Q. 1. What is difference between primary and secondary batteries?
Ans. Batteries are either primary or secondary. Primary batteries can be used only once because the chemical reactions that supply the current are irreversible. Secondary batteries, sometimes called storage batteries or accumulators, can be used, recharged and reused. In these batteries, the chemical reactions that provide current from the battery are readily reversed when current is supplied to the battery.
Q. 2. Why terminal potential of a cell (or a battery) is always less than its emf?
Ans. Terminal potential of a cell (or a battery) is always less than its emf because some of the emf developed is used in overcoming the internal resistance of the cell (or, a battery) itself.
Q. 3. Lead acid battery can be charged whereas primary cell cannot be charged. Why?
Ans. Primary cells can be used only once because the chemical reactions that supply the electric current are irreversible but in lead acid battery, the chemical reactions that provide the current are readily reversed when current is supplied to the battery.
Q. 4. What is ECE?
Ans. Electro-chemical equivalent (ECE) of a substance is the amount deposited on passing an electric current of one ampere for one second through its solution.
Q. 5. Why are electrolytic plants installed near the power station?
Ans. Because of power requirements in huge amount and at 100% load factor, electrolytic plants are installed near the power station.
Q. 6. In what form is energy stored in a secondary battery?
Ans. Energy is stored in the form of electrical energy in a secondary battery.
Q. 7. Why Faure process is much suitable for manufacturing of negative plates rather than positive plates?
Ans. Faure process is much suitable for manufacturing of negative plates because negative active material (sponge lead) is quite tough (not hard and brittle as lead peroxide is) and moreover, it undergoes a comparatively negligible change in volume during charging and discharging processes.
Q. 8. Why in grouping of lead acid cells the negative group is made to have one plate more than positive group?
Ans. The negative group of the plates contains one more plate than that of the positive group so that when interleaved both the end plates become the negative. This is done so that all the positive plates can work equally well on both sides. If a positive plate were used at one end, one of its sides would remain inactive, when the other side expands, and thus the plate is likely to buckle.
Q. 9. Do we use ac or dc to charge a battery?
Ans. For charging of a battery dc is essential.
Q. 10. Why should the lead acid cells not be allowed to stand completely discharged?
Ans. Lead acid cells shall not be allowed to stand completely discharged because the lead sulphate formed on the plates crystallizes and the proper reforming of the plates becomes difficult.
Q. 11. What are the factors that affect the battery capacity?
Ans. The capacity of battery depends upon several factors, principal among which are area of plate surface, quantity, arrangement and porosity of the active material used in the manufacture of plates, quantity and specific gravity of the electrolyte used and the porosity of the separators, rate of charge and temperature.
Q. 12. Why the lead acid battery should not be allowed to discharge beyond 1.75 V?
Ans. The lead acid battery should never be allowed to discharge beyond 1.75 V otherwise lead sulphate will be formed on the electrodes which is hard, insoluble and increases the internal resistance of the battery.
Q. 13. Why is the watt-hour efficiency of a cell lower than its ampere-hour efficiency?
Ans. Watt-hour efficiency is given as
and Vd is always lower than Ve, therefore is always lower than its
Q. 14. Why the state of charge of a nickel-iron cell cannot be determined by measuring the specific gravity of the electrolyte?
Ans. The state of charge of a nickel-iron cell cannot be determined by the specific gravity of electrolyte as it does not change during charging and discharging.
Q. 15. Why the ampere-hour and watt-hour efficiencies of an Edison cell are lower than those for lead-acid cell?
Ans. Due to comparatively high internal resistance, the efficiencies of the Edison cell are lower than those of the lead acid cell.
Q. 16. Why and where will you recommend the use of nickel-iron batteries?
Ans. Nickel-iron batteries are used for the propulsion of industrial trucks, and mine locomotives and for railway car lighting and air-conditioning because of their rugged construction, compact size, lighter in weight, more durability, long service life and as they need little maintenance and there is no evolution of obnoxious fumes.
Q. 17. What is the basic difference between nickel-iron and nickel cadmium cells?
Ans. The most important difference between the nickel-cadmium and nickel-iron cells is the arrangement of plates. Where as in nickel-iron cells, the extreme plates are negative, but in the nickel-cadmium cells the extreme plates are positive.
Q. 18. Why use of nickel-cadmium accumulators is limited in spite of its numerous advantages over nickel-iron accumulators and lead acid accumulators?
Ans. The use of nickel-cadmium accumulators is limited as they are heavier in weight and expensive in cost.
Q. 19. What would happen if no salt bridge were used in an electrochemical cell (like Zn – Cu cell)?
Ans. The meta ions (Zn2+) formed by the loss of electrons will accumulate in one electrode and the negative ions (SO42-) will accumulate in the other. Thus, the solutions will develop charges and the current stops flowing. Moreover, inner circuit is not completed.
Q. 20. What is the use of platinum foil in the hydrogen electrode?
Ans. It is used for the inflow and outflow of electrons.
Q. 21. If an electrochemical cell is set up as usual but there is no flow of current. What do you conclude?
Ans. The electrode potentials of both the electrodes are equal which depend upon the concentrations of the solutions in the half cells.
Q. 22. What is the role of ZnCl in a dry cell?
Ans. ZnCl2 combines with the NH3 produced to form the complex salt [Zn(NH3)2Cl2] as otherwise the pressure developed due to NH3 would crack the seal of the cell.
Q. 23. Why a mercury cell gives a constant voltage throughout its life?
Ans. This is because the electrolyte KOH is not consumed in the reaction.
Q. 24. Why does a dry cell become dead after a long time even if it has not been used?
Ans. This is because acidic NH4Cl corrodes the zinc container continuously even when the cell is not being use.
Q. 25. Which type of cells are rechargeable?
Ans. Those cells are rechargeable in which the product formed during discharge are deposited on the electrodes and these can be decomposed to give the original substances when electrical energy is supplied.
- A primary cell consists of a vessel containing a liquid known as electrolyte and two dissimilar metal plates, known as electrodes immersed in the liquid:
Since in these cells the energy is obtained after the primary action of placing the plates in the electrolyte, these cells are called the primary cells.
- Current delivered by a cell and pd across the load circuit are given respectively by
- Current delivered by a battery consisting of parallel-connected m series, each consisting of n identical series connected cells, is given by
and the delivered current will be maximum when mR = nr.
- According to Faraday’s laws of electrolysis
Mass of ions liberated, m = ECE * current strength * time duration = Zit
- The Weston cell is such a cell where emf remains constant for a longer period provided no appreciable current is drawn from the cell, therefore, such cells are never used as a source of energy but are used as a secondary standard of voltage for electrical measurements.
- The charging and discharging of a lead acid cell may be represented by a single reversible equation given below:
PbO2 (Positive plate) + 2H2SO4 + Pb (Negative plate)
PbSO4 + 2H2O + PbSO4 + Electrical Energy.
- The efficiency of a cell can be given in two ways as given bellow:
Quantity or ampere-hour efficiency,
Energy or watt-hour efficiency,
- Sealed lead cells and batteries are rugged in construction, compact in size, have long life, excellent performance, simplified charging, design flexibility and need little maintenance.
Starved-electrolyte batteries have a wide range of applications. These batteries are employed as standby units in computers, emergency lights, medical equipment, security alarm systems, telecommunication equipment and uninterruptible power supplies; as portable units in cellular phones, toys, cordless appliances and portable lighting, and as alternate units in computers, consumer electronics and instrumentation.
- The electrochemical process in the nickel-iron accumulators can be expressed by the equation
2Ni(OH)3[positive plate] + 2KOH + Fe[negative plate]
2Ni(OH2) [positive plate] + 2KOH + Fe(OH)2 [negative plate] + Electrical energy
- Nickel-iron batteries are comparatively lighter in weight, rugged in construction, more durable and mechanically stronger, they need little maintenance, they have long service life and they do not evolve obnoxious fumes. But they have the disadvantages of high initial cost, high internal resistance, lower emf per cell and lower operating efficiency. Edison battery may be considered as a special-service battery to employed where lightness, freedom from acid fumes or ability to stand severity of vibrations and shocks are particularly important, and where momentary high discharge currents are not required.
- The electrochemical process in the nickel-cadmium cells for charge and discharge may be given as
2Ni(OH)3 [Positive plate] + 2KOH + Cd [negative plate]
2Ni(OH)2 [Positive plate] + 2KOH + 2Cd(OH)2 [Negative plate] + Electrical energy
- The nickel-cadmium accumulators are superior to lead acid accumulators as well as nickel-iron accumulators for all applications except those in which weight is important criterion of selection of accumulator.
- The charging and discharging of silver zinc cell may be represented by a single reversible equation given below:
Ag2O [Positive plate] + Zn [Negative plate]
2Ag [positive plate] + Zno [Negative plate] + electrical energy
- Fuel cells differ from storage batteries in that their electrode materials are not changed in chemical composition and, therefore, they do not need charging.