Final - Thermodyanmics Part 2

spontaneity

will it or won’t it happen?

spontaneous

doesn’t require energy

* proceed in one direction
* ex - rusting

non-spontaneous

requires energy

* if spontaneous in one direction, it is non-spontaneous in the other
* ex - removing rust via chemicals

thermodynamics vs. kinetics

thermodynamics - initial and final states; spontaneity

kinetics - intermediate states; speed

reversibility of process

a reversible process will proceed back and forth between two end conditions

* will reach equilibrium
* at equilibrium, there will be no change in free energy

factors influencing spontaneity

enthalpy and entropy

enthalpy change

difference between the sum of the internal energy and PV work energy of reactants and that of the products

* reactants - products

△H

enthalpy of change - units and signs

△H = kJ/mol

* decrease in △H = - = exo (weak bonds broken and strong bonds formed)
* increase in △H = + = endo (strong bonds are broken and weak bonds formed)

entropy change

the difference betwen the "randomness" or "disorder" of the reactants and that of the products

△S

entropy

S (J/K or J/mol K)

* the amount of chaos in a system
* increaseds as the number of energetically equivalent ways of arranging the components increaes
* ex - different colored balls

absolute entropy

the amount of energy it has due to dispersion of energy through its particles

* S° at 25 is always positive

systems and entropy

random systems require less energy than ordered systems

first law of thermodynamics

energy cannot be created nor destroyed, just transferred

△E(sys) = - △E(surr)

third law of thermodynamics

for a perfect crystal at absolute 0, S° is 0 J/mol K

* not a perfect crystal at absolute 0 = some energy from entropy
* S° of a system is always +

second law of thermodynamics

the total entropy change of the universe must be positive for a process to be spontaneous

* irreversible/spontaneous process = △S(univ) > 0
* △S(univ) = △S(sys) + △S(surr)
* △S(sys) not = - △S(surr)

entropy and second law

entropy of system decreases during spontaneous processes = entropy of surroundings increases by a larger amount

* - △S(sys) = +△S(surr) an d bigger for a spontaneous process

macrostate

a given set of conditions or a general configuration

microstate

a snapshot of the exact internal energy distribution among the particles

dispersion and entropy

more dispersed and randomized = more entropy

order of increasing entropy

solid, liquid, gas

entropy and dissolution

dissolved solids generally have larger entropy, distributing particles throughout the mixture

entropy and molar mass

the larger the molar mass = larger the entropy

* more electrons = more entropy

entropy and moelcular complexity

larger, more complex molecules = larger entropy

* if same molar mass, look at complexity

entropy and allotropes

more constrained structure = lower entropy

* solids have lower entropy than gas

change in entropy (△S)

△S = S(final) - S(initial)

* +△S = less ordered = favorable = spontaneous
* -△S = more ordered = non-favorable = non-spontaneous

changes that increase △S

* products more random state than reactants
* more product molecules than reactant molecules
* increase in emp
* solids dissociating into ions

changes in # of ions and molecules and entropy

increase in # of molecules = increase entropy

decrease in # = decrease entropy

△S°rxn

S°products - S°reactants

△H and △S relationship

\-△H (exo) = + △S(sur)

\+△H (endo) = - △S(sur)

tempearture dependence of △S(sur)

higher og temp = less effect the heat has

equation of △S°(sur)

△S°(sur) = -△H(sys)/T

* inversely proportional to △S°(sur)

gibbs free energy

the amount of energy available to do work

△G = △H - T△S

* -△G = spontaneous
* +△G = non-spontaneous
* △G = 0 (equilibrium)

gibbs free energy = -△H and +△S

low temp

* - △G = spontaneous

high temp

* -△G = spontaneous

gibbs free energy = +△H and -△S

low temp

* +△G = non-spontaneous

high temp

* -△G = non-spontaneous

gibbs free energy = -△H and -△S

low temp

* -△G = spontaneous

high temp

* +△G = non-spontaneous

gibbs free energy = +△H and +△S

low temp

* +△G = non-spontaneous

high temp

* -△G = spontaneous

finding if the reaction is spontaneous at high or low temperatures

1. △G = 0
2. plug in values and find T

calculating △G(rxn) from △G(f)

products - reactants

gibbs free energy - stepwise reactions

like hess’s law