2TRANSFER REACTIONS Atom/Group transfer Electron transfer HCl + H2O ---> Cl H3O+Electron transferCu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
3Electron Transfer Reactions Electron transfer reactions are oxidation-reduction or redox reactions.Redox reactions can result in the generation of an electric current or be caused by imposing an electric current.Therefore, this field of chemistry is often called ELECTROCHEMISTRY.
4Why Study Electrochemistry? BatteriesCorrosionIndustrial production of chemicals such as Cl2, NaOH, F2 and AlBiological redox reactionsThe heme group
5Review of Terminology for Redox Reactions OXIDATION—loss of electron(s) by a species; increase in oxidation number.REDUCTION—gain of electron(s); decrease in oxidation number.OXIDIZING AGENT—electron acceptor; species is reduced.REDUCING AGENT—electron donor; species is oxidized.
6Review of Cheesy Ways to remember these terms OIL RIG(Oxidation Is Losing, Reduction Is Gaining)LEO (the lion) goes GERLosing Electrons Oxidation, Gaining Electrons ReductionOLE-Oxidation Losing Electrons
7But now feeling positive Oxidized is cool! Oxidation Haiku!Lost an electronBut now feeling positiveOxidized is cool!
8OXIDATION-REDUCTION REACTIONS Direct Redox ReactionOxidizing and reducing agents in direct contact.Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)
10OXIDATION-REDUCTION REACTIONS Indirect Redox ReactionA battery functions by transferring electrons through an external wire from the reducing agent to the oxidizing agent.
11Electrochemical Cells An apparatus that allows a redox reaction to occur by transferring electrons through an external connector.Product favored reaction ---> voltaic or galvanic cell ----> electric currentReactant favored reaction ---> electrolytic cell ---> electric current used to cause chemical change.This is important to distinguish these types due to subtle differences!Batteries are voltaic cells
12Balancing Equations for Redox Reactions Some redox reactions have equations that must be balanced by special techniques.MnO Fe H > Mn Fe H2OThis is all review from Chapter 5, so you don’t need any review…. Right?Mn = +7Fe = +2Mn = +2Fe = +3
13Fine, lets look at some examples! Consider the reduction of Ag+ ions with copper metal.Cu + Ag give--> Cu Ag
14Balancing EquationsStep 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction.Ox Cu ---> Cu2+Red Ag+ ---> AgStep 2: Balance each for mass. Already done in this case.Step 3: Balance each half-reaction for charge by adding electrons.Ox Cu ---> Cu e-Red Ag+ + e- ---> Ag
15Balancing EquationsStep 4: Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires.Reducing agent Cu ---> Cu e-Oxidizing agent 2 Ag e- ---> 2 AgStep 5: Add half-reactions to give the overall equation.Cu Ag > Cu AgThe equation is now balanced for both charge and mass.
17Add H2O on O-deficient side and add H+ on other side for H-balance. Balancing EquationsBalance the following in acid solution—VO Zn ---> VO Zn2+Step 1: Write the half-reactionsOx Zn ---> Zn2+Red VO > VO2+Step 2: Balance each half-reaction for mass.Red2 H+ +VO > VO H2OAdd H2O on O-deficient side and add H+ on other side for H-balance.
18Balancing Equations Step 3: Balance half-reactions for charge. Ox Zn ---> Zn e-Red e H+ + VO > VO H2OStep 4: Multiply by an appropriate factor.Red 2e H VO > 2 VO H2OStep 5: Add balanced half-reactionsZn H VO > Zn VO H2O
19Tips on Balancing Equations Never add O2, O atoms, or O2- to balance oxygen.Never add H2 or H atoms to balance hydrogen.Be sure to write the correct charges on all the ions.Check your work at the end to make sure mass and charge are balanced.PRACTICE!