2.4 Free Energy Flashcards
What is the issue with the 2nd law of thermodynamics?
it focuses on the universe, but we are concerned with the system:
∆S(universe) = ∆S(sys) + ∆S(surr) > 0
What is free energy?
examines the system itself and relates enthalpy and entropy changes
How is ENTROPY related to ENTHALPY?
∆S(surr) = ∆H(surr)/T
entropy = heat change/temp
at constant pressure and temp
remember: ∆H(surr) = -∆H(sys)
so: ∆S(surr) = -∆H(sys)/T
and: ∆S(sys) - ∆H(sys)/T > 0
and: ∆H - T∆S < 0
How can a process in an isolated system be considered spontaneous? Why?
∆S(sys) > 0
since every spontaneous process increase in entropy, the entropy is not lost to surroundings and kept in the system: increases system’s entropy
What is Gibbs’ free energy?
amount of energy available to enable spontaneous change to occur at constant temperature and pressure
How can Gibb’s free energy be calculated?
∆G = ∆H - T∆S
or
∆G = ∑G(prod) - ∑G(react)
–> state function
How can Gibbs’ free energy explain the 2nd law?
for every process spontaneous at constant temp and pressure: ∆G < 0
What does ∆G < 0 mean?
reaction will proceed spontaneous as written
–> reaction favours the formation of products but doesn’t indicate that the reaction proceed to completion
–> high to low free energy
What does ∆G > 0 mean?
reaction will not proceed spontaneously as written: it will proceed spontaneously in the reverse process
–> reaction favours reactants with little or no products formed
–> low to high free energy
What does ∆G = 0 mean?
the system is at equilibrium: numbers of moles of reactants and products do not change over time
–> rate of forward reaction = rate of reverse reaction
What is the extent/completion of the reaction dependent on?
sign and magnitude of ∆G
Under what conditions will a reaction always be spontaneous?
exothermic and increase in entropy: enthalpically and entropically driven:
∆G = ∆H - T∆S
= (-) - (+)(+)
= (-) - (+)
= (-)
Under what condition will a reaction never be spontaneous)?
endothermic and decrease in entropy:
∆G = ∆H - T∆S
= (+) - (+)(-)
= (+) - (-)
= (+)
Is the reaction spontaneous when exothermic and decrease in entropy?
Enthalpically driven if spontaneous: depends on magnitude of enthalpy and temperature:
∆G = ∆H - T∆S
= (-) - (+)(-)
= (-) - (-)
= (-) + (+)
if enthalpy magnitude is greater than T∆S magnitude, it is spontaneous
Is the reaction spontaneous when endothermic and increase in entropy?
Entropically driven if spontaneous: depends on magnitude of entropy and temperature:
∆G = ∆H - T∆S
= (+) - (+)(+)
= (+) - (+)
if enthalpy is greater than T∆S magnitude, it is NOT spontaneous
How does exothermic reactions impact entropy of surroundings?
exothermic reactions = -∆H(sys)
so:
∆S(surr) = -∆H(sys)/T
= -(-)/(+)
= (+)
increases entropy of surroundings by giving off heat
What are enthalpically driven processes?
strong, large magnitude exothermic reactions that cause spontaneous reactions regardless of the ∆S sign
What is an example of an enthalpically driven process?
2H2(g) + O2(g) → 2H2O(l)
–> ∆S < 0 (more order),
but ∆H «_space;0
What are entropically driven processes?
large magnitude entropy increase that cause spontaneous reaction even with endothermic reactions
What is an example of an entropically driven process?
H2O(l) → H2O(g)
–> needs heat (endothermic) but less order: ∆S»_space; 0
Why does candle wax melt at an elevated temperate?
individual alkane chains are in solid state held together by intermolecular force
- breaking interactions needs heat = endothermic = ∆H > 0
also, from solid to liquid, ∆S > 0
What type of process is melting: entropically or enthalpically driven?
entropically driven: increasing heat causes T∆S to increase causing ∆G to become negative (while endothermic)
What happens when temperature causes ∆H = T∆S?
this is the melting point of temperature or boiling point of temperature where ∆G = 0
What is ∆G dependant on?
temp, pressure, concentration of reactants and products
What are the conditions of standard free energy change (∆G˚)?
- all gases present are at a partial pressure of 1 atm
- all species in Solution have a concentration of 1M
- system is at a temperature of 298.15K
∆Gº = ∆Hº - T∆Sº
What is the standard free energy of formation (∆Gº(f))?
free energy change under standard condition that occur when 1 mol of a compound is made from elements in their standard states:
∆Gº(f) (element in standard state)= 0
∆Gº(f) = ∑∆Gº(f)(products) - ∑∆Gº(f)(reactants)
