Thermochemistry

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Thermochemistry

• Chapter 6

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The Basics of Thermochemistry

• Enthalpy
• A means of quantifying chemical energy (heat)
• The first law of thermodynamics
• All energy must be accounted for
• Hesss Law
• Known reactions can be used to determine
  energetics of complex, unknown reactions
• Energy
• The capacity to do work or transfer heat
• Potential Energy Kinetic Energy

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Energy-The capacity to do work or transfer heat
First Law of Thermodynamics The Energy in the
Universe is Constant ?E q w q heat
w work
Common Forms of Energy

• Radiant Energy
• Electromagnetic Radiation
• Thermal Energy
• Associated with random motion of a molecule or
  atom
• Chemical Energy
• Energy stored within the structural limits of a
  molecule or atom

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Units

• Joule E kg m2/s2
• 1 Joule(J) 0.239 calories(cal)
• 4.184 cal 1 Joule
• 1 calorie 3.965 x 10-3 btu
• 1 kcal 1 Cal(food calorie)

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Endothermic and Exothermic Reactions
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Enthalpy - H (A state function)

• A description for heat gained () or lost (-) by
  a closed system (constant pressure)
  ?????????final??????initial?
• A state function - depends only on a systems
  present state, not how it got there.

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Heat Capacity - The heat absorbed by the system
is a function of the mass and the ability to
absorb heat.

• Specific Heat Capacity
• Joules/ C g

• Molar Heat Capacity
• Joules/ K mole

Note 1cal energy to raise 1 gram of water 1
C So, sp. Heat of water also, 1cal/C g
Heat lost - Heat gained
Heat sp.heat (mass)(DT)
Heat heat capacity(mole)(DT)
DT T(final) - T(initial) (Tf - Ti)
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Calorimetry
What is the final temperature of 50.0 mL of water
at 23.2C to which 200. J of energy has been
added?
Heat sp. Heat (mass) (DT)
200. J 2.03 J/C g(50.0 g) (Tf- 23.2C)
1.97 C Tf- 23.2C
Tf 25.2 C
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Bond Energies/Strengths

• The Bond Dissociation Energy is the ?H for the
  bond breaking reaction in the gas phase.
• CH4 (g) gt C (g) 4 H (g) ?H 1660 kJ

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Enthalpies of Formation -Heat of Formation

• In order to standardize a set of data, chemists
  have established a set of conditions under which
  ?Hfo are reported.
• Standard State - most stable form of an element
  at STP
• STP - standard temperature and pressure
• P 105 Pascals 1 atm
• T 298 K (25 oC) note difference from Gas Laws

For an element, the heat of formation is defined
as 0.0 kJ/mol under STP conditions.
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To Calculate ?Ho Reaction

• ?Horxn ? n?Hfo(prod) - ? m?Hfo(react)
• Remember to change the sign of ?Hfo if the
  reaction is reversed.
• If the formation reaction must be multiplied by
  an integer, ?Hfo must be as well.
• Elements in their standard states dont
  contribute to ?Ho.

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An example - Alternative Automobile Fuels
Methanol is sometimes used as a fuel alternative
to octane (gasoline) in high performance race
cars. Determine the enthalpy of combustion per
gram of each given the following standard
enthalpies of formation. ?Hfo CH3OH (l)
-239 kJ/mole ?Hfo C8H18 (l) -269 kJ/mole ?Hfo
CO2 (g) -394 kJ/mole ?Hfo H2O (l) -286
kJ/mole
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Methanol Combustion
2CH3OH (l) 3O2 (g) gt 2CO2 (g) 4H2O
(l) ?Horeaction ? n?Hfo(prod) - ?
m?Hfo(react) 2 x ?Hfo (CO2) 4 x ?Hfo (H2O) -
2 x ?Hfo (CH3OH) - 3 x ?Hfo (H2O) 2 x (-394
kJ) 4 x (-286 kJ) - 2 x (-239 kJ) -1454 kJ/
2 moles CH3OH
Conversion to ?Hfo/g
?Horeaction/g -1454 kJ/ 2 moles CH3OH
-1454 kJ/2 moles x (32.0 g/mol)
-22.7 kJ/g
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Standard Enthalpies of Formation

• Hydrogen sulfide gas is a poisonous gas with the
  odor of rotten eggs. The gas burns in oxygen as
  follows Calculate the
  standard enthalpy change for this reaction using
  standard enthalpies of formation.

2H2S(g) 3O2(g)
2H2O(l) 2SO2(g)
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Standard Enthalpies of Formation
2H2S(g) 3O2(g)
2H2O(l) 2SO2(g)
2(-20)
3(0)
2(-285.8)
2(-296.8)
(kJ)
?Ho ?n ?Ho (products) - ?m ?Ho (reactants)
2(-285.8) 2(-296.8)-2(-20) 3(0)
kJ -1125.2 -1125 kJ