Secondary cells are essential for the daily operation of many devices, given their rechargeable nature. Electrolytic cells, meanwhile, are fundamental to various electrochemical processes. These notes will elucidate the core principles behind these two crucial electrochemical concepts.
Secondary Cells
Understanding the Charging Process
The charging process of secondary cells, or rechargeable batteries, is essentially a reversal of the discharging process. The basic mechanism involves the following:
- External Voltage: For recharging to occur, an external voltage greater than the cell's voltage is applied. This external voltage drives the reactions in the opposite direction to that of the discharge cycle.
- Redox Reactions: During the charging process, reduction takes place at the cathode while oxidation occurs at the anode. As electrons move from the anode to the cathode through an external circuit, ions within the cell migrate to restore charge neutrality.
Image courtesy of Barrie Lawson
Advantages and Disadvantages of Different Types of Cells
Rechargeable batteries come in various forms, each with its unique advantages and drawbacks.
- Lithium-ion (Li-ion):
- Advantages:
- High Energy Density: Allows for longer usage between charges.
- Low Self-Discharge: Retains its charge for longer periods when not in use.
- Lightweight: Ideal for portable devices.
- Disadvantages:
- Cost: More expensive due to complex manufacturing processes.
- Safety Concerns: Risk of overheating or exploding under certain conditions.
- Lifecycle: Limited to a certain number of charge cycles before performance degrades.
- Advantages:
- Nickel-Metal Hydride (NiMH):
- Advantages:
- Capacity: Offers a larger energy capacity compared to NiCd.
- Eco-friendly: Contains fewer toxic components.
- Disadvantages:
- Memory Effect: Can lose its maximum energy capacity if repeatedly recharged before being fully discharged.
- Self-Discharge: Loses its charge faster than Li-ion batteries when not in use.
- Advantages:
- Nickel-Cadmium (NiCd):
- Advantages:
- Performance: Maintains consistent performance even in colder conditions.
- Durability: Generally rugged and can endure a high number of charge-discharge cycles.
- Disadvantages:
- Memory Effect: Can lose its maximum energy capacity if repeatedly recharged before being fully discharged.
- Toxicity: Contains toxic materials that can be harmful to the environment.
- Advantages:
Reversible Electrode Reactions
The hallmark of secondary cells is the reversibility of their electrode reactions.
- Example: Li-ion Battery:
- During discharge, the lithium ions move from the anode to the cathode.
- During charging, this process is reversed with lithium ions moving from the cathode back to the anode.
Li-ion battery- showing discharging of the battery, ions moving from the anode to the cathode. During charging, this process is reversed.
Image courtesy of Sdk16420
Electrolytic Cells
Explanation of Current Conduction
Electrolytic cells are devices that use an external power source to drive a non-spontaneous chemical reaction. The mechanism involves:
- Ion Migration: When an external voltage is applied to an electrolytic cell, cations migrate towards the cathode and undergo reduction, while anions head towards the anode and undergo oxidation.
- Electrode Reactions: At the cathode, cations gain electrons (reduction). At the anode, anions lose electrons (oxidation).
Deduction of Products from the Electrolysis of Molten Salts
Electrolysis in molten salts is pivotal for obtaining pure elements.
- Example: Electrolysis of Molten Sodium Chloride:
- At the Anode: 2Cl- -> Cl2 + 2e-. This results in the evolution of chlorine gas.
- At the Cathode: Na+ + e- -> Na. Sodium metal is produced and can be seen collecting at the cathode.
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Conditions Under Which Ionic Compounds Can Act as Electrolytes
The capacity for ionic compounds to act as electrolytes depends on certain conditions:
- Physical State: Ionic compounds should either be molten or in an aqueous solution to facilitate free ion movement, enabling them to act as electrolytes.
- Dissociation: In aqueous solutions, the ionic compound must dissociate into its constituent ions. The greater the degree of dissociation, the better the electrolytic conduction.
- Temperature: Raising the temperature can increase the ionic conductivity of certain salts. The added energy can break more ionic bonds, allowing ions to move freely.
Always remember, for an ionic compound to effectively conduct electricity and hence be an effective electrolyte, the free movement of its ions is paramount.
FAQ
In an electrolytic cell, the electrolyte often undergoes chemical changes as the electrolysis process progresses. Depending on the type of electrolyte and the reactions occurring at the electrodes, the concentration of the electrolyte may change, or the electrolyte itself may be consumed or transformed into another compound. For instance, in the electrolysis of water, the water acts as the electrolyte and is decomposed into oxygen and hydrogen gases, resulting in a decrease in the volume of the electrolyte over time. It's crucial to monitor and replace or replenish the electrolyte when necessary to ensure the efficient functioning of the cell.
Lithium is a preferred element for use in modern rechargeable batteries due to its numerous advantageous properties. Firstly, lithium is the lightest metal, which allows for lightweight battery designs. Secondly, it has a very high electrochemical potential, meaning it can result in higher voltages compared to other materials. This makes lithium-ion batteries able to store and deliver a significant amount of energy relative to their size. Additionally, they have a longer lifecycle, can hold their charge well (low self-discharge rate), and don't suffer from a memory effect, which can plague some other types of rechargeable batteries.
No, secondary cells, even though they are rechargeable, cannot last indefinitely. Each time a cell undergoes a charge-discharge cycle, tiny structural and chemical changes occur inside the battery, which may lead to reduced efficiency and capacity. Over time, after numerous charge-discharge cycles, the battery's ability to hold and deliver charge diminishes. Factors like deep discharges, overcharging, high temperatures, and internal short circuits can accelerate this degradation. The term "cycle life" refers to the number of complete charge-discharge cycles a battery can undergo before its capacity drops below a specified percentage of its initial capacity.
The salt bridge in an electrochemical cell plays a crucial role in maintaining electrical neutrality within the internal cell environment. As redox reactions proceed at the electrodes, cations and anions are formed, which can disrupt the neutrality of the solutions. The salt bridge contains a gel or porous paper soaked in an electrolyte, which allows for the movement of ions between the two half-cells. This ionic movement compensates for the charge imbalance that arises due to electron flow in the external circuit. Without a salt bridge or a similar setup, the cell's potential would rapidly decrease, stopping the electron flow.
Secondary cells are termed 'rechargeable' because they have the capacity to undergo multiple charge and discharge cycles. Unlike primary cells, which can only be used once and then must be discarded, secondary cells can be recharged when their stored energy is depleted. This is achieved through an external voltage source that forces the electrochemical reactions to proceed in the opposite direction, essentially 'refilling' the cell with energy. This ability to reverse the chemical reactions allows for extended usage, making secondary cells more sustainable and cost-effective in many applications compared to primary cells.
Practice Questions
A primary cell is designed for a single use and cannot be recharged, whereas a secondary cell, like a rechargeable battery, can undergo multiple charge and discharge cycles. In a lithium-ion battery, during the discharge phase, lithium ions move from the anode to the cathode, releasing energy in the process. Electrons travel through the external circuit from the anode to the cathode, providing power to the connected device. During the charging process, an external voltage is applied, driving the lithium ions to move back from the cathode to the anode, thus storing energy for future use.
During the electrolysis of molten sodium chloride, chlorine gas evolves at the anode as chloride ions lose electrons to form chlorine molecules: 2Cl- -> Cl2 + 2e-. At the cathode, sodium ions gain electrons to produce sodium metal: Na+ + e- -> Na. For an ionic compound to act as an electrolyte, it should be in a state that allows free movement of ions, either in a molten state or dissolved in an aqueous solution. Additionally, in aqueous solutions, the compound must dissociate into its constituent ions. The degree of dissociation and temperature can also affect its conductivity; higher temperatures can often enhance the ionic conductivity of certain salts.
