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Electro Chemistry Unit 5 Chemistry 12. Introduction  Electro chemistry is concerned with the conversion of chemical energy to electrical energy and visa.

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Presentation on theme: "Electro Chemistry Unit 5 Chemistry 12. Introduction  Electro chemistry is concerned with the conversion of chemical energy to electrical energy and visa."— Presentation transcript:

1 Electro Chemistry Unit 5 Chemistry 12

2 Introduction  Electro chemistry is concerned with the conversion of chemical energy to electrical energy and visa versa.  Electrochemical reactions involve the movement of electrons;one reagent gains electrons, the other reagent looses electrons  Like acid/base reactions you can think of electrochemical reactions as a combination of 2 half reactions

3 Acids/Bases (In the rearview mirror)  Arrhenius: Acid donates H + ions Base donates OH - ions  Bronstead-Lowery : Acid proton donor Base proton accepter  Lewis : Acid is an electron pair accepter Base an electron pair donor

4 Electro Chemical Reactions  2 types (oxidation and reduction)  Oxidation 1/2 reaction in which the reacting species looses an electro  Reduction 1/2 reaction in which the reacting species gains an electron

5 Memory Trick “ LEO the lion says GER”  LEO: loss of electrons = oxidation  GER: Gain of electrons = reduction We say Fe 2+ is oxidizedWe say Cr 3+ was reduced

6 More Definitions  Oxidizing Agent: is the species that gets reduced (because it is what causes the other species to loose electrons)  Reducing Agent: is the species that gets oxidized (because it is what causes the other species to gain electrons)

7 Identification of oxidized/reduced species  Species that have been oxidized become more positive, because they have lost electrons (** the oxidized species is the reducing agent)  Species that have been reduced become more negative, because they have gained electrons. (** the reduced species is the oxidizing agent)

8 Predictions  Positive species will likely undergo reduction  Negative species will likely undergo oxidation  Neutral species will either undergo oxidation or reduction depending on their electron negativity.

9 Oxidation numbers  Oxidation number: is the charge that an atom would posses if the species containing the atom were made up of ions  Oxidation numbers are a useful way of deciding if a species has undergone oxidation or reduction  If the oxidation number of a species increases, then the species was oxidized  If the oxidation number of a species decreases, then the species was reduced.

10 Guidelines for Assigning Oxidation Numbers  For ionic compounds (including acids/bases) the oxidation number is the charge on the ion  Group 1 have an oxidation # of plus 1 (hydrogen is sometimes an exception)  Group 2 have an oxidation # of plus 2.  Group 17 usually have an oxidation # of -1.  Group 16 usually have an oxidation # of -2.  THE SUM OF +VE and -VE CHARGES MUST BE EQUAL TO THE OVERALL CHARGE ON THE SPECIES.

11 Strengths of Reducing/oxidizing Agent  The relative strength of reducing/oxidizing agents are measured against the Standard Hydrogen 1/2 cell (often called the SHE electrode)  Standard potential tables are written from a reduction perspective

12 Spontaneity of Redox Reactions  For reagents to under go a redox reactions they must be on opposite sides of the std. reduction potential datasheet. (ie. One reagent must be able to under oxidation and the other reduction)  The reaction will be spontaneous if the reduction 1/2 cell is above the oxidation 1/2 cell in the reduction potential tables  The potential (voltage) of a spontaneous rx is always positive

13 Balancing Redox Reactions 1.Break the reaction into the two 1/2 cells 2.Balance each 1/2 cell 3.Use a stiochiometric ratio so that Total electrons gained=Total electrons lost 4.Add the two half reactions together canceling out species that appear on both sides of the reaction

14 Balancing 1/2 Cells (acid) MAJOR OH -  Balance Major ions first  Balance oxygen by adding H 2 O’s  Balance the hydrogen by adding H +  Balance the charge by adding e -

15 Balancing 1/2 Cells (basic) MAJOR OH - (+water) 1.Balance Major ions first 2.Balance oxygen by adding H 2 O’s 3.Balance the hydrogen by adding H + 4.Balance the charge by adding e - 5.Use the dissociation of water to convert H + into OH -

16 Preferred Redox Reactions  If more than one redox reaction is possible, the reaction which has the highest voltage potential will occur first. ( Farthest apart on the standard tables using “DISCO RULE”)  Example Rust protection using Zinc as a “sacrificial anode”  Zn -> Zn +2 +2e  Fe -> Fe +2 +2e  1/2 O 2(aq) + 2H + (10-7) +2e -> H 2 O

17 Galvanic/Voltaic/Electrochemical Cells  An-Ox Ca-red An-OX Ca-Red

18 Electrolysis  Definition: is the process of supplying electrical energy to a molten ionic compound or ionic solution to produce a chemical change. Electrolysis supplies energy to non-spontaneous electro chemical reactions in which E 0 is < O

19 Electrolytic or Electrolysis Cell  Electrolyte is a molten salt or ionic solution  For Electrolytic cells, The Redox reaction with the least negative voltage will occur first

20 Electrowinning  Electrowinning: Is an electrolytic cell that causes the electro deposition of metal from metallic ore/ liquid leach solution. The metal is deposited on the cathode. Usually oxygen is produced at the anode.

21 Electro-plating  Electro-plating is a process in which a metallic ion is reduced or “plated out” onto a cathode of a different material  The Electrolyte contains the metallic ion to be plated out.  The anode can be an inert electrode or the same material as the cathode

22 Electrorefining  Electrorefining is an electrolytic process in which impure metal is purified by oxidizing the impure metal at the anode and reduce the metallic ion produced at a cathode this process is voltage regulated such that only the reduction of the metal being purified plates out

23 Gottchas  Non-standard potentials  Aqeous solutions vs. molten salts


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