Chemistry Puzzle Cache #06 - Oxidation States Mystery Cache

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In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero (0).

The concept of oxidation state in its current meaning was introduced by W. M. Latimer in 1938. Oxidation itself was first studied by Antoine Lavoisier who then held the belief that oxidation was literally the results of reactions of the elements with oxygen and that the common bond in any salt was based on oxygen.

The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.

This is the IUPAC definition of oxidation state: “ Oxidation state: A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules: (l) the oxidation state of a free element (uncombined element) is zero; (2) for a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion; (3) hydrogen has an oxidation state of +1 and oxygen has an oxidation state of -2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of -1 in hydrides of active metals, e.g. LiH, and oxygen has an oxidation state of -1 in peroxides, e.g. H₂O₂; (4) the algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. For example, the oxidation states of sulfur in H₂S, S₈ (elementary sulfur), SO₂, SO₃, and H₂SO₄ are, respectively: -2, 0, +4, +6 and +6. The higher the oxidation state of a given atom, the greater is its degree of oxidation; the lower the oxidation state, the greater is its degree of reduction. ”

Assigning Oxidation States

The algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. This fact, combined with the fact that some elements almost always have certain oxidation states, allows one to compute the oxidation states for atoms in simple compounds. Some typical rules that are used for assigning oxidation states of simple compounds follow:
* Fluorine has an oxidation state of -1 in all its compounds, since it has the highest electronegativity of all reactive elements.
* Hydrogen has an oxidation state of +1 except when bonded to more electropositive elements such as sodium, aluminium, and boron, as in NaH, NaBH₄, LiAlH₄, where each H has an oxidation state of -1.
* Oxygen has an oxidation state of -2 except where it is -1 in peroxides,
* Alkali metals have an oxidation state of +1 in virtually all of their compounds (exception, see alkalide).
* Alkaline earth metals have an oxidation state of +2 in virtually all of their compounds.
* Halogens, other than fluorine have an oxidation state of -1 except when they are bonded to oxygen, nitrogen or with another halogen.

Example: In Cr(OH)₃, oxygen has an oxidation state of -2 (no fluorine, O-O bonds present), and hydrogen has a state of +1 (bonded to oxygen). So, each of the three hydroxide groups has a charge of -2 + 1 = -1. As the compound is neutral, Cr has an oxidation state of +3.

CrCl₃, A= (oxidation state of Cr)

SO₃, B= (oxidation state of S)

H₂O, C= (oxidation state of H)

NaCl, D= (oxidation state of Na)

KMnO₄, E= (oxidation state of Mn)

H₂SO₄, F= (oxidation state of S)

HNO₃ and BF₃, G= (oxidation state of N) + (oxidation state of B)

K₂Cr₂O₇, H= (sum of the oxidation states)

MnO₂ and CO₂, I= (oxidation state of Mn) + (oxidation state of C)

H₃PO₄, J= (oxidation state of P)

AlCl₃, K= (oxidation state of Al)

BeI₂, L= (oxidation state of Be)

FeO, M= (oxidation state of Fe)

Na₂Cr₂O₇, N= (oxidation state of Cr)

CsF, O= (oxidation state of Cs)

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