oxidation numbers worksheet and answers

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03-02-2025

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Determining oxidation numbers (oxidation states) is a fundamental skill in chemistry, crucial for understanding redox reactions and balancing chemical equations. This worksheet provides practice problems with detailed answers, helping you master this essential concept. Whether you're a high school student, undergraduate, or simply brushing up on your chemistry knowledge, this guide will be invaluable.

Understanding Oxidation Numbers

Before diving into the problems, let's review the key rules for assigning oxidation numbers:

1. The oxidation number of an element in its free (uncombined) state is always 0. Examples: O₂ (oxygen), Na (sodium), Fe (iron).

2. The oxidation number of a monatomic ion is equal to its charge. Examples: Na⁺ (+1), Cl⁻ (-1), Mg²⁺ (+2).

3. The oxidation number of hydrogen is +1, except in metal hydrides where it is -1. Examples: HCl (+1), NaH (-1).

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

5. The oxidation number of a Group 1 (alkali) metal is always +1.

6. The oxidation number of a Group 2 (alkaline earth) metal is always +2.

7. The sum of the oxidation numbers of all atoms in a neutral molecule is 0.

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

Oxidation Numbers Worksheet: Practice Problems

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

Section 1: Simple Compounds

1. NaCl
2. H₂O
3. CO₂
4. SO₃
5. NH₃
6. Fe₂O₃
7. KClO₃
8. H₂SO₄

Section 2: More Complex Compounds

9. KMnO₄
10. Cr₂O₇²⁻
11. MnO₄⁻
12. Na₂Cr₂O₇
13. (NH₄)₂SO₄
14. Fe₃O₄ (Hint: This contains both Fe²⁺ and Fe³⁺ ions)
15. H₂O₂

Section 3: Identifying Oxidation and Reduction

16. In the reaction: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻, which element is oxidized and which is reduced?

Oxidation Numbers Worksheet: Answers

Section 1: Simple Compounds

1. Na: +1, Cl: -1
2. H: +1, O: -2
3. C: +4, O: -2
4. S: +6, O: -2
5. N: -3, H: +1
6. Fe: +3, O: -2
7. K: +1, Cl: +5, O: -2
8. H: +1, S: +6, O: -2

Section 2: More Complex Compounds

9. K: +1, Mn: +7, O: -2
10. Cr: +6, O: -2
11. Mn: +7, O: -2
12. Na: +1, Cr: +6, O: -2
13. N: -3, H: +1, S: +6, O: -2
14. Fe: +8/3 (average oxidation state; a mix of +2 and +3) O: -2
15. H: +1, O: -1

Section 3: Identifying Oxidation and Reduction

16. Iron (Fe) is oxidized (its oxidation number increases from +2 to +3), and chlorine (Cl) is reduced (its oxidation number decreases from 0 to -1).

Further Practice and Resources

This worksheet provides a solid foundation in determining oxidation numbers. For further practice, you can find numerous online resources and textbooks offering more complex examples and explanations. Remember to consistently apply the rules outlined above to confidently solve any oxidation number problem. Understanding oxidation numbers is key to comprehending many important chemical processes.