Matter and Measurement

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• Enthalpy
• C6H12O6(s) 6O2 (g) --gt 6CO2 (g) 6H2O(l)
  2803 kJ
• 2C57H110O6 163O2 (g) --gt 114 CO2(g) 110
  H2O(l) 75,520 kJ
• The heat lost or gained by a system undergoing a
  process at constant pressure is related to the
  change in ENTHALPY (DH) of the system
• DH Hfinal - Hinitial qp Note DH is a state
  function
• Most physical and chemical changes take place
  under the constant pressure of the Earths
  atmosphere.

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For an exothermic process at constant pressure DH
lt 0 For an endothermic process at constant
pressure DH gt 0
DH gt 0
DH lt 0
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Enthalpy, H is defined as H E PV For a
system undergoing pressure-volume work, the
change in enthalpy in the system is DH DE
DPV If the pressure is a constant external
pressure, Pext, DH DE PextDV From the 1st
law DE q w For constant P DH qp w
PextDV Since w - Pext DV DH qp - Pext DV
PextDV qp
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If volume is held constant, no pressure-volume
work can be done on that system or by that
system. work - PextDV 0 at constant
volume DE q w, For constant volume
processes, DE qv A constant pressure
calorimeter measures DH A constant volume
calorimeter (like a bomb calorimeter) measures
DE.
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Relationship between DH and DE
C(s) 1/2 O2(g) --gt CO(g) DH -110.5
kJ Determine the change in internal energy
accompanying this reaction. DH DE D (PV) DE
DH - D (PV) D(PV) D (nRT) RT(Dng) DE
DH - RT(Dng) or DH DE RTDng For this
reaction Dng 0.5 mol hence DE -111.7 kJ
For reactions that do not involve gases DH DE
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Enthalpy of Physical Change
Phase transitions involve change in
energy Vaporization is endothermic condensation
exothermic. Since phase transitions typically
take place at constant pressure, the heat
transfer is the change in enthalpy. DHvap
Hvapor - Hliquid liquid --gt gas DHfusion
Hliquid - Hsolid solid --gt liquid DHfreezing -
DHfusion
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DHforward - DHreverse
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DHsublimation DHfusion DHvaporization
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Heating curve of water
H2O(s) --gt H2O(l) DH 6.01 kJ H2O(l) --gt H2O(g)
DH 40.7 kJ
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Enthalpy of Chemical Change The enthalpy change
for a chemical reaction is given by DH
SH(products) - SH(reactants) For example 2H2(g)
O2 (g) --gt 2H2O(g) DH - 483.6 kJ
Thermochemical reaction
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In a chemical reaction, the enthalpy change
during the reaction indicates whether the
reaction releases energy or consumes energy. If
DH lt 0, the reaction releases heat and is
EXOTHERMIC If DH gt 0, the reaction absorbs heat
and is ENDOTHERMIC
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• The magnitude of DH for a reaction is directly
  proportional to the amount of reactants consumed
  by the reaction.
• CH4(g) 2O2(g) --gt CO2(g) 2H2O(l) DH -890
  kJ
• Calculate the amount of heat that would be
  released when 4.50 g of CH4(g) is burned in an
  oxygen atmosphere at constant pressure.
• 4.50g CH4 gt 0.28 mole CH4
• gt (0.28 mol CH4)(-890 kJ/mol CH4) -250 kJ

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• The enthalpy change for a reaction is equal in
  magnitude but opposite in sign to DH for the
  reverse reaction
• CH4(g) 2 O2(g) --gt CO2(g) 2H2O(l) DH -890
  kJ
• CO2(g) 2H2O(l) --gt CH4(g) 2 O2(g) DH
  890 kJ
• The enthalpy change for a reaction depends on the
  phase of the reactants and products.
• 1) CH4(g) 2 O2(g) --gt CO2(g) 2H2O(l) DH
  -890 kJ
• 2) CH4(g) 2 O2(g) --gt CO2(g) 2H2O(g) DH
  -802 kJ
• 3) 2H2O(g) --gt 2H2O(l) DH -88kJ

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Standard Reaction Enthalpies CH4(g) 2 O2(g)
--gt CO2(g) 2H2O(l) DH -890 kJ CH4(g) 2
O2(g) --gt CO2(g) 2H2O(g) DH -802 kJ
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In order to compare DHs accompanying reactions
need to define a standard state
Standard State reactant and products in their
pure forms at a pressure of 1 atm (or 1 bar). A
solute in a liquid solution is in its standard
state when its concentration is 1 mol L-1
While temperature is not part of the definition
of the standard state, standard reaction
enthalpies are usually reported for a temperature
of 298.15 K.
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Standard Reaction Enthalpies (DHo) enthalpy
change accompanying a reaction when reactants in
their standard states change to products in their
standard states.
CH4(g) 2 O2(g) --gt CO2(g) 2H2O(l) DHo
-890 kJ
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• Hesss Law
• If a reaction is carried out in a series of
  steps, DH for the reaction will be equal to the
  sum of the enthalpy change for the individual
  steps.

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• The enthalpy of combustion of C to CO2 is -393.5
  kJ/mol, and the enthalpy of combustion of CO to
  CO2 is -283.0 kJ/mol CO.
• (1) C(s) O2 (g) --gt CO2 (g) DH -393.5 kJ

Calculate the enthalpy change of combustion of C
to CO
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• C(s) O2 (g) --gt CO2 (g) DH -393.5 kJ

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Standard Enthalpy of Combustion
Change in enthalpy per mole of a substance that
is burned in a combustion reaction under standard
conditions
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Calculate the mass of propane that you would need
to burn to obtain 350 kJ of heat which is just
enough energy to heat I L of water from room
temperature (20oC) to boiling at sea level.
Assume that all the heat generated is absorbed by
the water. C3H8(g) 5 O2(g) --gt 3 CO2(g)
4H2O(l) DHo -2220 kJ Combustion of 1 mole of
propane generates 2220 kJ of heat Mass or
propane required (350 kJ) (1 mole C3H8
/2220kJ) (44.09 g/mol) 6.95 g
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If you were to use butane instead of propane in
the previous example, how much butane (in grams)
would you need? 2 C4H10(g) 13 O2(g) --gt 8
CO2(g) 10 H2O(l) DHo -5756 kJ Answer 7.07
g If you were to use ethanol instead of propane
in the previous example, how much ethanol (in
grams) would you need? C2H5OH(l) 3 O2(g) --gt 2
CO2(g) 3 H2O(l) DHo -1368 kJ Answer 11.8 g
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• Enthalpies of Formation
• The enthalpy of formation, DHf, or heat of
  formation, is defined as the change in enthalpy
  when one mole of a compound is formed from its
  stable elements.
• The standard enthalpy of formation (DHfo) of a
  compound is defined as the enthalpy change for
  the reaction that forms 1 mole of compound from
  its elements, with all substances in their
  standard states.

2C(s) 1/2 O2(g) 3 H2 (g) --gt C2H5OH(l) DHfo
-277.69 kJ
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Standard enthalpies of formation of substance can
be used to determine standard reaction enthalpies.
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• The standard enthalpy of formation of the most
  stable form of an element under standard
  conditions is ZERO.
• DHfo for C(graphite), H2(g), O2(g) are zero

The stoichiometry for formation reactions
indicate the formation of 1 mole of the desired
compound, hence enthalpies of formation are
always listed as kJ/mol.