Oxidation State

suriburi

How do i calculate all the possible oxidation states of an element?

edison

Oxidation State

Oxidation state is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions, and balancing redox chemical reactions. You are learning the skill to assign oxidation states (or oxidation numbers) to a variety of compounds and ions.

When an oxidation number is assigned to the element, it does not imply that the element in the compound acquires this as a charge, but rather that it is a convenient number to use for balancing chemical reactions. The guidelines for assigning oxidation states (numbers) are given below:

The oxidation state of any element such as Fe, H₂, O₂, P₄, S₈ is zero (0).

The oxidation state of oxygen in its compounds is -2, except for peroxides like H₂O₂, and Na₂O₂, in which the oxidation state for O is -1.

The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, etc., in which the oxidation state for H is -1.

The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

The following elements usually have the same oxidation states in their compounds:

+1 for alkali metals - Li, Na, K, Rb, Cs;

+2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;

-1 for halogens except when they form compounds with oxygen or one another;

These rules are wordy because we have to point out the special cases such as H₂O₂ and Na₂O₂. Rule 3. deals with hydride. Other than these, you may simply remember the oxidation states for H and O are +1 and -2 respectively in a coumpound, and oxidation of other elements can be asigned by making the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

For your practice, we provide some examples below. Please study the following examples and derive the oxidation state for all elements. The oxidation numbers of the key element are given in case you need help.

Element Oxidation

state Compound

or ion

H +1 H⁺

Group 1 +1 H₂O

0 H₂

-1 NaAlH₄

Cl -1 Cl^-

Group 7 0 Cl₂

+1 ClO^-

+3 ClO₂^-

+4 ClO₂

+5 ClO₃^-

+7 ClO₄^-

N -3 NH₃

Group 5 -2 N₂H₄

-1 NH₂OH

0 N₂

+1 N₂O

+2 NO

+3 NO₂^-

+4 NO₂

+5 NO₃^-

The following are some common oxidants or reductants. Changes of oxidation states in redox reactions of the key elements are given in groups. Please justify the assigned oxidation state to your satisfaction as you read on, and assign the oxidation number to all element in the formulas.

Element Oxidation

state Compound

or ion

Fe +2 Fe²⁺ Fe = Fe²⁺ + 2 e^-

+3 Fe³⁺ Fe²⁺ = Fe³⁺+ e^-

Zn 0 Zn Zn is reducing agent

+2 Zn²⁺

O -1 H₂O₂ H₂O₂ = O₂ + H₂O

0 O₂

-2 H₂O

Cr +6 Cr₂O₇^2-

+3 Cr³⁺ Cr₂O₇^2- + 6 e = 2 Cr³⁺

+6 CrO₄^2-

Mn +7 MnO₄^-

+6 MnO₄^2-

+4 MnO₂ MnO₄^- + 3 e = MnO₂

+2 Mn²⁺ MnO₄^- + 5 e = Mn²⁺

C +3 H₂C₂O₄

+4 CO₂

+4 CO₃^2-

+2 CO

edison

Working out oxidation states

You don't work out oxidation states by counting the numbers of electrons transferred. It would take far too long. Instead you learn some simple rules, and do some very simple sums!

The oxidation state of an uncombined element is zero. That's obviously so, because it hasn't been either oxidised or reduced yet! This applies whatever the structure of the element - whether it is, for example, Xe or Cl2 or S8, or whether it has a giant structure like carbon or silicon.

The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.

The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.

The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state. Remember that fluorine is the most electronegative element with oxygen second.

Some elements almost always have the same oxidation states in their compounds:

element usual oxidation state exceptions
Group 1 metals always +1
Group 2 metals always +2
Oxygen usually -2 except in peroxides and F2O (see below)
Hydrogen usually +1 except in metal hydrides where it is -1 (see below)
Fluorine always -1
Chlorine usually -1 except in compounds with O or F (see below)

The reasons for the exceptions

Hydrogen in the metal hydrides

Metal hydrides include compounds like sodium hydride, NaH. In this, the hydrogen is present as a hydride ion, H-. The oxidation state of a simple ion like hydride is equal to the charge on the ion - in this case, -1.

Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero. Since Group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0).

Oxygen in peroxides

Peroxides include hydrogen peroxide, H2O2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.

Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to balance it.

Oxygen in F2O

The problem here is that oxygen isn't the most electronegative element. The fluorine is more electronegative and has an oxidation state of -1. In this case, the oxygen has an oxidation state of +2.

Chlorine in compounds with fluorine or oxygen

There are so many different oxidation states that chlorine can have in these, that it is safer to simply remember that the chlorine doesn't have an oxidation state of -1 in them, and work out its actual oxidation state when you need it. You will find an example of this below.

Examples of working out oxidation states

What is the oxidation state of chromium in Cr2+?

That's easy! For a simple ion like this, the oxidation state is the charge on the ion - in other words: +2 (Don't forget the + sign.)

What is the oxidation state of chromium in CrCl3?

This is a neutral compound so the sum of the oxidation states is zero. Chlorine has an oxidation state of -1. If the oxidation state of chromium is n:

n + 3(-1) = 0

n = +3 (Again, don't forget the + sign!)

What is the oxidation state of chromium in Cr(H2O)63+?

This is an ion and so the sum of the oxidation states is equal to the charge on the ion. There is a short-cut for working out oxidation states in complex ions like this where the metal atom is surrounded by electrically neutral molecules like water or ammonia.

The sum of the oxidation states in the attached neutral molecule must be zero. That means that you can ignore them when you do the sum. This would be essentially the same as an unattached chromium ion, Cr3+. The oxidation state is +3.

What is the oxidation state of chromium in the dichromate ion, Cr2O72-?

The oxidation state of the oxygen is -2, and the sum of the oxidation states is equal to the charge on the ion. Don't forget that there are 2 chromium atoms present.

2n + 7(-2) = -2

n = +6

What is the oxidation state of copper in CuSO4?

Unfortunately, it isn't always possible to work out oxidation states by a simple use of the rules above. The problem in this case is that the compound contains two elements (the copper and the sulphur) whose oxidation states can both change.

The only way around this is to know some simple chemistry! There are two ways you might approach it. (There might be others as well, but I can't think of them at the moment!)

You might recognise this as an ionic compound containing copper ions and sulphate ions, SO42-. To make an electrically neutral compound, the copper must be present as a 2+ ion. The oxidation state is therefore +2.

You might recognise the formula as being copper(II) sulphate. The "(II)" in the name tells you that the oxidation state is 2 (see below).

You will know that it is +2 because you know that metals form positive ions, and the oxidation state will simply be the charge on the ion.

rtiit

nice ans!!!

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