Week 13 Book Notes
Week 13 Book Notes Chem 143
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This 5 page Class Notes was uploaded by Cassidy Zirko on Sunday April 24, 2016. The Class Notes belongs to Chem 143 at University of Montana taught by Dr. Cracolice in Spring 2016. Since its upload, it has received 8 views. For similar materials see General Chemistry 2 in Chemistry at University of Montana.
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Date Created: 04/24/16
Week 13 Chem 143, Prof Cracolice Chapter 70: How Are Electron Transfer Reaction Equations Written? 4/18/16 70.1 How are Electron Transfer Reaction Equations Written? 1. Balance Oxidation numbers a. Identify the elements that are being oxidized and reduced base on the equation. Then write half reactions for the oxidation and reduction reactions b. Balance element that is being oxidized or reduced. Make sure there are the same number on each side before you do anything with oxidation numbers c. Balance the elements other than hydrogen or oxygen. These elements are the ones that don’t have a variation on oxidation number. This keeps the equation balanced d. Determine how the oxidation number changes. This tell you how many electrons are being produced in the half reaction. Remember if you have multiple of the element that is being oxidized or reduced that will affect the number of electrons that are being added to or produced e. Add the electrons so that the net oxidation number is the same on both sides of the equation 2. Balance Charges a. Add hydrogen ions (H ) when the reaction is occurring in an acidic solution. These will be added to the oxidation reaction and you are looking to increase the charge to make it equal. Ex. if the charge on one side is 2 and the charge on the other side is +5 then you will want to had 7H to the side of the equation with 2. This will make both sides of the equation have the same net charge. Also, when you are balancing your oxygens, the number of hydrogens will come based on the number of waters are needed in the equation b. Add hydroxide ions (OH) when the reaction occurs in a basic solution. This is the same procedure as 2a but you are working to making the net charge of the reaction more negative 3. Balance oxygen and hydrogens a. Add water molecules to balance the oxygens. This works because water has a net charge of zero so it doesn’t manipulate all the work you just did to make the charges balanced. When accounting for the number of oxygens, be sure to account for oxygens that are in other compounds in the reaction b. Check the number of hydrogens on each side. If you have done all the previous steps right then your amount of hydogens should already be equal 4. Check a. Final step is to check both the atom and the charge balance, both should be equal on both sides of the equation Week 13 Chem 143, Prof Cracolice Chapter 71: Why do Electrons Flow Through Voltanic Cells? 4/19/16 71.1 Why do Electrons Flow through Voltanic Cells? Strong oxidizing agent strong attraction for electrons Weak oxidizing agent attracts electrons only slightly Strong reducing agent releases electrons readily Weak reducing agent holds on to its electrons pH meter can be used to measure the electrical potential (voltage) fluorine is an extremely strong oxidizing agent so it is very easily reduced any element in the activity series will react with and replace dissolved ions of any element beneath the one in question double arrows indicate that the reaction is reversible, more redox reactions in solution forward reaction when reversible reaction equation is read from left to right reverse reaction when the reversible reaction equation is read from right to left strong oxidizing agents take electrons from strong reducing agents creating weaker oxidizing and reducing agents reversible reaction occurs simultaneously acids have the ability to release hydrogen gases upon reactions with certain metals metals that release hydrogens are reducers voltage or potential depends on temperature of system standard state conditions variable reactions that occur at arbitrary standards causes standard electrode potential, E° , at 25℃ , having pure elemental electrodes, gases at 1 bar pressure, solutions with one mole of its respective ions standard hydrogen electrode standard state potentials related to an arbitrary amount of voltage reduction of the hydrogen gas to hydrogen ions ° standard reduction potential E between the standard hydrogen electrode and the second electrode ° + E spontaneous in forward direction ° E spontaneous in reverse direction The sign of E ° tells which direction the reaction will spontaneously occur Voltages are not proportional to the quantity of chemicals involved It is not energy per electrons, no need to multiply E by a factor How do Electron Transfer reactions compare with Proton Transfer Reactions? o Acid/base reaction uses transfer of proton reactions, redox reactions transfer electrons Week 13 Chem 143, Prof Cracolice o Special names to each of the species involved, acid and base vs. oxidizing and reducing agents Acid proton donor, base proton acceptor Oxidizing agent electron acceptor, reducing agent electron donor o Certain species can act as both electron donor and acceptor o Acids/bases and oxidizing/reducing agents can be classified as weak or strong based on attraction tendencies o Acid/base reactions and redox reactions have a state of equilibrium in which direction of reaction can be predicted Chapter 72: What is the Relationship among Gibbs Free Energy, the Equilibrium Constant, and Electromotive Force? 4/22/16 72.1 How is Gibbs Free Energy Related to Electromotive Force? Electrical work is done by a voltaic cell which is the product of the quantity of charge (coulombs) transfer and voltage o w eletlombs∗volts= joules Electrochemical cell coulombs of charge found by letE where n= number of moles of electrons, FFaraday constant=69,485.34 coulombs/mole of e, E cell potential w elet ∆ G=−nFE ∆ rG°=−nFE ° Maximum work and (this second one is only at standard conditions) ∆ rG°=−nFE ° Make sure to convert to kJ/mol when using 72.2 How is Equilibrium Constant Related to Electromotive Force? ∆ rG°=−RTlnK+RTlnQ ∆G°=−nFE we get that −nFE=−RTlnK+RTlnQ RT RT Nernst Equation E= nF lnK− nF lnQ .0257 at standard conditions the = n lnK K can help confirm the direction of spontaneity of the system Large K value procceds in the forward direction Small K value proceeds in the reverse direction How do Nonstandard Concentrations Affect Cell Voltages? Week 13 Chem 143, Prof Cracolice E=E°− .0257lnQ o n for nonstandard conditions but still at 25 Chapter 73: How is Electrochemistry Applied in Modern Society? 4/25/16 73.1 How do Batteries and Fuel Cell Work? Primary Batteries based on reactions that are difficult to reverse, it is dead and done Secondary batteries based on reactions that are easily reversed, rechargeable How does an Alkaline Battery Work? o Electrolytes is potassium hydroxide creating a basic solution How does a Calculator or Watch Battery Work? o Button battery alkaline battery, being small has advantages o Anode has oxidation of powdered zinc o Electrons reduces silver oxide rather than the magnesium oxide How Does an Automobile Battery work? o Needs to produce a very large amount of energy and needs to be rechargeable o Most cost efficient – 12V lead acid battery o Has 6 cells, lead is oxidized, electrons reduce lead(IV) at cathode How Does a Lithium Ion Battery Work? o Cell phones, laptops computers, digital cameras o High energy to weight ratio o Can be recharged while holding a full charge o Anode is a form of carbon, cathode a metal ion o Electrolyte is lithium ion How Does a Fuel Cell Work? o Fuel Cell electrochemical cell that converts energy of a fuel into electricity o Both fuel cell and oxidizing agent are continuously supplied to cell o Hydrogen is fuel and oxygen is oxidizing agent o Key point: proton exchange membrane, thin, semipermeable membrane that allows passage of hydrogen ions but not electrons ( proton exchange membrane) o Hydrogen gas moves into cell on left, into anode o Electrons pass through electric current powers electronic devices o Major advantages: high energy efficiency, lack greenhouse gas emissions o Hydrogen acts as an energy storage, not an energy source 73.2 Why do Metals Corrode and How can it be Prevented? Corrosion oxidization of a metal by a substance in the environment producing an unwanted product Electron transfer reaction forming rust varies with environment Week 13 Chem 143, Prof Cracolice How can Corrosion be Prevented? o Corrosion protection can be protective coatings o Pain prevents direct contact between steel o Galvanizing practice that adds zinc coating to steel or iron o Zinc is more reactive iron acts as anode in electrochemical cell 73.3 How Does an Electrolyte Cell Operate? Electrolyte ionic solution in where electrode are immersed into Electrodes are connected by wires Electronic current flows through metallic parts of battery Anode has oxidation, has a negative charge Cathode has reduction, has positive charge Basic units of energy, ampere, volt, ohm, watt, coulomb, joule Coulomb quantity of electric charge, symbolize by the letter C, Ampere is rate of flow of charge, measured in coulombs/sec When multiplied by time it changes just to coulombs Faraday, F quantity of charge current by one mole of electrons How Does Quantity of Electric Charge Relate to the Mass of Metal Deposited in an Electrolytic Cell? o Faraday’s Law of Electrolysis quantity (mass) of an elemental substance released or deposited in electrolysis is proportional to the quantity of electrical charge that has passed through the system How Is Mass of Metal Deposited for an Electrolytic Process? o Half reactions can be used in stoichiometry problems o Quantity of charge amperes*seconds converted to moles of electrons with 9.65*10 A*S
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