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Week 12 Book Notes

by: Cassidy Zirko

Week 12 Book Notes Chem 143

Cassidy Zirko

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About this Document

Covers Entropy, Gibbs Free Energy and Redox reactions
General Chemistry 2
Dr. Cracolice
Class Notes
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This 6 page Class Notes was uploaded by Cassidy Zirko on Sunday April 17, 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 10 views. For similar materials see General Chemistry 2 in Chemistry at University of Montana.


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Date Created: 04/17/16
Week 12, Chem 143, Prof. Cracolice  Chapter 67: How is Energy Spread out Within Molecules? 4/11/16 67.1 How is Energy Spread out Within Molecules?  Translational energy­ molecule moving from location to another within its container,  energy of random motion   Rotational energy­ energy from tumbling or rotating   Vibrational energy­  diatomic molecule stores energy by vibrations within the bond,  similar to a spring stretching and contracting between 2 nuclei   Electronic energy­ attraction among the electrons and the nuclei and the repulsions  among the electrons of the molecule    All four times of energy are quantized­ set levels of energy for that molecule   Translational­ very small gaps between levels, rotational­ slightly more spread out but  still small gaps   Vibrational­ very large gaps, electronic­ extremely large gaps (usually at ground level of  energy)   Spectroscopy­ study of interaction of electromagnetic radiation with matter   Translational and rotational studies with microwaves, vibrational with inferred, electronic with visible and UV radiation  ∆ E=hv  , the change in energy is equal to planks constant multiplied by frequency   Total thermodynamic energy­ sum of all of the 4 energy types   Energy at particulate level  number of molecules in each energies are 1 for each type of  energy   How does Molecular Energy Relate To Entropy?  o Macroscopic level analysis of entropy­ pressure, volume, number of particles and  temperature  o Microscopic level analysis  considering behavior of particles  energy state of  each type of molecule for each molecule  o Total macroscopic energy­ total number of quantum states that correspond to  macroscopic energy of system   W­ the number of different combinations of particulate quantum states  o Boltzmann Equation­ S=KlnW, k­Boltzmann’s constant  o Assumption: all quantum states are equally probable  o Change in entropy depends on number of quantum state changes:  ∆ S=Kln ( W final inital     How is W determined? o Set up particulate level model of system  o One dimensional solids­ only consider horizontal motion of atoms   Atoms move left and right as bonds vibrate  Week 12, Chem 143, Prof. Cracolice   Vibrations are restricted to evenly spaced energy levels  o Large  ∆ S+¿  for a reaction means statistically favored reaction  o Most probable arrangement is one that represents the greatest number of quantum  states   How can we Make Qualitative Predictions About Entropy Changes? o An increase in entropy is an increase in molecular level energy dispersion and  vise versa  o Entropy increases when a solid or liquid changes to a gas   Gas has higher motional order, less energy levels to more energy levels  o Entropy decreases when a gas is dissolved in water   Gas with large spaces in between particles, great deal of energy  o Entropy increases with increasing mass   Heavier particle has more closely spaced quantum energy levels o Entropy increases with increasing number of ions in an otherwise similar ionic  compound and increasing number of atoms in otherwise similar molecular  compounds   More particles, more ways the compound’s energy can be organized  o Entropy increase when a solid or a liquid is dissolved in water   Energy levels become more spread out  o Entropy increases with increase temperature   At a high temperature there are many different speeds at which molecules  are moving  temperature increases, translational energy changes from a  small amount to a large amount   How Small can W become? o Entropy can be state on an absolute scale  o Third law of Thermodynamics­ the entropy, S, of a perfect crystalline solid of a  pure substance is zero at an absolute zero of temperature  o Only one such configuration of energy for a crystalline solid  ∆ S ° 67.2 How are  r  Values Determined?  Same as change in enthalpy: The sum of the enthalpy multiplied by the moles of each  product subtracted from the sum of the enthalpy multiplied by the moles of each of the  reactants   At a low temperature­ entropy change is independent of temperature   At °hich change occurs pressure dependent for reactions with gases   S  is 0 for all elements in an elemental state  How do Entropy Changes Compare for Reversible and Irreversible Processes? V 2 o ∆ S=Rln ( ) , R­ 8.315J/mol*K because entropy is measured in J/K V 1 Week 12, Chem 143, Prof. Cracolice  ∆ S>0 o V2 will be greater than V1,    o Spontaneous Irreversible­  the constant temperature expansion of an ideal gas  which results in an increase in entropy of a system and the surroundings  o Spontaneous Reversible­  the constant temperature expansion of an ideal gas that results in no change in entropy in a system and its surroundings  o Change in entropy is a state function­ it only depends on the final and initial states of the system  o Irreversible process at a constant temperature   heat supplied to surroundings ∆S = −q irreversible o Surroundings stored reversible to original state  changed T o q irreversiblent of heat absorbed by the system  q ∆ S> irrsoT ∆S>q irrev o because process is irreversible  T   o irreversible process,  ∆ S>0 for the system and surroundings,   T ∆S>q irre  o reversible process,  ∆ S=0  for system and surroundings,  T ∆S=q irrev  Chapter 68: How Can Reaction Spontaneity be Predicted? 4/13/16 68.1 How does Reaction chemical Potential Predict Spontaneity?   Need to know the 2 forces of spontaneity: 1) Minimization of energy 2) Maximization of  energy   Gibbs Free energy­ net effect of energy working together or against each other being  expressed in a thermodynamic state   Reaction chemical potential is zero at the point of minimum gibbs free energy (at  equilibrium)   Extent of reaction is the reaction progress of the reaction   Spontaneous reaction direction is toward equilibrium concentrations when extent of  ∆ G <0 reaction is less than the equilibrium,   r  Negative slope, positive chemical forces pushing reaction toward products= spontaneous   Positive slope, negative chemical forces pushing reaction toward reactants=  nonspontaneous   All points along curve to the right of zero reaction potential point along extent of reaction axis will have a negative chemical potential  o Spontaneous reaction direction toward formation of reactants   + slope= ­ chemical forces= toward reactants  ­ slope= + chemical forces= towards products Week 12, Chem 143, Prof. Cracolice  68.2 How is Gibbs Free Energy Related to Other Thermodynamic Quantities?  Amount of heat transferred between system and surroundings must be equal with  ∆H opposite signs:  ∆S total S system T  d ∆ G ­ change in free energy,  ∆ G=∆H−T ∆S  only used for a reaction in solution at constant temperature and pressure   Gibbs free energy represents the thermodynamic state of a system by combing enthalpy,  entropy and temperature  ° ° °  Standard state of Gibbs free energy of a reaction:  ∆ r =∆ H −r ∆ S r ∆ H ∧∆ S ° o Need to find both  r r as learned previously  o S in J/K= this is why you have to multiply by temperature in kelvin and then by  1kJ/1000J o Always watch units, they will lead you to where to go   ∆ G ° ­ change in free energy for a reaction in which  pure, stable elements react (at a  pressure of 1 bar if gases) to form 1 mole of products  68.3 What is the Relationship between Free energy and the Equilibrium Constant? ∆ r °  ­ conversion of pure reactants to pure products (idealized reaction)  ∆ G  Non idealized reaction­  r  slope of line at any point in the reaction   Reactant quotient Q­ same form as equilibrium constant for concentration or pressure  o Q/K­ both are dimensionless and effected by temperature  °  ∆ G=∆ G +rTlnQ  because RTlnQ is the adjustment for a reaction of non pure  substances  °  At equilibrium  ∆ r =0    ∆ G=−RTlnQ  or  ∆ G=−RTlnK p ¿∆n(gases)  K pindicating the pressure equilibrium  K =K pRT c   68.4 What is the Role of Temperature in Predicting Reaction Spontaneity?  °  ∆ r <0  reaction is spontaneous  ∆ r >0   reaction is nonspontaneous  ° ° °  T in  ∆ r =∆ H −r ∆ S r  is absolute  always positive and in Kelvin  ∆rH ∧∆ S r ° ∆ r °  1­ when   have opposite signs,   is independent of temperature  Week 12, Chem 143, Prof. Cracolice  −∆ Hr∧+∆ S r ° −∆ Gr ° o   and reaction is spontaneous in the forward  direction  +∆ Hr∧−∆ S r ° +∆ r ° o     and reaction is spontaneous in reverse direction  ° ° °  2­ when  ∆ r ∧∆ S r are both negative   ∆ r  is temperature dependent  ° o At high temperature   −∆ Gr  and reaction is spontaneous in the forward  direction  ° o At low temperature   +∆ r  and reaction is spontaneous in the reverse  direction  ° ° °  3­ when  ∆ r ∧∆ S r are positive   ∆ r  is temperature dependent  +∆ G ° o At high temperature   r  and reaction is spontaneous in the reverse  direction  −∆ G ° o At low temperature   r  and reaction is spontaneous in the forward  direction  Chapter 69: What are Electron Transfer Reaction? 4/15/16 69.1 What is a Voltanic Cell?  Voltanic Cell­ spontaneously generates flow of electrons as a result of a chemical change  Salt bridge­ has electrolyte where ions are not involved in the net chemical change   The greater the flow of electrons thourught the circuit  the stronger the current will be   Oxidation­ loss of electrons from  an anode to the  Half reaction­  singular oxidation/reduction reaction because both have to be present for  a reaction to occur   Reduction­ gain of electrons   Half reaction equations­ when combined, the amount of electrons will cancel on from  each of the oxidation and reduction reaction to create the final equation   Electron transfer reaction­ electrons transferred from one substance to a second  substance   Number of electrons lost by one species must be equal to the amount of electrons gained  by the second substance   How do Chemists Keep Track of Electrons in Electron Transfer Reactions? o Procedure for oxidation numbers:   1) oxidation number for any elemental substance is zero   2) oxidation number for a monatomic ion is the same as the charge of the  ion  Week 12, Chem 143, Prof. Cracolice   3) Oxidation number of combined oxygen in ­2, except in peroxides (­1),  superoxide’s (­1/2) or OF 2(+2)  4) Oxidation number of combined hydrogen is +1, except when  monatomic   5) any molecular or ionic species, the sum of the oxidation number of all  the atoms in a formula is equal to the overall charge of the formula  o Oxidation half reaction­ oxidation number of a substance increases  o Oxidation is also thought to be the increase of oxidation number due to the loss of electrons  o Reduction is a decrease in oxidation number, oxidation number is getting more  negative because of the addition of electrons  o One element in a redox reaction must increase in oxidation number and the other  must decrease  o Hints and tips  Elemental state must change: element on one side of equation  oxidation  number  is zero so the other substance’s oxidation number is not zero   Non elemental form H= +1 and O=­2, unless the elemental form of  oxygen is on one side of the equation, the oxidation number of oxygen  wont change   Group 1A/1 and 2A/2 elements only have one oxidation state besides zero  when not appearing as an element  their oxidation state wont change  o Sulfur oxidation state is +6  69.2 How do Simple Batteries Operate?  Electrical energy/electrical work is expended or absorbed as charged particles move  through a circuit   If one joule of energy is required or released in moving one coulomb of charge from one  point to the other in a circuit, the potential energy difference is one volt (J/c)  Volts sometimes considered emf or electromagnetic forces   Anode­oxidized­ substance that loses electrons (negatively charged)  Cathode­ reduced­ substance that gains electrons (positively charged)  Oxidizing agent­ species that accepts electrons and is being reduced   Reducing agent­ species that donates the electrons and is being oxidized   Species oxidized is the reducing agent   Species reduced is the oxidizing agent 


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