**the measure of disorder** in the dispersal of matter or energy in a sample of matter
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changes in entropy (š«S) measure
how dispersed the matter or energy is in a particular system
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entropy increases when
matter becomes more dispersed
in cases where:
* phase changes from solid to liquid and from liquid to gas * individual particles become more free to move and occupy a large volume * volume of a gas increases * gas molecules are able to move within a larger space at the same speed * number of moles of product > reactants * temperature increases * distribution of KE among gas particles broadens as temperature increases
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entropy change
š«S reaction = Ī£S products - Ī£S reactants
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entropy is measured in
joules
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at absolute entropy
every substance has a nonzero value
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when calculating entropy the
number of moles of each substance (in the balanced equation) must be considered
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signs of entropy change can be predicted
by the state and number of moles of reactants and products
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\+š«S
* solid ā liquid ā gas * number of moles increase from reactant to products
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\-š«S
* gas ā liquid ā solid * number of moles decreases from reactant to products
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gibbs free energy (G)
describes whether a reaction is thermodynamically favorable or unfavorable
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changes that are thermodynamically favorable
* proceed to equilibrium without external intervention * does not happen quickly just because it is favorable
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gibbs free energy change (š«G)
š«G reaction = Ī£G products - Ī£G reactants
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š«G < 0
thermodynamically favorable
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š«G > 0
thermodynamically unfavorable
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with š«G = ĪH - TĪS
thermodynamically favored can be predicted from the signs of ĪH and ĪS
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MEMORIZE FOR UNIT 9
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processes under kinetic control
* thermodynamically favorable * large activation energy and thus very slow * catalysts have no effect on thermodynamic favorability
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products are favored at equilibrium
* thermodynamically favored * ĪG° < 0 and K > 1
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reactants are favored at equilibrium
* thermodynamically unfavored * ĪG° > 0 and K < 1
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free energy and equilibrium constant relationship
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coupled reaction
* an unfavorable reaction can be coupled with a favorable one to make the process occur * can be coupled if the two reactions share a common intermediate * hessās law is applied to determine the ĪG * the sum must be negative making the reaction favorable
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electrochemical cell
a device that can convert energy released by a favorable reaction to electrical energy or can drive a unfavorable reaction
* contains and anode and cathode
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cathode
* where oxidation occurs * cations
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anode
* where reduction occurs * anions
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galvanic (voltaic) cells
* thermodynamically favorable reaction * anode and cathode in separate half cells * salt bridge needed - allows for movement between half cells * necessary for current to flow in the circuit * produces electrical energy * + voltage value * electrons flow from anode to cathode
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electrolytic cells
* thermodynamically unfavorable reaction * anode and cathode in the same chamber * power source needed (no salt bridge) * uses electrical energy * - voltage values * electrons flow * anode ā power source ā cathode * occurs in ionic solution or liquid * cations ā cathode and anions ā anode
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electric potential difference (voltage) involves
* a reaction occurring in an electrochemical cell
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E°red
* standard reduction potentials * 1 M solutions, 1 atm of pressure for gases, 25ā
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to get oxidation potentials
* the reduction half-reaction and sign of voltage must be reversed * do not multiply stoichiometric coefficients in the equation
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more positive the E°V
more favorable the reduction
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cellās standard potential (E°cell)
* calculated with E°cell = E°cathode - E°anode
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voltaic cells (favorable reactions)
positive overall cell potential
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electrolytic cells (unfavorable reactions)
negative overall cell potential
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ĪG° is proportional to
* - cell potential for the reaction when its constructed * moles of e- transferred