The term "oxidation number" (also known as oxidation state) represents the hypothetical charge an atom would have if all bonds to atoms of different elements were 100% ionic. This concept is crucial in understanding redox reactions (reduction-oxidation reactions) where electron transfer occurs. In this article, we will delve into determining the oxidation number of cobalt (Co) in cobalt(II) sulfate (CoSO₄), a common inorganic compound. Understanding this requires a fundamental grasp of oxidation number rules and the ionic nature of the compound.
1. Defining Oxidation Numbers:
Oxidation numbers are assigned to atoms within a molecule or ion according to a set of rules. These rules help us systematically determine the charge each atom "appears" to possess based on electron distribution. It's important to remember that oxidation numbers are not necessarily the true charges on atoms, especially in covalent compounds where electron sharing occurs. However, they are a valuable tool for balancing redox reactions and understanding electron transfer.
2. Rules for Assigning Oxidation Numbers:
Several rules guide the assignment of oxidation numbers. Key rules relevant to CoSO₄ include:
Rule 1: The oxidation number of an element in its free (uncombined) state is always 0. For example, the oxidation number of O₂ is 0.
Rule 2: The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of Na⁺ is +1, and Cl⁻ is -1.
Rule 3: The sum of the oxidation numbers of all atoms in a neutral molecule is 0.
Rule 4: The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
Rule 5: Group 1 metals always have an oxidation number of +1, and Group 2 metals always have an oxidation number of +2.
Rule 6: Oxygen usually has an oxidation number of -2 (except in peroxides like H₂O₂, where it's -1, and in compounds with fluorine, where it can be positive).
Rule 7: Hydrogen usually has an oxidation number of +1 (except in metal hydrides like NaH, where it's -1).
3. Determining the Oxidation Number of Cobalt in CoSO₄:
To determine the oxidation number of cobalt in CoSO₄, we apply the rules above:
1. Sulfate ion (SO₄²⁻): We know the sulfate ion has a charge of -2. Oxygen (Rule 6) usually has an oxidation number of -2. Let 'x' represent the oxidation number of sulfur (S). Using Rule 4:
x + 4(-2) = -2
x - 8 = -2
x = +6
Therefore, the oxidation number of sulfur in SO₄²⁻ is +6.
2. Cobalt(II) sulfate (CoSO₄): CoSO₄ is a neutral compound (Rule 3), meaning the sum of the oxidation numbers of all its constituent atoms must be 0. Let 'y' represent the oxidation number of cobalt (Co). We know the oxidation number of the sulfate ion is -2 (from step 1). Therefore:
y + (+6) + 4(-2) = 0
y + 6 - 8 = 0
y = +2
Therefore, the oxidation number of cobalt in CoSO₄ is +2. This is why the compound is named cobalt(II) sulfate – the Roman numeral II indicates the oxidation state of the cobalt ion.
4. Illustrative Examples:
Let's consider a few more examples to solidify our understanding:
Co₂O₃: In this compound, the oxidation number of oxygen is -2 (Rule 6). Let 'z' be the oxidation number of cobalt. Since the molecule is neutral:
2z + 3(-2) = 0
2z - 6 = 0
2z = 6
z = +3
Thus, cobalt has an oxidation number of +3 in Co₂O₃.
K₃Co(CN)₆: This is potassium hexacyanocobaltate(III). Potassium (K) has an oxidation number of +1 (Rule 5). The cyanide ion (CN⁻) has a charge of -1, therefore the oxidation number of CN⁻ is -1. Let 'w' be the oxidation number of cobalt. The overall charge of the compound is 0:
3(+1) + w + 6(-1) = 0
3 + w - 6 = 0
w = +3
Cobalt has an oxidation number of +3 in this complex ion.
5. Conclusion:
Determining the oxidation number of an element in a compound is a systematic process involving the application of established rules. In the case of CoSO₄, the oxidation number of cobalt is +2, a fact directly reflected in the compound's name. Understanding oxidation numbers is fundamental to comprehending redox reactions and electron transfer, crucial aspects of many chemical processes.
Frequently Asked Questions (FAQs):
1. Can an element have more than one oxidation number? Yes, many elements can exhibit multiple oxidation numbers depending on the compound they are part of. For example, cobalt can have oxidation states of +2, +3, and others.
2. What does a positive oxidation number signify? A positive oxidation number indicates that the atom has lost electrons compared to its neutral state.
3. What does a negative oxidation number signify? A negative oxidation number indicates that the atom has gained electrons compared to its neutral state.
4. Is the oxidation number always an integer? Generally, yes. However, in some complex cases involving fractional bonding, non-integer oxidation numbers are possible.
5. How are oxidation numbers used in balancing redox reactions? Oxidation numbers are used to track electron transfer in redox reactions, helping to ensure that the number of electrons lost during oxidation equals the number of electrons gained during reduction, enabling balanced chemical equations.
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