1. Electrochemistry is primarily concerned with:
ⓐ. heat changes accompanying acid-base and neutralisation reactions
ⓑ. separation of mixtures through differences in physical properties
ⓒ. conversion between chemical and electrical energy through redox reactions
ⓓ. conversion between nuclear and chemical energy during radioactive change
Correct Answer: conversion between chemical and electrical energy through redox reactions
Explanation: Electrochemistry connects redox reactions with electrical phenomena. A spontaneous chemical reaction may produce an electric current, as occurs in a working battery. In the reverse type of process, an external electrical source may drive a chemical change that would not occur spontaneously. Both situations involve electron transfer between species. Heat changes and mixture-separation methods may accompany other chemical processes, but they do not define electrochemistry.
2. An electrochemical reaction is classified as a redox reaction because it involves:
ⓐ. formation and movement of positive ions without electron transfer
ⓑ. gain of electrons by one species without a corresponding electron loss
ⓒ. movement of solvent molecules between regions of different concentration
ⓓ. oxidation of one species and reduction of another species
Correct Answer: oxidation of one species and reduction of another species
Explanation: A redox reaction contains oxidation and reduction as coupled processes. The species undergoing oxidation releases electrons, while another species accepts those electrons and is reduced. Electron loss cannot continue indefinitely unless a corresponding electron-accepting process is available. Electrochemical cells separate or control these two processes so that electron transfer can be used electrically. Merely forming ions or moving solvent molecules does not by itself establish a redox reaction.
3. Which substance forms an electrolyte when mixed with water under ordinary conditions?
ⓐ. sucrose, which dissolves mainly as neutral molecules
ⓑ. sodium chloride, which produces mobile ions in water
ⓒ. kerosene, which remains as neutral non-polar molecules
ⓓ. paraffin wax, which provides no mobile charged particles
Correct Answer: sodium chloride, which produces mobile ions in water
Explanation: An electrolyte produces mobile ions when dissolved in a suitable solvent or when melted. Aqueous \(NaCl\) contains hydrated \(Na^+\) and \(Cl^-\) ions that can move in response to an electric field. Sucrose may dissolve in water, but its molecules remain largely uncharged and do not provide comparable ionic conduction. Kerosene and paraffin wax also lack freely moving ions. Dissolution alone is not sufficient; the dissolved or molten substance must supply mobile charged particles.
4. Solid \(NaCl\) conducts electricity very poorly, whereas molten \(NaCl\) conducts. The best explanation is that:
ⓐ. melting releases mobile electrons from neutral sodium atoms
ⓑ. the ions in solid sodium chloride remain fixed at lattice positions
ⓒ. melting converts chloride ions into electrically neutral atoms
ⓓ. melting frees the oppositely charged ions to move through the liquid
Correct Answer: melting frees the oppositely charged ions to move through the liquid
Explanation: Solid \(NaCl\) already contains \(Na^+\) and \(Cl^-\) ions, but they are fixed at lattice positions. Because the ions cannot move over appreciable distances, the solid does not conduct through ionic motion. On melting, the ordered lattice breaks down and both types of ions become mobile. These ions carry charge through the liquid electrolyte. The conduction is ionic; melting does not create a sea of free electrons like that found in a metal.
5. In an electrochemical cell, an electrode is best described as:
ⓐ. a porous separator that allows ions but not electrons to cross
ⓑ. a metallic lead that carries current without contacting the electrolyte
ⓒ. a conducting interface where an electrode reaction transfers electrons
ⓓ. an electrolyte region that transfers electrons through the solution
Correct Answer: a conducting interface where an electrode reaction transfers electrons
Explanation: An electrode provides an interface between an electronic conductor and an electrolyte. Oxidation or reduction occurs at this interface through the transfer of electrons. The electrode also connects the chemical reaction to the external electrical circuit. It may participate chemically, as a metal electrode can, or act mainly as an inert conducting surface. An electrode is not simply a separator, temperature probe, or liquid source of electrons.
6. An electrochemical cell consists essentially of an arrangement in which:
ⓐ. heat transfer between substances without an electron-transfer reaction
ⓑ. redox reactions and charge flow convert chemical and electrical energy
ⓒ. solvent evaporation coupled only to movement of neutral molecules
ⓓ. electron flow through the bulk electrolyte without interfacial reactions
Correct Answer: redox reactions and charge flow convert chemical and electrical energy
Explanation: An electrochemical cell contains electrode regions in contact with an electrolyte. Oxidation and reduction occur at the electrodes, while charge moves through appropriate internal and external paths. This arrangement can generate electrical energy from a spontaneous reaction or use electrical energy to drive a chemical reaction. Electron transfer is linked to the electrode processes rather than to simple evaporation. Within an electrolyte, charge is normally carried by ions rather than by free electrons travelling through the solution.
7. Match each symbol in Column I with the corresponding quantity and SI unit in Column II.
| Column I | Column II |
|---|
| P. \(E\) | 1. Charge in \(C\) |
| Q. \(I\) | 2. Time in \(s\) |
| R. \(Q\) | 3. Potential difference in \(V\) |
| S. \(t\) | 4. Current in \(A\) |
ⓐ. P-4, Q-3, R-2, S-1
ⓑ. P-3, Q-4, R-1, S-2
ⓒ. P-3, Q-1, R-4, S-2
ⓓ. P-2, Q-4, R-3, S-1
Correct Answer: P-3, Q-4, R-1, S-2
Explanation: The symbol \(E\) represents potential difference or cell potential and is measured in \(V\). Electric current is denoted by \(I\) and measured in \(A\). Charge is represented by \(Q\) and measured in \(C\). Time is represented by \(t\) and measured in \(s\). Keeping the quantity symbols separate from their unit symbols prevents confusion between \(E\) and \(V\), or between \(I\) and \(A\).
8. A steady current of \(2.0\,A\) flows through a circuit for \(30\,s\). The charge transferred is:
ⓐ. \(15\,C\)
ⓑ. \(28\,C\)
ⓒ. \(32\,C\)
ⓓ. \(60\,C\)
Correct Answer: \(60\,C\)
Explanation: \( \textbf{Known data:} \) Current \(I=2.0\,A\) and time \(t=30\,s\).
\( \textbf{Required quantity:} \) Charge transferred, \(Q\).
\( \textbf{Applicable relation:} \)
\[
Q=It
\]
This relation applies because current is the charge transferred per unit time.
\( \textbf{Unit interpretation:} \)
\[
1\,A=1\,C\,s^{-1}
\]
\( \textbf{Substitution:} \)
\[
Q=(2.0\,C\,s^{-1})(30\,s)
\]
\( \textbf{Calculation:} \)
\[
Q=60\,C
\]
The unit \(s\) cancels, leaving the required unit of charge.
\( \textbf{Final answer:} \) The transferred charge is \(60\,C\); dividing the time by the current would not represent the definition of charge.
9. The same quantity of charge passes through two conductors. If it passes through conductor P in half the time required for conductor Q, then:
ⓐ. the current in P is twice the current in Q
ⓑ. the current in P is half the current in Q
ⓒ. both conductors carry the same current
ⓓ. the current ratio cannot be found from the given information
Correct Answer: the current in P is twice the current in Q
Explanation: Current is given by \(I=\frac{Q}{t}\). The charge \(Q\) is the same for both conductors, so current varies inversely with the time taken. If \(t_P=\frac{1}{2}t_Q\), then \(I_P=\frac{Q}{t_P}=2\frac{Q}{t_Q}\). Hence \(I_P=2I_Q\). A shorter transfer time for the same charge corresponds to a larger rate of charge flow.
10. Device P is a dry cell connected to a light-emitting diode. Device Q contains two electrodes in an electrolyte connected to an external direct-current source that causes a new substance to form. The energy conversions in P and Q are respectively:
ⓐ. electrical to chemical; chemical to electrical
ⓑ. thermal to electrical; electrical to mechanical
ⓒ. chemical to electrical; electrical to chemical
ⓓ. chemical to thermal only; chemical to electrical
Correct Answer: chemical to electrical; electrical to chemical
Explanation: Device P operates as a discharging battery, so a spontaneous redox reaction supplies electrical energy to the external circuit. Device Q receives electrical energy from an external source to drive a chemical change. The two devices therefore represent opposite directions of energy conversion. Both processes still involve oxidation and reduction at electrodes. The presence of an external source in Q distinguishes electrolysis from spontaneous battery discharge.
11. Assertion: Electrochemistry includes both battery operation and electrolysis.
Reason: Oxidation occurs at the anode in every electrochemical cell.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true and Reason is false, so Reason cannot explain Assertion
ⓓ. Assertion is false and Reason is true, so Reason cannot explain Assertion
Correct Answer: Both Assertion and Reason are true, but Reason does not explain Assertion
Explanation: Battery operation and electrolysis are both electrochemical because they connect redox reactions with electrical energy. A battery converts chemical energy into electrical energy, whereas electrolysis uses electrical energy to produce chemical change. Oxidation does occur at the anode in both kinds of cells, so the Reason is true. However, that electrode rule does not explain why both processes belong to electrochemistry. Their inclusion is explained by the interconversion between chemical and electrical energy through redox processes.
12. In a redox process, oxidation and reduction must occur together because:
ⓐ. every oxidised species must also gain the same electrons
ⓑ. oxidation always produces oxygen, while reduction removes oxygen
ⓒ. both processes must occur at the same physical location
ⓓ. electrons lost by one species must be gained by another species
Correct Answer: electrons lost by one species must be gained by another species
Explanation: Oxidation involves loss of electrons, whereas reduction involves gain of electrons. Electrons cannot accumulate indefinitely as free particles in an ordinary chemical reaction. A species that loses electrons therefore requires another species capable of accepting them. The two half-processes may occur at separate electrodes in an electrochemical cell, but they remain parts of one overall redox reaction. Oxidation does not necessarily involve oxygen, and reduction does not necessarily mean removal of oxygen.
13. Which equation represents a reduction half-reaction?
ⓐ. \(Fe^{2+}(aq)\rightarrow Fe^{3+}(aq)+e^-\)
ⓑ. \(Cu^{2+}(aq)+2e^-\rightarrow Cu(s)\)
ⓒ. \(2Cl^-(aq)\rightarrow Cl_2(g)+2e^-\)
ⓓ. \(Zn(s)\rightarrow Zn^{2+}(aq)+2e^-\)
Correct Answer: \(Cu^{2+}(aq)+2e^-\rightarrow Cu(s)\)
Explanation: In the copper half-reaction, \(Cu^{2+}\) accepts two electrons and forms neutral copper metal. Electron gain identifies the process as reduction. The oxidation number of copper decreases from \(+2\) to \(0\). In each of the other equations, electrons appear on the product side, showing that the reacting species loses electrons. The location of electrons in a balanced half-reaction provides a direct way to distinguish reduction from oxidation.
14. In the reaction \(Zn(s)+Cu^{2+}(aq)\rightarrow Zn^{2+}(aq)+Cu(s)\), the reducing agent is:
ⓐ. \(Zn(s)\), because it donates electrons to \(Cu^{2+}(aq)\)
ⓑ. \(Cu^{2+}(aq)\), because it accepts electrons from \(Zn(s)\)
ⓒ. \(Zn^{2+}(aq)\), because it is produced when \(Cu^{2+}\) is reduced
ⓓ. \(Cu(s)\), because it forms while \(Zn(s)\) is oxidised
Correct Answer: \(Zn(s)\), because it donates electrons to \(Cu^{2+}(aq)\)
Explanation: Zinc changes from oxidation number \(0\) in \(Zn(s)\) to \(+2\) in \(Zn^{2+}\). It therefore loses two electrons and undergoes oxidation. A reducing agent causes another species to be reduced by supplying electrons to it. Here, the donated electrons are accepted by \(Cu^{2+}\), which forms \(Cu(s)\). The reducing agent is the species oxidised during the reaction, not the species formed after reduction.
15. The oxidation half-reaction \(Al(s)\rightarrow Al^{3+}(aq)+3e^-\) is combined with the reduction half-reaction \(Ag^+(aq)+e^-\rightarrow Ag(s)\). The balanced overall reaction is:
ⓐ. \(Al(s)+Ag^+(aq)\rightarrow Al^{3+}(aq)+Ag(s)\)
ⓑ. \(Al(s)+3Ag(s)\rightarrow Al^{3+}(aq)+3Ag^+(aq)\)
ⓒ. \(3Al(s)+Ag^+(aq)\rightarrow 3Al^{3+}(aq)+Ag(s)\)
ⓓ. \(Al(s)+3Ag^+(aq)\rightarrow Al^{3+}(aq)+3Ag(s)\)
Correct Answer: \(Al(s)+3Ag^+(aq)\rightarrow Al^{3+}(aq)+3Ag(s)\)
Explanation: \( \textbf{Oxidation half-reaction:} \)
\[
Al(s)\rightarrow Al^{3+}(aq)+3e^-
\]
\( \textbf{Reduction half-reaction:} \)
\[
Ag^+(aq)+e^-\rightarrow Ag(s)
\]
The aluminium half-reaction releases \(3e^-\), whereas one silver-ion reduction consumes only \(1e^-\).
The reduction half-reaction must therefore be multiplied by \(3\).
\[
3Ag^+(aq)+3e^-\rightarrow 3Ag(s)
\]
Adding the two half-reactions gives:
\[
Al(s)+3Ag^+(aq)+3e^-\rightarrow Al^{3+}(aq)+3e^-+3Ag(s)
\]
The \(3e^-\) terms cancel because electrons are transferred, not produced in the final reaction.
\[
Al(s)+3Ag^+(aq)\rightarrow Al^{3+}(aq)+3Ag(s)
\]
Both charge and atoms are conserved in the balanced equation.
\( \textbf{Final answer:} \) One mole of aluminium transfers three moles of electrons to three moles of \(Ag^+\).
16. Match each description in Column I with the appropriate term in Column II.
| Column I | Column II |
|---|
| P. Loses electrons | 1. Oxidising agent |
| Q. Gains electrons | 2. Oxidation |
| R. Causes another species to be reduced | 3. Reducing agent |
| S. Causes another species to be oxidised | 4. Reduction |
ⓐ. P-2, Q-4, R-3, S-1
ⓑ. P-4, Q-2, R-1, S-3
ⓒ. P-2, Q-3, R-4, S-1
ⓓ. P-3, Q-1, R-2, S-4
Correct Answer: P-2, Q-4, R-3, S-1
Explanation: Loss of electrons is oxidation, so P matches 2. Gain of electrons is reduction, so Q matches 4. A reducing agent supplies electrons and causes another species to be reduced, so R matches 3. An oxidising agent accepts electrons and causes another species to be oxidised, so S matches 1. The agent itself undergoes the process opposite to the one it causes in the other reactant.
17. An iron nail is placed in a blue \(CuSO_4\) solution. A reddish-brown copper coating appears, and the solution gradually develops the pale-green colour associated with \(Fe^{2+}\). The electron-transfer interpretation is:
ⓐ. \(Cu(s)\) loses electrons to form \(Cu^{2+}\), while \(Fe^{2+}\) gains electrons
ⓑ. \(Fe^{2+}\) and \(Cu^{2+}\) exchange places without any redox change
ⓒ. \(Fe(s)\) loses electrons, while \(Cu^{2+}\) gains electrons
ⓓ. sulphate ions oxidise iron and are converted into sulphur
Correct Answer: \(Fe(s)\) loses electrons, while \(Cu^{2+}\) gains electrons
Explanation: Iron atoms enter the solution as \(Fe^{2+}\), so iron loses electrons and is oxidised. Copper ions accept those electrons and form copper metal on the nail. The relevant half-reactions are \(Fe(s)\rightarrow Fe^{2+}(aq)+2e^-\) and \(Cu^{2+}(aq)+2e^-\rightarrow Cu(s)\). Sulphate ions remain spectator ions in this displacement reaction. The visible copper deposit and colour change both support the transfer of electrons from iron to copper ions.
18. Consider the following statements about metallic and electrolytic conduction.
Statement I: Metallic conduction involves mobile electrons.
Statement II: Electrolytic conduction involves mobile ions.
Statement III: Electrolytic conduction may be accompanied by chemical changes at electrodes.
ⓐ. Statements I, II and III
ⓑ. Statements I and II only
ⓒ. Statements II and III only
ⓓ. Statement I only
Correct Answer: Statements I, II and III
Explanation: Mobile electrons are the principal charge carriers in metallic conductors. In electrolyte solutions or melts, charge is transported by cations and anions. When an electrolyte carries current between electrodes, oxidation and reduction may occur at the electrode surfaces. Such reactions can change the composition of the electrolyte or produce deposited substances and gases. This chemical transport distinguishes electrolytic conduction from ordinary electronic conduction in a metal.
19. A graph shows conductance on the vertical axis and temperature on the horizontal axis. Curve P slopes downward for a metal, while curve Q slopes upward for an electrolyte solution over the same moderate temperature range. The best interpretation is:
ⓐ. both conductors depend on free-electron motion, but their electron charges differ
ⓑ. metal ions become mobile in P, while ions become fixed in Q
ⓒ. increasing temperature creates ions in the metal and destroys ions in the electrolyte
ⓓ. heating hinders metallic electron flow but increases ionic mobility
Correct Answer: heating hinders metallic electron flow but increases ionic mobility
Explanation: In a metal, higher temperature increases lattice vibrations and promotes electron scattering, so conductance generally falls. In an electrolyte solution, higher temperature commonly lowers the resistance to ionic movement. Ions can move more rapidly through the solvent, so conductance generally rises. The two curves differ because their principal charge carriers and transport mechanisms are different. The upward electrolyte curve does not require the ions to become electrons or the metal ions to leave their lattice positions.
20. A liquid dissolves readily in water but the resulting solution shows negligible electrical conduction. The most reasonable conclusion is that the dissolved substance:
ⓐ. contains a metal but does not ionise appreciably in water
ⓑ. remains mainly molecular and supplies few mobile ions
ⓒ. contains ions that cannot interact with water
ⓓ. releases electrons but produces no mobile ions in solution
Correct Answer: remains mainly molecular and supplies few mobile ions
Explanation: Electrical conduction through a solution requires mobile charged particles. A substance may dissolve because of favourable solute-solvent interactions yet remain present as neutral molecules. Such a solution behaves as a non-electrolyte or a very poor electrolyte. Sugar solutions provide a familiar example: the molecules disperse in water but do not ionise appreciably. Solubility and electrolytic conduction are related to different properties and must not be treated as identical.
