krit.club logo

Electrochemistry - Batteries and Fuel Cells

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Primary batteries are non-rechargeable cells where the reaction occurs only once. A common example is the Mercury Cell, which provides a constant voltage of 1.35 V1.35 \text{ V} because the electrolyte concentration does not change during its life.

Schematic representation of a Mercury cell showing Zinc amalgam anode and Mercury(II) oxide cathode.
•

Secondary batteries are rechargeable because the chemical reactions can be reversed by passing an external electric current. In a Lead Storage Battery, the density of H2SO4H_2SO_4 decreases during discharge and increases during charging.

Lead storage battery setup with lead anode and lead dioxide cathode in sulfuric acid.
•

Fuel cells are galvanic cells designed to convert the chemical energy of fuels like H2H_2, CH4CH_4, or CH3OHCH_3OH directly into electrical energy. The H2−O2H_2-O_2 fuel cell is highly efficient (70%70\%) and pollution-free.

Hydrogen-Oxygen Fuel Cell diagram illustrating the flow of gases and the aqueous electrolyte.
•

The Nickel-Cadmium (Ni-Cd) cell is a secondary cell with a longer life than lead storage batteries but is more expensive to manufacture. The overall reaction is: Cd(s)+2Ni(OH)3(s)→CdO(s)+2Ni(OH)2(s)+H2O(l)Cd(s) + 2Ni(OH)_3(s) \rightarrow CdO(s) + 2Ni(OH)_2(s) + H_2O(l).

Nickel-Cadmium cell components showing cadmium and nickel oxide electrodes.

📐Formulae

ΔG=−nFEcell\Delta G = -nFE_{cell}

Efficiency (η) of Fuel Cell=ΔGΔH×100\text{Efficiency (}\eta\text{) of Fuel Cell} = \frac{\Delta G}{\Delta H} \times 100

Dry Cell Overall: Zn(s)+2MnO2(s)+2NH4+(aq)→Zn2+(aq)+Mn2O3(s)+2NH3(g)+H2O(l)\text{Dry Cell Overall: } Zn(s) + 2MnO_2(s) + 2NH_4^+(aq) \rightarrow Zn^{2+}(aq) + Mn_2O_3(s) + 2NH_3(g) + H_2O(l)

Lead Storage (Discharging): Pb(s)+PbO2(s)+2H2SO4(aq)→2PbSO4(s)+2H2O(l)\text{Lead Storage (Discharging): } Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)

Fuel Cell (Overall): 2H2(g)+O2(g)→2H2O(l)\text{Fuel Cell (Overall): } 2H_2(g) + O_2(g) \rightarrow 2H_2O(l)

💡Examples

Problem 1:

Write the cathode and anode reactions that occur during the discharging of a Lead-storage battery.

Solution:

Anode: Pb(s)+SO42−(aq)→PbSO4(s)+2e−Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^- Cathode: PbO2(s)+SO42−(aq)+4H+(aq)+2e−→PbSO4(s)+2H2O(l)PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)

Explanation:

During discharging, the lead-storage battery acts as a galvanic cell. Lead is oxidized at the anode, and Lead dioxide is reduced at the cathode. Both reactions produce PbSO4PbSO_4, which sticks to the electrodes.

Problem 2:

Calculate the theoretical efficiency of a H2−O2H_2-O_2 fuel cell if ΔGf∘\Delta G_f^{\circ} for H2O(l)H_2O(l) is −237.2 kJ mol−1-237.2 \text{ kJ mol}^{-1} and ΔHf∘\Delta H_f^{\circ} is −285.8 kJ mol−1-285.8 \text{ kJ mol}^{-1}.

Solution:

Efficiency η=ΔGΔH×100=−237.2−285.8×100≈83%\eta = \frac{\Delta G}{\Delta H} \times 100 = \frac{-237.2}{-285.8} \times 100 \approx 83\%

Explanation:

The efficiency of a fuel cell is defined as the ratio of the maximum useful work (Gibbs Free Energy change) to the total heat of combustion (Enthalpy change).

Problem 3:

Identify the anode and cathode materials in a standard Mercury cell and write the half-cell reaction occurring at the anode.

Mercury cell components for identification.

Solution:

In a Mercury cell: Anode: Zinc-Mercury amalgam (Zn(Hg)Zn(Hg)) Cathode: A paste of Mercury(II) oxide (HgOHgO) and carbon.

Anode reaction: Zn(Hg)+2OH−(aq)→ZnO(s)+H2O(l)+2e−Zn(Hg) + 2OH^-(aq) \rightarrow ZnO(s) + H_2O(l) + 2e^-

Explanation:

The Mercury cell is a primary cell. The anode involves the oxidation of Zinc in the presence of basic electrolyte OH−OH^- ions.

Problem 4:

Describe the chemical changes at the cathode during the charging process of a Lead storage battery.

Lead storage battery in charging mode showing the conversion of lead sulfate.

Solution:

During charging, the lead storage battery acts as an electrolytic cell. The discharge reactions are reversed.

At the Cathode (Reduction of Pb2+Pb^{2+} to PbPb): PbSO4(s)+2e−→Pb(s)+SO42−(aq)PbSO_4(s) + 2e^- \rightarrow Pb(s) + SO_4^{2-}(aq)

Note: The electrode that was the anode during discharge becomes the cathode during charging.

Explanation:

Charging reverses the chemical consumption of PbSO4PbSO_4. Electrons are supplied by an external source to reduce PbSO4PbSO_4 back to metallic Lead at the negative electrode.

Batteries and Fuel Cells Class 12 Notes & Examples