Primary and Secondary Batteries — Revision Notes

⚡ 30-Second Revision

Primary Batteries: — Non-rechargeable, irreversible reactions. Examples: Dry Cell, Mercury Cell.

* Dry Cell: Anode ($Zn$), Cathode ($MnO_2$, C rod), Electrolyte ($NH_4Cl$, $ZnCl_2$ paste). Voltage $approx 1.5,\text{V}$. * Mercury Cell: Anode ($Zn(Hg)$), Cathode ($HgO$, C), Electrolyte ($KOH$, $ZnO$ paste). Voltage $approx 1.35,\text{V}$ (constant).

Secondary Batteries: — Rechargeable, reversible reactions. Examples: Lead-Acid, Ni-Cd, Li-ion.

* Lead-Acid: Anode ($Pb$), Cathode ($PbO_2$), Electrolyte ($H_2SO_4$). Voltage $approx 2,\text{V}$/cell. * Discharge: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$ * Charge: $2PbSO_4 + 2H_2O \rightarrow Pb + PbO_2 + 2H_2SO_4$ * Ni-Cd: Anode ($Cd$), Cathode ($NiO(OH)$), Electrolyte ($KOH$).

Voltage $approx 1.2,\text{V}$. Suffers from 'memory effect'. * Li-ion: Anode (Graphite), Cathode ($LiCoO_2$), Electrolyte (non-aqueous Li salt). Voltage $approx 3.7,\text{V}$. High energy density, no memory effect.

2-Minute Revision

Batteries convert chemical energy to electrical energy via redox reactions. They are categorized into primary (non-rechargeable) and secondary (rechargeable) types. Primary batteries, like the Dry Cell (Leclanché) and Mercury Cell, are single-use.

The Dry Cell uses a zinc anode and a carbon rod with $MnO_2$ paste as cathode, yielding 1.5V. The Mercury Cell, with a zinc-mercury amalgam anode and $HgO$ cathode in an alkaline electrolyte, provides a stable 1.

35V, ideal for watches and pacemakers. Secondary batteries, such as the Lead-Acid battery, Nickel-Cadmium (Ni-Cd), and Lithium-ion (Li-ion), can be recharged. The Lead-Acid battery, common in cars, uses lead and lead dioxide electrodes in sulfuric acid, producing 2V per cell, with its charge state indicated by electrolyte density.

Ni-Cd batteries (1.2V) are known for the 'memory effect.' Li-ion batteries (3.7V) are highly favored for modern electronics due to their high energy density and absence of the memory effect. Key aspects for NEET include knowing the anode/cathode materials, electrolytes, specific reactions, voltages, and applications for each type.

5-Minute Revision

A battery is an electrochemical cell that transforms chemical energy into electrical energy through spontaneous redox reactions. The two main types are primary and secondary batteries.

Primary Batteries (Non-Rechargeable): These batteries have irreversible chemical reactions and are designed for single use. Once the reactants are consumed, the battery is 'dead'.

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Dry Cell (Leclanché Cell):

* Anode: Zinc container ($Zn \rightarrow Zn^{2+} + 2e^-$) * Cathode: Carbon rod surrounded by $MnO_2$ and carbon powder paste ($2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O$) * Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$. * Voltage: $approx 1.5,\text{V}$. * Uses: Flashlights, radios.

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Mercury Cell:

* Anode: Zinc-mercury amalgam ($Zn(Hg) + 2OH^- \rightarrow ZnO + H_2O + 2e^-$) * Cathode: Paste of $HgO$ and carbon ($HgO + H_2O + 2e^- \rightarrow Hg + 2OH^-$) * Electrolyte: Concentrated paste of $KOH$ and $ZnO$. * Voltage: Constant $1.35,\text{V}$ (due to stable reactant/product concentrations). * Uses: Hearing aids, pacemakers, watches.

Secondary Batteries (Rechargeable): These batteries have reversible chemical reactions and can be recharged by applying an external current, allowing for multiple cycles.

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Lead-Acid Battery:

* Anode (Discharge): Lead ($Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-$) * Cathode (Discharge): Lead dioxide ($PbO_2 + SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4 + 2H_2O$) * Overall Discharge: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$ * Overall Charge: $2PbSO_4 + 2H_2O \rightarrow Pb + PbO_2 + 2H_2SO_4$ * Electrolyte: Aqueous $H_2SO_4$.

Density changes with charge state. * Voltage: $approx 2,\text{V}$ per cell (e.g., 12V car battery has 6 cells). * Uses: Automobile starter, UPS, inverters.

1
Nickel-Cadmium (Ni-Cd) Battery:

* Anode: Cadmium ($Cd + 2OH^- \rightarrow Cd(OH)_2 + 2e^-$) * Cathode: Nickel(III) oxyhydroxide ($2NiO(OH) + 2H_2O + 2e^- \rightarrow 2Ni(OH)_2 + 2OH^-$) * Electrolyte: Alkaline $KOH$ solution. * Voltage: $approx 1.2,\text{V}$. * Characteristic: Suffers from 'memory effect'. * Uses: Older portable electronics, power tools.

1
Lithium-ion (Li-ion) Battery:

* Principle: Intercalation of $Li^+$ ions between electrode layers. * Anode: Graphite ($Li_x C_6 \rightleftharpoons xLi^+ + xe^- + C_6$) * Cathode: Lithium metal oxide (e.g., $LiCoO_2$) ($Li_{1-x}CoO_2 + xLi^+ + xe^- \rightleftharpoons LiCoO_2$) * Electrolyte: Non-aqueous lithium salt solution. * Voltage: $approx 3.7,\text{V}$. * Characteristics: High energy density, no memory effect, low self-discharge. * Uses: Smartphones, laptops, electric vehicles.

For NEET, focus on the specific reactions, electrode materials, electrolytes, voltages, and key applications/characteristics of each battery type. Pay attention to oxidation states in redox reactions.

Prelims Revision Notes

Batteries: — Electrochemical cells converting chemical energy to electrical energy via spontaneous redox reactions.
Primary Batteries: — Non-rechargeable, single-use. Reactions are irreversible.

* Dry Cell (Leclanché Cell): * Anode: Zinc ($Zn$). Oxidation: $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$. * Cathode: Carbon rod surrounded by $MnO_2$ and carbon powder. Reduction: $2MnO_2(s) + 2NH_4^+(aq) + 2e^- \rightarrow Mn_2O_3(s) + 2NH_3(g) + H_2O(l)$.

* Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$. * Voltage: $approx 1.5,\text{V}$. * Uses: Flashlights, radios, remote controls. * Mercury Cell: * Anode: Zinc-mercury amalgam ($Zn(Hg)$). Oxidation: $Zn(Hg)(s) + 2OH^-(aq) \rightarrow ZnO(s) + H_2O(l) + 2e^-$.

* Cathode: Paste of $HgO$ and carbon. Reduction: $HgO(s) + H_2O(l) + 2e^- \rightarrow Hg(l) + 2OH^-(aq)$. * Overall Reaction: $Zn(Hg)(s) + HgO(s) \rightarrow ZnO(s) + Hg(l)$. * Electrolyte: Concentrated alkaline paste of $KOH$ and $ZnO$.

* Voltage: Constant $1.35,\text{V}$. * Uses: Hearing aids, pacemakers, watches (constant voltage critical).

Secondary Batteries: — Rechargeable, multiple-use. Reactions are reversible.

* Lead-Acid Battery: * Anode (Discharge): Lead ($Pb$). $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$. * Cathode (Discharge): Lead dioxide ($PbO_2$). $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$.

* Overall Discharge: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$. * Overall Charge: $2PbSO_4(s) + 2H_2O(l) \rightarrow Pb(s) + PbO_2(s) + 2H_2SO_4(aq)$. * Electrolyte: Aqueous $H_2SO_4$.

Density decreases on discharge, increases on charge. * Voltage: $approx 2,\text{V}$ per cell. * Uses: Automobile batteries, inverters, UPS. * Nickel-Cadmium (Ni-Cd) Battery: * Anode: Cadmium ($Cd$).

$Cd(s) + 2OH^-(aq) \rightarrow Cd(OH)_2(s) + 2e^-$. * Cathode: Nickel(III) oxyhydroxide ($NiO(OH)$). $2NiO(OH)(s) + 2H_2O(l) + 2e^- \rightarrow 2Ni(OH)_2(s) + 2OH^-(aq)$. * Electrolyte: Alkaline $KOH$ solution.

* Voltage: $approx 1.2,\text{V}$. * Characteristic: Exhibits 'memory effect'. Contains toxic cadmium. * Lithium-ion (Li-ion) Battery: * Principle: Intercalation of $Li^+$ ions. * Anode: Graphite ($Li_x C_6$).

* Cathode: Lithium metal oxide (e.g., $LiCoO_2$). * Electrolyte: Non-aqueous lithium salt solution. * Voltage: $approx 3.7,\text{V}$. * Characteristics: High energy density, no memory effect, low self-discharge.

* Uses: Smartphones, laptops, electric vehicles, medical devices.

Key Differences: — Primary (irreversible, single-use, lower initial cost) vs. Secondary (reversible, multi-use, higher initial cost).

Vyyuha Quick Recall

Primary Never Recharges, Secondary Reverses Reactions.

Primary: Dry Mercury (Dry cell, Mercury cell) Secondary: Lead Ni-Cd Li-ion (Lead-acid, Nickel-Cadmium, Lithium-ion)

Lead-Acid: Lead Lead Dioxide Sulfuric Acid (Anode, Cathode, Electrolyte) Mercury: Maintains Constant Voltage (1.35V)