The concept of oxidation number or oxidation state have the right to be very useful for expertise what"s walking 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 entrust oxidation numbers to atoms and also use lock to recognize whether or not the reaction is a oxidization reaction, and which atoms space oxidized and reduced in it.

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Remember that oxidation is a ns of electrons and reduction is a gain of electrons. Ns know, the doesn"t seem very logical for "reduction" to median "gain." The hatchet "oxidation" is an old one and also refers to a time once what we now refer to together oxidation was just observed to take place in the existence of oxygen. At the moment we didn"t really understand the part about move of electrons. Choose a the majority of terminology in science, we"re stuck to oxidation and reduction.


Example 1

In order come tell if a reaction is a reduction-oxidation (redox) reaction, us assign oxidation number to every atom. By the way, the reaction doesn"t should be well balanced for this oxidation-number analysis, however this one is.

In this reaction, the Mg is presumed to it is in in the metallic elemental type because that doesn"t have a charge, for this reason its oxidation number is zero. Hydrogen bound to a nonmetal has actually a +1 ox. Number, and also the Cl it s okay a -1 because the amount of oxidation numbers of a neutral compound has to be zero.

Mg is a team 2A element, for this reason it has a +2 oxidation number, and also the hydrogens the H2 have actually zero ox. Numbers because that"s hydrogen in its element form.


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


Example 2

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


But this time once we to compare oxidation number on the right and left, nothing has changed. No electrons have actually been transferred, for this reason this is not a oxidation reaction.


Example 3

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


Comparing left and also right sides, we view that the sulfur is oxidized; it"s ox. Number goes from -2 come zero. And also the nitrogen is decreased (+4+2). This is a oxidation reaction.


In an oxidation reaction, the chemical component (sometimes referred to generically as a "species") the is decreased is likewise called one oxidizing agent or one oxidizer due to the fact that its presence oxidizes one more component. In this reaction, nitric acid (HNO3) is an oxidizer and also H2S is a reducing agent (note that us don"t usually contact it a "reducer").

Example 4

Here"s an additional example in which we have to assign one oxidation number based just on the reality that the sum of oxidation numbers of neutral compounds needs to be zero (the +4 ~ above the C of CaCO3). The resulting evaluation shows the this is not a oxidation reaction.

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Notice that the evaluation of oxidation claims in a chemistry equation go not count on the number coefficients (number that moles) of any reactant or product.