Free Redox Reactions MCQs with Answers

1. In the reaction Zn + CuSO4 gives ZnSO4 + Cu, the substance oxidised is

• A. the sulphate ion
• B. copper ions
• C. zinc
• D. copper metal

Explanation: Zinc goes from an oxidation number of zero as the metal to plus 2 in the sulphate, so it has lost electrons and is oxidised, which also makes it the reducing agent. The copper ions gain those electrons, fall from plus 2 to zero and are reduced. The sulphate ion is a spectator, unchanged on both sides of the equation.

2. A reaction in which the same element is both oxidised and reduced is called

• A. a neutralisation reaction
• B. a displacement reaction
• C. a precipitation reaction
• D. a disproportionation reaction

Explanation: In disproportionation one species with an intermediate oxidation state changes into two products, one at a higher and one at a lower state, as when chlorine reacts with cold dilute alkali to give chloride and hypochlorite. The element must therefore have an accessible oxidation state on both sides of its starting value. This is why sodium, always plus 1 in compounds, cannot disproportionate.

3. In an electrochemical cell, oxidation always takes place at the

• A. cathode
• B. salt bridge
• C. anode
• D. voltmeter

Explanation: The anode is by definition the electrode at which oxidation occurs and reduction occurs at the cathode, and this holds for both galvanic and electrolytic cells. What changes between the two is the sign, since the anode is negative in a galvanic cell but positive in electrolysis. Remembering the definitions by process rather than by sign avoids the usual confusion.

4. The function of the salt bridge in a galvanic cell is to

• A. allow electrons to pass between the two solutions
• B. complete the circuit by allowing ions to migrate and keeping both solutions electrically neutral
• C. prevent the cell from producing any current
• D. act as a catalyst for the reaction

Explanation: As the cell runs, positive charge builds up in one half cell and negative charge in the other, which would stop the reaction almost immediately, so ions flow through the bridge to cancel this build up. Electrons travel through the external wire, not through the bridge, which is the point of the first distractor. A saturated potassium chloride or potassium nitrate solution is normally used because its ions move at similar rates.

5. During the electrolysis of molten sodium chloride, sodium metal is produced at the cathode because sodium ions

• A. lose electrons there
• B. gain electrons and are reduced there
• C. are attracted to the anode
• D. react with chlorine there

Explanation: Positive sodium ions migrate to the negative cathode, each accepting one electron to become a sodium atom, which is reduction. Chloride ions travel to the anode, give up electrons and are oxidised to chlorine gas. This is the Down's process, and the salt must be molten because the ions in the solid cannot move.

6. Which of the following is NOT a redox reaction?

• A. Zn + 2HCl gives ZnCl2 + H2
• B. 2H2 + O2 gives 2H2O
• C. NaOH + HCl gives NaCl + H2O
• D. 2Mg + O2 gives 2MgO

Explanation: In neutralisation no element changes its oxidation number, since hydrogen stays at plus 1, oxygen at minus 2 and both sodium and chlorine keep their values throughout, so only ions are rearranged. In the other three reactions a metal or hydrogen changes from zero to a positive state while another element is reduced. Checking oxidation numbers on both sides is the reliable test.

7. In the electrochemical series, a metal higher in the series will

• A. displace a metal lower in the series from its salt solution
• B. be displaced by a metal lower in the series
• C. never react with acids
• D. always be a poor conductor

Explanation: A more reactive metal gives up electrons more readily, so it reduces the ions of a less reactive metal and takes their place in solution, which is why zinc displaces copper from copper sulphate but copper cannot displace zinc. The same order predicts which metals react with dilute acid to release hydrogen, namely those above hydrogen in the series. Displacement reactions are always redox reactions.

8. Which of the following situations most clearly demonstrates a key characteristic of a redox reaction?

• A. Water boiling to steam
• B. Hydrogen gas reacting with chlorine to form hydrogen chloride gas
• C. Sodium chloride dissolving in water
• D. Ethanol evaporating at room temperature

Explanation: Hydrogen goes from zero to plus 1 and chlorine from zero to minus 1, so electrons are transferred and oxidation numbers change, which is the defining feature of a redox reaction. Boiling, dissolving and evaporating are physical changes in which no oxidation number moves at all. Checking oxidation numbers on both sides is the reliable test.