Spontaneity, Entropy, and Free Energy

A ball rolls spontaneously down a hill but not up….

Spontaneous

A reaction that will occur without outside intervention; product favored

  • Most reactants are converted to products
  • Says nothing about the rate of the reaction- KINETICS
  • Thermodynamics (tells us the direction in which a reaction will occur)
  • Kinetics describes the pathway

Thermodynamics

1st law- Energy of the universe is constant: keeps track of how much energy is involved in the change

Why do certain processes occur in 1 direction and not the other???

Early idea: exothermicity

Conclusion: the characteristic common to all spontaneous reactions and the driving force is an increase in entropy

Entropy- (S) is the disorder or randomness; natural tendency to go from order to disorder thus from a lower to higher entropy

2nd law- The entropy of the universe increases

  • Defined in terms of probability
  • Substances take the arrangement that is most likely which is the most RANDOM
  • See overhead for possible arrangements for a system

S solid < S liquid < S gas

  • More ways for molecules to be arranged as a liquid than a solid; gases have even more ways
  • Solutions form b/c there are many more possible arrangements of dissolved pieces than if they stay separate
  • In a gas, if the products have a smaller number of molecules, then the entropy has decreased

Examples 1-10

Second Law of Thermodynamics

ΔSuniverse = ΔSsystem + ΔSsurroundings

  • If ΔSuniverse is positive, the process is spontaneous
  • If ΔSuniverse is negative, the process is spontaneous in the opposite direction
  • If ΔSuniverse is zero, the process is at equilibrium

Entropy in the surroundings is primarily controlled by enthalpy

Recall: If ΔH is negative, heat is released into surroundings SO

ΔSsurroundings is positive ( more disorder due to faster movement)

If ΔH is positive, heat is absorbed from the surroundings SO

ΔSsurroundings is negative ( less disorder due to slower movement)

BUT WHAT HAPPENS WHEN..

H2O(l) H2O(g)

ΔSsys = positiveΔSsurr = negative

Which one controls it? ΔSuniv will be dependent on temperature

Temperature and Spontaneity

  • An exothermic process is favored because by giving up heat, the entropy of the surroundings increases
  • The sign of ΔSsurr depends on tenperature : at constant temp/pressure

ΔSsurr = - ΔH/ T

Units: J/K

ΔSsys / ΔSsurr / ΔSuniv / Spontaneous?
+ / + / + / Yes
- / - / - / No, reverse
+ / - / ? / At high temp
- / + / ? / At low temp

*****From now on no subscript next to the delta sign refers to the system*****

Example 13

Third law of Thermodynamics

  • Entropy of a pure crystal at 0K is zero. All others must be > zero.
  • Standard entropy S˚ at 298 K and 1 atm of substances- see appendix for values
  • It is a state function
  • Can use Σ ΔS products – Σ ΔS reactants

Example 11 and 12