oxidation number worksheet with answers pdf

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04-02-2025

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This worksheet provides a thorough practice in determining oxidation numbers, a fundamental concept in chemistry. Understanding oxidation numbers is crucial for balancing redox reactions and comprehending the behavior of elements in chemical compounds. This guide includes example problems and answers to help solidify your understanding.

What is an Oxidation Number?

The oxidation number (also called oxidation state) of an atom is a number assigned to it to represent the number of electrons it has gained or lost in a chemical compound. It's a bookkeeping tool that helps us track electron transfer in chemical reactions. While not a true physical charge, it's extremely useful in predicting chemical behavior.

Key Rules for Assigning Oxidation Numbers:

1. Free Elements: The oxidation number of an element in its free (uncombined) state is always 0. Examples: O₂ (O = 0), Na (Na = 0), Cl₂ (Cl = 0).

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

3. Hydrogen: Hydrogen usually has an oxidation number of +1, except in metal hydrides (e.g., NaH) where it is -1.

4. Oxygen: Oxygen usually has an oxidation number of -2, except in peroxides (e.g., H₂O₂) where it is -1, and in superoxides (e.g., KO₂) where it is -1/2.

5. Group 1 (Alkali Metals): Always +1.

6. Group 2 (Alkaline Earth Metals): Always +2.

7. Fluorine: Always -1.

8. Other Halogens (Cl, Br, I): Usually -1, but can have positive oxidation numbers when combined with more electronegative elements like oxygen.

9. The Sum of Oxidation Numbers: In a neutral compound, the sum of the oxidation numbers of all atoms must equal zero. In a polyatomic ion, the sum of the oxidation numbers must equal the charge of the ion.

Practice Problems:

Here are some practice problems to test your understanding. Try to solve them before looking at the answers below.

Problem 1: Find the oxidation number of sulfur in H₂SO₄.

Problem 2: Determine the oxidation number of manganese in KMnO₄.

Problem 3: What is the oxidation number of chromium in Cr₂O₇²⁻?

Problem 4: Find the oxidation number of nitrogen in HNO₃.

Problem 5: Determine the oxidation number of phosphorus in PO₄³⁻.

Problem 6: What is the oxidation number of iron in Fe₂O₃?

Problem 7: Find the oxidation number of carbon in CO₂.

Problem 8: Determine the oxidation number of chlorine in KClO₃.

Problem 9: What is the oxidation number of nitrogen in NH₃?

Problem 10: Find the oxidation number of sulfur in SO₃²⁻.

Answers:

Problem 1: S = +6

Problem 2: Mn = +7

Problem 3: Cr = +6

Problem 4: N = +5

Problem 5: P = +5

Problem 6: Fe = +3

Problem 7: C = +4

Problem 8: Cl = +5

Problem 9: N = -3

Problem 10: S = +4

This worksheet should provide a solid foundation in calculating oxidation numbers. Remember to practice regularly to master this essential skill. Further practice problems can be found in most general chemistry textbooks and online resources. Consistent practice will build your confidence and improve your problem-solving skills. Good luck!