Batteries — Revision Notes

⚡ 30-Second Revision

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: $38$. 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.

2-Minute Revision

Batteries are electrochemical cells converting chemical energy to electrical energy via redox reactions. They are categorized into primary (non-rechargeable) and secondary (rechargeable) types. Primary batteries, like the Leclanché cell (zinc anode, $MnO_2$ cathode, $NH_4Cl$ electrolyte) and mercury cell (zinc-mercury anode, $HgO$ cathode, $KOH$ electrolyte), are single-use because their reactions are irreversible.

Leclanché cells show voltage drop, while mercury cells maintain a constant voltage. Secondary batteries, such as the lead-acid battery (lead anode, $PbO_2$ cathode, $H_2SO_4$ electrolyte) and nickel-cadmium (cadmium anode, $NiO(OH)$ cathode, $KOH$ electrolyte), can be recharged by reversing their chemical reactions using an external current.

Lithium-ion batteries are a modern secondary type known for high energy density, using graphite and lithium metal oxides as electrodes with a non-aqueous electrolyte. Key aspects for NEET include understanding the specific electrode reactions, voltage characteristics, and applications of each battery type.

5-Minute Revision

Batteries are self-contained electrochemical power sources. They operate on the principle of spontaneous redox reactions, where electrons flow from the anode (oxidation) to the cathode (reduction) through an external circuit, generating electrical current. The internal circuit is completed by ion flow through the electrolyte.

Primary Batteries (Non-rechargeable):

1
Leclanché Cell (Dry Cell):

* Anode: Zinc container ($Zn \rightarrow Zn^{2+} + 2e^-$) * Cathode: Carbon rod in $MnO_2$ and carbon paste ($MnO_2 + NH_4^+ + e^- \rightarrow MnO(OH) + NH_3$) * Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$. * Overall: $Zn(s) + 2NH_4Cl(aq) + 2MnO_2(s) \rightarrow Zn(NH_3)_2Cl_2(s) + Mn_2O_3(s) + H_2O(l)$ * Voltage: $1.5,\text{V}$, drops during use due to $NH_3$ accumulation and reactant depletion.

1
Mercury Cell:

* Anode: Zinc-mercury amalgam ($Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O(l) + 2e^-$) * Cathode: Paste of $HgO$ and carbon ($HgO(s) + H_2O(l) + 2e^- \rightarrow Hg(l) + 2OH^-$) * Electrolyte: Paste of $KOH$ and $ZnO$. * Overall: $Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$ * Voltage: Constant $1.35,\text{V}$ because the overall reaction does not involve species whose concentrations change significantly.

Secondary Batteries (Rechargeable):

1
Lead-Acid Battery:

* Anode (Discharge): $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$ (Lead grid) * Cathode (Discharge): $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$ (Lead dioxide grid) * Electrolyte: $38$. Density decreases on discharge, increases on charge. * Overall Discharge: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$ * Charging: External current reverses the reactions. * Voltage: $2,\text{V}$ per cell.

1
Nickel-Cadmium (Ni-Cd) Cell:

* Anode (Discharge): $Cd(s) + 2OH^-(aq) \rightarrow Cd(OH)_2(s) + 2e^-$ * Cathode (Discharge): $NiO(OH)(s) + H_2O(l) + e^- \rightarrow Ni(OH)_2(s) + OH^-(aq)$ * Electrolyte: $KOH$ solution. * Overall Discharge: $Cd(s) + 2NiO(OH)(s) + 2H_2O(l) \rightarrow Cd(OH)_2(s) + 2Ni(OH)_2(s)$ * Voltage: $1.2,\text{V}$. Known for 'memory effect' in older versions.

1
Lithium-ion (Li-ion) Cell:

* High energy density, high voltage ($3.7,\text{V}$), no memory effect, low self-discharge. * Anode: Graphite. Cathode: Lithium metal oxide (e.g., $LiCoO_2$). Electrolyte: Non-aqueous lithium salt solution. * Ions move between electrodes during charge/discharge (rocking chair mechanism).

Key Takeaways: Memorize reactions, identify primary vs. secondary, understand voltage characteristics, and know common applications and environmental concerns (e.g., toxicity of Hg, Cd, Pb).

Prelims Revision Notes

For NEET, a strong grasp of battery types, their components, and the underlying chemical reactions is paramount.

1. Primary Batteries (Non-rechargeable):

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

$MnO_2$ acts as a depolarizer. * Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$. * Voltage: Approximately $1.5,\text{V}$, but drops during use due to product accumulation and reactant depletion. * Applications: Flashlights, radios.

* Mercury Cell: * Anode: Zinc-mercury amalgam ($Zn(Hg)$). Oxidation: $Zn(Hg) + 2OH^-(aq) \rightarrow ZnO(s) + H_2O(l) + 2e^-$. * Cathode: Paste of mercury(II) oxide ($HgO$) and carbon. Reduction: $HgO(s) + H_2O(l) + 2e^- \rightarrow Hg(l) + 2OH^-(aq)$.

* Electrolyte: Paste of $KOH$ and $ZnO$. * Overall Reaction: $Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$. * Voltage: Constant $1.35,\text{V}$ throughout its life due to non-changing electrolyte concentration in the overall reaction.

* Applications: Watches, hearing aids. Contains toxic mercury.

2. Secondary Batteries (Rechargeable):

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

* Electrolyte: $38$ Sulfuric acid ($H_2SO_4$). Density decreases during discharge, increases during charge. * Overall Discharge: $Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)$.

* Charging: The reverse of discharge, converting $PbSO_4$ back to $Pb$ and $PbO_2$, and regenerating $H_2SO_4$. Acts as an electrolytic cell. * Voltage: $2,\text{V}$ per cell. Typically $12,\text{V}$ for car batteries (6 cells in series).

* Applications: Car batteries, inverters. * Nickel-Cadmium (Ni-Cd) Cell: * Anode (Discharge): Cadmium ($Cd$). $Cd(s) + 2OH^-(aq) \rightarrow Cd(OH)_2(s) + 2e^-$. * Cathode (Discharge): Nickel(III) oxide hydroxide ($NiO(OH)$).

$NiO(OH)(s) + H_2O(l) + e^- \rightarrow Ni(OH)_2(s) + OH^-(aq)$. * Electrolyte: $KOH$ solution. * Overall Discharge: $Cd(s) + 2NiO(OH)(s) + 2H_2O(l) \rightarrow Cd(OH)_2(s) + 2Ni(OH)_2(s)$. * Voltage: $1.

2, ext{V}$. Known for 'memory effect' in older versions. Contains toxic cadmium. * Applications: Cordless phones, power tools. * **Lithium-ion (Li-ion) Cell:** * High energy density, high voltage ($3.

7, ext{V}$), no memory effect, low self-discharge. * Electrodes: Graphite (anode), Lithium metal oxide (cathode). Electrolyte: Non-aqueous lithium salt solution. * Applications: Mobile phones, laptops, electric vehicles.

Key Points for Recall:

Distinguish primary vs. secondary based on rechargeability and reaction reversibility.
Memorize anode, cathode, and electrolyte for each battery type.
Be able to write the balanced half-reactions and overall reactions for discharge (and charge for secondary batteries).
Understand the voltage characteristics (constant vs. dropping) and the reasons behind them.
Be aware of the environmental impact of toxic components (Hg, Cd, Pb).

Vyyuha Quick Recall

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!)