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Oxidation–Reduction Reactions

Oxidation–Reduction Reactions
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.  Metallicfree 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.comTable of Oxidation HalfReactions www.ThesisScientist.com 45Electrical Current • When we talk about the current of a liquid in a stream, we are discussing the amount of water that passes by in a given period of time. • When we discuss electric current, we are discussing the amount of electric charge that passes a point in a given period of time.  Whether as electrons flowing through a wire or ions flowing through a solution. Tro's Introductory Chemistry, Chapter 50 www.ThesisScientist.com 16Redox Reactions and Current • Redox reactions involve the transfer of electrons from one substance to another. • Therefore, redox reactions have the potential to generate an electric current. • In order to use that current, we need to separate the place where oxidation is occurring from the place that reduction is occurring. Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 51 16Electric Current Flowing Directly Between Atoms www.ThesisScientist.com 52Electric Current Flowing Indirectly Between Atoms Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 53 16Electrochemical Cells • Electrochemistry is the study of redox reactions that produce or require an electric current. • The conversion between chemical energy and electrical energy is carried out in an electrochemical cell. • Spontaneous redox reactions take place in a voltaic cell.  Also known as galvanic cells.  Batteries are voltaic cells. • Nonspontaneous redox reactions can be made to occur in an electrolytic cell by the addition of electrical energy. www.ThesisScientist.comElectrochemical Cells, Continued • Oxidation and reduction reactions kept separate.  Halfcells. • Electron flow through a wire, along with ion flow through a solution, constitutes an electric circuit. • Requires a conductive solid (metal or graphite) electrode to allow the transfer of electrons.  Through external circuit. • Ion exchange between the two halves of the system.  Electrolyte. www.ThesisScientist.comElectrodes • Anode Electrode where oxidation occurs. Anions attracted to it. Connected to positive end of battery in electrolytic cell. Loses weight in electrolytic cell. • Cathode Electrode where reduction occurs. Cations attracted to it. Connected to negative end of battery in electrolytic cell. Gains weight in electrolytic cell. Electrode where plating takes place in electroplating. www.ThesisScientist.comVoltaic Cell www.ThesisScientist.comCurrent and Voltage • The number of electrons that flow through the system per second is the current. Electrode surface area dictates the number of electrons that can flow. • The amount of force pushing the electrons through the wire is the voltage. The farther the metals are separated on the activity series, the larger the voltage will be. www.ThesisScientist.comCurrent The number of electrons that pass a point each second is called the current of the electricity. The amount of water that passes a point each second is called the current of the river. www.ThesisScientist.comVoltage Voltage is the force pushing the electrons down the wire. Gravity is the force pulling the water down the river. www.ThesisScientist.comLeClanché’s Acidic Dry Cell • Electrolyte in paste form. ZnCl + NH Cl. 2 4 Or MgBr . 2 • Anode = Zn (or Mg). 2+ Zn(s)  Zn (aq) + 2 e • Cathode = graphite rod. • MnO is reduced. 2 + 2 MnO (s) + 2 NH (aq) + 2 H O(l) + 2 e 2 NH OH(aq) + 2 Mn(O)OH(s) 2 4 2 4 • Cell voltage = 1.5 v. • Expensive, nonrechargeable, heavy, easily corroded. www.ThesisScientist.comAlkaline Dry Cell • Same basic cell as acidic dry cell, except electrolyte is alkaline KOH paste. • Anode = Zn (or Mg). 2+ Zn(s)  Zn (aq) + 2 e • Cathode = brass rod. • MnO is reduced. 2 + 2 MnO (s) + 2 NH (aq) + 2 H O(l) + 2 e 2 NH OH(aq) + 2 Mn(O)OH(s) 2 4 2 4 • Cell voltage = 1.54 v. • Longer shelf life than acidic dry cells and rechargeable; little corrosion of zinc. www.ThesisScientist.comLead Storage Battery • Six cells in series. • Electrolyte = 6 M H SO . 2 4 • Anode = Pb. 2 Pb(s) + SO (aq)  PbSO (s) + 2 e 4 4 • Cathode = Pb coated with PbO . 2 • PbO is reduced. 2 + 2 PbO (s) + 4 H (aq) + SO (aq) + 2 e PbSO (s) + 2 H O(l) 2 4 4 2 • Cell voltage = 2.09 v. • Rechargeable, heavy. www.ThesisScientist.comFuel Cells • Like batteries in which reactants are constantly being added.  So it never runs down • Anode and cathode both Ptcoated metal. – • Electrolyte is OH solution. – • Anode reaction: 2 H + 4 OH → 4 H O(l) + 4 e . 2 2 – • Cathode reaction: O + 4 H O + 4 e → 4 OH . 2 2 www.ThesisScientist.comElectrolysis • Electrolysis is the process of using electricity to break a compound apart. • Electrolysis is done in an electrolytic cell. • Electrolytic cells can be used to separate elements from their compounds.  Generate H from water for fuel cells. 2  Recover metals from their ores. www.ThesisScientist.comElectrolytic Cell • The + terminal of the battery = anode. • The terminal of the battery = cathode. • Cations attracted to the cathode; anions attracted to the anode. • Cations pick up electrons from the cathode and are reduced; anions release electrons to the anode and are oxidized. • In electroplating, the work piece is the cathode.  Cations are reduced at the cathode and plate onto the surface.  The anode is made of the plate metal, the anode oxidizes and replaces the metal cations lost from the solution. www.ThesisScientist.comElectrolytic Cell—Electroplating www.ThesisScientist.comCorrosion • Corrosion is the spontaneous oxidation of a metal by chemicals in the environment. • Since many materials we use are active metals, corrosion can be a very big problem. Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 70 16Preventing Corrosion • One way to reduce or slow corrosion is to coat the metal surface to keep it from contacting corrosive chemicals in the environment. Paint. Some metals, like Al, form an oxide that strongly attaches to the metal surface, preventing the rest from corroding. • Another method to protect one metal is to attach it to a more reactive metal that is cheap. Sacrificial electrode. Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 71 16
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