Assigning Oxidation Numbers, Oxidation vs Reduction

Assigning Oxidation Numbers
Oxidation and Reduction

Bromfield Honors Chemistry

Overview

​In this activity you will learn to:

Assign oxidation numbers for any atom in a molecule or ion
Classify reactions as redox or non-redox
Compare and contrast oxidation and reduction processes
Identify the species oxidized and reduced in a redox reaction
Identify the oxidizing agent and the reducing agent

​Take notes on key definitions and examples as you work through this activity

Rule #1

​Uncombined elements

Uncombined elements have an oxidation number of 0.

Fill in the Blanks

What is the oxidation number of P in a molecule of P₄?

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

Fill in the Blanks

What is the oxidation number of Te in the compound K₂TeF₈ ?

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

Multiple Choice

Oxidation is the _________ of electrons.

• loss

• gain

• sharing

• transfer

Multiple Choice

Reduction is the ___________ of electrons.

• loss

• gain

• sharing

• transfer

Multiple Choice

If an atom loses electrons during a chemical reaction, the atom was:

• oxidized

• reduced

• neutralized

• precipitated

Multiple Choice

Which statement is true:
Mg → Mg²⁺ + 2e^–

• Mg is reduced

• Mg²⁺ is oxidized

• Mg²⁺ is reduced

• Mg is oxidized

Multiple Choice

Cl₂ + 2e- ⟶ 2Cl ^- is an example of:

• a chemical reaction

• redox

• oxidation

• reduction

Multiple Choice

Which half-reaction correctly represents reduction?

• Ag → Ag⁺ + e^–

• F₂ → 2 F^– + 2e^–

• Au³⁺ + 3e^–→ Au

• Fe²⁺+ e^– → Fe³⁺

Multiple Choice

In a reaction, copper is reduced; its number of electrons has:

• Increased

• Decreased

• Remained Constant

• Varies Randomly

Multiple Choice

Which half-reaction shows conservation of charge?

• Cu + e^– → Cu⁺

• Cu⁺ → Cu + e^–

• Cu²⁺ + 2e^– → Cu

• Cu²⁺ → Cu + 2e^–

Multiple Choice

     In the reaction Zn + H₂O ⟶ ZnO₂ + H₂
 which element, if any, is oxidized? 

• Zinc
• Hydrogen
• Oxygen
• None

Multiple Choice

Given the reaction for the corrosion of aluminum:

4 Al + 3 O₂ → 2 Al₂O₃

Which half-reaction correctly represents the oxidation that occurs?

• Al + 3e–^ → Al³⁺

• Al → Al³⁺ + 3e^–

• O₂ + 4e^– → 2 O^2–

• O₂ → 2 O₂– + 4e^-

Multiple Choice

In a redox reaction, the species reduced

• gains electrons and is the oxidizing agent

• loses electrons and is the oxidizing agent

• loses electrons and is the reducing agent

• gains electrons and is the reducing agent

Multiple Choice

Substance that oxidizes another substance by accepting its electrons.

• reducing agent

• oxidation number

• combustion

• oxidizing agent

Multiple Choice

In the reaction
2Ca(s) + O₂(g) ⟶ 2CaO(s), calcium is __________

• reduced

• oxidized

• neither oxidized nor reduced

Multiple Choice

Given the balanced ionic equation representing a reaction:

2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s)

Which half-reaction represents the reduction that occurs?

• Al → Al³⁺ + 3e^-

• Al³⁺ + 3e^- → Al

• Cu → Cu²⁺ + 2e^-

• Cu²⁺ + 2e^- → Cu