chapter 20 - Chapter 20 Electrochemistry REVIEW MATERIAL...

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Chapter 20 Electrochemistry
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REVIEW MATERIAL Slides 2 – 27 are review material you learned in Chem 1 A
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Electrochemical Reactions In electrochemical reactions, electrons are transferred from one species to another.
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Oxidation Numbers In order to keep track of what loses electrons and what gains them, we assign oxidation numbers .
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Oxidation and Reduction A species is oxidized when it loses electrons. Here, zinc loses two electrons to go from neutral zinc metal to the Zn 2+ ion.
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Oxidation and Reduction A species is reduced when it gains electrons. Here, each of the H + gains an electron and they combine to form H 2 .
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Oxidation and Reduction What is reduced is the oxidizing agent . H + oxidizes Zn by taking electrons from it. What is oxidized is the reducing agent . Zn reduces H + by giving it electrons.
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Assigning Oxidation Numbers 1. Elements in their elemental form have an oxidation number of 0. 1. The oxidation number of a monatomic ion is the same as its charge.
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Assigning Oxidation Numbers 1. Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1. Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.
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Assigning Oxidation Numbers 1. Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. Fluorine always has an oxidation number of −1. The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.
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Assigning Oxidation Numbers 1. The sum of the oxidation numbers in a neutral compound is 0. 2. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.
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Balancing Oxidation-Reduction Equations Perhaps the easiest way to balance the equation of an oxidation-reduction reaction is via the half-reaction method .
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Balancing Oxidation-Reduction Equations This involves treating (on paper only) the oxidation and reduction as two separate processes, balancing these half reactions, and then combining them to attain the balanced equation for the overall reaction.
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Half-Reaction Method 1. Assign oxidation numbers to determine what is oxidized and what is reduced. 2. Write the oxidation and reduction half- reactions.
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Half-Reaction Method 1. Balance each half-reaction. a. Balance elements other than H and O. b. Balance O by adding H 2 O. c. Balance H by adding H + . d. Balance charge by adding electrons. 2. Multiply the half-reactions by integers so that the electrons gained and lost are the same.
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1. Add the half-reactions, subtracting things that appear on both sides. 2. Make sure the equation is balanced
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chapter 20 - Chapter 20 Electrochemistry REVIEW MATERIAL...

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