Zn (s) → Zn2+ + 2e-
Above reaction at anode is found in:
Leclanché cell
The question asks us to identify the electrochemical cell in which the reaction $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$ occurs at the anode. This reaction represents the oxidation of solid zinc metal ($\text{Zn (s)}$) into zinc ions ($\text{Zn}^{2+}$), releasing two electrons. Oxidation always takes place at the anode in an electrochemical cell.
Understanding Anode Reactions in Electrochemical Cells
The anode is the electrode where oxidation occurs. Different types of electrochemical cells use different materials as anodes, leading to different oxidation reactions. Let's examine the anode reactions in the options provided:
- Leclanché cell: This is a type of dry cell. The outer container of the cell is made of zinc and acts as the anode. The anode reaction in a Leclanché cell is the oxidation of zinc metal:
$\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$
These $\text{Zn}^{2+}$ ions then react further with ammonium ions from the electrolyte. - Mercury cell: This is a primary cell often used in small devices. The anode in a mercury cell is typically a zinc amalgam (an alloy of zinc and mercury). The oxidation reaction involves zinc, but it reacts with hydroxide ions from the electrolyte:
$\text{Zn (Hg) + 2OH}^- \text{ → ZnO (s) + H}_2\text{O (l) + 2e}^{-}$
While zinc is oxidized, the form of the reactant is different (amalgam) and the products involve zinc oxide and water, not free $\text{Zn}^{2+}$ ions in the initial step shown. - Ni-Cd Cell: This is a rechargeable secondary cell. The anode is made of cadmium. The anode reaction is the oxidation of cadmium:
$\text{Cd (s) + 2OH}^- \text{ → Cd(OH)}_2\text{ (s) + 2e}^{-}$
This reaction involves cadmium, not zinc. - Fuel cell: A fuel cell is an electrochemical cell that converts the chemical energy from a fuel (like hydrogen or methanol) into electricity through an electrochemical reaction with an oxidizing agent (usually oxygen). The anode reaction depends on the fuel used. For example, in a hydrogen fuel cell with an alkaline electrolyte, the anode reaction is:
$\text{H}_2\text{ (g) + 2OH}^- \text{ → 2H}_2\text{O (l) + 2e}^{-}$
In a hydrogen fuel cell with an acidic electrolyte, it is:
$\text{H}_2\text{ (g) → 2H}^+ \text{ (aq) + 2e}^{-}$
Fuel cells do not typically use zinc as a fuel or anode material in the way shown in the question.
Comparing Reactions and Identifying the Cell
Let's compare the given reaction $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$ with the anode reactions in each type of cell:
| Cell Type | Anode Material | Anode Reaction | Matches $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$? |
|---|---|---|---|
| Leclanché cell | Zinc (container) | $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$ | Yes |
| Mercury cell | Zinc amalgam | $\text{Zn (Hg) + 2OH}^- \text{ → ZnO (s) + H}_2\text{O (l) + 2e}^{-}$ | No (Reactant form/products differ) |
| Ni-Cd Cell | Cadmium | $\text{Cd (s) + 2OH}^- \text{ → Cd(OH)}_2\text{ (s) + 2e}^{-}$ | No (Involves Cadmium, not Zinc) |
| Fuel cell | Varies (e.g., Hydrogen) | Varies (e.g., $\text{H}_2 \text{ reaction}$) | No (Typically not Zinc oxidation) |
Based on this comparison, the Leclanché cell is the cell type among the options where the anode reaction is explicitly $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$. This oxidation of solid zinc forms the basis of the current production in this type of dry cell.
Conclusion
The reaction $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$ represents the oxidation of solid zinc, which acts as the anode. This specific reaction is found in the Leclanché cell.
The final answer is Leclanché cell.
Revision Table: Electrochemical Cells and Anode Reactions
| Cell Type | Anode Material | Key Anode Reaction | Type |
|---|---|---|---|
| Leclanché cell (Dry cell) | Zinc (Zn) | $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$ | Primary |
| Mercury cell | Zinc Amalgam (Zn(Hg)) | $\text{Zn (Hg) + 2OH}^- \text{ → ZnO (s) + H}_2\text{O (l) + 2e}^{-}$ | Primary |
| Ni-Cd Cell | Cadmium (Cd) | $\text{Cd (s) + 2OH}^- \text{ → Cd(OH)}_2\text{ (s) + 2e}^{-}$ | Secondary (Rechargeable) |
| Hydrogen Fuel cell (alkaline) | Porous electrode with catalyst | $\text{H}_2\text{ (g) + 2OH}^- \text{ → 2H}_2\text{O (l) + 2e}^{-}$ | Fuel Cell |
Additional Information: Zinc Electrochemistry and Cell Types
Zinc is a common anode material in various electrochemical cells due to its relatively low standard electrode potential, meaning it is easily oxidized. Let's delve a bit deeper into some points:
- Leclanché Cell Details: The $\text{Zn}^{2+}$ ions produced at the anode in a Leclanché cell react with $\text{NH}_4^+$ ions to form complex ions, or with $\text{OH}^-$ ions present to form $\text{Zn(OH)}_2$, which further reacts. This keeps the concentration of $\text{Zn}^{2+}$ low, driving the forward reaction at the anode. The cathode typically involves manganese dioxide ($\text{MnO}_2$).
- Primary vs. Secondary Cells: Primary cells (like Leclanché and Mercury cells) are designed for single use because the electrode reactions are not easily reversible. Secondary cells (like Ni-Cd) are rechargeable because the electrode reactions can be reversed by applying an external voltage.
- Fuel Cells: Fuel cells are unique in that they are supplied with fuel and oxidant from external sources, allowing them to generate electricity continuously as long as these are supplied. They are distinct from batteries which store the reactants internally.
- Oxidation States: In the reaction $\text{Zn (s) → Zn}^{2+} \text{ + 2e}^{-}$, the oxidation state of zinc changes from 0 (in solid metal) to +2 (in the ion). This increase in oxidation state is characteristic of oxidation.
Important Questions from Electrochemistry
Identify transition metal complexes which are not octahedral in shape.
(A) [Co(NH₃)₆]³⁺
(B) [Ni(CO)₄]
(C) [CoCl(NH₃)₅]²⁺
(D) [CoCl₂(NH₃)₄]⁺
(E) [PtCl₄]²⁻
Choose the correct answer from the options given below:
The product of complete hydrolysis of XeF₆ in the following reaction is:
XeF₆ + H₂O → ? HF
In a reaction A and B react to form product. The initial rate of reaction (ro) was determined using different initial concentrations of A and B as shown below:
A/mol L-1 B/mol L-1 ro/mol L-1 s-1 0.10 0.30 6.81 × 10-4 0.10 0.10 2.27 × 10-4 0.20 0.30 13.62 × 10-4 What is the initial rate of reaction (ro) when the critical concentration of A and B is 0.50 mol/L and 0.50 mol/L, respectively?
In which of the following actinoid elements 6d subshell is vacant?
Which of the following shows both, Frenkel and Schottky defect?
