oxidation numbers worksheet answer key

Lemon

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04-02-2025

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This worksheet provides answers and a detailed explanation of oxidation numbers, a crucial concept in chemistry. Understanding oxidation numbers is essential for balancing redox reactions and predicting the reactivity of chemical species. This guide will not only provide the answers to a typical oxidation numbers worksheet but also delve into the underlying principles to solidify your understanding.

What is an Oxidation Number?

An oxidation number (also called oxidation state) is a number assigned to an atom in a molecule or ion that represents the number of electrons it has gained or lost compared to its neutral state. It's a bookkeeping tool to track electron transfer in chemical reactions. While not a true charge, it helps us understand electron distribution within a compound.

Rules for Assigning Oxidation Numbers:

Before we jump into the answers, let's review the fundamental rules for assigning oxidation numbers:

1. Free Elements: The oxidation number of an atom in its elemental form is always 0 (e.g., O₂ , Fe, Cl₂).

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

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

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

5. Hydrogen: Usually +1, except in metal hydrides (e.g., NaH), where it's -1.

6. Oxygen: Usually -2, except in peroxides (e.g., H₂O₂) where it's -1 and in superoxides (e.g., KO₂) where it's -1/2. Also, in compounds with fluorine, oxygen can have a positive oxidation state.

7. Fluorine: Always -1.

8. The Sum of Oxidation Numbers: In a neutral molecule, the sum of oxidation numbers of all atoms is 0. In a polyatomic ion, the sum of oxidation numbers equals the charge of the ion.

Example Worksheet & Answer Key:

Let's consider a sample worksheet with a few example problems and their solutions:

Problem 1: Determine the oxidation number of Mn in KMnO₄.

Answer: Let x be the oxidation number of Mn. K is +1, and O is -2. Since the molecule is neutral, the sum of oxidation numbers is 0:

(+1) + x + 4(-2) = 0 x = +7

Therefore, the oxidation number of Mn in KMnO₄ is +7.

Problem 2: Find the oxidation number of Cr in Cr₂O₇²⁻.

Answer: Let x be the oxidation number of Cr. O is -2, and the overall charge is -2.

2x + 7(-2) = -2 2x = +12 x = +6

Therefore, the oxidation number of Cr in Cr₂O₇²⁻ is +6.

Problem 3: Determine the oxidation number of S in H₂SO₄.

Answer: Let x be the oxidation number of S. H is +1, and O is -2.

2(+1) + x + 4(-2) = 0 x = +6

Therefore, the oxidation number of S in H₂SO₄ is +6.

Problem 4: What is the oxidation number of P in PO₄³⁻?

Answer: Let x be the oxidation number of P. O is -2 and the overall charge is -3.

x + 4(-2) = -3 x = +5

Therefore, the oxidation number of P in PO₄³⁻ is +5.

Problem 5: Find the oxidation number of Fe in Fe₂O₃.

Answer: Let x be the oxidation number of Fe. O is -2.

2x + 3(-2) = 0 2x = +6 x = +3

Therefore, the oxidation number of Fe in Fe₂O₃ is +3.

Advanced Considerations:

Remember that oxidation numbers are assigned based on a set of rules, and sometimes fractional oxidation numbers are possible, especially in complex compounds or materials. This worksheet focuses on common cases and provides a solid foundation for understanding this concept. Further exploration into coordination chemistry and organometallic compounds may reveal more complex scenarios.

This comprehensive guide, including the answer key and detailed explanations, should significantly enhance your understanding of oxidation numbers. Remember to practice regularly to master this essential chemical concept.