Class 12 Electrochemistry Mock Test (CBSE)

Topics in Electrochemistry

• Electrochemical cells
• Galvanic cells
• Electrode potential
• Nernst equation
• Equilibrium constant from Nernst equation
• Gibbs energy and cell potential
• Conductance of electrolytic solutions
• Molar conductivity and its variation with concentration
• Kohlrausch's law
• Electrolytic cells and electrolysis
• Batteries
• Fuel cells
• Corrosion

Electrochemistry practice questions with answers

1. Q1Multiple choice·1 mark
   During the electrolysis of concentrated aqueous sodium chloride (brine) using platinum electrodes, the gas liberated at the anode is:

   1. (a)$O_2(g)$
   2. (b)$H_2(g)$
   3. (c)$Cl_2(g)$
   4. (d)$SO_2(g)$
   Show answer and step marks

   1. Correct option: (c)+1

   Answer: (c)
2. Q2Multiple choice·1 mark
   Which of the following substances is used as an electrolyte in a mercury cell?

   1. (a)Paste of $\text{NH}_4\text{Cl}$ and $\text{ZnCl}_2$
   2. (b)Molten $\text{NaCl}$ and $\text{CaCl}_2$
   3. (c)Aqueous $\text{H}_2\text{SO}_4$
   4. (d)Moist paste of $\text{KOH}$ and $\text{ZnO}$
   Show answer and step marks

   1. Correct option: (d)+1

   Answer: (d)
3. Q3Short answer·2 marks
   What is a fuel cell? Write the chemical reactions taking place at the anode and cathode of a hydrogen-oxygen fuel cell.
   Show answer and step marks

   1. Definition: a fuel cell is a galvanic cell designed to convert the energy of combustion of fuels such as hydrogen, methane or methanol directly into electrical energy.+1
   2. Anode reaction: $2\text{H}_2\text{(g)} + 4\text{OH}^-\text{(aq)} \rightarrow 4\text{H}_2\text{O}\text{(l)} + 4e^-$ (with an acidic electrolyte, $2\text{H}_2 \rightarrow 4\text{H}^+ + 4e^-$ also earns the mark).+0.5
   3. Cathode reaction: $\text{O}_2\text{(g)} + 2\text{H}_2\text{O}\text{(l)} + 4e^- \rightarrow 4\text{OH}^-\text{(aq)}$ (with an acidic electrolyte, $\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}$ also earns the mark).+0.5

   Answer: A fuel cell is a galvanic cell that converts the energy of combustion of fuels like hydrogen directly into electrical energy. Anode: $2\text{H}_2\text{(g)} + 4\text{OH}^-\text{(aq)} \rightarrow 4\text{H}_2\text{O}\text{(l)} + 4e^-$, Cathode: $\text{O}_2\text{(g)} + 2\text{H}_2\text{O}\text{(l)} + 4e^- \rightarrow 4\text{OH}^-\text{(aq)}$
4. Q4Short answer·3 marks
   Explain the electrochemical mechanism of rusting of iron. Write the reactions taking place at the anode and cathode, and mention one method to prevent corrosion.
   Show answer and step marks

   1. Mechanism: a spot on the iron surface is oxidised and acts as the anode; the electrons flow through the metal to another spot where oxygen is reduced (cathode), and water containing dissolved $\text{CO}_2$ (or salts) acts as the electrolyte, forming a tiny electrochemical cell.+1
   2. Anode reaction: $2\text{Fe(s)} \rightarrow 2\text{Fe}^{2+}\text{(aq)} + 4e^-$+0.5
   3. Cathode reaction: $\text{O}_2\text{(g)} + 4\text{H}^+\text{(aq)} + 4e^- \rightarrow 2\text{H}_2\text{O(l)}$+0.5
   4. The $\text{Fe}^{2+}$ ions are further oxidised by atmospheric oxygen to $\text{Fe}^{3+}$, forming rust, hydrated ferric oxide $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$.+0.5
   5. Prevention, any one of: painting, oiling or greasing the surface; galvanisation (coating with zinc); coating with tin or another metal; sacrificial protection with a more active metal such as magnesium or zinc; applying anti-rust chemicals such as bisphenol; or any other valid method.+0.5

   Answer: Iron is oxidised at anodic spots, $2\text{Fe} \rightarrow 2\text{Fe}^{2+} + 4e^-$, and oxygen is reduced at cathodic spots, $\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}$, with moist air containing $\text{CO}_2$ as electrolyte; $\text{Fe}^{2+}$ is oxidised further to rust, $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$. Prevention: e.g. galvanisation (or painting, sacrificial protection).
5. Q5Short answer·3 marks
   For the redox reaction $\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$: (i) Represent the galvanic cell. (ii) Write the individual half-cell reactions at the anode and cathode. (iii) State the function of the salt bridge.
   Show answer and step marks

   1. (i) Cell representation: $\text{Zn(s)} | \text{Zn}^{2+}\text{(aq)} || \text{Cu}^{2+}\text{(aq)} | \text{Cu(s)}$+1
   2. (ii) Anode reaction (oxidation): $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^-$+0.5
   3. (ii) Cathode reaction (reduction): $\text{Cu}^{2+}\text{(aq)} + 2e^- \rightarrow \text{Cu(s)}$+0.5
   4. (iii) Function of salt bridge: it completes the electrical circuit by letting ions move between the two solutions and keeps both solutions electrically neutral.+1

   Answer: (i) $\text{Zn(s)} | \text{Zn}^{2+}\text{(aq)} || \text{Cu}^{2+}\text{(aq)} | \text{Cu(s)}$ (ii) Anode: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$; Cathode: $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$ (iii) It completes the circuit through ion flow and maintains electrical neutrality of the two solutions.

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