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CHEM 163 Chapter 20. Spring 2009. 3-minute exercise. Is each of the following a spontaneous change? Water evaporates from a puddle A small amount of sugar dissolves in hot tea Methane burns in air A hamburger becomes uncooked. Thermodynamics. First Law: Law of Conservation of Energy.

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3 minute exercise
3-minute exercise

Is each of the following a spontaneous

change?

  • Water evaporates from a puddle
  • A small amount of sugar dissolves in hot tea
  • Methane burns in air
  • A hamburger becomes uncooked
thermodynamics
Thermodynamics

First Law:

Law of Conservation of Energy

Limitation:

Internal E of a system

Explains change, but not direction

heat

work

Second Law:

Systems change towards more disorder

spontaneous change
Spontaneous Change
  • Change that occurs without continuous E input
  • Change can only be spontaneous in one direction, under a given set of conditions
  • Enthalpy (∆H):
    • heat gained or lost at constant P
    • Sign of ∆H
      • Exothermic or endothermic
      • No information about spontaneity
  • Entropy (∆S):
    • Freedom of particle motion (dispersed E of motion)
energy levels
Energy Levels
  • Each atom or molecule has quantized E levels
    • Electronic
    • Kinetic
      • Vibrational, rotational, translational
  • Microstate: combined E at any given point
    • each microstate is equally possible (equal E)

for a given set of conditions

Number of microstates

entropy

Boltzmann constant

(J/K)

= R/NA

= 1.38 x 10-23 J/K

entropy change heat changes
Entropy Change: Heat Changes
  • Remove 1 grain of sand
  • Gas does work on piston
    • absorbs heat to maintain E
  • Works for tiny changes (totally reversible)
always increasing entropy
Always Increasing Entropy

All real processes occur spontaneously in direction that increases the entropy of the universe.

  • Perfect crystal at T = 0 K has S = 0
    • 1 microstate
  • Standard Molar Entropy (S°)
    • S increase from 0 to standard state
      • 1 atm (gases)
      • 1 M (solutions)
      • Pure substance, most stable form (liquids/solids)
what affects s
What affects S°?

As # of microstates (or kinetic E) increases, S increases

  • Temperature change

↑ T ↑ S

  • Phase change

absorb heat ↑ S

slide10

Dissolution

Ions: increased S, except small, highly charged ions

Molecules (solid or liquid): ∆S ≈ 0

Gases: decreased S

  • Atomic Size
    • heavier atoms
    • allotropes

 closer E levels

 more microstates

Molecular Complexity

more complex

 more types of movement

 more microstates

Only applies to molecules in same physical state

3 minute practice
3-minute practice

What is the sign of ∆Ssys?

  • A pond freezes in winter
  • Atmospheric CO2 dissolves in the ocean
  • 2 K (s) + F2 (g)  2KF (s)
slide12

Standard Entropy of Reaction

Increasing disorder:

Decreasing disorder:

Predict ∆Srxn:

Change in # moles of gas

Calculate ∆Srxn:

N2 (g) + 3H2 (g)  2 NH3 (g)

s universe
∆Suniverse

Decrease in ∆Ssys only if greater increase ∆Ssurr

System acts as heat sink or drain

  • Exothermic
  • Endothermic

at constant P

Measure ∆Hsys to determine ∆Ssurr

spontaneous at 298 k
Spontaneous at 298 K?

3

2

Balance equation!

Calculate ∆Ssys

Calculate ∆Hsys

Calculate ∆Ssurr

Calculate ∆Suniv

N2 (g) + H2 (g)  NH3 (g)

entropy at equilibrium
Entropy at Equilibrium

Approaching equilibrium:

At equilibrium:

No net change

5 minute practice
5-minute Practice

Calculate ∆Ssys for the combustion reaction of ammonia (producing nitrogen dioxide and water vapor).

gibbs free energy
Gibbs Free Energy
  • Measure of spontaneity
  • Combines enthalpy and entropy

Spontaneous if…

∆Suniv > 0

∆Gsys < 0

T∆Suniv > 0

-T∆Suniv < 0

slide18
∆G

Spontaneous process

Nonspontaneous process

Process at equilibrium

Standard Free Energy Change

Standard Free Energy of Formation :

E change when 1 mol of compound is made from its elements in standard states

Element in standard state:

g and work constant t p
∆G and work (constant T & P)
  • Nonspontaneous process:
    • Process may occur if work is done to the system
    • How much work is needed?

w = ∆G

  • Spontaneous process:
    • ∆G = maximum useful work done by the system
  • wmax only if process is totally reversible
  • Actually does less w <wmax
    • Extra E lost as heat
useful work
Useful Work
  • Excludes work done by or on atmosphere
  • Some free energy is always lost to heat

∆Hsys?

< 0

∆Ssys?

> 0

∆Gsys = wmax that can be done by system

∆Gsys?

< 0

∆Gsys > w actually done by system

In some multistep reactions, ∆G from one reaction can cause an otherwise nonspontaneous reaction to occur.

“coupling of reactions”

what about t
What about T?
  • Typically ∆H > T∆S
  • For ∆G to be negative, need ∆H to be…
  • What about at high T?

negative

  • T∆S term can dominate
  • Sign of ∆S becomes important

4 situations:

∆H < 0 & ∆S > 0

∆H > 0 & ∆S < 0

∆H > 0 & ∆S > 0

∆H < 0 & ∆S < 0

∆G < 0

∆G > 0

∆G > 0 at low T;

∆G < 0 at high T

∆G < 0 at low T;

∆G > 0 at high T

high t v low t
High T v. Low T?

Spontaneous “limit” at what temperature?

equilibria and g
Equilibria and ∆G
  • If Q < K, reaction…
  • If Q > K, reaction…
  • If Q = K, at equilibrium

Q/K

< 1

∆G < 0

Q/K

> 1

∆G > 0

Q/K

= 1

∆G = 0

proportional

0

Make Q standard state (all values = 1)

homework due monday may 11 th
Homework due MONDAY, May 11th

Chap 20: #19, 26, 30, 41, 50, 52, 58, 70, 78, 106

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