Unit 9: Thermodynamics and Electrochemistry

entropy

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

positive entropy

indicates the system has increased in entropy/disorder

negative entropy

indicates the system has decreased in entropy/disorder

perfect crystal entropy

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

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

more microstates

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

less microstates

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

solid, liquid, gas

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

more particles

means that there is more disorder

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)

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)

increased number of microstates

increased temperature, volume, and number of independently moving molecules

second law of thermodynamics

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

entropy during spontaneous process

greater than 0

entropy at equilibrium

equal to 0

ΔS°reaction = ΣS°products - ΣS°reactants

entropy formula

Gibbs Free Energy

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

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)

standard state

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

ΔG°reaction = ΣG°products - ΣG°reactants

Gibbs Free Energy formula with given values of reactants and products

ΔG°= ΔH° - TΔS°

Gibbs Free Energy formula without given values of reactants and products

- ΔG°

process is thermodynamically favorable

+ ΔG°

process is thermodynamically unfavorable

ΔG°=0

process is in thermodynamic equilibrium

driving force

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

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

- ΔH°, + ΔS°

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

+ ΔH°, - ΔS°

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

+ ΔH°, + ΔS°

thermodynamically favored (- ΔG°) at high temperatures

- ΔH°, - ΔS°

thermodynamically favored (- ΔG°) at low temperatures

negative ΔH°

favored sign of ΔH°

positive ΔS°

favored sign of ΔS°

kinetic control

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

factors that lead to kinetic control

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

catalysts

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

K>1

products are favored at equilibrium

K<1

reactants are favored at equilibrium, indicates thermodynamic unfavorability

- ΔG°, K>1, products favored

indicates thermodynamically favorable

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

indicates equilibrium

+ ΔG°>0, K<1, reactants favored

indicates thermodynamically unfavorable

ΔG°=-RTlnK, K=e^-ΔG°/RT

used to convert between ΔG° and K

8.314J/molK

what R in ΔG° and K conversions equals

inverse relationship

ΔG° and K relationship

J/mol K

ΔS units

kJ/mol

ΔG and ΔH units

ways to force thermodynamically unfavorable reaction to occur

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

coupled reactions

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

Hess’s Law

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

anode

where oxidation occurs

oxidation

loss of electrons

electrolyte solution

solution present in galvanic cells

half cell

one half (or side) of a galvanic cell

salt bridge

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

cathode

where reduction takes place

reduction

gaining of electrons

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

electromotive force (EMF)

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

lowest voltage/better at oxidation

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

negative voltage

means electrolytic cell overall reaction is thermodynamically favorable

positive voltage

means galvanic cell overall reaction is thermodynamically favorable

galvanic/volcanic cells

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

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

anode electrode

mass decreases as reaction proceeds

cathode electrode

mass increases as reaction proceeds

both types of cells

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

Q

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

Q=(P/M products)^coefficients/

(P/M reactants)^coefficients

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

cations

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

anions

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

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)

Q>K

reaction shifts towards the reactants

Q<K

reaction will shift towards the products

negative voltage

means electrolytic cell overall reaction is thermodynamically favorable