ch 20 entropy and free energy
Download
Skip this Video
Download Presentation
Ch. 20: Entropy and Free Energy

Loading in 2 Seconds...

play fullscreen
1 / 19

Ch. 20: Entropy and Free Energy - PowerPoint PPT Presentation


  • 196 Views
  • Uploaded on

Section 20.1 . ?Thermodynamics tells us NOTHING about the rate of reaction. The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.). Section 20.2 Entropy . Entropy, S: Measure of dispersal or disorder. ? Can be measured with a calorimeter. Ass

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 'Ch. 20: Entropy and Free Energy' - naomi


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
ch 20 entropy and free energy
Ch. 20: Entropy and Free Energy
  • Thermodynamics: the science of energy transfer
    • Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored
section 20 1
Section 20.1
  • ØThermodynamics tells us NOTHING about the rate of reaction.
    • The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.)
section 20 2 entropy
Section 20.2 Entropy
  • Entropy, S:Measure of dispersal or disorder.
  • ØCan be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no disorder and S = 0.
  • ØIf temperature change is very small, can calculate entropy change, DS = q/T (heat absorbed / T at which change occurs)
  • ØSum of DS can give total entropy at any desired temperature. See Table 20.1
section 20 2 entropy4
Section 20.2 Entropy
  • In general, the final state is more probable than the initial one if:
  • (1)energy can be dispersed over a greater number of atoms and molecules (hot  cold)
  • (2)the atoms and molecules can be more disordered (dissolving, diffusion of gas)
section 20 2 entropy5
Section 20.2 Entropy
  • More specifically,
  • (1)if energy and matter are both more dispersed, it is definitely product-favored
  • (2)if only energy or matter is dispersed, then quantitative information is necessary to decide which effects are greater
  • (3)if neither matter nor energy is more dispersed, then the process will be reactant-favored
entropy examples positive d s
Entropy Examples (positive DS)
  • Boiling water
  • Melting ice
  • Preparing solutions
  • CaCO3 (s)  CaO (s) + CO2 (g)
entropy examples negative d s
Entropy Examples (negative DS)
  • Molecules of gas collecting
  • Liquid converting to solid at room temp
  • 2 CO (g) + O2 (g)  2 CO2 (g)
  • Ag+ (aq) + Cl-(aq)  AgCl (s)
entropy generalizations
Entropy Generalizations
  • Sgas > S liquid > Ssolid
  • Entropies of more complex molecules are larger than those of simpler molecules (Spropane > Sethane>Smethane)
  • Entropies of ionic solids are higher when attraction between ions are weaker.

ØEntropy usually increases when a pure liquid or solid dissolves in a solvent.

Entropy increases when a dissolved gas escapes from a solution

laws of thermodynamics
Laws of Thermodynamics
  • First law: Total energy of the universe is a constant.
  • Second law: Total entropy of the universe is always increasing.
  • Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.
calculating d s o system
Calculating DSosystem
  • DSosystem =  So (products) -  So (reactants)

Can also relate surroundings to the system!

  • DSosurroundings = q surroundings / T

= - DHsystem / T

calculating d s o universe
Calculating DSouniverse
  • DSouniverse = DSosurroundings +DSosystem
  • DSouniverse = - DHsystem / T +DSosystem
  • Can use 2nd law to predict whether a reaction is product-favored or reactant-favored!
      • The higher the temperature, the less important the enthalpy term is!
slide12
Roald Hoffmann (1981 Nobel prize): “One amusing way to describe synthetic chemistry, the making of molecules that is at the intellectual and economic center of chemistry, is that it is the local defeat of entropy.”
20 3 gibbs free energy
20.3 Gibbs Free Energy
  • DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!
20 3 gibbs free energy14
20.3 Gibbs Free Energy
  • DGsystem = - T DSuniverse

= DHsystem - TDSsystem

  • DGosystem = DHosystem - T DSosystem
  • DGorxn = DHorxn - T DSorxn
20 3 gibbs free energy15
20.3 Gibbs Free Energy
  • DGosystem or DGorxn If negative, then product-favored. If positive, then reactant-favored.
  • DGoreaction =  Gfo (products) -  Gfo (reactants)
20 3 gibbs free energy16
20.3 Gibbs Free Energy
  • DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!
  • Know the meaning of enthalpy-driven vs. entropy-driven reactions.

DGs are additive!

20 4 thermodynamics and k
20.4 Thermodynamics and K

If not at standard conditions,

DG = DGo + RT ln Q

 (Equilibrium is characterized by the inability to do work.)

At equilibrium, Q = K and DG = O

Therefore, substituting into previous equation gives 0 = DGo + RT ln K and

DGo = - RT ln K (can use Kp or Kc)

slide18
2020.5      Thermodynamics and Time
  • First law: Total energy of the universe is a constant.
  • Second law: Total entropy of the universe is always increasing.
  • Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.
  • Entropy : time’s arrow
  • Absolutely MUST learn table in Chapter highlights!
20 4 thermodynamics and k19
20.4 Thermodynamics and K
  • ØUnderstand relationship between DGo, K, and product-favored reactions!
  • DGo<0 K>1 product-favored
  • DGo=0 K=1
  • DGo>0 K<1 reactant-favored
ad