Tuesday, 21 February 2012

Oxidation states and the Chemical Chameleon

Redox (reduction-oxidation) reactions play a major role in chemistry. In essence, redox reactions involve the transfer of one or more electrons from one reactant to another; the reactant that loses electrons is oxidized, and the reactant that gains electrons is reduced.

Students generally use mnemonics to remember the definitions of oxidation and reduction, and below are two more popular ones:

L - Loss of
E - Electrons is
O - Oxidation

G - Gain of
E - Electrons is
R - Reduction

(= Leo (lion) goes: 'GER' (grr))

the other being:

O - Oxidation
I - Is
L - Loss (of electrons)

R - Reduction
I - Is
G - Gain (of electrons)

(= oil rig).

How much an element/chemical can be reduced or oxidized is shown by its oxidation number, which generally ranges from -3 to +7. Some elements can only have two oxidation numbers (i.e. one oxidation state while bonded in a compound, and one neutral (charge 0) state), while others can have multiple oxidation numbers. These elements are generally the heavier elements starting from Period 3 of the Periodic Table as they have more electrons to 'play' with as they form compounds.

In general, the heavier Group 17 halogens (Cl, Br, I) and a class of metals known as the transition or d-block metals have multiple oxidation states. Many common metals such as iron and nickel are considered as transition metals, and as metals are all electron donors in chemical reactions, they can only be oxidized from their neutral state to form chemicals with a positive oxidation number. For ease of differentiating compounds with the same metal but with different oxidation states, Roman numerals (I to VII) are used to denote the oxidation state of transition metals in their compounds. For example, iron forms compounds with a +2 and +3 oxidation state; so iron chloride can either be iron (II) chloride (iron is +2) or iron (III) chloride (iron is +3).

One interesting aspect of transition metals is that their compounds are all colored, and the color of the compound depends on the oxidation state of the metal. In the illustration above,  for example, iron (II) chloride and all other iron (II) compounds have a greenish color; iron (III) chloride and all other iron (III) compounds have a reddish-brown color similar to that of rust (which, by the way, is just iron (III) oxide).

To demonstrate this phenomenon an experiment called the 'Chemical Chameleon' is conducted. The transition metal used is manganese, Mn; it has the distinction of being one of a few metals with oxidation states up to +7. The manganese compound used is potassium permanganate, aka potassium manganate (VII), chemical formula KMnO4. In solution form it is a purple colour.

In the experiment we add KMnO4 solution to a reducing agent, to reduce it until Mn has a +2 oxidation state. In between are a few transition stages, each with its own colors. Watch the video below to see what they are.

To summarize, the glucose reduces the manganate (VII) ion (MnO4-) with the aid of sodium hydroxide as a catalyst. Manganese undergoes the following reduction stages:

MnO4- + e- ---> MnO4 2- (+6)

MnO4 2- + 2 e- ---> MnO2 (+4) + O2

MnO2 + 2e- ---> Mn 2+ (+2) + O2

And that is the mechanism of the chemical chameleon, a simple experiment explaining a major concept in chemistry.

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