Todays goals

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Todays goals

• DHrxn
• Hesss law
• Product vs. reactant favored

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Admin stuff

• Next OWLs due Today, Thur.
• (More OWLs are added after every lecture)

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Enthalpy of reaction, DHrxn

• Virtually every chemical process involves heat
• CH4 2O2 -gt CO2 2H2O Heat
• H2O(s) Heat -gt H2O(l)
• Knowing the energetics of these processes are
  important for understanding the world around us,
  and making predictions about as-yet undiscovered
  chemical compounds and processes.

DHrxn -802kJ
DHrxn 44kJ
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Reaction enthalpies

• CH4(g) 2O2(g) -gt CO2(g) 2H2O(g) Heat
• exothermic, DHrxn -802 kJ

1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
802 kJ
DHrxn is the heat gained or lost in the course
of a reaction where the coefficients denote the
number of moles of each chemical species
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Reaction enthalpies

• CH4(g) 2O2(g) -gt CO2(g) 2H2O(g) 802 kJ
  heat
• exothermic, DHrxn -802 kJ

1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
802 kJ
What happens when the reaction is reversed?
CO2(g) 2H2O(g) 802 kJ -gt CH4(g) 2O2(g)
endothermic, DHrxn 802 kJ
DHrxn changes sign, magnitude unchanged (802kJ)
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DH is a state function
Energy
We are describing the relative energies of the
different arrangements of atoms
CH4 2O2
-802 kJ / mol
802 kJ / mol
CO2 2H2O
1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
(DH -802 kJ)
1 mol CO2 2 mol H2O -gt 1 mol CH4 2 mol O2
(DH 802 kJ)
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Reaction enthalpies

• CH4 2O2 -gt CO2 2H2O Heat
• exothermic, DHrxn -802 kJ

1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
802 kJ
What happens when the amount of products /
reactants are doubled?
2 mol CH4 4 mol O2 -gt products 1604 kJ heat
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Hesss Law DHrxn is additive
1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
(DH -802 kJ)
1 mol CH4 2 mol O2 -gt 1 mol CO2 2 mol H2O
(DH -802 kJ)
2 mol CH4 4 mol O2 -gt 2 mol CO2 4 mol H2O
(DH -1604 kJ)
2 mol CH4 1 mol O2 -gt 2 mol CH3OH (DH -252
kJ)
2 mol CH3OH 3 mol O2 -gt 2 mol CO2 4 mol H2O
(DH -1352 kJ)
2 mol CH4 4 mol O2 -gt 2 mol CO2 4 mol H2O
(DH -1604 kJ)
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Energy
Energy difference between start and finish is a
constant
2CH4 4O2
-252 kJ
-1604 kJ
2 CH3OH
-1352 kJ
2CO2 4H2O
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Which is more stable, diamond or graphite?
-393.5 kJ/mole to produce CO2 from graphite
-395.4 kJ/mole to produce CO2 from diamond
C(diamond)
C(graphite)
-1.9 kJ
Energy
-395.4 kJ
-393.5 kJ
CO2
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Calculating unknowns from DHrxn

• Want DHrxn of 2B(s) 3H2(g) -gt B2H6(g)
• Know DHrxn for related reactions
• 4B(s) 3O2(g) -gt 2B2O3(s) DH -2543.8 kJ
• H2(g) ½O2(g) -gt H2O(g) DH -241.8 kJ
• B2H6(g) 3O2(g) -gt B2O3(s) 3H2O(g) DH
  -2032.9 kJ

DHrxn 35.6 kJ
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Its all relative, but to what?

• We can define a DH for each of the components in
  a reaction - DHf

• DHf is the enthalpy change for the formation of
  1 mol of a compound directly from its component
  elements in their standard states
• -gt standard temp and pressure, 25 C and 1
  bar pressure

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Standard heat of formation DHf

• DHf O2(g) 0 kJ/mol
• DHf Ar(g) 0 kJ/mol
• DHf Fe(s) 0 kJ/mol
• DHf Hg(l) 0 kJ/mol

Zero for all elements in their standard form
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Standard Heats of Formation
DHf -74.8 kJ/mol CH4

• 1 mole C(s) 2 mol H2(g) -gt 1 mol CH4(g)
• 1 mole O2(g) -gt 1 mole O2(g)
• 1 mole C(s) 1 mole O2(g) -gt 1 mole CO2(g)
• 1 mole H2(g) ½ mole O2(g) -gt 1 mole H2O (l)

DHf 0 kJ/mol O2
DHf -393.5 kJ/mol CO2
DHf -241.8 kJ/mol H2O
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Its all relative, but to what?

• We can use DHf for the products and reactants to
  calculate DHrxn

DHrxn DHf(products) - DHf(reactants)
1 mol CH4(g) 2 mol O2(g) -gt 1 mol CO2(g) 2
mol H2O(g)
-74.8 kJ/mol 0 kJ / mol -393.5 kJ/mol
-241.8 kJ/mol (x2) lt- DHf
-74.8 kJ
-877.1 kJ
calc DHrxn to be -802 kJ same value as before
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Its all relative

• We can use Hesss law to calculate unknown DHf
  if DHrxn is measured.

DHrxn DHf(products) - DHf(reactants)
1 mol C10H8(g) 12 mol O2(g) -gt 10 mol CO2(g)
4 mol H2O(l)
DHf ??? 0 kJ / mol -393.5
kJ/mol -285.8 kJ/mol

• What is the enthalpy of formation of C10H8
  (napthalene) if we measure DHrxn for the above
  reaction using a calorimeter to be -5156 kJ?

78 kJ/mol
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Product-favored vs reactant-favored
Lower energy states are generally favored
1 mol CO2 2 mol H2O -gt 1 mol CH4 2 mol O2
(DH 802 kJ) Are the products or reactants
favored in this reaction?
A
B
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Unused slides
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Chemical energy - DHrxn
Same energy concept as before, however the
chemical energy will be contained within chemical
bonds, and will be released or gained during
chemical reactions.
A chemical reaction is run in which 440 Joules of
heat are generated and 635 Joules of work are
done on the system. Calculate the change in the
internal energy of the chemical system.
DE q w
previous lectures
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PRS

• Calorimeters always measure this
• Changes in internal energy (DE)
• Changes in enthalpy (DH)
• Exothermic processes
• Endothermic processes

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What can we measure in a calorimeter?
Enthalpies -- DH

• How much energy does ice absorb when it melts
  (DHfus)?
• How much energy is released when a hydrocarbon
  burns (octane rating of gasoline - DHrxn)?
• How much energy is heat is absorbed when ammonium
  nitrate dissolves? (cold packs - DHsol)

physical
q
chemical
chemical