WAEC 2010 Chemistry · Question 23

What is the change in oxidation number of manganes in the reaction represented by the following equation? MnO-4(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)

Options

A.+3 to + 2
B.+ 4 to + 2
C.+ 5 to + 2
D.+ 7 to 2

Answer: D. + 7 to 2

Explanation written with AI and checked against the answer key

Analyze the question: We need the oxidation number of Mn in MnO₄⁻ (on the left) and in Mn²⁺ (on the right). Concept: The oxidation number of an ion's atoms adds up to the charge on the ion. Oxygen is usually −2. Step 1: Mn in MnO₄⁻ Let the oxidation number of Mn be x. x + 4(−2) = −1 x − 8 = −1 x = +7 Step 2: Mn in Mn²⁺ For a simple monatomic ion, the oxidation number equals the charge, so Mn is +2. Step 3: Change Mn goes from +7 to +2, a decrease of 5. This is reduction, which matches the 5 electrons gained (5e⁻) on the left side of the equation. Check: Charge balance on the left: (−1) + 8(+1) + 5(−1) = +2. On the right: +2 for Mn²⁺ and 0 for the water. The equation balances. Answer: The change is +7 to +2, which is option D. Tip: In the permanganate ion MnO₄⁻, Mn is always +7. In acidic solution it is reduced to Mn²⁺. The number of electrons gained in the half-equation equals the drop in oxidation number (7 − 2 = 5), so you can use the electrons to check your answer quickly.

AI can make mistakes. Report anything that looks wrong.