Calculate the oxidation number of a substance

Video: How To Calculate Oxidation Numbers - Basic Introduction

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Part 1 of 2: Assigning oxidation numbers based on chemical rules
Part 2 of 2: Assigning numbers to atoms without the rules for oxidation numbers
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In chemistry, the terms "oxidation" and "reduction" mean those reactions in which an atom (or group of atoms) loses or gains electrons, respectively. Oxidation numbers are numbers assigned to atoms (or groups of atoms) to assist chemists in controlling how many electrons are available to be displaced and whether given reactants oxidize or reduce during a reaction. The process of assigning oxidation numbers to atoms can range from very simple to more complex, depending on the charge of the atoms and the chemical composition of the molecules of which they are a part. To make things more complicated, some atoms can have multiple oxidation numbers. Fortunately, the assignment of oxidation numbers is governed by clearly defined, easy-to-follow rules, but a basic understanding of chemistry and algebra will make using these rules a lot easier.

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Part 1 of 2: Assigning oxidation numbers based on chemical rules

Determine if the substance in question is elemental. Free, unbound atoms always have an oxidation number of 0. This is true for both atoms that consist of a single atom and atoms whose elemental form is diatomic or polyatomic.
- For example, Al_(s) and Cl₂ both have the oxidation number of 0 because they are not compound atoms.
- Note that sulfur in its elemental form, S.₈ (octasulfur), although irregular, also has an oxidation number of 0.
Determine if the substance in question is an ion. Ions have oxidation numbers equal to their charge. This is true of unbound ions as well as ions that are part of a composite ion.
- For example, the ion Cl has an oxidation number of -1.
- The Cl ion still has an oxidation number of -1 when it is part of the compound NaCl. Since the Na ion, by definition, has a charge of +1, we know that the Cl ion has a charge of -1, so that the oxidation number is still -1.
In the case of metal ions, it is good to remember that multiple oxidation numbers are possible. Many metals can have more than one landing. For example, the metal iron (Fe) can be an ion with charge +2 or +3. the charge of metal ions (and thus their oxidation numbers) can be determined in relation to the charge of the other atoms in the composition of which they are part, or, when written as text, by the notation in Roman numerals (such as in the sentence: "The iron (III) ion has a charge of +3.").
- For example, let's take a closer look at a compound that contains an aluminum ion. The compound AlCl₃ has a charge of 0. Because we know that Cl ions have a charge of -1 and 3 Cl ions are present in the compound, the Al-ion must have a charge of +3, so that the charge of all ions added together Is 0. So, the oxidation number of Al is +3.
Assign an oxidation number -2 to oxygen (with exceptions). In almost in all cases, oxygen atoms have an oxidation number of -2. There are a few exceptions to this rule:
- When oxygen is in the elemental state (O₂), then the oxidation number is equal to 0, which is the case for all elementary atoms.
- When oxygen is a part of peroxide, then the oxidation number is -1. Peroxides are a class of compounds that have an oxygen-oxygen bond (or the peroxide anion O₂). For example, in the molecule H₂O₂ (hydrogen peroxide), oxygen has an oxidation number (and a charge) of -1. Also, when oxygen is part of a superoxide, its oxidation number is -0.5.
- When oxygen is bound to fluorine, the oxidation number is +2. See the fluor rule below for more information. In (O₂F.₂) this is +1.
Assign an oxidation number of +1 to hydrogen (with exceptions). As with oxygen, the oxidation number of hydrogen depends on exceptional cases. Generally, hydrogen has an oxidation number +1 (except in elemental form, H.₂). But in the case of a special compound called hybrids, hydrogen has an oxidation number of -1.
- For example, from H₂Oh, we know that hydrogen has an oxidation number of +1 because oxygen has a charge of -2 and we need 2 +1 charges to make a compound with a total charge of zero. But with the substance sodium hydride, NaH, hydrogen has an oxidation number -1 because the Na ion has a charge of +1 and, to make the total charge of the compound 0, hydrogen has a charge (and thus an oxidation number) of -1.
Fluorine always an oxidation number of -1. As indicated above, the oxidation numbers of certain elements can vary due to various factors (metal ions, oxygen atoms in peroxides, etc.). Fluorine, on the other hand, has an oxidation number of -1, and that never changes. This is because fluorine is the most electro-negative element, or in other words, it is the element that is least willing to give up electrons and most likely to take over electrons from other atoms. Therefore, the oxidation number will not change.
The oxidation numbers in a compound are equal to the charge of the compound. The oxidation numbers of all atoms in a compound are equal to the charge of that compound. For example, if a compound has no charge, then the sum of all oxidation numbers will be zero; if the compound is a polyatomic ion with a charge of -1, then the oxidation numbers added together must be -1, etc.
- This is a good way to check your answer - if the oxidation numbers of a compound added together don't equal the charge of that compound, then you know you've made a mistake.

Part 2 of 2: Assigning numbers to atoms without the rules for oxidation numbers

Find atoms without oxidation number rules. Some atoms don't follow the rules for finding oxidation numbers. If an atom does not comply with the above rules and you are not sure what its charge is (for example, if it is part of a larger compound so that the individual charge is unknown), you can find the oxidation number of that atom by elimination. First you determine what the oxidation is of every other atom in the compound. Then you solve the sum for the unknown in the equation, based on the total charge of the compound.
- For example, in the compound Na₂SO₄, the charge of sulfur (S) is unknown - it's not in its elemental form, so it's not 0, but that's all we know. This is a good candidate for applying this method to determine the oxidation number algebraically.
Determine the known oxidation numbers of the other elements in the compound. Using the oxidation number assignment rules, we determine which oxidation numbers the other atoms in the compound have. Be aware of exceptions such as O, H, etc.
- In Na₂SO₄, we know, based on our set of rules, that the Na ion has a charge (and thus an oxidation number) of +1 and that the oxygen atoms have oxidation numbers of -2.
Multiply the number of each atom by the oxidation number. Now that we know the oxidation numbers of all atoms except the unknown, we will have to consider that some of these atoms may occur more than once. Multiply each coefficient (written in subscript after the symbol of the atom in the compound) by the oxidation number.
- As for Na₂SO₄, we know that there are 2 Na atoms and 4 O atoms. Now we do the following calculation, 2 × +1, to get the oxidation number of Na, 2, and we multiply 4 × -2, the oxidation number of O, -8.
Add up the results. Adding the results of these multiplications gives the oxidation number of the compound, without taking into account the oxidation number of the unknown atom.
- In our example with Na₂SO₄, we add 2 to -8 to get -6.
Calculate the unknown oxidation number based on the charge of the compound. You now have all the data to find the unknown oxidation number using some simple algebra. We will use an equation and the answer from the previous step, plus the charge of the compound. In other words: (Sum of the unknown oxidation numbers) + (the unknown oxidation number you want to know) = (charge of the compound).
- In the example of Na₂SO₄, we solve this as follows:
  - (Sum of known oxidation numbers) + (unknown oxidation number you want to solve) = (charge of the compound)
  - -6 + S = 0
  - S = 0 + 6
  - S = 6. S has an oxidation number or 6 in Na₂SO₄.

Tips

Atoms in their basic form always have an oxidation number of 0. An ion that consists of 1 atom has an oxidation number equal to the charge. Group 1A metals such as hydrogen, lithium and sodium have an oxidation number +1; Group 2A metals, such as magnesium and calcium, have an oxidation number of +2. Both hydrogen and oxygen can have 2 different oxidation numbers, depending on their bond.
In a compound the sum of all oxidation numbers should be equal to 0. If there is an ion with 2 atoms, then the sum of the oxidation numbers should be equal to the charge of the ion.
It is very helpful to know how to read the periodic table and where to find metals and non-metals.