Heat flows from a system of higher temperature to one of lower temperature until the two systems are

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Lower Temperature
Higher Temperature
Heat flows from a system of higher temperature to
one of lower temperature until the two systems
are in thermal equilibrium (i.e., are at the same
temperature)
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Heat is transferred to the system from the
surroundings in an ENDOTHERMIC reaction Heat is
transferred to the surroundings from the system
in an EXOTHERMIC reaction
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Heat is transferred to the system from the
surroundings in an ENDOTHERMIC reaction so the
surroundings become cooler (due to loss of heat
to the system)
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Heat is transferred to the surroundings from the
system in an EXOTHERMIC reaction so the
surroundings become warmer (due to the addition
of heat from the system)
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State functions are independent of the pathway
(process) used to arrive at the state under study
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Because state functions are independent of
process, we can calculate differences in states
using real or imagined processes Imagined
processes are frequently easy to evaluate real
processes frequently are not easily evaluated
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Heats of reaction can be measured in a bomb
calorimeter, which measures a change in
temperature of a known quantity of water
surrounding a reaction vessel (the bomb)
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Sodium chlorine explosion!!
energy is released
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The formation of 1 mol of NaCl from 1 mol of Na
and 1 mol of Cl releases 639 kJ of energy. In
practice the amount of energy released (or
absorbed) during a chemical reaction can be
determined experimentally however, these values
can also be predicted by calculations of net
energy change for another process that starts and
ends at the same states
1 mol NaCl
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Bond forming steps in this process (E released!)
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Fomation of ionic compounds
Na(g) ? Na(g) e- Ne 3s1
Ne Cl(g) e- ? Cl(g)
Ne3s23p5 Ne3s23p6 Sum Na(g)
Cl(g) e- ? Na(g) e- Cl(g) Net Na(g)
Cl(g) ? Na(g) Cl(g)
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Ionization energy Electron affinity
Step 1
Step 2a
Step 2b
Step 1 ionization energy electron affinity
(147 kJ/mol) Step 2a formation of a single ion
pair (-493 kJ/mol) Step 2b
formation of crystal lattice (-293
kJ/mol) from many ion pairs TOTAL energy
change -639 kJ/mol
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Bond formation releases energy

• when a covalent or non-covalent (e.g., ionic,
  dipole-dipole, etc.) bond is formed between atoms
  (or ions), energy is released (the value of DH is
  -)
• to break bonding interactions, energy is required
  (absorbed) (the value of DH is )

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Bond Dissociation Energy (BDE)
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CH4
Cl2
CH3Cl
HCl
Is this reaction likely to be exothermic or
endothermic? You can answer this question by
calculating bond energies for the reactants and
products
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H
H

C


Cl
H
Cl

H
ENERGY
CH3Cl
HCl

Energy required to break all bonds 1884
kJ/mol Energy released by bond formation - 1988
kJ/mol Energy change for reaction
-104 kJ/mol Therefore reaction is predicted to be
exothermic.
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The practical aspects of understanding Bond
Dissociation Energies
Fuels Coal 30.6 kJ/g Methane 50.0
kJ/g Octane 44.4 kJ/g Nutrients (metabolic
fuels) Glucose 15.6 kJ/g (4 Calories/g)
Fat 38.5 kJ/g (9 Calories/g)
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The rules of stoichiometry apply to DH for
chemical reactions
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6.53
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Heat capacity the heat required to raise the
temperature of a substance by 1 K (or 1 C)
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Online HW problem 18

•  The specific heat of iron is 0.449 J/(g . oC)
  and the specific heat of water is 4.18 J/(g .
  oC).
• A piece of iron was heated to 92.7oC and dropped
  into a constant pressure calorimeter containing
  226 grams of water at 34.0oC.
• The final temperature of the water and iron was
  53.1oC.
• Assuming that the calorimeter itself absorbs a
  negligible amount of heat, what was the mass, in
  g, of the piece of iron?

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Online HW problem 26

•  A 16.5-mL sample of 0.919 M NaOH is mixed with
  39.0 mL of 0.918 M HCl in a coffee-cup
  calorimeter.
• The enthalpy of the reaction, written with the
  lowest whole number coefficients, is -54.4 kJ.
• Both solutions are at 29.7oC prior to mixing and
  reacting.
• What is the final temperature of the reaction
  mixture?
• When solving this problem, assume that no heat is
  lost from the calorimeter to the surroundings,
  the density of all solutions is 1.00 g/mL, the
  specific heat of all solutions is the same as
  water, and that volumes are additive.

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2C(graphite) O2 (g) ? 2CO (g) What is DH for
this reaction?

• Given the following info
• C (graphite) O2 (g) ? CO2 (g) DH -393.5 kJ
• 2CO (g) O2 (g) ? 2CO2 (g) DH -566.0 kJ
• Since DH is a state function, we can construct
  an alternate pathway to get from reactants to
  products. This alternate pathway will allow us
  to calculate the enthalpy change for the reaction
  when we sum DH for the individual steps in the
  alternate process (Hesss Law of Heat Summation)

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2C(graphite) O2 (g) ? 2CO (g) What is DH for
this reaction?
Step 1 Step 2 Step 3 thus, DH1 DH2 DH3
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Hesss Law of Heat Summation

• The alternate pathway MUST be equivalent to the
  reaction of interest (i.e., the sum of the
  balanced chemical equations for each step must
  equal the balanced chemical equation for the
  reaction of interest)
• Whatever mathematical operation is carried out on
  a chemical equation must also be carried out on
  the corresponding value of DH for that equation.

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Text problem 6.75

• Given the following info
• C2H4 (g) 3O2 (g) ? 2CO2 (g) 2H2O (l) DH
  -1411 kJ
• 2C2H6 (g) 7O2 (g) ? 4CO2 (g) 6H2O (l) DH
  -3120 kJ
• 2H2 (g) O2 (g) ? 2H2O (l) DH -572 kJ
• Calculate the enthalpy change for
• C2H4 (g) H2 (g) ? C2H6 (g)

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Table 6.2 Standard Enthalpies of Formation
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CH4
Cl2
CH3Cl
HCl
Is this reaction likely to be exothermic or
endothermic? You can answer this question by
calculating bond energies for the reactants and
products
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H
H

C


Cl
H
Cl

H
ENERGY
CH3Cl
HCl

Energy required to break all bonds 1884
kJ/mol Energy released by bond formation - 1988
kJ/mol Energy change for reaction
-104 kJ/mol Therefore reaction is predicted to be
exothermic.
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Tables of Thermodynamic Data (including DHf
values) can be found at http//webbook.nist.gov/
chemistry/
Compound DHf (kJ/mol) CH4 -74.87 Cl2
0 CH3Cl -83.68 HCl -92.31
CH4
Cl2
CH3Cl
HCl
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Tables of Thermodynamic Data (including DHf
values) can be found at http//webbook.nist.gov/
chemistry/
Compound DHf (kJ/mol) CH4 (g) -74.87 Cl2 (
g) 0 CH3Cl (g) -83.68 HCl (g) -92.
31 C6H5COOH (s) -385.1 CO2 (g) -393.5 H2O
(l) -285.8