Electrochemistry

 involves oxidation reduction reactions that can be brought about by electricity or used to produce electricity  it is concerned with the conversion of chemical energy to electrical energy, and vise versa An electrochemical cell is a system which produces electrical energy.

Electrochemistry in Everyday Life  The rusting of iron, the photosynthesis that takes place in the leaves of green plants, and the conversion of foods to energy in the body are all examples of chemical changes that involve the transfer of electrons from one chemical species to another.  When such reactions can be made to cause electrons to flow through a wire or when a flow of electrons makes a redox reaction happen, the processes are referred to as electrochemical changes. The study of these changes is called electrochemistry.

An Electrochemical Cell Where reduction occurs voltmeter Where Oxidation occurs

 The voltmeter reveals that e  ’s are flowing through the wire, from the Zn strip to the Cu strip.  The rxn; Cu 2+ (aq) + Zn (s)  Cu (s) + Zn 2+ (aq) only involves the loss and gain of e  ’s.

Electrochemical Cell is Split into Two Half-cells  Left half-cell (or half rxn.) Cu 2+ (aq) + 2e   Cu (s)  Right half-cell (or half rxn.) Zn (s)  Zn 2+ (aq) + 2e   if the salt bridge is removed, rxn stops like flicking off a switch

REDOX Oxidation Reaction is a half- rxn in which a species loses e  ’s reduction Reaction is a half- rxn in which a species gains e  ’s A rxn involving the loss & gain of e  ’s is called reduction- oxidation rxn or REDOX rxn.

LEO the lion says GER LEO = Loss of Electrons is Oxidation GER = Gain of Electrons is Reduction

rxn. again... Cu 2+ (aq) + Zn (s)  Cu (s) + Zn 2+ (aq) Cu 2+ (aq) + 2e   Cu (s)  Gains e , thus Reduction rxn., Cu 2+ is reduced, thus it’s the oxidizing agent Zn (s)  Zn 2+ (aq) + 2e   Loses e , thus Oxidation rxn., Zn is oxidized, thus it’s the reducing agent

IMPORTANT!!  The oxidizing agent is reduced.  The reducing agent is oxidized.

Sample Questions: 1.How can you tell which species is being oxidized & which is being reduced?  A species being oxidized (loses e  ) becomes more positively charged (or less negatively charged)  A species being reduced (gains e  ) becomes more negatively charged (or less positively charged )

2. Indicate the following for questions to come: i) species oxidizied ii) Species reduced iii) oxidizing agent iv) reducing agent

a) Zn 2+ + Mg  Zn + Mg 2+ Zn e   Zn Zn 2+ is reduced, thus it’s an oxidizing agent Mg  Mg e  Mg is oxidized, thus it’s a reducing agent

b) H 2 + Sn 4+  2H + + Sn 2+ H 2  2H + + 2e  H 2 is oxidized, thus it’s a reducing agent Sn e   Sn 2+ Sn 4+ is reduced, thus it’s an oxidizing agent

c) 2Li + F 2  2Li + + 2F  2Li  2Li + + 2e  Li is oxidized, thus it’s a reducing agent F 2 + 2e   2F  F 2 is reduced, thus it’s an oxidizing agent

d) 2Fe 2+ + Sn 4+  Sn Fe 3+ 2Fe 2+  2Fe e  Fe 2+ is oxidized, thus it’s a reducing agent Sn e   Sn 2+ Sn 4+ is reduced, thus it’s an oxidizing agent

Spontaneous Redox Rxns  using Reduction Potentials we can predict whether a rxn. is spontaneous or not  imagine a redox rxn. as a competition for e  b/t the 2 half-rxn.’s  each half-rxn. has a natural tendency to proceed as a reduction rxn.  the magnitude of the tendency to go through reduction is expressed by its reduction potential or standard reduction 25°C, 1 M & 1 atm  the rxn. with the larger reduction potential (i.e. the one with the greater tendency to undergo reduction) acquires e  ’s from the other half-rxn.

Examples: Cu: oxidized Ag + : reduced, thus oxid. agent Ag + Cu Spont. 1.

2. Zn (s) + Cu 2+ (aq)  Zn 2+ (aq) + Cu (s) Zn: oxidized, thus reducing agent Cu 2+ : reduced, thus oxidizing agent Cu 2+ Zn Spont.

3. Cu (s) + Zn 2+ (aq)  Cu 2+ (aq) + Zn (s) Cu: oxidized (reducing agent) Zn 2+ : reduced (oxidizing agent) Cu Zn 2+  Not Spont.

 A rxn. will be spontaneous if & only if there is a reactant to be reduced which is ABOVE a reactant to be oxidized  Basically oxidizing agent must be above reducing agent on chart

Examples: Are the following reactions spontaneous or not? 1.Ni 2+ + Ag  2.Zn 2+ + Li 3.Sn 4+ + Au  4.Sn 2+ + Co 5.Cu + + Sn 6.Al 3+ + Ni 

Examples: Which is the stronger oxidizing agent? 1. Cu + or Sn 2+ Cu + 2. Cr 2+ or Fe 2+ Fe 2+

Examples: Which is the stronger reducing agent? 1.Zn 2+ or Ca 2+ Ca 2+ 2.Cr 3+ or Cu 2+ Cr 3+ 3.Br 2 or I 2 I2I2