redox - Solomons Study Notes General Chemistry CHE 132...

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S olomon’s Study Notes General Chemistry CHE 132 Spring 2009 Midterm 3 Solomon Weiskop PhD [ Redox Reactions ] These Study Notes cover the material of Ch. 19
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Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomon’s CHE 1 32 Tutoring © Copyright 2009
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1 1. Introduction Redox Reactions are chemical reactions that involve the transfer of electrons ( 𝒆 ) They are also called “electron transfer” reactions. Reduction = gain of 𝒆 ‘s “GER” Oxidation = loss of 𝒆 ‘s “LEO” These are actually “half - reactions”. Eq. ( ? ) is an oxidation half-reaction. Eq.( ? ) is a reduction half-reaction Oxidation and Reduction always occur together You can’t have one without the other. If some species is being Reduced (gaining electrons), some other species has to be losing those electrons i.e. some other species is being Oxidized
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2 The species that is losing electrons (being Oxidized) is making it possible for the other species to gain those electrons. That is, the species that is being Oxidized is making it possible for another species to be Reduced. So the species that is being Oxidized is called the Reducing Agent Likewise, the species that is being Reduced is called the Oxidizing Agent To get the full balanced Redox reaction : add together the two half-reactions: Notice that the electrons cancel out. This must always happen. You never “see” electrons in a full balanced Redox reaction . (You only “see” them in the half -reactions.) In other words: [# 𝒆 ‘s lost in Oxidation] must equal [# 𝒆 ‘s gained in Reduction] In this first example, when we add the two half-reactions the cancellation of electrons happens automatically. Later on we will encounter situations where this cancellation of electrons will not happen automatically. In such cases, there ’ll be something we’ll have to do to make the electrons cancel. (More on this later.)
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3 2. Oxidation Numbers (Ox#) also called “Oxidation State” Redox reactions involve the transfer of electrons. But, as we’ve just seen, a full balanced Redox reaction doesn’t “show” the electrons! So it can sometimes be difficult to identify whether a particular chemical reaction is or is not a Redox reaction. For this reason, it is convenient to use an “electron book - keeping device” called Oxidation Number (Ox#) . With the help of Oxidation Numbers, there is a simple way to decide whether any chemical reaction is or is not Redox. We will discuss this simple way soon. But, before doing that, let’s first introduce Oxidation Numbers themselves and how to assign them: Oxidation Number (Ox#) is a generalization of the concept of Ionic Charge The Ionic Charge for an ion (in an ionic compound) tells you how many electrons the ion lacks or has more than” compared to its neutral atom.
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