2 IntroductionElectro 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 electronsLike 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+ ionsBase donates OH- ionsBronstead-Lowery : Acid proton donorBase proton accepterLewis : Acid is an electron pair accepterBase an electron pair donor
4 Electro Chemical Reactions 2 types (oxidation and reduction)Oxidation 1/2 reaction in which the reacting species looses an electroReduction 1/2 reaction in which the reacting species gains an electron
5 Memory Trick “LEO the lion says GER” LEO: loss of electrons = oxidationGER: Gain of electrons = reductionWe say Fe2+ is oxidizedWe say Cr3+was reduced
6 More DefinitionsOxidizing 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 oxidationNeutral species will either undergo oxidation or reduction depending on their electron negativity.
9 Oxidation numbersOxidation number: is the charge that an atom would posses if the species containing the atom were made up of ionsOxidation numbers are a useful way of deciding if a species has undergone oxidation or reductionIf the oxidation number of a species increases, then the species was oxidizedIf 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 ionGroup 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 tablesThe potential (voltage) of a spontaneous rx is always positive
13 Balancing Redox Reactions Break the reaction into the two 1/2 cellsBalance each 1/2 cellUse a stiochiometric ratio so that Total electrons gained=Total electrons lostAdd 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 firstBalance oxygen by adding H2O’sBalance the hydrogen by adding H+Balance the charge by adding e-
15 Balancing 1/2 Cells (basic) MAJOR OH-(+water) Balance Major ions firstBalance oxygen by adding H2O’sBalance the hydrogen by adding H+Balance the charge by adding e-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 +2eFe -> Fe+2 +2e1/2 O2(aq) + 2H+(10-7) +2e -> H2O
18 ElectrolysisDefinition: 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 E0 is < O
19 Electrolytic or Electrolysis Cell Electrolyte is a molten salt or ionic solutionFor Electrolytic cells, The Redox reaction with the least negative voltage will occur first
20 ElectrowinningElectrowinning: 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-platingElectro-plating is a process in which a metallic ion is reduced or “plated out” onto a cathode of a different materialThe Electrolyte contains the metallic ion to be plated out.The anode can be an inert electrode or the same material as the cathode
22 ElectrorefiningElectrorefining 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 GottchasNon-standard potentialsAqeous solutions vs. molten salts