What is the fundamental difference between a primary and a secondary battery?

The core difference lies in their reversibility. Primary batteries are designed for single use; their electrochemical reactions are irreversible or practically irreversible. Once the reactants are consumed, the battery cannot be recharged and must be discarded. Examples include the common Leclanché (dry cell) and mercury cells. Secondary batteries, conversely, are rechargeable. Their chemical reactions can be reversed by applying an external electrical current, allowing them to be used and recharged multiple times. Lead-acid, nickel-cadmium, and lithium-ion batteries are prominent examples of secondary batteries.

Why does the voltage of a Leclanché cell drop during continuous use, unlike a mercury cell?

In a Leclanché cell, the electrolyte ($NH_4Cl$) is consumed, and ammonia ($NH_3$) gas is produced at the cathode. The accumulation of $NH_3$ can form a layer around the carbon rod, increasing internal resistance and decreasing the effective surface area for reaction, leading to a voltage drop. Additionally, the concentration of reactants changes significantly. In contrast, the overall reaction in a mercury cell ($Zn(Hg) + HgO(s) ightarrow ZnO(s) + Hg(l)$) does not involve species whose concentrations change significantly in the electrolyte, leading to a more constant voltage output throughout its operational life.

Explain the role of sulfuric acid in a lead-acid battery during discharge and charge.

During discharge, sulfuric acid ($H_2SO_4$) acts as the electrolyte and is consumed in the overall reaction: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) ightarrow 2PbSO_4(s) + 2H_2O(l)$. This leads to a decrease in the concentration of $H_2SO_4$ and an increase in water content, which lowers the density of the electrolyte. During charging, an external current reverses this process. Water is consumed, and $H_2SO_4$ is regenerated, increasing the electrolyte's concentration and density. Monitoring the specific gravity of the electrolyte is a common way to check the battery's state of charge.

What is the 'memory effect' in batteries, and which battery type is most associated with it?

The 'memory effect' primarily affects older Nickel-Cadmium (Ni-Cd) batteries. It describes a phenomenon where a battery, if repeatedly recharged after only being partially discharged, 'remembers' the shallower discharge point and subsequently delivers only that reduced capacity, even if it was originally capable of a full discharge. This is believed to be due to the formation of larger, harder-to-reduce cadmium hydroxide crystals. Modern Ni-Cd batteries and other chemistries like NiMH and Li-ion are largely immune to this effect, making full discharge cycles less critical for their longevity.

Why are lithium-ion batteries preferred for modern portable electronic devices despite their higher cost and potential safety concerns?

Lithium-ion batteries offer several significant advantages that outweigh their drawbacks for portable electronics. Foremost is their exceptionally high energy density, meaning they can store a large amount of energy in a small, lightweight package, crucial for devices like smartphones and laptops. They also provide a high cell voltage (typically $3.7, ext{V}$), have a low self-discharge rate, and do not suffer from the 'memory effect'. While safety concerns (overheating, fire risk) exist, continuous advancements in battery management systems and cell chemistry have significantly mitigated these risks, making them the preferred choice for high-performance applications.

Batteries

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Batteries are fundamentally electrochemical cells, or combinations thereof, that convert stored chemical energy into electrical energy through spontaneous redox reactions. They are self-contained power sources, distinct from electrolytic cells which require an external power input to drive non-spontaneous reactions. The efficiency and utility of a battery are determined by factors such as its volt…

Quick Summary

Batteries are electrochemical devices that convert chemical energy into electrical energy through spontaneous redox reactions. They consist of an anode (where oxidation occurs), a cathode (where reduction occurs), and an electrolyte (which allows ion flow).

Batteries are broadly classified into primary (non-rechargeable, like Leclanché and mercury cells) and secondary (rechargeable, like lead-acid, nickel-cadmium, and lithium-ion cells). Primary cells are 'use and throw' as their reactions are irreversible.

Secondary cells can be recharged by reversing the chemical reactions using an external electrical current. Key characteristics include voltage, capacity, energy density, and cycle life. Understanding the specific chemical reactions at each electrode and the role of the electrolyte is crucial for comprehending their operation and applications.

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Primary Batteries: — Non-rechargeable. Irreversible reactions.

- Leclanché Cell: Anode: $Zn$ . Cathode: $C$ rod in $MnO_2, C$ . Electrolyte: $NH_4Cl, ZnCl_2$ . Voltage: $1.5,\text{V}$ (drops). $Zn(s) + 2NH_4Cl(aq) + 2MnO_2(s) \rightarrow Zn(NH_3)_2Cl_2(s) + Mn_2O_3(s) + H_2O(l)$ . - Mercury Cell: Anode: $Zn(Hg)$ . Cathode: $HgO, C$ . Electrolyte: $KOH, ZnO$ . Voltage: $1.35,\text{V}$ (constant). $Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$ .

Secondary Batteries: — Rechargeable. Reversible reactions.

- Lead-Acid Battery: Anode: $Pb$ . Cathode: $PbO_2$ . Electrolyte: $38% H_2SO_4$ . Voltage: $2,\text{V}$ /cell. Discharge: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$ . Charging reverses this.

- Ni-Cd Cell: Anode: $Cd$ . Cathode: $NiO(OH)$ . Electrolyte: $KOH$ . Voltage: $1.2,\text{V}$ . Discharge: $Cd(s) + 2NiO(OH)(s) + 2H_2O(l) \rightarrow Cd(OH)_2(s) + 2Ni(OH)_2(s)$ . Charging reverses this.

- Li-ion Cell: Anode: Graphite. Cathode: $Li$ metal oxide. Electrolyte: Non-aqueous $Li$ salt. Voltage: $3.7,\text{V}$ . High energy density. No memory effect.

Lead Me Now, Please Sir! (Types of Batteries)**

Leclanché (Primary)

Mercury (Primary)

Nickel-Cadmium (Secondary)

Pb-Acid (Lead-Acid) (Secondary)

Secondary (General category for rechargeable)

(For Li-ion, remember it's the 'new kid on the block' for high-tech secondary batteries!)