Chapter 16
Download
1 / 27

Chapter 16 Spontaneity - PowerPoint PPT Presentation


  • 1119 Views
  • Updated On :

Chapter 16. Spontaneity, entropy and free energy. Spontaneous. A reaction that will occur without outside intervention. We can’t determine how fast. We need both thermodynamics and kinetics to describe a reaction completely. Thermodynamics compares initial and final states.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Chapter 16 Spontaneity' - andrew


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Chapter 16

Chapter 16

Spontaneity, entropy and free energy


Spontaneous
Spontaneous

  • A reaction that will occur without outside intervention.

  • We can’t determine how fast.

  • We need both thermodynamics and kinetics to describe a reaction completely.

  • Thermodynamics compares initial and final states.

  • Kinetics describes pathway between.


Thermodynamics
Thermodynamics

  • 1st Law- the energy of the universe is constant.

  • Keeps track of thermodynamics doesn’t correctly predict spontaneity.

  • Entropy (S) is disorder or randomness

  • 2nd Law the entropy of the universe increases.


Entropy
Entropy

  • Defined in terms of probability.

  • Substances take the arrangement that is most likely.

  • The most likely is the most random.

  • Calculate the number of arrangements for a system.





Gases
Gases

  • Gases completely fill their chamber because there are many more ways to do that than to leave half empty.

  • Ssolid <Sliquid <<Sgas

  • there are many more ways for the molecules to be arranged as a liquid than a solid.

  • Gases have a huge number of positions possible.


Entropy1
Entropy

  • Solutions form because there are many more possible arrangements of dissolved pieces than if they stay separate.

  • 2nd Law

  • DSuniv = DSsys + DSsurr

  • If DSuniv is positive the process is spontaneous.

  • If DSuniv is negative the process is spontaneous in the opposite direction.


  • For exothermic processes DSsurr is positive.

  • For endothermic processes DSsurr is negative.

  • Consider this process H2O(l)® H2O(g)

  • DSsys is positive

  • DSsurr is negative

  • DSuniv depends on temperature.


Temperature and spontaneity
Temperature and Spontaneity

  • Entropy changes in the surroundings are determined by the heat flow.

  • An exothermic process is favored because by giving up heat the entropy of the surroundings increases.

  • The size of DSsurr depends on temperature

  • DSsurr = -DH/T


-

+

?

DSsurr

DSuniv

Spontaneous?

DSsys

-

-

-

No, Reverse

+

+

+

Yes

+

-

?

At High temp.

At Low temp.


Gibb s free energy
Gibb's Free Energy

  • G=H-TS

  • Never used this way.

  • DG=DH-TDS at constant temperature

  • Divide by -T

  • -DG/T = -DH/T-DS

  • -DG/T = DSsurr + DS

  • -DG/T = DSuniv

  • If DG is negative at constant T and P, the Process is spontaneous.


Let s check
Let’s Check

  • For the reaction H2O(s) ® H2O(l)

  • DSº = 22.1 J/K mol DHº =6030 J/mol

  • Calculate DG at 10ºC and -10ºC

  • Look at the equation DG=DH-TDS

  • Spontaneity can be predicted from the sign of DH and DS.


D g d h t d s

Spontaneous?

DS

DH

DG=DH-TDS

+

-

At all Temperatures

At high temperatures,

“entropy driven”

+

+

At low temperatures,

“enthalpy driven”

-

-

Not at any temperature,

Reverse is spontaneous

-

+


Third law of thermo
Third Law of Thermo

  • The entropy of a pure crystal at 0 K is 0.

  • Gives us a starting point.

  • All others must be>0.

  • Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed.

  • Products - reactants to find DSº (a state function).

  • More complex molecules higher Sº.


Free energy in reactions
Free Energy in Reactions

  • DGº = standard free energy change.

  • Free energy change that will occur if reactants in their standard state turn to products in their standard state.

  • Can’t be measured directly, can be calculated from other measurements.

  • DGº=DHº-TDSº

  • Use Hess’s Law with known reactions.


Free energy in reactions1
Free Energy in Reactions

  • There are tables of DGºf .

  • Products-reactants because it is a state function.

  • The standard free energy of formation for any element in its standard state is 0.

  • Remember- Spontaneity tells us nothing about rate.


D g d h t d s1

Spontaneous?

DS

DH

DG=DH-TDS

+

-

At all Temperatures

At high temperatures,

“entropy driven”

+

+

At low temperatures,

“enthalpy driven”

-

-

Not at any temperature,

Reverse is spontaneous

-

+


Third law of thermo1
Third Law of Thermo

  • The entropy of a pure crystal at 0 K is 0.

  • Gives us a starting point.

  • All others must be>0.

  • Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed.

  • Products - reactants to find DSº (a state function)

  • More complex molecules higher Sº.


Free energy in reactions2
Free Energy in Reactions

  • DGº = standard free energy change.

  • Free energy change that will occur if reactants in their standard state turn to products in their standard state.

  • Can’t be measured directly, can be calculated from other measurements.

  • DGº=DHº-TDSº

  • Use Hess’s Law with known reactions.


Free energy in reactions3
Free Energy in Reactions

  • There are tables of DGºf

  • Products-reactants because it is a state function.

  • The standard free energy of formation for any element in its standard state is 0.

  • Remember- Spontaneity tells us nothing about rate.


Free energy and pressure
Free energy and Pressure

  • DG = DGº +RTln(Q) where Q is the reaction quotients (P of the products /P of the reactants).

  • CO(g) + 2H2(g) ® CH3OH(l)

  • Would the reaction be spontaneous at 25ºC with the H2 pressure of 5.0 atm and the CO pressure of 3.0 atm?

  • DGºf CH3OH(l) = -166 kJ

  • DGºf CO(g) = -137 kJ DGºf H2(g) = 0 kJ


How far
How far?

  • DG tells us spontaneity at current conditions. When will it stop?

  • It will go to the lowest possible free energy which may be an equilibrium.

  • At equilibrium DG = 0, Q = K

  • DGº = -RTlnK


D g k
DGº K

=0 =1

<0 >0

>0 <0


Temperature dependence of k
Temperature dependence of K

  • DGº= -RTlnK = DHº - TDSº

  • ln(K) = DHº/R(1/T)+ DSº/R

  • A straight line of lnK vs 1/T


Free energy and work
Free energy And Work

  • Free energy is that energy free to do work.

  • The maximum amount of work possible at a given temperature and pressure.

  • Never really achieved because some of the free energy is changed to heat during a change, so it can’t be used to do work.


ad