CHM 11600 Final Exam (New Material) Study Guide
CHM 11600 Final Exam (New Material) Study Guide CHM 116
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This 5 page Study Guide was uploaded by Gayatri on Friday May 1, 2015. The Study Guide belongs to CHM 116 at Purdue University taught by Dr. Nash in Winter2015. Since its upload, it has received 156 views. For similar materials see CHM 116 in Chemistry at Purdue University.
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Date Created: 05/01/15
CHM 11600 Final Exam New Material Describe what information the First Law of Thermodynamics provides about the directionality or tendency of physical and chemical changes 0 The First Law of Thermodynamics states that the internal energy of a system which is the sum of the kinetic and potential energies of its particles changes when heat or work are absorbed or released So it accounts for energy but not directionality of a process cannot tell whether a process is spontaneous or not Describe quotspontaneousquot processes 0 A process that occurs in a system without any input of energy from the surroundings Describe what entropy is 0 Degree of randomness or disorder To predict the sign of the entropy change for the system ASsys for physical and chemical changes 0 When a phase change involves going from fewer to more microstates entropy increases e g going from solid to liquid or liquid to gas This is true even in the same physical state if the number of microstates increases e g expanding of a gas from a 1L container to a 2L container 0 As for chemical changes the as the freedom of motion increases entropy increases e g going from a solid plus a liquid to ions or going from solids to gases plus ions in solution Describe why the entropy of a gas is greater than that of a liquid and a solid and why the entropy of a liquid is greater than that of a solid 0 A liquid has more freedom of motion than a solid because the particles are not as tightly packed together and a gas has more freedom of motion than a liquid because the particles are more dispersed State the Third Law of Thermodynamics 0 As the temperature approaches absolute zero 0 K the entropy of a system approaches a constant and mnimum value The entropy of a perfect crystalline state is zero at 0 K Calculate the standard entropy change ASo o ASoreaction ASoproducts ASoreactants State the Second Law of Thermodynamics o The state of entropy of the entire universe will always increase over time and that changes in the entropy in the universe can never be negative 0 This tells us that all real processes occur spontaneously in the direction that increase the entropy of the universe sys surr To quantify entropy changes using a statistical approach and heat changes 0 Particles have certain energy levels when they are con ned in a box of changeable volume When this box is opened or expanded there are more energy levels and they are closer together giving them more options for their placement increasing entropy 0 Change in entropy of the system is equal to the heat absorbed in a reversible reaction divided by the temp at which the heat change occurs Describe the obstruction to the Second Law of Thermodynamics that makes life possible 0 This obstruction is referred to as activation energy Ea This is the amount of extra energy needed to be put into any set of chemicals to initiate the reaction This needs to be added even though the chemicals may contain much more energy than the products they would form Describe the meaning of a positive value and a value of zero for ASuniv o ASuniv gt 0 means that a spontaneous change occurred and the entropy of the system decreases only if the entropy of the surroundings increases even more 0 ASuniv 0 for a system at equilibrium and here ASsys 2ASsurr Describe the change in free energy of the system for a physical or chemical process in terms of the changes in enthalpy and entropy of the system A lith Frpp pnprov FA id 2 tprrn 111an with Hop nnmhinn rinn nFn QVQ rpm q pn rhnlnv and pn rrnnV39 Describe the meaning of a positive value a negative value and a value of zero for AG AGo o AGo lt 0 for a spontaneous process 0 AGo gt 0 for a nonspontaneous process 0 AGo 0 for a process at equilibrium Describe the relationship between the change in free energy and the maximum amount of work that can be done by the system 0 AG is the maximum useful work done by a system during a spontaneous process at constant T and P 0 AG wmax 0 Similarly AG is the minimum useful work done to a system to make a nonspontaneous process occur at constant T and P Describe why a physical or chemical change is spontaneous only if AG is negative 0 Looking at the equation AGsys AHsys TASsys we know that TASuniv AGsys and since absolute temperature is always positive the sign of AG depends on entropy AS of the system Therefore if AG is negative then AS has to be positive therefore a physical or chemical change is spontaneously only if AG is negative Calculate standard free energy changes by using standard free energies of formation 0 AGorxn AGoproducts AGoreactants Describe why a system is at equilibrium if AG is equal to zero 0 A system is at equilibrium if there is no net change in the entropy of the universe ASuniv 0 and since TASuniv AGsys this makes AG is equal to zero 0 wmax is also equal to zero at equilibrium because there is no work needed since the reaction is not proceeding spontaneously or nonspontaneously Describe the difference between AG and AGo o AGo is the standard free energy change which occurs when all of the components of the system are in their standard states while AG is the free energy of a particular reaction with specific conditions and it varies with variations in variables Describe why chemical reactions occur 0 Chemical reactions take place because different types of molecules come in contact and interact with others and as a result new substances are created A lot of these occur naturally and are not man made They occur due to the breaking of old bonds between molecules and formation of new ones when molecules collide with each other Driving forces that are responsible for physical and chemical changes 0 The main driving force for any chemical reaction is the urge to move toward a greater stability of atoms and molecules To do this atoms tend to react in such a way that lowers their energy to make them more stable and increases their entropy Describe the key chemical event in an oxidationreduction reaction 0 In a redox reaction the main event that occurs is the movement of electrons from one chemical species to another Assigning oxidation numbers to atoms in elements compounds and ions 0 Each pure element has ON 0 For monoatomic ions the ON their charge Flourine is always ON l in compounds Cl Br and I are always ON l in compounds except with O or F H is usually ON l in compounds with exceptions O is usually ON 2 in compounds with exceptions o The sum of ON 0 for compounds unless they have a charge Describe oxidation and reduction A nvirlofinn 1390 flan Inca n F nlnnfrnno 111Man nnno 11n OOOOO Determine whether or not a chemical reaction is an oxidationreduction reaction 0 If the oxidation numbers in a reaction change as it proceeds then it is an oxidationreduction reaction if the same numbers of electrons lost by one reactant are gained by another Identify species that are oxidized and or reduced and identify those species that are oxidizing agents andor reducing agents in an oxidationreduction reaction 0 In a reaction the species that loses electrons ON decreases is being oxidized and is the reducing agent 0 The species that gains electrons ON increases is being reduced and is the oxidizing agent 0 The reducing and oxidizing agents on the left side of the reaction are always stronger than the right side Balancing oxidationreduction reactions in neutral acidic or basic solution by using the halfreaction method 0 Separate the reaction into two half reactions one reduction and one oxidation 0 Balance the mass and charge in each half reaction Balance atoms besides O and H Balance 0 by adding H20 Balance H by adding H Balance charge with e 0 Multiply each half reaction to get same number of electrons in each one and add them together 0 If the solution is basic add OH to it for every H to neutralize the reaction Describe an overall oxidationreduction reaction in terms of a balanced oxidation halfreaction and a balanced reduction halfreaction 0 An overall oxidationreduction reaction is the sum of two half reactions a reduction reaction and an oxidation reaction When the two are combined extra entities get cancelled and we are left with the overall reaction which shows a net transfer of electrons from one species to another Describe why it is necessary that a salt bridge or porous disk be added to a voltaic cell in order for the cell to produce an electrical current 0 The salt bridge in a voltaic cell is needed to maintain the charge neutrality of the two half cells by letting the ions ow in between them with minimal mixing of the solution so the reaction can be drawn out rather than letting it go by really fast and reaching equilibrium right away List and describe the components needed for the operation of a voltaic cell 0 A voltaic cell converts the free energy change of a spontaneous reaction into the kinetic energy of electrons moving through an external circuit electrical energy This energy is proportional to the different in electric potential between the two electrodes called the cell potential Electrodes an anode and a cathode Two solutions A salt bridge 0 A wire conduction between the solution Identify the anode and cathode and the direction of electron ow in a voltaic cell 0 The anode is the part of the cell where oxidation takes place 0 The cathode is the part of the cell where reduction takes place 0 Electrons ow from the anode to the cathode Describe cell potential as a measure of the potential of a voltaic cell to do work on the surroundings 0 Cell potential refers to the energy available to do the work of moving a charge electrons between electrodes 0 SI Unit of electric potential is volt V and that of electric charge is the coulomb C o If two electrodes differ by 1 volt of electric potential 1 joule of energy is released lJ work can be done for each C of charge that moves between the electrodes So 1 V l JC OOO improves kinetics of the reaction and it adsorbs hydrogen well at its surface It also catalyzes the reaction of proton reduction Describe how halfcell potentials are experimentally measured 0 Usually when we want to nd an unknown standard electrode potential we construct a voltaic cell consisting of the unknown half cell and a known half cell for reference The standard reference half cell is the standard hydrogen electrode which consists of a platinum electrode that has hydrogen has at 1 atm in it and is immersed in l M strong acid H The cell potential of this is 000 V Since it is zero the overall standard cell potential of the reaction gives us the standard cell potential of the unknown substance To predict whether or not a given reaction oxidationreduction or otherwise is spontaneous under non standard and standard conditions by using a Table of Standard Reduction Potentials o For standard conditions a spontaneous reaction E cell gt 0 will occur between an oxidizing agent and any reducing agent that lies below it on the table So the half reaction higher in the list acts as the cathode and gets reduced and the lower one acts as the anode and gets oxidized This ensures that the stronger OA and stronger RA will be the reactants o For nonstandard conditions we can still write a spontaneous reaction by making sure the following criteria are followed One of the half reactions must be reversed so that E cell gt 0 BUT the signs should not be reversed Make sure that the stronger OA and RA appear on the left as reactants the larger the E value the stronger the 0A The smaller the E value the stronger the RA Calculate the standard cell potential for an overall voltaic cell reaction using a Table of Standard Reduction Potentials o E cellrxn E cellcathode E cellanode Nemst equation to calculate cell potentials under nonstandard conditions 0 Ecell E cell RTnFan Describe concentration cells and how they are constructed 0 A concentration cell uses the same solution in both half cells but with different concentrations 0 It generates electrical energy through the simple spontaneous change that occurs when you mix a concentrated solution with a dilute solution of it to create a final solution that has an intermediate concentration Calculate the cell potential for a concentration cell 0 E cell for a concentration cell is zero because the standard electrode potentials are both based on the same substance so they cancel each other out Describe why metals such as aluminum and copper corrode more slowly than iron 0 Metals such as Al and Cu corrode slower than iron because even though they do go through oxidation rusting just like iron aluminum for example forms a thin hard coating on itself when it is converted into aluminum oxide This sticks on the metal and prevents further oxidation and corrosion As for the iron it akes off and corrodes easily Describe galvanizing and cathodic protection and how these methods are used to prevent corrosion o Corrosion of a metal such as iron decreases when iron behaves as the cathode o In cathodic protection the most effective way to prevent corrosion is by letting iron make contact with a more active metal one that is a stronger reducing agent lower than iron on the list such as zinc Thus the iron acts as the cathode and stays intact while the zinc acts as the anode and loses electrons o Galvanizing is when steel is coated with a sacrificial layer of zinc and the zinc is sacrificed oxidized instead of the iron 0 Mg and A1 are often used for this purpose Describe the difference between an oxidation potential and a reduction potential 0 Standard oxidation potential is the tendency for a species to be oxidized at standard conditions and Describe what information the cell potential provides about the speed of the oxidationreduction reaction 0 The larger E cell is the faster the reaction will proceed because more electricity is generated Describe the differences between voltaic and electrolytic cells 0 Voltaic cells use a spontaneous reaction to generate electrical energy Here the difference in free energy between higher energy reactants and lower energy products is converted to electrical energy 0 Electrolytic cells use electrical energy to drive a nonspontaneous reaction Here an external source supplies free energy to convert lower energy reactants into higher energy products Faraday s Law of Electrolysis the amount of substance consumed or produced at one of the electrodes in an electrolytic cell is directly proportional to the amount of electricity that passes through the cell To perform calculations for electrolytic cells e g time amount of substance amount of charge etc 0 To perform calculation for electrolytic cells we need to be mindful of the following variables Mass of the substance solid m Molar mass of the substance M The current electric charge supplied Q use conversion factor if It Q where I is coulumbs per second and t is time converted into seconds Time taken for solid to form t Moles of electrons transferred in reaction 96485 Cmol e 0 To find mass of a substance that is formed use the formula QMnF 0 To find time taken for a given mass of a solid to form use the formula t mnFIM Describe the construction and general operation of a lead storage battery used in automobiles 0 Lead acid batteries are constructed of several single cells connected in series It consists of one positive plate covered with a paste of lead dioxide and there is an insulator between them The plates are inside a plastic case and submerged in an electrolyte solution of water and sulfuric acid
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