The concept of oxidation number or oxidation state can be very useful for understanding what"s going on in a reaction beneath the balanced equation. In another section we"ll deal with reduction-oxidation ("redox") reactions, but in this section, we"ll assign oxidation numbers to atoms and use them to determine whether or not the reaction is a redox reaction, and which atoms are oxidized and reduced in it.

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Remember that oxidation is a loss of electrons and reduction is a gain of electrons. I know, it doesn"t seem very logical for "reduction" to mean "gain." The term "oxidation" is an old one and refers to a time when what we now refer to as oxidation was only observed to occur in the presence of oxygen. At the time we didn"t really understand the part about transfer of electrons. Like a lot of terminology in science, we"re stuck with oxidation and reduction.


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Example 1

In order to tell if a reaction is a reduction-oxidation (redox) reaction, we assign oxidation numbers to each atom. By the way, the reaction doesn"t need to be balanced for this oxidation-number analysis, but this one is.

In this reaction, the Mg is presumed to be in its metallic elemental form because it doesn"t have a charge, so its oxidation number is zero. Hydrogen bound to a nonmetal has a +1 ox. number, and the Cl gets a -1 because the sum of oxidation numbers of a neutral compound has to be zero.

Mg is a group 2A element, so it has a +2 oxidation number, and the hydrogens of H2 have zero ox. numbers because that"s hydrogen in its elemental form.

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Now let"s look at the results. Mg goes from an oxidation state of 0 to +2. It"s oxidation number increases, so Mg is oxidized in this reaction and we refer to HCl as an oxidizing agent or an oxidizer. Likewise, the oxidation number of H is reduced from +1 to zero, so hydrogen is reduced in this reaction and Mg is referred to as a reducing agent.

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Example 2

This is an acid-base neutralization reaction. Here are the assignments of the oxidation numbers; they"re pretty straightforward:

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But this time when we compare oxidation numbers on the right and left, nothing has changed. No electrons have been transferred, so this is not a redox reaction.

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Example 3

In this example, we have two cases where we assign a -2 oxidation number to oxygen, and we are left to assign the oxidation number of the nitrogen so that the sum of all of the ox. numbers is zero because these are neutral molecules.

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Comparing left and right sides, we see that the sulfur is oxidized; it"s ox. number goes from -2 to zero. And the nitrogen is reduced (+4+2). This is a redox reaction.

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In an oxidation reaction, the chemical component (sometimes referred to generically as a "species") that is reduced is also called an oxidizing agent or an oxidizer because its presence oxidizes another component. In this reaction, nitric acid (HNO3) is an oxidizer and H2S is a reducing agent (note that we don"t usually call it a "reducer").


Example 4

Here"s another example in which we have to assign an oxidation number based only on the fact that the sum of oxidation numbers of neutral compounds has to be zero (the +4 on the C of CaCO3). The resulting analysis shows that this is not a redox reaction.

See more: The Product Of The Means Equals The Product Of The Extremes, Ratios And Proportions

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Notice that the analysis of oxidation states in a chemical equation does not depend on the numerical coefficients (number of moles) of any reactant or product.