0.24 Bis2a 06.appendix a review of red/ox reactions  (Page 35/31)

This module is a review and supplemental information

This module is meant as a review to oxidation-reduction reactions, balancing red/ox equations and calculating red/ox states of atoms and simple molecules. In Bis2A, you will not need to balance equations or determine (calculate) the red/ox state of an atom or molecule. However, given a pair of compounds you will have to decide which one is the reduced form and which one is the oxidized form. This module may be of use and is provided as an appendix of sorts to the modules in group 6: Energy.

Humans interact with one another in various and complex ways, and we classify these interactions according to common patterns of behavior. When two humans exchange information, we say they are communicating. When they exchange blows with their fists or feet, we say they are fighting. Faced with a wide range of varied interactions between chemical substances, scientists have likewise found it convenient (or even necessary) to classify chemical interactions by identifying common patterns of reactivity. This module will provide an introduction to three of the most prevalent types of chemical reactions: precipitation, acid-base, and oxidation-reduction.

Oxidation-reduction reactions

Earth’s atmosphere contains about 20% molecular oxygen, O ₂ , a chemically reactive gas that plays an essential role in the metabolism of aerobic organisms and in many environmental processes that shape the world. The term oxidation    was originally used to describe chemical reactions involving O ₂ , but its meaning has evolved to refer to a broad and important reaction class known as oxidation-reduction (redox) reactions . A few examples of such reactions will be used to develop a clear picture of this classification.

Some redox reactions involve the transfer of electrons between reactant species to yield ionic products, such as the reaction between sodium and chlorine to yield sodium chloride:

It is helpful to view the process with regard to each individual reactant, that is, to represent the fate of each reactant in the form of an equation called a half-reaction    :

These equations show that Na atoms lose electrons while Cl atoms (in the Cl ₂ molecule) gain electrons , the “ s ” subscripts for the resulting ions signifying they are present in the form of a solid ionic compound. For redox reactions of this sort, the loss and gain of electrons define the complementary processes that occur:

In this reaction, then, sodium is oxidized and chlorine undergoes reduction    . Viewed from a more active perspective, sodium functions as a reducing agent (reductant) , since it provides electrons to (or reduces) chlorine. Likewise, chlorine functions as an oxidizing agent (oxidant) , as it effectively removes electrons from (oxidizes) sodium.