CHAPTER  20“Oxidation-Reduction Reactions”                      EO SAYS                     LEO SAYS GER                  ...
Section 20.1        The Meaning of Oxidation and Reduction (called “redox”)         •   OBJECTIVES             o Define ox...
Oxidation and Reduction (Redox)Each sodium atom loses one electron:Each chlorine atom gains one electron:LEO says GER :___...
Oxidation and Reduction (Redox)•      It is easy to see the loss and gain of electrons in ionic compounds, but what about ...
Section 20.2                              Oxidation NumbersOBJECTIVES     • Determine the oxidation number of an atom of a...
NO3            SO4Example Problem: What is the oxidizing number of each kind of atom in the followingcompounds?   a.   SO2...
10.1 and 10.2 Section Review1. State the characteristics of a redox reaction, and explain how to identify the oxidizingage...
•   Balance a redox equation by breaking the equation into oxidation and             reduction half-reactions, and then us...
•  Fortunately, two systematic methods are available, and are based on the              fact that the total electrons gain...
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Chemistry - Chp 20 - Oxidation Reduction - Notes

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Chemistry - Chp 20 - Oxidation Reduction - Notes

  1. 1. CHAPTER 20“Oxidation-Reduction Reactions” EO SAYS LEO SAYS GER GER
  2. 2. Section 20.1 The Meaning of Oxidation and Reduction (called “redox”) • OBJECTIVES o Define oxidation and reduction in terms of the loss or gain of oxygen, and the loss or gain of electrons. o State the characteristics of a redox reaction and identify the oxidizing agent and reducing agent.Oxidation and Reduction (Redox)• Early chemists saw “oxidation” reactions only as the combination of a material with oxygen to produce an oxide. • For example, when methane burns in air, it oxidizes and forms oxides of carbon and hydrogen, as shown CH4 + 2O2  CO2 + 2H2O• But, not all oxidation processes that use oxygen involve burning: • Elemental iron slowly oxidizes to compounds such as iron (III) oxide, commonly called “rust” • Bleaching stains in fabrics • Hydrogen peroxide also releases oxygen when it decomposes • A process called _______________________________ is the opposite of oxidation, and originally meant the loss of oxygen from a compound • Oxidation and reduction always occur simultaneously • The substance gaining oxygen (or losing electrons) is _______________________, while the substance losing oxygen (or gaining electrons) is __________________________. • Today, many of these reactions may not even involve oxygen • Redox currently says that electrons are __________________________________ Mg + S→ Mg2+ + S2- (MgS) • The magnesium atom (which has zero charge) changes to a magnesium ion by losing 2 electrons, and is __________________________ to Mg2+ • The sulfur atom (which has no charge) is changed to a sulfide ion by gaining 2 electrons, and is _________________________to S2- 2
  3. 3. Oxidation and Reduction (Redox)Each sodium atom loses one electron:Each chlorine atom gains one electron:LEO says GER :___ose ___lectrons = ___xidation 0 +1Na → Na + e − Sodium is ______________________________ain ___lectrons = ___eduction 0 −1Cl + e − → Cl Chlorine is __________________________• Losing electrons is oxidation, and the substance that loses the electrons is called the ________________________________________.• Gaining electrons is reduction, and the substance that gains the electrons is called the ________________________________________.Example Problem:What is oxidized and what is reduced in this single replacement reaction? What is theoxidizing agent? What is the reducing agent? 2AgNO3 + Cu  Cu(NO3)2 + Ag 3
  4. 4. Oxidation and Reduction (Redox)• It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds?• In water, we learned that oxygen is highly electronegative, so:  the oxygen gains electrons (is _______________________ and is the oxidizing agent), and the hydrogen loses electrons (is ___________________________ and is the reducing agent)Not All Reactions are Redox Reactions• Reactions in which there has been ___________________________________________________________________________________________ are NOT redox reactions.Examples: +1 +5 −2 +1 −1 +1 −1 +1 +5 −2Ag N O3 (aq ) + Na Cl (aq ) → Ag Cl ( s ) + Na N O3 (aq ) +1 −2 +1 +1 +6 −2 +1 +6 −2 +1 −22 Na O H (aq ) + H 2 S O 4 (aq ) → + Na 2 S O 4 (aq ) + H 2 O(l ) 4
  5. 5. Section 20.2 Oxidation NumbersOBJECTIVES • Determine the oxidation number of an atom of any element in a pure substance. • Define oxidation and reduction in terms of a change in oxidation number, and identify atoms being oxidized or reduced in redox reactions.Assigning Oxidation Numbers • An “oxidation number” is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction. • Generally, a bonded atom’s oxidation number is the charge it would have if the electrons in the bond were assigned to the atom of the more electronegative elementRules for Assigning Oxidation Numbers1) The oxidation number of any uncombined element is ________________2) The oxidation number of a monatomic ion _________________________________. 2Na + Cl2  2NaCl3) The oxidation number of oxygen in compounds is ________, except in peroxides, suchas H2O2 where it is -1.4) The oxidation number of hydrogen in compounds is ________, except in metalhydrides, like NaH, where it is -1. H2O5) The sum of the oxidation numbers of the atoms in the compound _________________ H20 Ca(OH)26) The sum of the oxidation numbers in the formula of a polyatomic ion is __________________________________________________________. 5
  6. 6. NO3 SO4Example Problem: What is the oxidizing number of each kind of atom in the followingcompounds? a. SO2 b. CO32- c. K2SO4Reducing Agents and Oxidizing Agents • An increase in oxidation number = ____________________________________ • A decrease in oxidation number = _____________________________________ Na  Na + e-Sodium is ____________________________ - it is the ___________________________ Cl + e-  ClChlorine is ____________________________- its is the __________________________Trends in Oxidation and ReductionActive metals:• Lose electrons easily• Are easily oxidized• Are strong reducing agentsActive nonmetals:• Gain electrons easily• Are easily reduced• Are strong oxidizing agentsExample Problem:Use the changes in oxidation number to identify which atoms are oxidized and which arereduced in each reaction. a. Cl2 + 2HBr  2HCl + Br2 b. C + O2  CO2Name ___________________________________ Date __________________________ 6
  7. 7. 10.1 and 10.2 Section Review1. State the characteristics of a redox reaction, and explain how to identify the oxidizingagent and the reducing agent?2. Determine which reactant is oxidized and which reactant is reduced. a. H2 + Cl2  2HCl b. S + Cl  SCl2 c. N2 + 2O2  2NO2 d. 2Li + F2  2LiF e. H2 + S  H2S3. In the equations above a-e, determine which reactant is the reducing agent and which isthe oxidizing agent. Section 20.3 Balancing Redox Equations OBJECTIVES • Describe how oxidation numbers are used to identify redox reactions. • Balance a redox equation using the oxidation-number-change method. 7
  8. 8. • Balance a redox equation by breaking the equation into oxidation and reduction half-reactions, and then using the half-reaction method.Identifying Redox EquationsIn general, all chemical reactions can be assigned to one of two classes: 1. oxidation-reduction, in which electrons are transferred: • Single-replacement, combination, decomposition, and combustion 2. this second class has no electron transfer, and includes all others: • Double-replacement and acid-base reactionsIdentifying Redox Equations• In an electrical storm, nitrogen and oxygen react to form nitrogen monoxide: N2(g) + O2(g) → 2NO(g) • Is this a redox reaction? • If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction; therefore, the reaction as a whole must be a _______________________.Example ProblemUse the change in oxidation number to identify whether each reaction is a redox reactionor a reaction of some other type. If a reaction is a redox reaction, identify the elementreduced, the element oxidized, the reducing agent, and the oxidizing agent. a. N2O4  2NO2 b. Cl2 +NaBr  2NaCl + Br2 c. PbCl2 + K2SO4  2KCl + PbSO4 d. 2NaOH + H2SO4  Na2SO4 + 2H2OBalancing Redox Equations • It is essential to write a correctly balanced equation that represents what happens in a chemical reaction 8
  9. 9. • Fortunately, two systematic methods are available, and are based on the fact that the total electrons gained in reduction _________________ the total lost in oxidation. The two methods: 1. Use oxidation-number changes 2. Use half-reactions (you only need to know this method)Using half-reactions • A half-reaction is an equation showing just the oxidation or just the reduction that takes place • they are then balanced separately, and finally combined • Step 1: write unbalanced equation in ionic form • Step 2: write separate half-reaction equations for oxidation and reduction • Step 3: balance the atoms in the half-reactions • Step 4: add enough electrons to one side of each half-reaction to balance the charges • Step 5: multiply each half-reaction by a number to make the electrons equal in both • Step 6: add the balanced half-reactions to show an overall equation • Step 7: add the spectator ions and balance the equation • Rules shown on page 665 – 666Example problem:Use the half reaction method to balance the equation for the following redox reaction. 9

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