Chem 130 Chapter 20 Notes!
Chem 130 Chapter 20 Notes! CHEM 130 - 003
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This 0 page Class Notes was uploaded by Izabella Nill Gomez on Monday November 30, 2015. The Class Notes belongs to CHEM 130 - 003 at University of Tennessee - Knoxville taught by Bin Zhao in Summer 2015. Since its upload, it has received 18 views. For similar materials see General Chemistry II in Chemistry at University of Tennessee - Knoxville.
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Date Created: 11/30/15
Chemistry Chapter 20 Notes Electrochemistry study of the relationships between electricity and chemical reactions Includes spontaneous and nonspontaneous processes By keeping track of oxidation numbers oxidation states one can determine whether a given reaction is oxidation reduction Elemental forms of molecules have an oxidation number of 0 oxygen always has 2unless it is in peroxide hydrogen is 1 and so forth By determining the change in oxidation state in a chemical reaction oxidizing agents oxidants and reducing agents reductants can be determined Oxidants are the substances that make it possible for other substances to be oxidized Reductants are the substances that make it possible for other substances to be reduced Reducing agent is therefore oxidized in the process and vice versa Whenever balancing a chemical equation one must obey the law of conservation of mass Gains and losses must also be balanced If a substance loses a certain amount of electrons during the process of a reaction another gains them 2 iaq 4Zaq2FeZ Ex 3Z iaQ Sn 2Zaq2FeZ Snz Separate the substances and their electron lossgain to make things easier Z 4Z2equot 2Zaq gtSnz Oxidation Snquot 2Zaqo Z gt2Fequot 3Zaq266 Reduction 2 Fe 2 Halfreactions illustrate either oxidationreduction alone To balance an equation via halfreaction 1 Divide the equation into 1 oxidation and 1 reduction halfreaction 2 Balance each half reaction 3 Multiply halfreactions by integers as needed to make the numbers of elections lost in the oxidation halfreaction equal to the number gained by the reduction 4 Add halfreactions and simplify if possible 5 Check to make sure atoms and charges are balanced If a redox reaction occurs in an acidic solution 1 Assign oxidation numbers to determine what is oxidized and what is reduced 2 Write the oxidation and reduction halfreactions 3 Balance each halfreaction a Balance elements other than H and O b Balance 0 by adding H20 c Balance H by adding H d Balance charge by adding electrons 4 Multiply the halfreactions by integers so that the electrons gained and lost are the same 5 Add the halfreactions subtracting things that appear on both sides 6 Check to make sure that atoms and charges are balanced If a redox reaction occurs in a basic solution the equation must be balanced by using OH and H20 rather than H and H20 One way is to balance the half reaction like in an acidic solution and count the number of H in each half reaction and add the same number of OH to each side for the halfreaction This way it is also mass balanced This way protons are neutralized to form water In spontaneous oxidationreduction redox reactions electrons are transferred and energy is released A voltaic cell is a device in which the transfer of electrons takes place through an external pathway rather than directly between reactants present in the same 2ZaqCus reaction vessel Ex when zinc is reacted with copper 2 2509Z S Zn Cuquot Electrons owing through a wire and ions moving in solution both constitute electrical current The ow of electrical charge can be used to complete electrical work The two solid materials connected by an external circuit are electrodes in a cell The anode negative is where oxidation occurs and the cathode positive is where reduction occurs Each compartment of the voltaic cell is called a halfcell One half cell is the site of oxidation the other for reduction Ex 2Z 2 3 anode oxidation halfreaction aq e Zns gtan Z gtCus cathode reduction halfreaction 2Z a926 Cuz Electrons become available as zinc is oxidized at the anode ow through external circuit to the cathode where copper is reduced For a voltaic cell to work the solutions in the two halfcells must remain electrically neutral To allow zinc cations to move out of the solution and anions to migrate in there must be a means to connect the two solutions For copper cations to leave the solution and excess anions to remain there must be something connecting the solution as well A salt bridge is composed of a Ushaped tube with an electrolyte solution Allows for anions to always migrate toward the anode and the cations toward the cathode Electrons ow from the anode through the external circuit to the cathode This is why the anode has a negative sign and the cathode has a positive sign Potential energy of electrons is higher in the anode than in the cathode allowing for spontaneous flow toward the electrode with a more positive electron potential The difference between potential energy per electric charge is measured in volts Joule 1 V Coulomb Cell potential is the potential difference between two electrodes of a voltaic cell AKA Electromotive force emf pushes electrons through the external circuit AKA voltage Cell potential of any voltaic cell is positive Magnitude depends on the reactions that occur at the cathode and anode concentrations of reactants and products and the temperature standard conditions25 deg C Standard cell E0 1139 E0 potentialemf 36 0 is the cell potential at standard conditions A standard reduction potentials E Bell E2 cathode E anode The greater the difference between oxidizing and reducing agents the greater the voltage of the cell The values of standard reduction potentials are referenced to a standard hydrogen electrode SHE Consists of a platinum wire connected to a piece of platinum foil covered with nely divided platinum that serves as an inert surface for the reaction and can operate as either the anode or cathode Whenever we assign an electrical potential to a halfreaction we write the reaction as a reduction Standard reduction potentials are intensive properties The more 0 positive the value of EL the greater the tendency for reduction under standard conditions AG for a redox reaction can be found by using the equation AG nFE n the number of moles of electrons transferred F Faraday constant 1 F 96485 Cmol 96485 JV mol E emf A positive value of E and a negative value of AG both indicate that a reaction is spontaneous Under standard conditions AG nFE 0 A G RTan 0 nF E RTnK 0 E nF an Finding the emf under nonstandard conditions use Nernst equation RT EE l nF nQ Nernst equation implies that cell could be created that has the same substance for anode and cathode as long as concentrations are different Electrolytic ce an electrochemical cell in which electric power is used to cause a nonspontaneous redox reaction to occur Ex 2 NaC 2 Na C2g electrolysis Since AG WW and AG nFE The maximum useful electrical work obtained from a voltaic cell is wmax nFE When an external potential Eext is applied to a cell the amount of work performed is wanEext
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