Free Electrochemistry MCQs with Answers
24 Electrochemistry MCQs from Chemistry, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
24 questions · page 3 of 3
21. Oxidation numbers of X, Y, Z are +6, -2 and -1 respectively. Possible molecular formula when atoms combine:
- A. X2YZ
- B. XY2Z2
- C. XYZ2
- D. XY2Z
Explanation: A neutral compound must have oxidation numbers summing to zero, and XY2Z2 gives plus 6, then two at minus 2 and two at minus 1, that is 6 minus 4 minus 2, which is exactly zero. Testing each option the same way disposes of the others: XYZ2 gives 6 minus 2 minus 2, which is plus 2 rather than zero. Summing to zero is the only test needed for a neutral formula.
Correct answer: XY2Z222. The order of ease of reduction of H+1, Cu+2 and Ag+1:
- A. H+1 > Cu+2 > Ag+
- B. H+1 > Ag+ > Cu+2
- C. Ag+ > Cu+2 > H+1
- D. H+1 > Cu+2 > Ag+
Explanation: The more positive the standard electrode potential, the more readily the ion accepts electrons, and the values are plus 0.80 V for silver, plus 0.34 V for copper and zero for hydrogen by definition. Silver is therefore reduced most easily and hydrogen least, which is also why copper and silver do not displace hydrogen from dilute acids. Note that options (a) and (d) were printed identically in the original paper.
Correct answer: Ag+ > Cu+2 > H+123. 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.
Correct answer: Hydrogen gas reacting with chlorine to form hydrogen chloride gas24. Which of the following best identifies the redox reaction:
- A. Transfer of proton
- B. Transfer of electron
- C. Absorption of light
- D. Exchange of ions
Explanation: Oxidation is loss of electrons and reduction is gain, so a redox reaction is defined by electron transfer and by the change in oxidation number that accompanies it. Proton transfer defines an acid base reaction in the Bronsted sense, which is the closest distractor. Simple exchange of ions, as in a precipitation, involves no change in oxidation number at all.
Correct answer: Transfer of electron