Assigning Oxidation Numbers

Assigning Oxidation Numbers

Bromfield Honors Chemistry

Rule #1

​Uncombined elements

Uncombined elements have an oxidation number of 0.

Multiple Choice

What is the oxidation state of H in H₂?

• +2

• +1

• 0

• None of the above

Rule #2

​Monatomic ions

The charge of a monatomic ion is its oxidation number​

Monatomic ions: examples

Li⁺ oxidation # = +1

​

N^3- oxidation # = -3

​

Fe²⁺ oxidation # = +2​

​

Cl- oxidation # = -1

Multiple Choice

Give the oxidation number for aluminum in aluminum oxide, Al₂O₃.

• +3

• +6

• -3

• -6

Rule #3

​F in compounds

In compounds, the oxidation number of F is -1.

Multiple Choice

What is the oxidation number of F in the compound OF₂?

• +2

• -1

• 0

• +1

Rule #4

​H in compounds

Hydrogen atoms in a compound almost always have an oxidation number of +1

Multiple Choice

What is the oxidation number of H in H₂O?

• 0
• -2
• -1
• +1

Rule #4, continued

​Metal hydrides

In metal hydrides, the oxidation number of H is -1

Multiple Choice

What is the oxidation number of H in sodium hydride, NaH ?

• +1

• 0

• -1

• None of the above

Rule #5

​Oxygen in a compound

Oxygen in a compound usually has an oxidation number of -2.

Multiple Choice

What is the oxidation number of O in CO₂?

• +2
• -1
• +4
• -2

Rule #5, continued

​Oxygen in peroxides

In a peroxide, oxygen atoms have an oxidation number of -1.​

Rule #5, continued

​O in compounds with F

In a compound with F, O has an oxidation number of +2

Multiple Choice

What is the oxidation number of O in hydrogen peroxide, H₂O₂?

• +1

• -1

• +2

• -2

Multiple Choice

What is the oxidation number of O in OF₂?

• +1

• -1

• -2

• +2

Rule #6

​Neutral compounds

In a neutral compound, the sum of the oxidation numbers is 0.

Drag and Drop

What is the oxidation number of C in carbon dioxide, CO₂?

Placeholder

Drag these tiles and drop them in the correct blank above

+4

-4

+2

-2

+1

-1

+3

-3

Multiple Choice

Determine the oxidation number of the sulfur atom in H₂S.

• +1

• +2

• -2

• -1

Multiple Choice

Determine the oxidation number of phosphorus in the molecule H₃PO₄.

• +5

• -5

• +3

• -3

Rule #7

​Polyatomic ions

In a polyatomic ion, the sum of the oxidation numbers equals the overall charge of the ion.​

Multiple Choice

What is the oxidation number of S in SO₄ ^2- ?

• -2

• +2

• +6

• -4

Multiple Choice

What is the oxidation number of N in NO₂^1- ?

• -3

• +4

• -2

• +3

Redox reactions

In redox reactions, the oxidation numbers change.

*One species has an increase in its oxidation number.

*One species has a decrease in its oxidation number.

Multiple Choice

The following reaction is an example of a redox reaction:

Mg + 2 AgNO₃ ⟶ 2 Ag + Mg(NO₃)₂

• True

• False

Multiple Choice

The following is an example of a redox reaction:

MnCl₂ + K₂SO₄ ⟶ 2KCl + MnSO₄

• True

• False

Multiple Select

Which of the following are redox reactions?

• CaCO₃(s) ⟶ CaO + CO₂

• 2Na(s) + Cl₂(g) ⟶ 2 NaCl

• C₆H₁₂O₆ + 6 O₂ ⟶ 6CO₂ + 6 H₂O

• Zn + H₂SO₄ ⟶ ZnSO₄ + H₂

• CaCl₂ + Na₂CO₃ ⟶ CaCO₃ + 2 NaCl