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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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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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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• 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


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• 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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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.


of

o

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• 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.


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• 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


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• Using the summation law:

- Be careful of arithmetic signs as they are a

likely source of mistakes.


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

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General

Problem

6.83

122-9 hesses law

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