AP Chem - Unit 9

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entropy (S)

entropy (S)

the measure of disorder in the dispersal of matter or energy in a sample of matter

changes in entropy (𝚫S) measure

how dispersed the matter or energy is in a particular system

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

entropy change

𝚫S reaction = ΣS products - ΣS reactants

entropy is measured in

joules

at absolute entropy

every substance has a nonzero value

when calculating entropy the

number of moles of each substance (in the balanced equation) must be considered

signs of entropy change can be predicted

by the state and number of moles of reactants and products

+𝚫S

• solid → liquid → gas

• number of moles increase from reactant to products

-𝚫S

• gas → liquid → solid

• number of moles decreases from reactant to products

gibbs free energy (G)

describes whether a reaction is thermodynamically favorable or unfavorable

changes that are thermodynamically favorable

• proceed to equilibrium without external intervention

• does not happen quickly just because it is favorable

gibbs free energy change (𝚫G)

𝚫G reaction = ΣG products - ΣG reactants

𝚫G < 0

thermodynamically favorable

𝚫G > 0

thermodynamically unfavorable

with 𝚫G = ΔH - TΔS

thermodynamically favored can be predicted from the signs of ΔH and ΔS

MEMORIZE FOR UNIT 9

processes under kinetic control

• thermodynamically favorable

• large activation energy and thus very slow

• catalysts have no effect on thermodynamic favorability

products are favored at equilibrium

• thermodynamically favored

• ΔG° < 0 and K > 1

reactants are favored at equilibrium

• thermodynamically unfavored

• ΔG° > 0 and K < 1

free energy and equilibrium constant relationship

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

• hess’s law is applied to determine the ΔG

• the sum must be negative making the reaction favorable

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

cathode

• where oxidation occurs

• cations

anode

• where reduction occurs

• anions

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

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 → power source → cathode

• occurs in ionic solution or liquid

• cations → cathode and anions → anode

electric potential difference (voltage) involves

• a reaction occurring in an electrochemical cell

E°red

• standard reduction potentials

• 1 M solutions, 1 atm of pressure for gases, 25℃

to get oxidation potentials

• the reduction half-reaction and sign of voltage must be reversed

• do not multiply stoichiometric coefficients in the equation

more positive the E°V

more favorable the reduction

cell’s standard potential (E°cell)

• calculated with E°cell = E°cathode - E°anode

voltaic cells (favorable reactions)

positive overall cell potential

electrolytic cells (unfavorable reactions)

negative overall cell potential

ΔG° is proportional to

• • cell potential for the reaction when its constructed

• moles of e- transferred

ΔG° = -nFE°cell

• +E°cell = -ΔG°

• -E°cell = +ΔG°

F = faraday’s constant = 96485 C/m e-