Energy

1
Energy Disorder
2
Thermochemistry

• Thermodynamics - the study of energy its
  transformations
• Thermochemistry- studies the relationship between
  chemical rxns E changes involving heat.

3
The Nature of Energy

• System- the limited well-defined part of the
  universe th well look at
• Closed system - can exchange E w/ surroundings,
  but not matter
• Surroundings- everything else

4
First Law of Thermodynamics

• Energy is conserved
• Any E lost by a system must be gained by the
  surroundings, visa versa
• Internal E the sum of all the potential
  kinetic E of all of the components of the system
• Rarely know the exact value
• Can know the ?E E final - E initial
• Reactants -gt products
• ?E is negative, E lost to surroundings
• ?E is positive, E gained by system

5
First Law of Thermody-cont

• State Function- a property of a system th is
  determined by specifying its condition or state
  (temp, press, location, etc.)
• Depends only on the present condition
• ?E

6
Enthalpy (H)

• Accounts for heat flow in chemical changes
  occurring at constant pressure
• ?H endothermic rxn
• ?H - exothermic rxn
• ?H is a state function

7
Enthalpies of Reaction

• The enthalpy change for a rxn
• ?H H (products) - H (reactants)
• ?Hrxnheat of a rxn
• Thermochemical equation
• 2H2 O2 -gt 2H2O ?H -483.6 kJ
• Balanced chem equation th shows the associated
  enthalpy change
• Reversing a rxn changes the sign, but not the
  magnitude of ?Hrxn

8
Hesss Law

• ?Hrxn depends upon the amount of matter
  undergoing a change the initial final states
• Hesss Law - the enthalpy of a rxn th occurs in
  steps is a sum of the enthalpy changes of the
  individual steps.

9
Enthalpies of Formation

• ?Hf - the enthalpy change associated w/ the
  formation of 1 mole of a sub fr free elements
  kJ/mol
• Standard conditions - 1 atm 25C (298 K)

10
Using ?Hf to calc ?Hrxn

• Using Hesss Law and the ?Hf values for all of
  the reactants products, we can calculate the
  ?Hrxn for a rxn
• ?Hrxn ??Hf (products) - ??Hf (reactants)

11
Calorimetry

• The measurement of heat flow
• Calorimeter -apparatus used
• Heat capacity - temp change experienced by an
  object when it absorbs E
• The amt. of heat required to raise the temp by 1
  K (or 1 C)
• Molar heat capacity - heat capacity of 1 mole of
  sub
• Specific heat - heat capacity of 1 g of sub

12
Calorimetry -cont.

• Specific heat (quantity of heat transferred)
• (grams of sub) x (temp change)
• ____q____
• m x ?T
• Of water 4.18 J/g K 1 calorie
• - very high
• maintains relative ocean temp body temp
• q (specific heat) x (grams of a sub) x ?T

13
Calorimetry -cont.

• Constant - Pressure Calorimetry - can be
  conducted in a coffee-cup calorimeter
• Atm press is constant in this open sy
• Heat of rxn is absorbed by soln
• Dilute aqueous soln have a Cp 4.18 J/gK
• Bomb Calorimetry ( Constant- Vol)

14
Spontaneous Processes-cont

• Reversible Irreversible Processes
• Reversible processes can be restored exactly to
  their original state
• In a chemical sy at equilibrium, R P can
  interconvert reversibly.
• Irreversible processes cannot be restored to
  their original state
• In a spontaneous process, the path between R P
  is irreversible
• A spontaneous rxn can be fast or slow
  thermodynamics will tell us about the direction
  extent, but not the speed

15
Entropy the 2nd Law of Thermodynamics

• Entropy (disorder) (S) is a driving force in
  spontaneous processes
• Even if a process is not energetically (enthalpy)
  favorable, the drive towards randomness may be
  enough for a spontaneous rxn to occur
• Entropy is a state function
• Depend only upon the initial final states
• ?S S final - S initial

16
Relating Entropy to heat Transfer/Temp

• Entropy in the universe increases in any
  spontaneous process
• Exothermic process entropy of the surroundings
  increases
• For an isolated sy, surroundings are not
  affected.

17
Molecular Interpretation of Entropy

• Entropy generally increases w/ increasing temp
• S solidlt S liquid lt S gas

18
Entropy Changes in Chem Rxns

• Standard molar enthalpies (S)
• Free elements are not zero (appendix C)
• Gases are greater than liquids or solids
• Generally increase w/ increasing molar mass
• Generally increase w/ increasing of atoms in
  the formula of a sub

19
Entropy Changes in Chemical Rxns-cont

• ?S ?SProducts - ?SReactants

20
Spontaneous Processes

• Occur w/out any ongoing outside intervention Has
  as definite direction Can depend on temp Ice
  melts when Tgt 0 Water freezes when Tlt0

21
Spontaneous Processes-cont

• Reversible Irreversible Processes
• Reversible processes can be restored exactly to
  their original state
• In a chemical sy at equilibrium, R P can
  interconvert reversibly.
• Irreversible processes cannot be restored to
  their original state
• In a spontaneous process, the path between R P
  is irreversible
• A spontaneous rxn can be fast or slow
  thermodynamics will tell us about the direction
  extent, but not the speed

22
Gibbs Free Energy

• Used to determine spontaneity of a rxn at a given
  temp
• ?G ?H -T?S
• ?G is negative, the rxn is spontaneous in the
  forward direction
• ?G is zero, the rxn is at equilibrium
• ?G is positive, the rxn in the forward direction
  is nonspontaneous work must be supplied by the
  surroundings reverse rxn is spontaneous

23
Gibbs Free Energy-cont

• Indicates whether or not a rxn will occur
• ?G ?H ?S
• - - Always spontaneous
• or - Spon _at_ high temps
• or - - - Spon _at_ low temps
• - Never spontanteous
• ?H T?S 0 Equilibrium
• Exergonic rxn -?G is negative spontaneous
• Endergonic rxn ?G is positive nonspontaneous
• All spontaneous rxns proceed toward
  equilibrium

24
Gibbs Free Energy-cont

• ?G ?GProducts - ?GReactants
• ?G lt 0 spontaneous
• ?G gt 0 non spontaneous
• ?G 0 equilibrium