SELECTED PRINCIPLES: REDOX REACTIONS (1) The word redox is a contraction of 'reduction-oxidation'. Historically, oxidation was used to describe the addition of oxygen to a reactant; e.g., in this first reaction, magnesium has been oxidized by dioxygen (which is acting as the oxidizing agent or oxidant):
Conversely, reduction described the removal of oxygen from a reactant; e.g., in this second reaction, copper(II) oxide has been reduced by dihydrogen (which is acting as the reducing agent or reductant):
Close inspection of the above reactions reveals two important points. First, when one reactant is reduced, the other is oxidized (... redox). And second, when a particle is oxidized it loses electrons, and when a particle is reduced it gains electrons; e.g.,
This second point focuses on electron transfer in redox reactions, and this is the basis for one modern definition of oxidation and reduction: namely, oxidation is the loss of electrons (Olé!), and reduction is the gain of electrons.
Most recently, another definition of oxidation and reduction has found widespread favour: namely, one based on changes in oxidation numbers (often called oxidation states). Because a comprehensive presentation of oxidation numbers is beyond the scope of this text, a brutal summary will have to suffice: oxidation is an increase in oxidation number, and reduction is a decrease in oxidation number ... as illustrated below.
The oxidation number or state, (in Roman numerals), is placed immediately after the name of the element concerned
I = 1 II = 2 III = 3 IV = 4 V = 5 VI = 6 VII = 7
1. Oxidizing agents are substances which cause oxidation; examples of these reagents used in laboratories include potassium dichromate(VI), concentrated tetraoxosulfuric(VI) acid, and potassium manganate(VII). (a) Potassium dichromate(VI), dissolved in aqueous sulfuric acid, is a convenient reagent for the oxidation of alcohols to their corresponding carboxylic acids; e.g., propan-1-ol is oxidized to propanoic acid:
In this reaction, the orange dichromate(VI) ions are reduced to green chromium(III) ions; a simplified redox half equation is as follows:
Recalling that the number of electrons equals the number of protons in a neutral atom of an element, state the number of electrons there are in each of these particles: 24Cr __________ 24Cr3+ __________ 24Cr6+ __________ 18Ar ___________  (b) Concentrated tetraoxosulfuric(VI) acid, better known as sulfuric acid, oxidizes hydrogen bromide as follows:
State the change in oxidation number of sulfur. _______________________  Suggest two observations which should be made during this reaction. ___ _______________________________________________________________________  (c) Potassium manganate(VII), dissolved in aqueous potassium hydroxide, is a reagent sometimes used to test for the presence of a C=C double bond; e.g., cis-but-2-ene is oxidized to butane-2,3-diol:
In this reaction, the purple manganate(VII) ions are reduced to brown manganese(IV) ions; a simplified redox half equation is as follows:
State the number of electrons there are in each of these particles: 25Mn __________ 25Mn4+ __________ 25Mn7+ __________ 18Ar ___________  2. Reducing agents are substances which cause reduction; examples of these reagents used in laboratories include magnesium(s), methane(g), and carbon-graphite(s). Construct a symbol equation for the reduction of lead(IV) oxide to lead(0) with each of these named reducing agents. _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________  3. Lead(II) ions are very toxic to most living organisms because they inhibit the 'active sites' of a wide variety of enzymes; in particular, those which are catalysts in the biosynthesis of compounds involved, directly or indirectly, in the redox processes of either photosynthesis or cellular respiration or both (e.g., chlorophylls, cytochromes, and haemoglobin). Name the condition caused by a shortage of haemoglobin, the dioxygen-carrying pigment in mammalian red blood cells. ___________ 
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