Free Oxidation and Reduction MCQs with Answers

1. Oxidation is best defined as

• A. loss of electrons and an increase in oxidation number
• B. gain of electrons and a decrease in oxidation number
• C. gain of hydrogen only
• D. loss of oxygen only

Explanation: The modern definition is in terms of electrons, so oxidation is loss and reduction is gain, remembered as OIL RIG. Definitions based on adding oxygen or removing hydrogen are older special cases that fail for reactions in which neither element is present. Every oxidation must be accompanied by a reduction, since the electrons lost by one species are gained by another.

2. The oxidation number of sulphur in sulphuric acid, H2SO4, is

• A. plus 2
• B. plus 6
• C. minus 2
• D. plus 4

Explanation: Hydrogen contributes plus 1 each and oxygen minus 2 each, so 2 plus x minus 8 equals zero and x is plus 6. The value plus 4 belongs to sulphur in sulphur dioxide and in sulphurous acid, which is why it is the closest distractor. Sulphur at plus 6 is already at its maximum oxidation state, so concentrated sulphuric acid can act as an oxidising agent but not as a reducing one.

3. An oxidising agent is a substance that

• A. loses electrons and is itself oxidised
• B. gains electrons and is itself reduced
• C. neither gains nor loses electrons
• D. always contains oxygen

Explanation: An oxidising agent oxidises something else by taking its electrons, so it is itself reduced in the process, and the same reasoning in reverse defines a reducing agent. Potassium permanganate and acidified potassium dichromate are common oxidising agents that contain oxygen, but chlorine is a strong oxidising agent with none, which disposes of the last option.

4. The oxidation number of oxygen in hydrogen peroxide, H2O2, is

• A. minus 2
• B. zero
• C. minus 1
• D. plus 2

Explanation: Hydrogen is plus 1, so the two oxygens must total minus 2 between them, giving minus 1 each, which is the exception to the usual rule of minus 2 for oxygen. This intermediate value is why hydrogen peroxide can act both as an oxidising agent, going to minus 2, and as a reducing agent, going to zero. In compounds with fluorine, oxygen takes a positive oxidation number.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.