Free Electrochemistry MCQs with Answers

11. 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.

12. 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.

13. A metal with a large negative standard electrode potential is

• A. a strong reducing agent that is easily oxidised
• B. a strong oxidising agent
• C. chemically unreactive
• D. unable to form ions

Explanation: A large negative value means the metal releases electrons readily and prefers to exist as its ion, so it is easily oxidised and therefore acts as a powerful reducing agent, as lithium and potassium do. Metals at the positive end of the series, such as gold and silver, hold their electrons tightly and are unreactive. The electrochemical series is simply the reactivity series expressed in volts.

14. The oxidation number of chromium in the dichromate ion, Cr2O7 2-, is

• A. plus 3
• B. plus 12
• C. plus 6
• D. plus 7

Explanation: Seven oxygens contribute minus 14, and the overall charge is minus 2, so the two chromium atoms must total plus 12, giving plus 6 each. Quoting plus 12 forgets to divide between the two atoms, which is the commonest slip. Acidified dichromate is reduced to the green chromium three ion, and that colour change is used as a test for a reducing agent.

15. 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.

16. In the reaction 2Na + Cl2 gives 2NaCl, chlorine acts as

• A. a reducing agent because it gains electrons
• B. neither an oxidising nor a reducing agent
• C. a catalyst
• D. an oxidising agent because it accepts electrons from sodium

Explanation: Each chlorine atom accepts one electron and falls from zero to minus 1, so chlorine is reduced and is therefore the oxidising agent, while sodium is oxidised and acts as the reducing agent. The confusion in the first option comes from linking gaining electrons with the word reducing, when in fact the species that gains electrons is the oxidising agent. Naming the agent always means describing what it does to the other substance.

17. The standard electrode potential of a half cell is measured under conditions of

• A. 1 molar solution, 298 K and 1 atmosphere pressure
• B. any concentration at 273 K
• C. 0.1 molar solution at 373 K
• D. pure water at room temperature

Explanation: Standard conditions fix the concentration of every solution at 1 molar, the pressure of any gas at 1 atmosphere and the temperature at 298 K, so that values from different laboratories can be compared. Departing from these conditions changes the measured potential, which the Nernst equation quantifies. Note that 298 K is 25 degrees Celsius, not the 273 K of standard temperature for gases.

18. 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.

19. 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.

20. The oxidation number of an element in its free, uncombined state is

• A. plus 1
• B. equal to its group number
• C. minus 1
• D. zero

Explanation: An uncombined element has not gained or lost any electrons, so its oxidation number is zero, and this holds for atoms such as helium and for molecules such as O2, P4 and S8 alike. This rule is the starting point for every oxidation number calculation and for recognising which species has been oxidised in a reaction. The group number gives the usual oxidation state in compounds, not in the free element.