Unit 9: Thermodynamics and Electrochemistry

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73 Terms

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entropy

a measure of thermal energy of a system divided by a unit in temperature/a measure of the energy lost to molecular disorder or randomness in a sample

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positive entropy

indicates the system has increased in entropy/disorder

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negative entropy

indicates the system has decreased in entropy/disorder

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perfect crystal entropy

zero at 0K as stated by the third law of thermodynamics

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microstate

specific energy states and arrangement of molecules or atoms in a system in a given instant, particles are more likely to be spread out with one in each section

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more microstates

corresponds to more disorder/entropy due to energy being distributed more

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less microstates

corresponds to less disorder/entropy due to energy being more contained

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solid, liquid, gas

entropies lowest to highest based on state, entropy increases when particles are able to move more freely and are less organized

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more particles

means that there is more disorder

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entropy increases

decompostion reaction, increased temperature, s → aq, s→l→g (endothermic phase changes), increased moles of gas (more product gas), gas expansion (decreasing pressure, increasing volume)

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entropy decreases

composition reaction, decreased in temperature, aq→s, g→l→s (exothermic phase changes), decreased moles of gas (more reactant gas), gas compression (increasing pressure, decreasing volume)

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increased number of microstates

increased temperature, volume, and number of independently moving molecules

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second law of thermodynamics

entropy of the universe increases in a spontaneous process and remains unchanged when a system is at equilibrium

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entropy during spontaneous process

greater than 0

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entropy at equilibrium

equal to 0

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ΔS°reaction = ΣS°products - ΣS°reactants

entropy formula

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Gibbs Free Energy

energy available in a system that can be used to do work (energy “free” to do work)

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thermodynamicaly favorable characteristics

change will occur without input of energy, energy is not required for the reaction to happen, the reaction will happen on its own (spontaneous), can be determined by enthalpy and entropy (these affect rate as well)

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standard state

used for Gibbs Free Energy calculations, pure substance, 1.0M solution, gases at 1 atm

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ΔG°reaction = ΣG°products - ΣG°reactants

Gibbs Free Energy formula with given values of reactants and products

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ΔG°= ΔH° - TΔS°

Gibbs Free Energy formula without given values of reactants and products

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- ΔG°

process is thermodynamically favorable

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+ ΔG°

process is thermodynamically unfavorable

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ΔG°=0

process is in thermodynamic equilibrium

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driving force

factors that cause change in a system or process that tend to propel a system toward a new state or equilibrium

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favorable signs (- ΔH and +ΔS)

dominant/favorable H and S signs, if one is true, that is the only driving force, if both are true, both are the driving force, if neither is true, neither is the driving force

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- ΔH°, + ΔS°

thermodynamically favored (- ΔG°) at all temperatures, no calculation needed since both enthalpy and entropy are favored

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+ ΔH°, - ΔS°

thermodynamically favored (- ΔG°) at no temperatures, no calculation needed since both enthalpy and entropy are unfavorable

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+ ΔH°, + ΔS°

thermodynamically favored (- ΔG°) at high temperatures

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- ΔH°, - ΔS°

thermodynamically favored (- ΔG°) at low temperatures

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negative ΔH°

favored sign of ΔH°

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positive ΔS°

favored sign of ΔS°

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kinetic control

processes are under this when they are thermodynamically favored but do not appear to make products at a measruable rate

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factors that lead to kinetic control

high activation energy, unfavorable orientations, high numbers of particles must collide simultaneously

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catalysts

can decrease activation energy and increase reaction rate but will have no effect on thermodynamic favorability

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K>1

products are favored at equilibrium

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K<1

reactants are favored at equilibrium, indicates thermodynamic unfavorability

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- ΔG°, K>1, products favored

indicates thermodynamically favorable

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ΔG°=0, K=1, reactants and products are equally favored

indicates equilibrium

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+ ΔG°>0, K<1, reactants favored

indicates thermodynamically unfavorable

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ΔG°=-RTlnK, K=e-ΔG°/RT

used to convert between ΔG° and K

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8.314J/molK

what R in ΔG° and K conversions equals

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inverse relationship

ΔG° and K relationship

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J/mol K

ΔS units

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kJ/mol

ΔG and ΔH units

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ways to force thermodynamically unfavorable reaction to occur

external energy source (battery, photosynthesis, light, energy, electrical energy) or coupling

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coupled reactions

two reactions that share a common intermediate, produce a thermodynamically favorable reaction when combined, typically used to drive unfavorable reactions

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Hess’s Law

used to calculate the free energy change for the overall reaction that results from coupling

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anode

where oxidation occurs

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oxidation

loss of electrons

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electrolyte solution

solution present in galvanic cells

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half cell

one half (or side) of a galvanic cell

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salt bridge

a device which allows for the movement of ions between half-cells and in necessary for a reaction to occur

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cathode

where reduction takes place

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reduction

gaining of electrons

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table of standard reduction potentials

places substance that are best at reduction/most likely to be reduced at the top, places substance that are the best at oxidation/ most likely to oxidate at the bottom

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electromotive force (EMF)

also known as reduction potential, gives the voltage that is generated from that half-reaction

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lowest voltage/better at oxidation

flip this equation, and the voltage sign, to get the thermodynamically favored balance overall reaction

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negative voltage

means electrolytic cell overall reaction is thermodynamically favorable

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positive voltage

means galvanic cell overall reaction is thermodynamically favorable

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galvanic/volcanic cells

ideal, work themselves, thermodynamically favorable, anods and cathod in separate chambes (half-cells)

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electrolytic cells

not ideal, don’t work themselves, thermodynamically unfavorable, require input of energy/force to occur, no salt bridge, anode and cathode in same chamber

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anode electrode

mass decreases as reaction proceeds

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cathode electrode

mass increases as reaction proceeds

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both types of cells

oxidation at anode, reduction at cathode, requires ion flow in the cell for reaction to occur

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Q

represents the ratio or amounts of products and reactants in a reaction at any given time

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Q=(P/M products)coefficients/

(P/M reactants)coefficients

used to compare pressure or molarities of reacts and products to determine if at equilibrium

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cations

flow through salt bridge into the cathode (reduction) half cell

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anions

flow through the salt bridge into the anode (oxidation) half cell

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opposite process

do the opposite steps of galvanic cells to solve electrolytic cells (negative voltage is produced, so flip the higher half-reaction voltage and match that to anode)

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Q>K

reaction shifts towards the reactants

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Q<K

reaction will shift towards the products

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negative voltage

means electrolytic cell overall reaction is thermodynamically favorable