Chemical Thermodynamics

1
Chemical Thermodynamics

• Chapter 19 in Brown, LeMay, and Bursten

2
Thermodynamics vs Kinetics

• Thermodynamics answers the question Will the
  reaction proceed spontaneously as written?
• Kinetics answers the question How fast will it
  proceed?
• Ice melting at room temperature vs. nail rusting.

3
Exothermic Reactions

• Most of these are spontaneous at room temperature
  and pressure (nail).
• Why? System releases energy to the surroundings
  and ends up at a lower energy state.
• What about ice melting?
• Something else must be involved.

4
Entropy

• Nature tends to move spontaneously from more
  ordered to more random states.
• Many examples in life
• Chemistry example (two gases)
• Sometimes called degrees of freedom

5
S

• Symbolized S
• The more random, the greater the value of S
• ?S Sfinal Sinitial (state function)
• Increasing entropy solid to liquid to gas
• Third Law of Thermodynamics Entropy of a pure
  crystalline solid at 0 K is 0
• What was the first law?

6
Standard Molar Entropy S

• Can be determined precisely
• 1 mole at 1 atm and any temperature (usually 25
  )
• Elements are not 0!!
• All values of elements and molecules are
  positive!
• Ions are calculated relative to H(aq) 0, so
  ions may be negative.

7
Trends

• Solid to liquid to gas increase in entropy
• Same structure, entropy increases with increasing
  mass (CH4 vs. C2H6)
• You can use the table of standard values the same
  way that you did ?H problems.
• Can also predict sign.

8
Second Law of Thermodynamics

• In a spontaneous process, there is a net increase
  in entropy, taking into account both system and
  surroundings.
• Example Using the table for rusting, you get a
  ?S, but the change in entropy of the
  surroundings must be greater than that value
  because it is spontaneous.
• Very difficult to calculate

9
Gibbs Free Energy

• Combines H and S
• Symbolized G
• Represents energy that is free (released from
  system) to do useful work.
• G H TS

10
Sign

• - ?G spontaneous reaction
• ?G reverse will occur spontaneously
• ?G 0 means that the system is at equilibrium
• Reactions at constant temperature and pressure go
  in such a direction as to decrease the free
  energy of the system.

11
Gibbs-Helmholtz Equation

• ?G ?H T ?S
• Two forces that contribute to making a reaction
  spontaneous
• Exothermic (forming a strong bond)
• Increase in disorder
• ?G ?H T ?S (standard conditions, 1 mole, 1
  atm pressure

12
Calculations

• Can use equation to find ?G if you use a table
  to calculate the other two.
• This will tell about spontaneity.
• Usually we do not use tables to do ?G directly
  because they are temperature-dependent and
  difficult to calculate.

13
?G at Other Temperatures

• Only change T in the equation because S and H
  vary little.
• ?G varies greatly with temperature.
• What would a plot of ?G vs T look like?
• Line with ?S slope and y intercept of ?H.

14
Four Situations
?G ?H T ?S
15
?G Under Any Condition

• General equation under any conditions
• ?G ?G RT(lnQ)
• What is Q
• Called reaction Quotient
• T in Kelvin, R 8.31 J/K, atm and molarity
• Example

16
?G and Equilibrium

• Look at the equation again
• ?G ?G RT(lnQ)
• What is ?G for equilibrium?
• ?G -RT(lnK)
• You can use this to calculate K
• Read about coupling reactions at the end of
  Masterton chapter.