2.4 Free Energy

Question 1

What is the issue with the 2nd law of thermodynamics?

Answer 1

it focuses on the universe, but we are concerned with the system:

∆S(universe) = ∆S(sys) + ∆S(surr) > 0

Question 2

What is free energy?

Answer 2

examines the system itself and relates enthalpy and entropy changes

Question 3

How is ENTROPY related to ENTHALPY?

Answer 3

∆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

Question 4

How can a process in an isolated system be considered spontaneous? Why?

Answer 4

∆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

Question 5

What is Gibbs’ free energy?

Answer 5

amount of energy available to enable spontaneous change to occur at constant temperature and pressure

Question 6

How can Gibb’s free energy be calculated?

Answer 6

∆G = ∆H - T∆S

or

∆G = ∑G(prod) - ∑G(react)
–> state function

Question 7

How can Gibbs’ free energy explain the 2nd law?

Answer 7

for every process spontaneous at constant temp and pressure: ∆G < 0

Question 8

What does ∆G < 0 mean?

Answer 8

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

Question 9

What does ∆G > 0 mean?

Answer 9

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

Question 10

What does ∆G = 0 mean?

Answer 10

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

Question 11

What is the extent/completion of the reaction dependent on?

Answer 11

sign and magnitude of ∆G

Question 12

Under what conditions will a reaction always be spontaneous?

Answer 12

exothermic and increase in entropy: enthalpically and entropically driven:

∆G = ∆H - T∆S
= (-) - (+)(+)
= (-) - (+)
= (-)

Question 13

Under what condition will a reaction never be spontaneous)?

Answer 13

endothermic and decrease in entropy:

∆G = ∆H - T∆S
= (+) - (+)(-)
= (+) - (-)
= (+)

Question 14

Is the reaction spontaneous when exothermic and decrease in entropy?

Answer 14

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

Question 15

Is the reaction spontaneous when endothermic and increase in entropy?

Answer 15

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

Question 16

How does exothermic reactions impact entropy of surroundings?

Answer 16

exothermic reactions = -∆H(sys)

so:

∆S(surr) = -∆H(sys)/T
= -(-)/(+)
= (+)

increases entropy of surroundings by giving off heat

Question 17

What are enthalpically driven processes?

Answer 17

strong, large magnitude exothermic reactions that cause spontaneous reactions regardless of the ∆S sign

Question 18

What is an example of an enthalpically driven process?

Answer 18

2H2(g) + O2(g) → 2H2O(l)

–> ∆S < 0 (more order),
but ∆H &laquo_space;0

Question 19

What are entropically driven processes?

Answer 19

large magnitude entropy increase that cause spontaneous reaction even with endothermic reactions

Question 20

What is an example of an entropically driven process?

Answer 20

H2O(l) → H2O(g)

–> needs heat (endothermic) but less order: ∆S&raquo_space; 0

Question 21

Why does candle wax melt at an elevated temperate?

Answer 21

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

Question 22

What type of process is melting: entropically or enthalpically driven?

Answer 22

entropically driven: increasing heat causes T∆S to increase causing ∆G to become negative (while endothermic)

Question 23

What happens when temperature causes ∆H = T∆S?

Answer 23

this is the melting point of temperature or boiling point of temperature where ∆G = 0

Question 24

What is ∆G dependant on?

Answer 24

temp, pressure, concentration of reactants and products

Question 25

What are the conditions of standard free energy change (∆G˚)?

Answer 25

• 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

Question 26

What is the standard free energy of formation (∆Gº(f))?

Answer 26

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)