Presentation on theme: "Chapter 21. the study of the production of ___________ during chemical rxns and the changes produced by ___________ ___________. Electrochemical reactions."— Presentation transcript:
the study of the production of ___________ during chemical rxns and the changes produced by ___________ ___________. Electrochemical reactions are __________________ reactions. Oilrig: 1. Oxidation = loss of electrons a. the substance oxidized is the reducing agent 2. Reduction = gain of electrons a. the substance reduced is the oxidizing agent
___________ ___________ - nonspontaneous chemical reactions ___________ ___________ - spontaneous chemical reactions The two parts of the reaction are physically separated. –oxidation occurs at one cell –reduction occurs in the other cell
___________ - electrode at which reduction occurs (red cat) ___________ - electrode at which oxidation occurs (an ox) ___________ electrodes do not react with the liquids or products of the electrochemical reaction. Graphite and Platinum are common inert electrodes.
Use electrical energy to force ______________ (non thermodynamically favored) chemical reactions to occur. Process called ___________. Used in: –plating of jewelry and auto parts –electrolysis of chemical compounds Electrolytic cells consist of a: –container for reaction mixture –electrodes immersed in the reaction mixture –source of direct current
In all electrolytic cells, ___________ ___________ ___________ ___________ ___________ ___________ ___________ In all ___________ cells the most easily reduced species is reduced and the most easily oxidized species is oxidized.
The amount of substance undergoing chemical reaction at each electrode during electrolysis is directly proportional to the amount of electricity that passes through the electrolytic cell. During electrolysis, one ___________ of electricity (96,487 coulombs) reduces and oxidizes, respectively, one equivalent of the oxidizing agent and the reducing agent. Corresponds to the passage of ___________ _______ through the electrolytic cell.
Amount of electricity that reduces one equivalent of a species at the cathode and oxidizes one equivalent of a species at the anode. 1 faraday of electricity = 6.022x10 23 e - 1 faraday = 6.022x10 23 e - = 96487 coulombs 1 eq. of oxidizing agent= gain of 6.022x10 23 e - 1 eq. of reducing agent = loss of 6.022x10 23 e -
Ex. 1) Calculate the mass of palladium produced by the reduction of palladium (II) ions during the passage of 3.20 amperes of current through a solution of palladium (II) sulfate for 30.0 minutes. (1 ampere = 1 coulomb per second)
11 Ex. 2) Calculate the volume of oxygen (measured at STP) produced by the oxidation of water in Ex. 1.
Electrochemical cells in which a ___________ chemical reaction produces electrical energy. In all voltaic cells, ___________ ___________ ___________ ___________ ___________ ___________ ___________ ___________ Cell halves are physically separated so that electrons (from redox reaction) are forced to travel through wires and creating a potential difference Examples: Car & flashlight batteries
13 Half-cell contains the ___________ and ___________ forms of an element (or other chemical species) in contact with each other. Simple cells consist of: two pieces of metal immersed in solutions of their ions wire to connect the two half-cells salt bridge to ▪ complete circuit ▪ maintain neutrality ▪ prevent solution mixing
14 Cell components: Cu strip immersed in 1.0 M copper (II) sulfate Zn strip immersed in 1.0 M zinc (II) sulfate wire and a salt bridge to complete circuit Initial voltage is 1.10 volts How do we find the initial voltage?
15 There is an establish ___________ standard to measure potentials of a variety of electrodes ___________ ___________ ___________(SHE) assigned an arbitrary voltage of 0.000000… V
16 Electrodes that force the SHE to act as an anode are assigned ___________ standard reduction potentials. Electrodes that force the SHE to act as the cathode are assigned ___________ standard reduction potentials. Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written.
17 Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written. For example, the half-reaction for the standard potassium electrode is: The large ___________ value tells us that this reaction will occur only under extreme conditions.
18 Compare the potassium half-reaction to fluorine’s half-reaction: The large ___________ value denotes that this reaction occurs readily as written. Positive E 0 values tell us that the reaction tends to occur to the right larger the value, greater tendency to occur to the right Opposite for negative values
1. Choose the appropriate half-reactions from a table of standard reduction potentials. 2. Write the equation for the half-reaction with the more positive E 0 value first, along with its E 0 value. 3. Write the eqn for the other half-reaction as an oxidation with its oxidation potential, reverse the tabulated reduction half-reaction and change the sign of the tabulated E 0. 4. Balance the electron transfer. Do NOT change the E o values 5. Add the reduction and oxidation half-reactions and their potentials. This produces the equation for the reaction for which E 0 cell is positive, which indicates that the forward reaction is spontaneous.
20 Step 1) Find Copper and Zinc on the SRP sheet
21 Steps 2 & 3) What are the two half reactions? (V) red: ox:
22 Steps 4 & 5) Make sure e - are balanced and add the volts together (V) red: ox:
23 In all ___________, electrons flow spontaneously from the negative electrode (anode) to the positive electrode (cathode).
30 Compare the Zn-Cu cell to the Cu-Ag cell Cu electrode is cathode in Zn-Cu cell Cu electrode is anode in Cu-Ag cell Whether a particular electrode behaves as an anode or as a cathode depends on what the other electrode of the cell is.
31 Demonstrates that Cu 2+ is a stronger oxidizing agent than Zn 2+ Cu 2+ oxidizes metallic Zn to Zn 2+ Ag + is is a stronger oxidizing agent than Cu 2+ Ag + oxidizes metallic Cu to Cu 2+ Arrange these species in order of increasing strengths
32 Use standard electrode potentials to predict whether an electrochemical reaction at standard state conditions will occur spontaneously. Ex. 5) Will silver ions, Ag +, oxidize metallic zinc to Zn 2+ ions, or will Zn 2+ ions oxidize metallic Ag to Ag + ions? What is the overall value for E o ? E 0 values are not multiplied by any stoichiometric relationships in this procedure.
33 Ex.6) Will tin(IV) ions oxidize iron (II) ions to iron (III) ions, or will iron (III) ions oxidize tin(II) ions to tin(IV) ions in acidic solution? What is the overall value for rxn?
34 Standard electrode potentials are determined at thermodynamic standard conditions. 1 M solutions 1 atm of pressure for gases liquids and solids in their standard states temperature of 25 0 C Potentials change if conditions are nonstandard. ___________ equation describes the electrode potentials at nonstandard conditions.
36 Substitution of the values of the constants into the Nernst equation at 25 0 C gives:
used to calculate electrode potentials and cell potentials for concentrations other than standard –state values. (pg 877) Ex. 7) Calculate the cell potential for the following electrochemical cell. If the [Sn 2+ ] = 4.5 x 10 -1 M and [Ag + ] = 0.110 M Sn (s) + 2Ag + Sn 2+ + 2Ag (s)
38 The Nernst equation can also be used to calculate the potential for a cell that consists of two nonstandard electrodes. Ex. 8) Calculate the initial potential of a cell that consists of an Fe 3+ /Fe 2+ electrode in which [Fe 3+ ]=1.0 x 10 -2 M and [Fe 2+ ]=0.1 M connected to a Sn 4+ /Sn 2+ electrode in which [Sn 4+ ]=1.0 M and [Sn 2+ ]=0.10 M. A wire and salt bridge complete the circuit.
39 From previous chapters we know the relationship of G 0 and K for a reaction.
40 The relationship between G 0 and E 0 cell is also a simple one.
41 You can combine these two relationships into a single relationship to relate E 0 cell to K.
42 Ex. 9) Calculate the standard Gibbs free energy change, G 0, at 25 0 C for the following reaction.
43 Calculate E 0 cell using the appropriate half-reactions.
44 Now that we know E 0 cell, we can calculate G 0. The negative value tells us that the reaction is spontaneous as written.
45 Ex. 10) Calculate the thermodynamic equilibrium constant for the reaction in Ex. 9 at 25 0 C.
46 Metallic corrosion is the oxidation-reduction reactions of a metal with atmospheric components such as CO 2, O 2, and H 2 O.
The oxidation of most metals by oxygen is spontaneous. Many metals develop a thin coating of metal oxide on the outside that prevents further oxidation The presence of a salt accelerates the corrosion process by increasing the ease with which electrons are conducted from anodic to cathodic regions
48 Some examples of corrosion protection. 1 Plate a metal with a thin layer of a less active (less easily oxidized) metal. 2. Connect the metal to a sacrificial anode, a piece of a more active metal.
49 3 Allow a protective film to form naturally.
50 4 Galvanizing, coating steel with zinc, a more active metal. 5. Paint or coat with a polymeric material such as plastic or ceramic.
51 As a voltaic cell discharges, its chemicals are consumed. Once chemicals are consumed, further chemical action is impossible. Electrodes and electrolytes cannot be regenerated by reversing current flow through cell.
52 One example is flashlight, radio, etc. batteries. Container is made of zinc acts as an electrode Graphite rod is in center of cell acts as the other electrode Space between electrodes is filled with a mixture of: ammonium chloride, NH 4 Cl manganese (IV) oxide, MnO 2 zinc chloride, ZnCl 2 porous inactive solid
53 As current is produced, Zn dissolves and goes into solution as Zn 2+ ions. Zn electrode is negative (anode).
54 Secondary cells are reversible, rechargeable. Electrodes can be regenerated One example is the lead storage or car battery.
55 Electrodes are two sets of lead alloy grids (plates). Holes in one grid are filled with lead (IV) oxide, PbO 2. Other holes are filled with spongy lead. Electrolyte is dilute sulfuric acid. When battery is discharging, spongy lead is oxidized to lead ions and the plate becomes negatively charged.
56 Cell reaction for a discharging lead storage battery is
57 This battery can be recharged by passing electricity through the cell in the opposite direction. Voltaic cell is converted to galvanic cell. At lead electrode, lead ions are reduced to lead atoms.
58 Rechargeable cell used in calculators, cameras, watches, etc. Discharge half-reactions are:
59 Fuel cells are batteries that must have their reactants continuously supplied in the presence of appropriate catalysts. Hydrogen-oxygen fuel cell is used in the space shuttle.
60 The reaction combines hydrogen and oxygen to form water. Drinking water supply for astronauts. Very efficient energy conversion 60-70%
61 1. What are the explosive chemicals in the fuel cell that were aboard Apollo 13? Which tank exploded? 2. Some of the deadliest snakes in the world, for example the cobra, have venoms that are neurotoxins. Neurotoxins have an electrochemical basis. How do neurotoxins disrupt normal chemistry and eventually kill their prey?