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Purdue - CHM 116 - Study Guide - Midterm

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Schools > Purdue University > Chemistry > CHM 116 > Purdue - CHM 116 - Study Guide - Midterm

Purdue - CHM 116 - Study Guide - Midterm

School: Purdue University
Department: Chemistry
Course: CHM 116
Professor: Nash
Term: Winter 2015
Tags:
Name: CHM 116 Final Exam New Material
Description: Final exam new material
Uploaded: 05/02/2016
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background image 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. 
o  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 "spontaneous" processes  o  A process that occurs in a system without any input of energy from the surroundings  • Describe what “entropy” is  o  Degree of randomness or disorder  • To predict the sign of the entropy change for the system (ΔSsys) for physical and chemical changes  o  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). 
o  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 
o  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   o  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 (ΔSo)   o  ΔSo(reaction) = ΔSo(products) – ΔSo(reactants)  • 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.  o  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”  o  Particles have certain energy levels when they are confined 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. 
o  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  o  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 ΔSuniv  o  ΔSuniv > 0 means that a spontaneous change occurred and the entropy of the system decreases only if  the entropy of the surroundings increases even more.  o  ΔSuniv = 0 for a system at equilibrium, and here. ΔSsys = 2ΔSsurr  • 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.  
o  Gibbs free energy (G) is a term made with the combination of a system’s enthalpy and entropy:  
o  ΔGsys = ΔHsys – TΔSsys 
background image • Describe the meaning of a positive value, a negative value, and a value of zero, for ΔG (ΔGo)  o  ΔGo < 0 for a spontaneous process 
o  ΔGo > 0 for a nonspontaneous process 
o  ΔGo = 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 
o  ΔG is the maximum useful work done by a system during a spontaneous process at constant T and P: 
o  ΔG = w(max) 
o  Similarly, ΔG 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 ΔG is negative  o  Looking at the equation ΔGsys = ΔHsys – TΔSsys, we know that –TΔSuniv = ΔGsys, and since absolute  temperature is always positive, the sign of ΔG depends on entropy, ΔS of the system. Therefore, if ΔG is 
negative, then ΔS has to be positive, therefore a physical or chemical change is spontaneously only if 
ΔG is negative. 
• Calculate standard free energy changes by using standard free energies of formation   o  ΔGo(rxn) = ΔGo(products) – ΔGo(reactants)  • Describe why a system is at equilibrium if ΔG is equal to zero  o  A system is at equilibrium if there is no net change in the entropy of the universe (ΔSuniv = 0) , and  since –TΔSuniv = ΔGsys, this makes ΔG is equal to zero.  o  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 ΔG and ΔGo  o  ΔGo is the standard free energy change, which occurs when all of the components of the system are in  their standard states, while ΔG is the free energy of a particular reaction with specific conditions, and it 
varies with variations in variables. 
• Describe why chemical reactions occur  o  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 force(s) that are responsible for physical and chemical changes   o  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 oxidation-reduction reaction 
o  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  o  Each pure element has O.N.  = 0 
o  For monoatomic ions, the O.N.  = their charge 
o  Flourine is always O.N. = 1 in compounds 
o  Cl, Br, and I are always O.N. = 1 in compounds except with O or F 
o  H is usually O.N. = 1 in compounds (with exceptions) 
o  O is usually O.N. = -2 in compounds (with exceptions) 
o  The sum of O.N. = 0 for compounds, unless they have a charge 
•  Describe “oxidation” and “reduction”  o  Oxidation is the loss of electrons (number goes up) 
o  Reduction is the gain of electrons (number goes down) 

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School: Purdue University
Department: Chemistry
Course: CHM 116
Professor: Nash
Term: Winter 2015
Tags:
Name: CHM 116 Final Exam New Material
Description: Final exam new material
Uploaded: 05/02/2016
5 Pages 81 Views 64 Unlocks
  • Better Grades Guarantee
  • 24/7 Homework help
  • Notes, Study Guides, Flashcards + More!
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