Kapnayan 101 Day 3

What is the oxidation number (or oxidation state) of hydrogen (H) in the hydronium ion (H₃O⁺)?

• 0

• -1

• +1

• -2

Answer explanation

According to our rules, H would generally have an oxidation number of +1.

What is the oxidation number (or oxidation state) of oxygen (O) in the hydronium ion (H₃O⁺)?

• 0

• -1

• +1

• -2

Answer explanation

We know the oxidation number (O.N.) of hydrogen is +1. The sum of the oxidation numbers of all atoms in a compound should be equal to its formal charge, which in this case is +1.

There are three hydrogens with +1 O.N. each.

So oxygen O.N. = +1 - 3(+1) = -2

One form of copper oxide is cuprite (Cu₂O).

Over time, cuprite can be oxidized to a new form known as tenorite (CuO).

What are the oxidation numbers of copper in cuprite and tenorite?

• Cuprite: +1

  Tenorite: +1

• Cuprite: +2

  Tenorite: +4

• Cuprite: +1

  Tenorite: +2

• Cuprite: +2

  Tenorite: +1

Answer explanation

Recall that the O.N. of oxygen is -2. You can solve for the O.N. of the coppers by equating the sum of the oxidation numbers for both minerals to 0, since the compounds are neutral.

(Extra hint: The question indicated that cuprite is oxidized to tenorite, so the copper O.N. should increase)

Which species is reduced in the following redox reaction:

Zn + Cu²⁺ --> Zn²⁺ + Cu

• Zn

• Cu²⁺

• Zn²⁺

• Cu

Answer explanation

Copper's oxidation state went from +2 to 0.

It's reduced because it lost electrons (O.N. decreased).

What is the oxidizing agent in the following redox reaction:

Zn + Cu²⁺ --> Zn²⁺ + Cu

• Zn

• Cu²⁺

• Zn²⁺

• Cu

Answer explanation

Recall that the oxidizing agent is the species being reduced.

Cu²⁺ is reduced, so it is also the oxidizing agent.

What is the oxidized species in the following chemical reaction?

2 Sr + O₂ --> 2 SrO

• Sr

• O₂

• SrO

Answer explanation

Sr changed from 0 to +2 oxidation state.

What is the oxidizing agent in the following reaction?

Na + H₂O --> NaOH + H₂

• Na

• H₂O

• NaOH

• H₂

Answer explanation

The H in H2O got reduced from +1 to 0 oxidation number.

Alternatively, you can see that Na got oxidized from 0 to +1. As such, the other reagent (water) is the oxidizing agent.

Acid rain is propagated when sulfur dioxide (SO₂) reacts with oxygen (O₂) to form sulfur trioxide (SO₃):

SO₂ + O₂ --> SO₃

What is the coefficient of SO₃ when this redox reaction equation is balanced?

• 1

• 2

• 3

• 4

Answer explanation

The balanced reaction is:

2 SO₂ + O₂ --> 2 SO₃

Sulfur (S) went from +4 to +6 oxidation state.

Meanwhile, oxygen (O) went from 0 in oxygen gas to -2 in SO₃.

Total of 2 electrons is transferred.

[CHALLENGE]

The permanganate ion (MnO₄^-) is a common oxidizing agent in organic reactions. How many moles of electrons would be needed to reduce 2 mol of MnO₄^- to Mn²⁺ in acidic conditions?

• 2

• 4

• 6

• 10

Answer explanation

The element that changes O.N. here is the manganese (Mn).

In MnO₄^-, Mn has an O.N. of +7.

In Mn2+, Mn has an O.N. of +2.

Per MnO₄^- to be reduced, it needs 7-2 = 5 electrons. So, 2 mol of MnO₄^- will require:

5 x 2 = 10 mol e-

(Kudos! This is a common subproblem in olympiad chemistry!)

What is the reducing agent in the following redox reaction:

2Fe³⁺ + 3Sn²⁺ --> 2Fe²⁺ + 3Sn⁴⁺

• Fe³⁺

• Sn²⁺

• Fe²⁺

• Sn⁴⁺

Answer explanation

Sn got oxidized from +2 to +4. So it's the reducing agent.