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36 Cards in this Set
- Front
- Back
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System and Surroundings
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System- body under study
Surroundings-everything else -system definitions based on mass and energy exchange with surroundings -Open systems: mass and energy exchange -Closed systems: exchange energy but not mass -Isolated Systems: exchange nothing |
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State Functions: extensive and intensive properties
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-properties that describe the macroscopic state of a system
extensive: proportional to the size of the system (volume and moles) -intensive: independent of the size of the system (pressure and temperature) |
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State Functions Definition
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properties that describe the state of the function independent of the pathway it took to get there. "pathway independent"
-three state properties, one being extensive describe the state of the system unambigouisly Opposite of state functions: path functoins ex. work + heat |
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Heat
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-"q"
-natural transfer of energy from a warmer to cooler body -any energy that is not heat is work -three forms : conduction, convection, radiation |
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Heat: Conduction
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-thermal energy transfer via molecular collisions
-requires direct physical contact -higher energy molecules transfer energy to lower energy molecules -object's ability to conduct heat = thermal conductivity K Q/t = kA[(Th-Tc)/L] or Change in Temperature= I R I = heat curren R = Resistance to heat flow -in a steady state system the rate of heat flow is consistent among any number of slabs between heat reservoirs -higher conductivity = lower temp difference across different lenghts of slabs |
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Heat: Convection
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-thermal energy transfer via fluid movements
-differences in pressures or densities drive warm fluid in direction of cold fluid -ex: ocean and air currents, air above land has quicker convection rate than air about water..rises above |
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Radiation
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- thermal energy tranver via electromagnetic waves
rate at which object radiates electromagnetic radiation: Stefan-Boltzman law: pressure=boltzman constant X emissivity of object x Surface Area X Temp^4 boltzman constant= 5.67 x 10^-8 -net rate of heat transfer: substitute T for ΔT or difference between hot and cold temperatures -emissivity: fraction of radiation absorbed and not reflected: an object that radiates heat faster also absorbs heat faster |
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Work: Thermodynamics style
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-energy transfer that is not heat
-a system may be at rest but may change its size and shape so is able to do work.."PV work" -pathway dependent w=P(ΔV) @ constant pressure -Pressure vs Volume graph: work is area under curve |
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First Law of Thermodynamics
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Change in Energy = q + w
energy of a system and surroundings is always conserved, any change in energy must be work or heat * -w if work is done by the system + w if work is done on the system |
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Heat Engines
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-a machine that converts heat to work
-this is an example of the second law of thermodynamics which says heat cannot be changed completely into work in cyclical processes ex. piston in cylinder, requires work to expand the gas but less work to compress it..you still have a positive amount of work but the point is not ALL the heat can be converted into work qh = w + qc net work entering engine equals the work done by engine (w) + heat leaving engine (qc) |
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Carnot Engine
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-most effective cyclical conversion of heat into work
-hypothetical -efficiency e = 1-Tc/Th -most efficient when there is a large difference between Tc and Th |
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Reverse heat engine
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-this is a refrigerator
-remember qh=qc + w from heat engine and second law of thermodynamics however in a fridge, qc is coming in and the fridge requires work to generate qh -more heat is generated then the heat it removes from cold reservoir |
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Thermodynamic Functions
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Seven State Functions
-U: internal energy -T: temperature -P: Pressure -V: Volume -H: Enthalpy -S: Entropy -G: Gibbs Energy |
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Internal Energy
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-all forms of energy on a molecular scale:
a) vibrational energy: created by the atoms vibrating within a molecule b) rotational energy: molecular movement where spatial orientation of the body changes while the center of mass remains fixed c) translational energy: movement of the center of mass of a molecule d) electronic energy: the potential electrical energy created by the attractions between electrons and nuclei e) intermolecular PE: energy by intermolecular forces of moving diples f) rest mass energy: energy from E=mc2 Internal energy for a group of molecules = sum of these energies |
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Internal energy in a closed system
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-no electric or magnetic fields
-we can say change in U = q + w -if there is no volume change, no work is done then Change in U =q |
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Temperature
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-zeroth law states that it exists
-measurement of how fast molecules are moving -an intensive propertiey -KE avg = 3/2 kT, formula for the average kinetic energy of a single molecule in any fluid k= Boltzmann constant 1.38 X 10^-23 |
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Pressure
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random translational kinetic energy per volume
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Enthalpy H
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-state function
-not conserved -man made property that accounts for extra capacity to do PV work H = U + PV no enthalphy values, base compounds on standard state -not constant change in heat of formation |
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standard state
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-it is the reference form of a substante at a chosen temperature T and pressure of 1 barr or 750 torr
25C = 0j/m enthalpy -using this we can assign standard enthalpies of formation |
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Standard enthalpy of formation
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change in enthalpy for a reaction that creates one mole of that compound from its raw elements in their standard state
-for any molecule in pure elemental form change in H= 0 |
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Change in enthalpy = q (heat)
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this occurs when there is constant pressure, closed system, and PV work only.
-occurs if gas is not part of reaction |
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Heat of reaction
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change in enthalpy from reactants to products
ΔHreaction= ΔHproducts - ΔHreactants |
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Hess's Law
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Sum of enthalpy changes for each step is equal to the total enthalpy change regardless of the path
-forward reaction has exactly the opposite change in enthalpy as the reverse enthalpy change is + =endothermic enthalpy change is - = exothermic |
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Graphical representation of Δ H of a reaction
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-enthalpy is loosely used with internal energy becasue of the close relationship
ΔH forward=-ΔHreverse -increase in energy is called activation energy, -peak of energy (activation energy) is transition state where old bonds are broken and new bonds are forming ΔH is difference between starting PE of reactants and ending PE of products Ea forward = difference between PE of reactants and peak/transition state Ea reverse= difference between PE of products and peak/transition state Ea forward does not equal Ea reverse but ΔH forward is same magnitude but opposite sign of Δreverse. |
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Ea forward vs. Ea reverse
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Activation energy for a forward reaction is much less than that of a reverse reaction
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Entropy S
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-nature's tendency to create the most probable situation that can occur within a system
-it is never negative, always zero or positive -Δsystem + Δsurroundings = Δuniverse -state function (entropy change of forward reaction is =-entropy change of reverse reaction -only ideal reactions are reversible,all real reactions are irreversible -extensive property (increases with amount) -increases with #moles, volume and temperature -if reaction increases # of molecules, reaction has positive entropy -things do not start magically moving like a reverse reaction bc then change in entropy would be negative |
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Second Law of thermodynamics
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-entropy of an isolated system will never decrease
-entropy not energy is the driving force which dictates whether a reaction will proceed (must increase entropy of universe to proceed) |
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Entropy and equilibrium
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-equillibrium= max entropy for a reaction
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Third Law of Thermodynamics
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-assigns a zero entropy value to any pure substance either an element or compound at absolute zero and in internal equilibrium.
-all other substances have positive entropy value (zero kelvin unattainable) |
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Gibbs Free Energy
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Δ G= ΔH - TΔS
-variables refer to systems and not surrounding -good for only constant temperature reactions -equilibrium achieved when change in Gibbs=O - negative ΔG = spontaneous reaction which requires constant temperature and pressure, PV work only, and reversible process -represents the maximum non-PV work available from a reaction -negative enthalpy and positive entropy = always spontaneous + enthalpy and - entropy = never spontaneous |
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Question 49: What are the most efficient conditions for purifying nickel when using the Mond process?
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1) first decide which side of the reaction you want to favor
2) for pressure adjust pressure to favor the amount of moles, high pressure pushes equation to side with lower moles vice versa, -for temperature decide if its exothermic or endothermic using enthalpy, and then adjust temperature using Lechatlier's principle as well |
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Question 56: Which of the following would allow a carnot engine to operate at 100% efficiency where e=1
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1) first ask yourself what you need for 100 percent efficiency: you need W/Qh to equal to 1, so top has to equal bottom
2) if W=Qh-Qc then Qc needs to be zero so w=Qh and then e=1 3) cold reservoir must be at absolute zero |
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Question 61: Which reaction is most kinetically favored
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The most kinetically favored reactions are those with the lowest Ea
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Question 62:Which reaction is most thermodynamically favored?
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The most thermodynamically favored reactions are those where the products are at a lower PE then the reactants
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Question 63: if a catalyst were added to reaction 1, what would happen?
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Remember a catalyst lowers activation energy, but dont confuse it with changing the energy between products and reactants. Catalysts dont effect equillibrium concentrations.
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Question 68: A metal rod is in thermal contact with two heat reservoirs both at constant temperature, one at 100K the other at 200K. The rod conducts 1000 J of heat from the warmer to the colder reservoir. What is the total change of entropy if no energy is exchanged with surroundings?
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1) off the bat know that entropy is positive, eliminate zero and -5.
2) Use formula deltaS= q/T 3) for 200K reservoir, s=-1000/200 = -5, for cold reservoir 1000/100 = 10 , net = +5 |
