1. "The mass of an ion liberated at an electrode is directly proportional to the quantity of electricity".
The above statement is associated with
A. Newton's law
B. Faraday's law of electromagnetic induction
C. Faraday's law of electrolysis
D. Gauss's law
Answer: C. Faraday's law of electrolysis
Explanation:
Faraday’s – First Law of Electrolysis
It is one of the primary laws of electrolysis. It states, during electrolysis, the amount of chemical reaction which occurs at any electrode under the influence of electrical energy is proportional to the quantity of electricity passed through the electrolyte.
Faraday’s – Second Law of Electrolysis
Faraday’s second law of electrolysis states that if the same amount of electricity is passed through different electrolytes, the masses of ions deposited at the electrodes are directly proportional to their chemical equivalents.
2. The charge required to liberate one gram equivalent of any substance is known as constant
A. time
B. Faraday's
C. Boltzmann
D. Faraday's and Boltzmann
Answer: B. Faraday's
Explanation:
Faraday number is defined as the quantity of electric current which liberates one gram equivalent of a substance at an electrode during electrolysis.
3. The capacity of a cell is measured in
A. amperes
B. ampere-hours
C. watts
D. watt-hours
Answer: B. ampere-hours
Explanation:
The capacity of a cell is measured by the discharging at a constant electric current until it fully drains out for that particular time. Hence, the capacity of the cell/battery is measured by the ampere-hour rating.
4. Active materials of a lead acid cell are
A. spongy lead
B. lead peroxide
C. dilute H2SO4
D. all of the above
Answer: D. all of the above
Explanation:
Lead-acid battery:
The battery which uses a spongy lead and lead peroxide for the conversion of the chemical energy into electrical power, such type of battery is called a lead-acid battery. The lead-acid battery is most commonly used in power stations and substations because it has higher cell voltage and lower cost. It is a reversible battery.
The active elements of the lead-acid are
Lead peroxide (PbO2):
It forms the positive active material; The PbO2 is dark chocolate brown in color.
Spongy lead:
It forms the negative active material; It is grey in color
Dilute Sulfuric Acid (H2SO4):
It is used as an electrolyte; It contains 31% of sulfuric acid
- Lead peroxide (PbO2) = Positive active material
- Sponge lead (Pb) = Negative active material
- Dilute sulfuric acid = Electrolyte
- Separators are thin sheets of non-conducting materials made of porous rubbers, or molecules of glass fiber place between positive and negative, to insulate them from each other.
- Battery terminals: It has two terminals ie. A positive terminal with a diameter of 17.5 mm at the top is slightly larger than the negative terminal which is 16 mm in diameter.
5. Sulphation in a lead-acid battery occurs due to
A. heavy charging
B. fast charging
C. trickle charging
D. incomplete charging
Answer: D. incomplete charging
Explanation:
- Sulfation occurs in lead–acid batteries when they are subjected to insufficient charging during normal operation. It impedes recharging; sulfate deposits ultimately expand, cracking the plates and destroying the battery.
- Sulfation occurs when a battery is deprived of a full charge, it builds up and remains on battery plates. When too much sulfation occurs, it can impede the chemical to electrical conversion.
A buildup of sulfates i.e. over sulfation can cause
- longer charging times
- excessive heat build-up
- shorter running times between charges
- dramatically shorter battery life
- complete battery failure
The best way to prevent sulfation is to practice proper battery maintenance and follow the best charging practices. Ex: Trickle charge
Trickle charge:
- Trickle charging is designed to compensate for the self-discharge of the battery. The charge rate varies according to the frequency of discharge.
- Not suitable for some battery chemistries, e.g. NiMH and Lithium, which are susceptible to damage from over-charging
- In some applications, the charger is designed to switch to trickle charging when the battery is fully charged
6. During the charging of a lead-acid cell
A. its voltage increases
B. it gives out energy
C. its cathode becomes dark chocolate brown in colour
D. specific gravity of H2SO4 decreases
Answer: A. its voltage increases
Explanation:
During charge, the lead sulfate of the positive plate becomes lead dioxide. As the battery reaches full charge, the positive plate begins generating dioxide causing a sudden rise in voltage due to decreasing internal resistance.
The chemical changes that occur during recharging:
- The positive plate (anode) is converted into PbO2 which is in dark chocolate brown colour and the negative plate (cathode) into Pb
- H2SO4 is formed in the reactions; Therefore, the specific gravity of the electrolyte (H2SO4) is raised
- When the cell is fully charged, the specific gravity of H2SO4 rises to about 1.28
- The emf of cell raises; The emf of a fully charged lead-acid cell is about 2 V
- Electrical energy supplied is converted into chemical energy which is stored in the cell; So, during charging, energy is absorbed by the cell
The chemical changes that occur during discharging:
- Both the plates are converted into lead sulfate (PbSO4) which is whitish in colour
- Water is formed which lowers the specific gravity of the electrolyte (H2SO4)
- When the cell is fully discharged, the specific gravity of H2SO4 falls to about 1.18
- The emf of the cell falls; The lead-acid cell should not be discharged beyond the point where its emf falls to about 1.8 volts
- The chemical energy stored in the cell is converted into electrical energy; So, during discharging, energy is delivered by the cell
7. The capacity of a lead-acid cell does not depend on its
A. temperature
B. rate of charge
C. rate of discharge
D. quantity of active material
Answer: B. rate of charge
Explanation:
The rate of charge is used to indicate how fast the battery is getting charged and then the capacity is expressed in ampere-hours (AH). If the quantity of the active materials is more, then the capacity of the cell is more. Hence, the capacity of a lead-acid cell does not depend on its rate of charge.
8. During charging the specific gravity of the electrolyte of a lead-acid battery
A. increases
B. decreases
C. remains the same
D. becomes zero
Answer: A. increases
Explanation:
During charging, the specific gravity of the electrolyte of a lead-acid battery INCREASES
- The lead-acid battery is the oldest and most mature among the all-battery technologies. Because of their large applications, lead-acid batteries have the lowest cost of all-battery technologies.
- This battery operates at an ambient temperature and has an aqueous electrolyte. Even though the lead-acid battery is relatively inexpensive, it is very heavy, with limited usable energy by weight (specific energy).
- The state of charge of a lead-acid battery can be determined by the specific gravity of the electrolyte (its weight compared to water). The specific gravity can be measured directly with a hydrometer or determined by the stabilized voltage.
- A hydrometer is a bulb-type syringe that will extract electrolytes from the cell. A glass float in the hydrometer barrel is calibrated to read in terms of specific gravity. A common range of specific gravity used on these floats is 1.160-1.325. The lower the float sinks in the electrolyte, the lower its specific gravity is.
- Measuring the specific gravity of a battery’s electrolyte with a hydrometer is the most accurate way to test the state of charge. This is true because the concentration of sulfuric acid changes as the battery is charged or discharged, and the change in concentration corresponds with a change in electrolyte density. Specific gravity is a measurement of the density of a liquid. The specific gravity of 1.260 in a lead-acid battery means that the battery is fully charged. As the battery is discharged, the specific gravity of the electrolyte falls to 1.120 for a state of charge of 0 percent.
- A fully charged battery has all of the sulfates in acid. As the battery discharges, some of the sulfates begin to appear on the plates. The acid becomes more dilute and its specific gravity drops as water replaces more of the sulfuric acid. A fully discharged battery has more sulfate in the plates than in the electrolyte. Please note that the hydrometer float sinks lower in the electrolyte as the specific gravity becomes lower. A hydrometer reading should never be taken immediately after water is added to the cell. The water must be thoroughly mixed with the underlying electrolyte.
Important Notes:
- The electrolyte in a lead-acid cell is dilute sulphuric acid (H2SO4) solution mixed in such a proportion so that with a fully charged battery, its specific gravity is about 1.28
- Dilute sulphuric acid is used as electrolyte by adding Sulphuric acid to water
- Spongy lead is used for negative plate
- Lead peroxide paste is used for positive plates
- It has low internal resistance
- It requires more charging time
- Gases are evolved during charging
- During the normal life of a battery that is properly cared for, the electrolyte loses none of the acids
- During the charging process, the specific gravity of the electrolyte (H2SO4) increases and provides an important indication to the state of charge of the cell
- The specific gravity of the electrolyte of a fully charged lead-acid cell is about 1.28; This can be measured by means of a hydrometer
In practice, the state of charge of a lead-acid cell (or battery) is determined from the specific gravity of the electrolyte. The following table is useful in this regard.
The specific gravity of H2SO4 | State of charge |
1.13 | Discharged |
1.19 | 25% charged |
1.22 | 50% charged |
1.25 | 75% charged |
1.28 | 100% charged |
9. The active materials on the positive and negative plates of a fully charged lead-acid battery are
A. lead and lead peroxide
B. lead sulphate and lead
C. lead peroxide and lead
D. none of the above
Answer: C. lead peroxide and lead
Explanation:
- A Lead Acid cell is a battery that uses sponge lead and lead peroxide for the conversion of chemical energy into electric power.
- It is commonly used in power stations and substations.
- The container is made of glass, lead-lined wood.
- The plate of load acid cell consists of some form of grid mode of lead and active material.
Active material: The material in a cell that takes active participation in a chemical reaction during charging or discharging.
For a lead-acid cell they are:
- Lead peroxide (PbO2) = Positive active material
- Sponge lead (Pb) = Negative active material
- Dilute sulfuric acid = Electrolyte
Important Notes:
- Separators are thin sheets of non-conducting materials made of porous rubbers, or molecules of glass fiber place between positive and negative, to insulate them from each other.
- Battery terminals: It has two terminals ie. A positive terminal with a diameter of 17.5 mm at the top is slightly larger than the negative terminal which is 16 mm in diameter.
10. When a lead-acid battery is in fully charged condition, the colour of its positive plate is
A. dark grey
B. brown
C. dark brown
D. none of above
Answer: C. dark brown
Explanation:
Lead-acid battery:
- Positive plate: PbO2, deposited on a grid frame of antimony lead alloy. When battery is fully charged condition, the positive plate is in dark brown in colour.
- Negative plate: Pb, deposited on a grid frame. When the battery is fully charged condition, the negative plate is grey in colour. When the battery has fully discharged the colour of negative plate is pure white.