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44 Cards in this Set
- Front
- Back
Thermodynamics
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Study of energy and its relationship to macroscopic properties of chemical systems
divides universe into 2: 1. surroundings 2. system |
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System
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macroscopic body under study
3 types: 1. open 2. closed 3. isolated |
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Surroundings
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everything other than macroscopic body under study (system)
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Open system
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exchange both mass and energy with surroundings
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Close system
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exchange energy but do not exchange mass with surroundings
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Isolated system
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doesn't exchange mass or energy with surroundings
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Extensive properties
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chance with amount
Examples: volume (V) and # of moles (n) describes macroscopic state of a system proportional to size of system |
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Intensive properties
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describes macroscopic state of a system
independent of size of system examples: Pressure (P) and temperature (T) |
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State functions
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properties that describe the state of a system
pathway independent state properties describe the state of a system change in a state property going from one state to another is the same regardless of the process via which the system was changed 3 state properties describe the state of a system (one being extensive property) unambiguously |
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2 ways to transfer energy between systems
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1. Heat (Q)
2. Work (W) |
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Heat (Q)
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Natural transfer of energy from a warmer body to a cooler body
3 forms: 1. conduction 2. convection 3. radiation movement of energy via conduction, convection or radiation; always from hot to cold |
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Work (W)
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any energy transfer between systems that is not heat
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Conduction
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thermal energy transfer via molecular collisions
requires physical contact higher energy molecules of one system transfer some of their energy to lower energy molecules of other system via molecular collisions |
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Energy flow
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heat flow:
change in T = IR T: temperature I: heat current (Q/t) R: resistance to heat flow Current flow: V = IR V: voltage I: electrical current R: resistance to electrical flow Fluid flow: change in P = QR P: pressure Q: heat R: resistance to flow thicker conduits = greater flow longer conduits = less flow |
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Convection
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thermal energy transfer via fluid movements
differences in pressure or density drive warm fluid in direction of cooler fluid examples: ocean and air currents |
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Radiation
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thermal energy transfer via electromagnetic waves
only type of heat that transfer though a vacuum |
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PV work
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system at rest, no gravitational potential energy and no kinetic energy may still be able to do PV work
at constant pressure, work is equal to pressure multiplied by change in volume W = P(change in V) [at constant pressure] No PV work is done if volume is constant PV work takes place when a gas expands against a force regardless of whether or not the pressure is constant |
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1st Law of Thermodynamics
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energy of system and surroundings is always conserved
any energy change to a system must equal heat flow into system + work done by system Change in E = Q + W Work done on the system is positive and work done by the system is negative |
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2nd Law of Thermodynamics
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Heat cannot be changed completely into work in a cyclical process
Qh = W + Qc Qh: heat entering engine W: work Qc: heat leaving engine |
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Carnot's efficiency
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e = 1 - (Tc/Th)
e: efficiency Tc: temperature of cold reservoir Th: temperature of hot reservoir |
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7 state functions
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1. internal energy (U)
2. temperature (T) 3. pressure (P) 4. volume (V) 5. enthalpy (H) 6. entropy (S) 7. Gibbs free energy (G) |
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Internal energy (U)
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all possible forms of energy imaginable on a molecular scale
examples: vibrational, rotational, translational, electronic, intermolecular potential and rest mass energy heat energy, thermal energy and or heat |
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Temperature
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thermodynamic property described by zeroth law:
2 systems in thermal equilibrium with 3rd system are in thermal equilibrium with each other any increase in thermal energy, increases temperature KE = (3/2)kT KE: kinetic energy T: temperature k: boltzmann constant (1.38e-23 J/K) measurement of how fast molecules are moving or vibrating |
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Enthalpy (H)
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H = U + PV
H: enthalpy (joules) U: internal energy PV: Pressure x Volume not conserved, constantly changing Enthalpy is a state function and en extensive property Depends only on temperature Change in H = (change in U) + P(change in V) [constant pressure] |
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Standard State
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do not confuse with STP (standard temperature and pressure)
an element at its standard state at 25 degrees Celsius is arbitrarily assigned an enthalpy value of 0 J/mol Any chosen temperature 1 bar of pressure = 750 torr = 10^5 Pascals |
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Reference form
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standard state for a pure solid or liquid
any chosen temperature 1 bar of pressure = 750 torr = 10^5 Pascals form that is most stable at the values |
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Standard enthalpy of formation (change in Hf)
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Change in enthalpy for a reaction that creates 1 mole of that compound from its raw elements in their standard state
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Change in enthalpy at constant pressure
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Change in H = Q
constant pressure, closed system at rest, PV work only H: enthalpy Q: heat |
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Heat of reaction
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change in enthalpy from reactants to products
Change in Hreaction = (Change in Hfproducts) - (change in Hfreactants) |
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Hess' Law
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Sum of enthalpy changes for each step is equal to total enthalpy change regardless of path chosen
when you add reactions, you can add their enthalpies because enthalpy is a state function |
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Endothermic
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if enthalpy change is positive
absorbs heat, making reaction system cold at constant P, where change in H = Q, endothermic reaction produces heat flow to system |
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Exothermic
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if enthalpy change is negative
release heat, making reaction system hot at constant P, where change in H = Q, exothermic reaction produces heat flow to surroundings |
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Activation energy
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initial increase in energy
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Transition state
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peal of energy hill represents molecules in transition state
old bonds are breaking and new bonds are forming occurs during reaction collision do not confuse with intermediates (products of 1st step in 2 step reaction) |
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Catalyst
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lowers activation energy of forward and reverse reactions
equilibrium and enthalpy change is unaffected Affects the rate of reaction |
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Entropy (S)
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Nature's tendency to create the most probably situation that can occur within a system
nature's tendency toward disorder state function, which means that entropy change of forward reaction is equal to negative entropy change of reverse reaction |
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2nd law of thermodynamics
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entropy of an isolated system will never decrease without some outside intervention (work)
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Entropy change of universe
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Change Ssystem + change Ssurroundings = Change Suniverse > or = 0
Sum of entropy changes of any system and its surroundings equals entropy change of universe, which is always equal to or greater than zero (positive) entropy of system can decrease, only if, at the same time, entropy of surroundings increases by greater or equal magnitude |
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Reversible
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Only ideal reactions are reversible because only ideal reactions create zero change in entropy of universe
on a microscopic scale, all real chemical reactions are reversible |
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Irreversible
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all reactions are irreversible on a macroscopic scale
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what drives the direction of a reaction
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it is entropy, not energy, that drives the direction of a reaction
entropy is nature's effort to spread energy evenly between system, from high to low entropy increases with number, volume and temperature reaction must increase the entropy of the universe in order to proceed |
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Equilibrium
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point in reaction where universe has achieved maximum entropy
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3rd law of thermodynamics
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assigns by convention a zero entropy value to any pure substance (either element or compound) at absolute zero and in internal equilibrium
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Gibbs Free energy (G)
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change G = (change H) = T(change S)
G: gibbs free energy H: enthalpy T: temperature (constant) S: entropy All variables refer to the system and not surroundings only good for constant T & P reactions negative change G = spontaneous reactions extensive property and state function not conserved, an isolated system can change its gibbs free energy Maximum non-PV work available from a reaction positive change in H and negative change in S, can never equal negative change in G, which means can never be spontaneous negative change in H and positive change in S = negative change in G = spontaneous if both change in H and S are same sign, than change in G depends on T |