oxidation number worksheet and answers

Lemon

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04-02-2025

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Determining oxidation numbers (oxidation states) is a fundamental concept in chemistry, crucial for understanding redox reactions and balancing chemical equations. This worksheet provides a range of exercises to solidify your understanding, progressing from simple to more complex examples. Each problem includes a detailed solution, allowing you to check your work and pinpoint areas needing further attention.

Understanding Oxidation Numbers

Before we dive into the worksheet, let's briefly review the rules for assigning oxidation numbers:

1. The oxidation number of an element in its free (uncombined) state is zero. For example, the oxidation number of O₂ is 0, and the oxidation number of Fe (iron) is 0.

2. The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of Na⁺ is +1, and the oxidation number of Cl⁻ is -1.

3. The oxidation number of hydrogen is +1, except in metal hydrides where it is -1. For example, in HCl, hydrogen has an oxidation number of +1; in NaH, it's -1.

4. The oxidation number of oxygen is -2, except in peroxides (e.g., H₂O₂) where it is -1, and in superoxides (e.g., KO₂) where it is -1/2.

5. The sum of the oxidation numbers of all atoms in a neutral molecule is zero.

6. The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.

Oxidation Number Worksheet: Problems

Instructions: Determine the oxidation number of the underlined element in each compound or ion.

Problem 1: H₂O

Problem 2: CO₂

Problem 3: NH₃

Problem 4: SO₄²⁻

Problem 5: KMnO₄

Problem 6: Cr₂O₇²⁻

Problem 7: H₂SO₄

Problem 8: FeCl₃

Problem 9: PO₄³⁻

Problem 10: Na₂Cr₂O₇

Oxidation Number Worksheet: Answers and Explanations

Problem 1: H₂O

• Oxygen typically has an oxidation number of -2.
• Since there are two hydrogen atoms and the overall charge is 0, each hydrogen atom has an oxidation number of +1. Therefore, the oxidation number of H is +1.

Problem 2: CO₂

• Oxygen has an oxidation number of -2.
• There are two oxygen atoms, resulting in a total of -4.
• For the molecule to be neutral (overall charge of 0), the carbon atom must have an oxidation number of +4. Therefore, the oxidation number of C is +4.

Problem 3: NH₃

• Hydrogen has an oxidation number of +1.
• There are three hydrogen atoms, resulting in a total of +3.
• To maintain a neutral molecule, the nitrogen atom must have an oxidation number of -3. Therefore, the oxidation number of N is -3.

Problem 4: SO₄²⁻

• Oxygen has an oxidation number of -2.
• Four oxygen atoms contribute -8.
• The overall charge of the sulfate ion (SO₄²⁻) is -2.
• Therefore, the sulfur atom must have an oxidation number of +6 to balance the charge: (+6) + (-8) = -2. The oxidation number of S is +6.

Problem 5: KMnO₄

• Potassium (K) has an oxidation number of +1.
• Oxygen has an oxidation number of -2.
• Four oxygen atoms contribute -8.
• The overall charge is 0.
• Therefore, the manganese (Mn) must have an oxidation number of +7 to balance the charges: (+1) + (+7) + (-8) = 0. The oxidation number of Mn is +7.

Problem 6: Cr₂O₇²⁻

• Oxygen has an oxidation number of -2.
• Seven oxygen atoms contribute -14.
• The overall charge is -2.
• Therefore, the two chromium (Cr) atoms must have a combined oxidation number of +12 to balance the charge: (+12) + (-14) = -2. Each chromium atom has an oxidation number of +6. The oxidation number of Cr is +6.

Problem 7: H₂SO₄

• Hydrogen has an oxidation number of +1 (two hydrogens contribute +2).
• Oxygen has an oxidation number of -2 (four oxygens contribute -8).
• The overall charge is 0.
• Therefore, the sulfur (S) must have an oxidation number of +6: (+2) + (+6) + (-8) = 0. The oxidation number of S is +6.

Problem 8: FeCl₃

• Chlorine (Cl) has an oxidation number of -1.
• Three chlorine atoms contribute -3.
• The overall charge is 0.
• Therefore, the iron (Fe) must have an oxidation number of +3: (+3) + (-3) = 0. The oxidation number of Fe is +3.

Problem 9: PO₄³⁻

• Oxygen has an oxidation number of -2.
• Four oxygen atoms contribute -8.
• The overall charge is -3.
• Therefore, the phosphorus (P) must have an oxidation number of +5: (+5) + (-8) = -3. The oxidation number of P is +5.

Problem 10: Na₂Cr₂O₇

• Sodium (Na) has an oxidation number of +1 (two sodiums contribute +2).
• Oxygen has an oxidation number of -2 (seven oxygens contribute -14).
• The overall charge is 0.
• Therefore, the two chromium (Cr) atoms must have a combined oxidation number of +12: (+2) + (+12) + (-14) = 0. Each chromium atom has an oxidation number of +6. The oxidation number of Cr is +6.

This worksheet and its answers provide a strong foundation for understanding oxidation numbers. Remember to practice regularly to master this crucial chemical concept. Further practice problems can be found in most general chemistry textbooks.