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Oxidation–Reduction Reactions

Oxidation–Reduction Reactions
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Dr.SamuelHunt,United Arab Emirates,Teacher
Published Date:21-07-2017
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Chapter 16 Oxidation and ReductionOxidation–Reduction Reactions • Oxidation–reduction reactions are also called redox reactions. • All redox reactions involve the transfer of electrons from one atom to another. • Spontaneous redox reactions are generally exothermic, and we can use their released energy as a source of energy for other applications.  Convert the heat of combustion into mechanical energy to move our cars.  Use electrical energy in a car battery to start our car engine. www.ThesisScientist.comCombustion Reactions • Combustion reactions are always exothermic. • In combustion reactions, O combines with all the 2 elements in another reactant to make the products. 4 Fe(s) + 3 O (g) → 2 Fe O (s) + energy 2 2 3 CH (g) + 2 O (g) → CO (g) + 2 H O(g) + energy 4 2 2 2 www.ThesisScientist.comReverse of Combustion Reactions • Since combustion reactions are exothermic, their reverse reactions are endothermic. • The reverse of a combustion reaction involves the production of O . 2 energy + 2 Fe O (s) → 4 Fe(s) + 3 O (g) 2 3 2 energy + CO (g) + 2 H O(g) → CH (g) + 2 O (g) 2 2 4 2 • Reactions in which O is gained or lost are 2 redox reactions. www.ThesisScientist.comOxidation and Reduction: One Definition • When an element attaches to an oxygen during the course of a reaction it is generally being oxidized.  In CH (g) + 2 O (g) → CO (g) + 2 H O(g), C is being 4 2 2 2 oxidized in this reaction, but H is not. • When an element loses an attachment to oxygen during the course of a reaction, it is generally being reduced.  In 2 Fe O (s) → 4 Fe(s) + 3 O (g), the Fe is being reduced. 2 3 2 • One definition of redox is the gain or loss of O, but it is not the best. www.ThesisScientist.comAnother Oxidation–Reduction • Consider the following reactions: 4 Na(s) + O (g) → 2 Na O(s) 2 2 2 Na(s) + Cl (g) → 2 NaCl(s) 2 • The reaction involves a metal reacting with a nonmetal. • In addition, both reactions involve the conversion of free elements into ions. + – 4 Na(s) + O (g) → 2 Na O (s) 2 2 + – 2 Na(s) + Cl (g) → 2 Na Cl (s) 2 www.ThesisScientist.comOxidation and Reduction: Another Definition • In order to convert a free element into an ion, the atoms must gain or lose electrons.  Of course, if one atom loses electrons, another must accept them. • Reactions where electrons are transferred from one atom to another are redox reactions. • Atoms that lose electrons are being oxidized, atoms that gain electrons are being reduced. Ger + – 2 Na(s) + Cl (g) → 2 Na Cl (s) 2 + – Na → Na + 1 e (oxidation) – – Cl + 2 e → 2 Cl (reduction) 2 www.ThesisScientist.com LeoPractice—Identify the Element Being Oxidized and the Element Being Reduced. • 2 C(s) + O (g) → 2 CO(g) 2 • Mg(s) + Cl (g) → MgCl (s) 2 2 2+ 2+ • Mg(s) + Fe (aq) → Mg (aq) + Fe(s) www.ThesisScientist.comPractice—Identify the Element Being Oxidized and the Element Being Reduced, Continued. • 2 C(s) + O (g) → 2 CO(g) 2 C is oxidized because it is gaining an attachment to O. O is reduced; there has to be reduction and it’s the only other element. 2+ − 0 0 • Mg(s) + Cl (g) → MgCl (s) 2 2 Mg is oxidized because it is becoming a cation by losing electrons. Cl is reduced because it is becoming an anion by gaining electrons. 2+ 2+ • Mg(s) + Fe (aq) → Mg (aq) + Fe(s) Mg is oxidized because it is becoming a cation by losing electrons. 2+ Fe is reduced because it is gaining electrons to become neutral. www.ThesisScientist.comOxidation–Reduction • Oxidation and reduction must occur simultaneously.  If an atom loses electrons, another atom must take them. • The reactant that reduces an element in another reactant is called the reducing agent.  The reducing agent contains the element that is oxidized. • The reactant that oxidizes an element in another reactant is called the oxidizing agent.  The oxidizing agent contains the element that is reduced. + – 2 Na(s) + Cl (g) → 2 Na Cl (s) 2 Na is oxidized, Cl is reduced. Na is the reducing agent, Cl is the oxidizing agent. 2 www.ThesisScientist.comElectron Bookkeeping • For reactions that are not metal + nonmetal, or do not involve O , we need a method for determining 2 how the electrons are transferred. • Chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reaction.  Although they look like them, oxidation states are not ion charges Oxidation states are imaginary charges assigned based on a set of rules. Ion charges are real, measurable charges. www.ThesisScientist.comRules for Assigning Oxidation States, Continued 5. In their compounds, nonmetals have oxidation states according to the table below.  Nonmetals higher on the table take priority. Nonmetal Oxidation state Example F -1 CF 4 H +1 CH 4 O -2 CO 2 Group 7A -1 CCl 4 Group 6A -2 CS 2 Group 5A -3 NH 3 www.ThesisScientist.comPractice—Assign an Oxidation State to Each Element in the Following: • F 2 2+ • Mg • KCl • SO 2 3− • PO 4 • BaO 2 www.ThesisScientist.comPractice—Assign an Oxidation State to Each Element in the Following, Continued: • F F = 0 (Rule 1) 2 2+ • Mg Mg = +2 (Rule 2) • KCl K = +1 (Rule 4a) and Cl = -1 (Rule 5) • SO O = -2 (Rule 5) and S = +4 (Rule 3a) 2 3− • PO O = -2 (Rule 5) and P = +5 (Rule 3b) 4 • BaO Ba = +2 (Rule 4b) and O = -2 www.ThesisScientist.comOxidation and Reduction: A Better Definition • Oxidation occurs when an atom’s oxidation state increases during a reaction. • Reduction occurs when an atom’s oxidation state decreases during a reaction. CH + 2 O → CO + 2 H O 4 2 2 2 -4 +1 0 +4 –2 +1 -2 oxidation reduction www.ThesisScientist.comPractice—Assign Oxidation States and Identify the Oxidizing and Reducing Agents in Each of the Following: – + • 3 H S + 2 NO + 2 H 3 S + 2 NO + 4 H O 2 3 2 • MnO + 4 HBr MnBr + Br + 2 H O 2 2 2 2 www.ThesisScientist.comPractice—Assign Oxidation States and Identify the Oxidizing and Reducing Agents in Each of the Following, Continued: reducing oxidizing agent agent – + • 3 H S + 2 NO + 2 H 3 S + 2 NO + 4 H O 2 3 2 +1 -2 +5 -2 +1 0 +2 -2 +1 -2 oxidation Oxidizing reduction reducing agent agent • MnO + 4 HBr MnBr + Br + 2 H O 2 2 2 2 +4 -2 +1 -1 +2 -1 0 +1 -2 oxidation reduction www.ThesisScientist.comTendency to Lose Electrons • Some metals have a greater tendency to lose electrons than others.  Metallic-free elements are always oxidized.  The greater the tendency of a metal to lose electrons, the easier it is to oxidize.  The greater the tendency of a metal to lose electrons, the harder it is to reduce its cations. • If Metal A has a greater tendency to lose electrons than Metal B, then: + + A(s) + B (aq)  A (aq) + B(s), + but: A (aq) + B(s)  no reaction. Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 42 16displace H displace H displace H2 2 2 displace H 2 K K K K from Ba from Ba from Ba Activity Series of Metals from Ba Sr cold Sr Sr cold cold Sr cold Ca Ca H O Ca H O H2O Ca 2 2 H O • Listing of metals by Na 2 Na Na Na Mg Mg Mg Mg reactivity. from Al from Al from Al from Al Mn steam Mn Mn steam steam Mn steam • Free metal higher on the Zn Zn Zn Zn Cr Cr Cr list displaces metal cation Cr Fe Fe Fe Fe Cd lower on the list. Cd Cd Cd Co Co from Co Gold is at the from from Co from Ni Ni Ni • Metals above H will acids acids Ni acids bottom, so it is acids Sn Sn Sn Sn Pb dissolve in acid: Pb Pb very unreactive. Pb H H 2+ 2+ H H Zn + Fe Fe + Zn Sb Sb Sb Sb As 2+ As As Cu + Fe no reaction As Bi Bi Bi Bi + 2+ Cu Zn + 2 H H + Zn Cu Cu 2 Cu Hg Hg Hg Hg F F ee is ab is be ove low Z Cu, so C n, so Zu me n metal tal Ag Ag Zn is above H, Ag Ag 2+. 2+ Pd Pd will will not di displa splac ce Fe e Fe Pd Pd so Zn will react with acids Pt Pt Pt www.ThesisScientist.com Pt Au Au Au Au react with O in the air to make oxides react with O 2 in the air to make oxides react with O in the air to make oxides react with O 2 in the air to make oxides 2 2Mg is above Cu on the Mg will react with activity series. Cu will not 2+ 2+ Cu to form Mg 2+ react with Mg . and Cu metal. www.ThesisScientist.com
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