Chapter-200 - Chapter 20 Electrochemistry Please review section 4.4 in BLB page 137 140 Electrochemistry is the study of the relationships between

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1 Chapter 20 Electrochemistry Electrochemistry is the study of the relationships between electricity and chemical reactions. In an electrochemical reactions, electrons are transferred from one species to another. LEO goes GER L osing E lectrons is O xidation G aining E lectrons is R eduction Please review section 4.4 in BLB, page 137 – 140. Redox reaction : Both Oxidation and Reduction occurs in a single electrochemical reaction.
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2 20.1 Oxidation Numbers (Oxidation State) In order to keep track of what loses electrons and what gains them, we assign oxidation state or oxidation numbers . For example, If you put a piece of zinc metal in an acid solution, you will see brisk effervescence of H 2 gas. The redox reaction that take place in this experiment can be written as follows: Zn(s) + 2H + (aq) → Zn 2+ (aq) + H 2 (g) Before we proceed further, how do we assign oxidation state?
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3 4.4 Assigning Oxidation Numbers 1. Elements in their elemental form have an oxidation number of 0 H atom in H 2 molecule has an oxidation number 0. P atom in P 4 molecule has an oxidation number 0. 2. The oxidation number of a monatomic ion is the same as its charge. K + ==> oxidation number +1 S 2- ==> oxidation number -2 Al 3+ ==> oxidation number +3 Please review section 4.4 in BLB, page 139.
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4 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 (O 2 2- ion) in which it has an oxidation number of −1. Hydrogen is −1 when bonded to a metal (NaH), +1 when bonded to a nonmetal (HCl). Fluorine always has an oxidation number of −1. The other halogens have an oxidation number of −1 in most binary compounds (HCl). When combined with oxygen, as in oxyanions (HOCl), they have positive oxidation states. 4.4 Assigning Oxidation Numbers – Contd…
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5 1. The sum of the oxidation numbers in a neutral compound is 0. (NaCl) 2. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion. (H 3 O + ) 4.4 Assigning Oxidation Numbers – Contd… What is the oxidation state of the Red element in each of the following? (a) P 2 O 5 (b) Na H (c) Cr 2 O 7 2- (d) Sn Br 4 (e) Ba O 2 2(x) + 5(-2) = 0 x = +5 (+1) + (x) = 0 x = -1 x = +6 x = +4 x = -1
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6 20.1 Oxidation State – Contd… A species is oxidized when it loses electrons. Here, zinc loses two electrons to go from neutral Now, we will go back to slide-15 and write the oxidation state of each species in the reaction as shown below: 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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7 What is reduced is the oxidizing agent . H
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This note was uploaded on 05/05/2008 for the course CHEM 178 taught by Professor Gupta during the Spring '08 term at N. Arizona.

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Chapter-200 - Chapter 20 Electrochemistry Please review section 4.4 in BLB page 137 140 Electrochemistry is the study of the relationships between

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