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7/27/2019 122-9 hesses law
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• Hess’s law of heat summation states that
for a chemical equation that can be written
as the sum of two or more steps, theenthalpy change for the overall equation is
the sum of the enthalpy changes for the
individual steps.
Hess’s Law
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A B + C H = x
B + C D H = y
A D H = ?
H = x + y
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Figure 6.7: Campsite to illustrate altitude.
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Figure 6.13: Enthalpy diagram
illustrating Hess’s law.
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• For example, suppose you are given the
following data:
Hess’s Law
kJ-297H );g(SO)g(O)s(So
22
kJ198H );g(O)g(SO2)g(SO2o
223
• Could you use these data to obtain the enthalpychange for the following reaction?
?H );g(SO2)g(O3)s(S2o
32
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• If we multiply the first equation by 2 and
reverse the second equation, they will sum
together to become the third.
Hess’s Law
(2)kJ)-297(H );g(SO2)g(O2)s(S2 o22
(-1)kJ)198(H );g(SO2)g(O)g(SO2 o322
kJ -792H );g(SO2)g(O3)s(S2o
32
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Standard Enthalpies of
Formation• The term standard state refers to the
standard thermodynamic conditions chosen
for substances when listing or comparingthermodynamic data: 1 atmosphere pressure
and the specified temperature (usually 25 oC).
– The enthalpy change for a reaction in whichreactants are in their standard states is denoted
Ho (“delta H zero” or “delta H naught”).
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Standard Enthalpies of
Formation• The standard enthalpy of formation of a
substance, denoted Hf o, is the enthalpy
change for the formation of one mole of asubstance in its standard state from its
component elements in their standard
state. – Note that the standard enthalpy of formation for
a pure element in its standard state is zero.
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Standard Enthalpies of
Formation• The law of summation of heats of
formation states that the enthalpy of a
reaction is equal to the total formationenergy of the products minus that of the
reactants.
S is the mathematical symbol meaning “the
sum of”, and m and n are the coefficients of the
substances in the chemical equation.
)reactants(Hm)products(HnHo
f
o
f
o
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A Problem to Consider
• Large quantities of ammonia are used to
prepare nitric acid according to the
following equation:
- What is the standard enthalpy change for thisreaction? Use Table 6.2 for data.
)g(OH6)g(NO4)g(O5)g(NH4 223
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A Problem to Consider
• You record the values of Hf o under the
formulas in the equation, multiplying them
by the coefficients in the equation.
- You can calculate Ho by subtracting the values
for the reactants from the values for the
products.
)9.45(4 )0(5 )3.90(4 )8.241(6
)g(OH6)g(NO4)g(O5)g(NH4 223
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A Problem to Consider
• Using the summation law:
- Be careful of arithmetic signs as they are a
likely source of mistakes.
)reactants(Hm)products(HnHof
of
o
kJ)]0(5)9.45(4[kJ)]8.241(6)3.90(4[Ho
kJ906Ho
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Fuels
• A fuel is any substance that is burned to
provide heat or other forms of energy.
• In this section we will look at:- Foods as fuels
- Fossil fuels
- Coal gasification and liquefaction
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Suggested problems for Ch. 6:
33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 55, 59, 63, 65, 67, 69, 71,
73, 81, 83, 87, 89, 91, 103
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Standard Enthalpies of
Formation• The law of summation of heats of
formation states that the enthalpy of a
reaction is equal to the total formationenergy of the products minus that of the
reactants.
S is the mathematical symbol meaning “the
sum of”, and m and n are the coefficients of the
substances in the chemical equation.
)reactants(Hm)products(HnHof
of
o
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A Problem to Consider
• Large quantities of ammonia are used to
prepare nitric acid according to the
following equation:
- What is the standard enthalpy change for thisreaction? Use Table 6.2 for data.
)g(OH6)g(NO4)g(O5)g(NH4 223
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A Problem to Consider
• You record the values of Hf o under the
formulas in the equation, multiplying them
by the coefficients in the equation.
- You can calculate Ho by subtracting the values
for the reactants from the values for the
products.
)9.45(4 )0(5 )3.90(4 )8.241(6
)g(OH6)g(NO4)g(O5)g(NH4 223
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A Problem to Consider
• Using the summation law:
- Be careful of arithmetic signs as they are a
likely source of mistakes.
)reactants(Hm)products(HnHof
of
o
kJ)]0(5)9.45(4[kJ)]8.241(6)3.90(4[Ho
kJ906Ho
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How is the heat of sublimation, Hsub, the enthalpychange for the reaction:
H2O(s) H2O(g)
related to Hfis
and
Hvap
?
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Fuels
• A fuel is any substance that is burned to
provide heat or other forms of energy.
• In this section we will look at:
- Foods as fuels
- Fossil fuels- Coal gasification and liquefaction
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Fuels
• Food fills three needs of the body:
- It supplies substances for the growth and repair
of tissue.- It supplies substances for the synthesis of
compounds used in the regulation of body
processes.
- It supplies energy. About 80% of the energy we
need is for heat. The rest is used for muscular
action and other body processes
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Figure 6.15:
Sources of energy
consumed in
the United
States
(1996).
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Fuels
• A typical carbohydrate food, glucose (C6H12O6)undergoes combustion according to the followingequation.
)g(O6)s(OHC 26126
kJ-2803H );l(OH6)g(CO6o
22
• One gram of glucose yields 15.6 kJ (3.73 kcal)
when burned.
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Fuels
• A representative fat is glyceryl trimyristate,
C45H86O6. The equation for its combustion is:
)g(O)s(OHC 2212768645
kJ -27,820H );l(OH43)g(CO45o
22
• One gram of fat yields 38.5 kJ (9.20 kcal) when burned. Note that fat contains more than twice the
fuel per gram than carbohydrates contain.
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Figure6.16:
Thelaunchingof theColumbia
spaceshuttle.Photocourtesyof NASA.
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Fuels
• Fossil fuels account for nearly 90% of the
energy usage in the United States.
– Anthracite, or hard coal, the oldest variety of coal,
contains about 80% carbon.
– Bituminous coal, a younger variety of coal,
contains 45% to 65% carbon.
– Fuel values of coal are measured in BTUs (BritishThermal Units).
– A typical value for coal is 13,200 BTU/lb.
– 1 BTU = 1054 kJ
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Fuels
• Natural gas and petroleum account for nearly
three-quarters of the fossil fuels consumed per year.
– Purified natural gas is primarily methane, CH4
, but
also contains small quantities of ethane, C2H6,
propane, C3H8, and butane, C4H10.
– We would expect the fuel value of natural gas to be
close to that for the combustion of methane.
)g(OH2)g(CO)g(O2)g(CH 2224 kJ 802H ; o
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Fuels
• Petroleum is a very complicated mixture of
compounds.
– Gasoline, obtained from petroleum, contains many
different hydrocarbons, one of which is octane,C8H18.
)g(O)l(HC 2225
188
kJ -5,074H );g(OH9)g(CO8 o22
– This value of Ho is equivalent to 44.4 kJ/gram.
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Fuels• With supplies of petroleum estimated to be 80%
depleted by the year 2030, the gasification of coal has become a possible alternative.
– First, coal is converted to carbon monoxide usingsteam.
)g(H)g(CO)g(OH)s(C 22
– The carbon monoxide can then be used to producea variety of other fuels, such as methane.
)g(OH)g(CH)g(H3)g(CO 242
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General
Problem
6.84
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Operational Skills• Calculating kinetic energy.
• Writing thermochemical equations.
• Manipulating thermochemical equations.
• Calculating the heat of reaction from thestoichiometry.
• Relating heat and specific heat.
• Calculating H from calorimetric data.
• Applying Hess’s law.
• Calculating the enthalpy of reaction from standardenthalpies of formation.
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Practice Problem 6.45
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Practice Problem 6.46