Oxidationreduction PPT学习课件PPT课件

1、Oxidation and Reduction第1页/共67页 Oxidation-reduction (redox) reactions involve transfer of electrons Oxidation loss of electrons Reduction gain of electrons Both half-reactions must happen at the same time Can be identified through understanding of oxidation numbers第2页/共67页Oxidation States Oxidation

2、number assigned to element in molecule based on distribution of electrons in molecule There are set rules for assigning oxidation numbers第3页/共67页第4页/共67页 Chromium gives great example of different oxidation numbers Different oxidation states of chromium have different colors Chromium (II) chloride =

3、blue Chromium (III) chloride = green Potassium chromate = yellow Potassium dichromate = orange第5页/共67页Oxidation Oxidation reactions in which the atoms or ions of an element experience an increase in oxidation state Ex. combustion of metallic sodium in atmosphere of chlorine gas第6页/共67页 Sodium ions a

4、nd chloride ions made during exothermic reaction form cubic crystal lattice Sodium cations are ionically bonded to chloride anions第7页/共67页 Formation of sodium ions shows oxidation b/c each sodium atom loses an electron to become sodium ion Oxidation state represented by putting oxidation number abov

5、e symbol of atom and ion第8页/共67页 Oxidation state of sodium changed from 0 (elemental state) to +1 (state of the ion)A species whose oxidation number increases is oxidized Sodium atom oxidized to sodium ion第9页/共67页Reduction Reduction reactions in which the oxidation state of an element decreases Ex.

6、Chlorine in reaction with sodium Each chlorine atom accepts e- and becomes chloride ion Oxidation state decreases from 0 to -1第10页/共67页A species that undergoes a decrease in oxidation state is reduced The chlorine atom is reduced to the chloride ion第11页/共67页Oxidation and Reduction as a Process Elect

7、rons are made in oxidation and acquired in reduction For oxidation to happen during chemical reaction, reduction must happen as well Number of electrons made in oxidation must equal number of electrons acquired in reduction Conservation of mass第12页/共67页 Transfer of e- causes changes in oxidation sta

8、tes of one or more elements Oxidation-reduction reaction any chemical process in which elements undergo changes in oxidation number Ex. When copper oxidized and NO3- from nitric acid is reduced第13页/共67页Part of the reaction involving oxidation or reduction alone can be written as a half-reaction Over

9、all equation is sum of two half-reactions Number of e- same of oxidation and reduction, they cancel and dont appear in overall equation第14页/共67页 Electrons lost in oxidation appear on product side of oxidation half-reaction Electrons gained in reduction appear as reactants in reduction half-reaction第

10、15页/共67页 When copper reacts in nitric acid 3 copper atoms are oxidized to Cu+2 ions as two nitrogen atoms are reduced from a +5 oxidation state to a +2 oxidation state第16页/共67页 If no atoms in reaction change oxidation state, it is NOT a redox reaction Ex. Sulfur dioxide gas dissolves in water to for

11、m acidic solution of sulfurous acid第17页/共67页 When solution of NaCl is added to solution of AgNO3, an ion-exchange reaction occurs and white AgCl precipitates第18页/共67页Redox Reactions and Covalent Bonds Substances with covalent bonds also undergo redox reactions Unlike ionic charge, oxidation number h

12、as no physical meaning Oxidation number based on electronegativity relative to other atoms to which it is bonded in given molecule NOT based on charge第19页/共67页 Ex. Ionic charge of -1 results from complete gain of one electron by atom An oxidation state of -1 means increase in attraction for a bondin

13、g electron Change in oxidation number does not require change in actual charge第20页/共67页 When hydrogen burns in chlorine a covalent bond forms from sharing of two e- Two bonding e- in hydrogen chloride not shared equally The pair of e- is more strongly attracted to chlorine atom because of higher ele

14、ctronegativity第21页/共67页 As specified by Rule #3, chlorine in HCl is assigned oxidation number of -1 Oxidation number for chlorine atoms changes from 0 So chlorine atoms are reduced第22页/共67页 From Rule #1, oxidation number of each hydrogen atom in hydrogen molecule is 0 By Rule #6, oxidation state of

15、hydrogen atom in HCl is +1 Hydrogen atom oxidized第23页/共67页 No electrons totally lost or gained Hydrogen has donated a share of its bonding electron to chlorine It has NOT completely transferred that electron Assignment of oxidation numbers allows determination of partial transfer of e- in compounds

16、that are not ionic Increases/decreases in oxidation number can be seen in terms of completely OR partial loss or gain of e-第24页/共67页 Reactants and products in redox reactions are not limited to monatomic ions and uncombined elements Elements in molecular compounds or polyatomic ions can also be redo

17、xed if they have more than one non-zero oxidation state Example: copper and nitric acid第25页/共67页 Nitrate ion, NO3-, is converted to nitrogen monoxide, NO Nitrogen is reduced in this reaction Instead of saying nitrogen atom is reduced, we say nitrate ion is reduced to nitrogen monoxide第26页/共67页Balanc

18、ing Redox EquationsSection 2第27页/共67页 Equations for simple redox reactions can be balanced by looking at them Most redox equations require more systematic methods Equation-balancing process needs use of oxidation numbers Both charge and mass are conserved Half-reactions balanced separately then comb

19、ined第28页/共67页Half-Reaction Method Also called ion-electron method Made of seven steps Oxidation numbers assigned to all atoms and polyatomic ions to determine which species are part of redox process Half-reactions balanced separately for mass and charge Then added together第29页/共67页第30页/共67页 Sulfur c

20、hanges oxidation state from -2 to +6 Nitrogen changes from +5 to +4 Other substances deleted第31页/共67页 In this example, sulfur is being oxidized第32页/共67页 To balance oxygen, H2O must be added to left side This gives 10 extra hydrogen atoms on that side So, 10 H atoms added to right side In basic solut

21、ion, OH- ions and water can be used to balance atoms第33页/共67页 Electrons added to side having greater positive net charge Left side has no net charge Right side has +8 Add 8 electrons to product side (oxidation of sulfur from -2 to +6 involves loss of 8 e-)第34页/共67页 Nitrogen reduced from +5 to +4第35页

22、/共67页 H2O added to product side to balance oxygen atoms 2 hydrogen ions added to reactant side to balance H atoms第36页/共67页 Electrons added to side having greater positive net charge Left side has net charge of +1 1 e- added to this side balancing the charge第37页/共67页第38页/共67页 This ratio is already in

23、 lowest terms If not, need to reduce Multiply oxidation half-reaction by 1 Multiple reduction half-reaction by 8 Electrons lost = electrons gained第39页/共67页 Each side has 10H+, 8e-, and 4H2O They cancel第40页/共67页 The NO3- ion appeared as nitric acid in original equation Only 6 H ions to pair with 8 ni

24、trate ions So, 2 H ions must be added to complete this formula If 2 H ions added to left side, then 2 H ions must be added to the right side第41页/共67页 Sulfate ion appeared as sulfuric acid in original equation H ions added to right side used to complete formula for sulfuric acid第42页/共67页Sample Proble

25、m第43页/共67页第44页/共67页第45页/共67页第46页/共67页第47页/共67页第48页/共67页第49页/共67页 The iron (II), iron (III), manganese (II), and 2 H ions in original equation are paired with sulfate ions Iron (II) sulfate requires 10 sulfate ions Sulfuric acid requires 8 sulfate ions To balance equation, 18 sulfate ions must be add

26、ed to each side第50页/共67页 On product side, 15 of these form iron (III) sulfate, and 2 form manganese (II) sulfate Leaves 1 sulfate unaccounted for Permanganate ion requires the addition of 2 potassium ions to each side These 2 K ions form potassium sulfate on product side第51页/共67页Oxidizing and Reduci

27、ng AgentsSection 3第52页/共67页 Reducing agent substance that has the potential to cause another substance to be reduced They love electrons Attain a positive oxidation state during redox reaction Reducing agent is oxidized substance第53页/共67页 Oxidizing agent substance that has the potential to cause ano

28、ther substance to be oxidized Gain electrons Attain a more negative oxidation state during redox reactions Oxidizing agent is reduced substance第54页/共67页Strength of Oxidizing and Reducing Agents Different substances compared and rated on relative potential as reducing/oxidizing agents Ex. Activity se

29、ries related to each elements tendency to lose electrons Elements lose electrons to positively charged ions of any element below them in series第55页/共67页 The more active the element the greater its tendency to lose electrons Better a reducing agent it is Greater distance between two elements in list

30、means more likely that a redox reaction will happen between them第56页/共67页 Fluorine atom most highly EN atom Is also most active oxidizing agent b/c of strong attraction for its own e-, fluoride ion is weakest reducing agent Negative ion of strong oxidizing agent is weak reducing agent第57页/共67页 Posit

31、ive ion of strong reducing agent is weak oxidizing agent Ex. Li Strong reducing agents b/c Li is very active metal When Li atoms oxidize they produce Li+ ions Li+ ions unlikely to reacquire e-, so its weak oxidizing agent第58页/共67页 Left column of each pair also shows relative abilities of metals listed to displace other metals Zinc, ex., is above copper so is more active reducing agent Displaces copper ions from solutions of copper compounds Copper ion is more active oxidizing agent than Zn第59页/共67页 Nonmetals and others are included in series Any reducing agent is oxidized by oxid