Causes of Change

1
Causes of Change

• Ch.11

2
(11-1) Governing Principles

• Nature favors rxns that proceed toward lower E
  greater disorder
• Heat total KE of particles
• Quantity of thermal E
• Joules
• Temp. avg. KE of particles
• Intensity of thermal E
• Kelvin

3
Calorimeter

• Measures heat absorbed or released in a rxn
• Exo E released
• Endo E absorbed

4
(No Transcript)
5
(No Transcript)
6
Specific Heat Capacity

• (Cp) amt of heat needed to raise the T of 1 g of
  a substance by 1ºC
• Units J/g ºC
• Water 4.18 J/g ºC
• q Cpm?T
• m mass (g)
• ?T Tfinal - Tinitial temp. change (ºC)
• q heat (J)

7
Specific Heat Practice

• How many joules are needed to raise the T of 300
  g of Al from 20 C to 70C if the Cp of Al is
  0.902 J/g C ?
• List the eq.
• q Cpm?T
• 2. Substitute solve
• q (0.902 J/g C)(300 g)(70C - 20C)
• 13,530 J

8
Law of Heat Exchange

• Heat flows from hot to cold
• The law
• Heat lost heat gained
• Heat lost by a metal will be gained by the
  surrounding H2O (measured w/ calorimeter)

9
Heat Exchange Practice

• Find the Cp of 100 g of an unknown metal when
  its removed from H2O at 100C placed into 200
  g of H2O at 20C. The final T of the mixture is
  23.5C.
• Write eq.
• Heat lost (by metal) heat gained (by water)
• q(metal) q(water)
• Cpm?T Cpm?T

10
Heat Exchange Practice

• Substitute solve
• Cp(100 g)(100C 23.5C)
• (4.18 J/g C)(200 g)(23.5C - 20C)
• Cp 0.382 J/g C

Note this side must remain , Therefore ?T Ti
- Tf (only in these types of problems)
11
Molar Heat Capacity

• (C) heat required to inc. the T of 1 mol of a
  substance by 1 K
• Units J/Kmol
• Table 11-1, p.389
• q nC?T
• n moles

12
Molar Heat Capacity Practice

• If C of H2O is 76 J/Kmol, calculate the amt of
  heat E needed to raise the T of 90.0 g of H2O
  from 35C to 45C.
• 1. List eq.
• q nC?T

13
Molar Heat Capacity Practice

• 2. Substitute (make sure to convert g to mol)
  solve
• q (90 g x 1 mol ) )(76 J/Kmol)(45C - 35C)
• 18.02 g
• 3,792 J

14
(11-2) Thermodynamics

• Study of E flow
• Thermo heat
• Dynamics motion

15
Entropy

• Total disorder in a substance or system
• Molar entropy (S) quantity of entropy in 1 mol
  of a subst.
• Units J/Kmol
• ?S gt O (), disorder inc.
• ?S lt O (-), disorder dec.

16
Enthalpy

• E inside an atom or molecule
• Molar enthalpy (H) total E content of a system
• Units kJ/mol or J/mol
• ?H gt O (), endo
• ?H lt O (-), exo
• ?H q nC?T C?T
• n n

17
Enthalpy Practice

• How much does the molar enthalpy change when a
  92.3 g block of ice is cooled from 0.2C to
  5.4C?
• List eq.
• ?H q C?T
• n

Cant use w/out q
18
Enthalpy Practice

• Find C for ice in Table 11-1
• 37.4 J/Kmol
• Convert C to K
• -0.2C 273 272.8 K
• -5.4C 273 267.6 K
• Subst. solve
• ?H (37.4 J/Kmol)(267.6 K 272.8 K)
• -194 J/mol

Most ?H are in kJ/mol
19
Properties of Matter

• Extensive property depends on amt. of subst.
• S, H, m, V, C
• Intensive property does not depend on amt. of
  material
• D, P, T

20
(11-3) Change of State

• S H change dramatically during a state change
• Heat of fusion (?Hfus) heat absorbed when 1 mol
  of a subst. melts
• Molar enthalpy of fusion
• Heat of vaporization (?Hvap) heat absorbed when
  1 mol of a liquid vaporizes
• Molar enthalpy of vaporization

21
(g)
?Hvap
(l)
(s)
?Hfus
22
Gibbs Energy

• Molar Gibbs E (G) free E determines
  spontaneity of a rxn
• Units kJ
• Spontaneous rxns occur w/out outside assistance
• ?G lt O (-), spont. rxn
• ?G gt O (), nonspont. rxn
• ?G ?H - T?S, (T in K)

23
Gibbs Practice

• If ?H is 41.2 kJ/mol ?S is 0.0418 kJ/K is the
  following rxn spontaneous at 25C?
• H2 CO2 ? H2O CO
• List the eq.
• ?G ?H T?S
• Subst. solve
• ?G 41.2 kJ/mol (298 K)(0.0418 kJ/K)
• 28.7 kJ, nonspontaneous

24
(11-4) Hesss Law

• Overall enthalpy change in a rxn is to the sum
  of the individual steps
• CH4(g) 2O2(g) ? CO2(g) 2H2O(l)
• Actually occurs in 2 steps
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g) DH -802
  kJ
• 2H2O(g) ? 2H2O(l) DH -88 kJ
• ?H -802 kJ -88 kJ -890 kJ

25
Heat of Rxn

• E absorbed or released during a chemical rxn
• Std. heat of formation (?Hºf) change in enthalpy
  when 1 mol of a cmpd is produced from free
  elements
• Table A-13, p.802 (check state of matter)

26
Standard Conditions

• () are generally
• Temp 298 K or 25C
• Pressure 1 atm or 760 mmHg

27
Equations

• ?Hº ??Hºf(products) - ??Hºf(reactants)
• ?Sº ??Sºf(products) - ??Sºf(reactants)
• ?Gº ??Gºf(products) - ??Gºf(reactants)

28
Enthalpy Practice

• Calculate ?Hº for the following rxn. Is the rxn
  exo. or endothermic?
• H2(g) CO2(g) ? H2O(g) CO(g)
• List the eq.
• ?Hº ??Hºf(products) - ??Hºf(reactants)

29
Enthalpy Practice

• Using Table A-13, subst. solve (account for
  of mols (coef.) of each cmpd)
• ?Hº (1 mol)(-241.8 kJ/mol) (1
  mol)(-110.5 kJ/mol)
• - (1 mol)(0 kJ/mol) (1 mol)(-393.5
  kJ/mol)
• 41.2 kJ, endothermic

30
Entropy Practice

• Calculate ?Sº for the following rxn. Does the
  rxn proceed toward a more ordered or disordered
  state?
• H2(g) CO2(g) ? H2O(g) CO(g)
• List the eq.
• ?Sº ??Sºf(products) - ??Sºf(reactants)

31
Entropy Practice

• Using Table A-13, subst. solve (account for
  of mols (coef.) of each cmpd)
• ?Sº (1 mol)(188.7 J/Kmol) (1 mol)(197.6
  J/Kmol)
• - (1 mol)(130.7 J/Kmol) (1 mol)(213.8
  J/Kmol)
• 41.8 J/K, disorder

32
Gibbs Practice

• Calculate ?Gº for the following rxn. Is the rxn
  spontaneous at 25C?
• H2(g) CO2(g) ? H2O(g) CO(g)
• List the eq.
• ?Gº ??Gºf(products) - ??Gºf(reactants)

33
Gibbs Practice

• Using Table A-13, subst. solve (account for
  of mols (coef.) of each cmpd)
• ?Gº (1 mol)(-228.6 kJ/mol) (1
  mol)(-137.2 kJ/mol)
• - (1 mol)(0 kJ/mol) (1 mol)(-394.4
  kJ/mol)
• 28.6 kJ, nonspontaneous