Again, the carbon rod does not involve the reaction just like it in the dry cells. The diagram of button cell is shown as below: Oxidation reaction at the anode ( up ): Zn (s) + 2OH- → Zn(OH)2 (s) + 2e- Reduction reaction at the cathode ( down ): Ag2O (s) + H2O (l) + 2e- → 2Ag (s) + 2OH- (aq) The overall equation is Zn (s) + Ag2O (s) + H2O (l) → 2Ag (s) + 2OH- (aq) + Zn(OH)2 (s). Calculation of the potential difference: Ag+ +  e− → Ag Eo = +0.8 V Zn → Zn2+ + 2e- Eo = +0.76 V So the net equation is 2Ag+ + Zn → Ag + Zn2+ Eo = 0.8 + 0.76 = 1.56 V It is similar to the normal given value (1.5V). Button cells are relatively expensive, but a long time in miniature, low-energy devices. Their tiny sized and good performance are worth the expense. The environmental impact is low because these cells contain so little material, and the relatively inert stainless steel and silver metal inside them is not a significant environmental hazard. It is not recommend use mercury oxide because it is toxic to the human nervous system although it is cheaper than silver. it has had a major contribution as a miniature power source in providing reliable energy output and minimal toxic waste products. This has allowed the development of potable miniature devices such as laser pointers, miniature cameras etc. It has also allowed for the development of medical devices such as heart pacemakers and hearing aids. There are serval summarised tables that compares the cost and practicality, impact on society and impact on the environment of the dry cell and the button cell. The cost and practicality (advantage) |Dry cell |Button cell | |Dry cell batteries are cheap to produce. |High energy capacity per unit weight. | |Small in size makes it practical for portable devices. |Long operating life (e.g. will keep a watch running non-stop | |Low cost means they are easily replaced. |for 3-5 years). | |It is widely used in portable appliances such as |Long shelf life. | |calculators. |Produces a very steady output of 1.5V. | |Zinc anode also forms the container. |Widely used in cameras, heart pacemakers, watches, hearing | | |aids. | The cost and practicality (disadvantage) | Dry cell |Button cell | |Non-rechargeable. |Non-rechargeable. | |it does not produce a very large amount of electricity for its |Poor low temperature performance. | |size. |High cost due to expensive materials (Ag2O). | |It possesses a relatively short shelf-life of around 1.5 years.| | |If current is drawn rapidly from the cell, a drop in voltage is| | |caused. | | |The battery is also prone to leakage as the zinc casing is | | |oxidised during use and through reactions with the NH4Cl. | | Impact on society |Dry cell |Button cell | |Widely used as it is suitable for common low drain appliances |Small size,light weight and stable voltage over a long period | |such as torches, toys and portable radios. |makes it useful in small appliances such as cameras, | |Short shelf life due to acidic paste attacking the zinc. |calculators and watches. | Impact on the environment |Dry cell |Button cell | |Weakly acidic paste and reaction products are not toxic and |KOH is however highly caustic and can cause burns if casing is | |pose little problem in dumps. |damaged. | |Being non-rechargeable means that it contributes to landfill. |Expensive silver needs to be recycled. | |The magnitude of its usage in society means that a very large |No highly toxic materials that will harm the environment. | |amount of these batteries end up in landfill. | | To summary and compare, dry cells are useful and quite adequate for some purposes such as toys and radios. The dry cell was an improvement over earlier cells as it contained no liquid. However, eventually the zinc casing may be used up and allow the battery to leak. Also, if used continuously, ammonia may be produced and cause the cell to burst. The button cell represents improvements in design as it also contains no liquid, it is smaller, has a longer shelf life and provides a more constant voltage for a longer period of time. These advantages have made possible the development of small watches. The reduction in size of these appliances has made them more portable and made them available to a greater percentage of the population. Hearing aids, which were previously bulky and obvious are now smaller and more convenient. At present, the button is more expensive than the dry cell however, and it usually contains the heavy metal like silver or sometimes even mercury. It required very carefully to deal these disposed because they are toxic. 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