Dr. S. M. Condren Chapter 6 Thermochemistry. Dr. S. M. Condren Thermite Reaction.

Dr. S. M. Condren

Chapter 6

Thermochemistry

Dr. S. M. Condren

Thermite Reaction

Dr. S. M. Condren

Thermite Reaction

Dr. S. M. Condren

Terminology

Energy• capacity to do work

Kinetic Energy• energy that something has because it is moving

Potential Energy• energy that something has because of its position

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Kinetic Energy

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Chemical Potential Energy

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Chemical Potential Energy

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Internal Energy

• The sum of the individual energies of all nanoscale particles (atoms, ions, or molecules) in that sample.

• E = 1/2mc2

• The total internal energy of a sample of matter depends on temperature, the type of particles, and how many of them there are in the sample.

Dr. S. M. Condren

Energy Units

• calorie - energy required to heat 1-g of water 1oC

• Calorie - unit of food energy; 1 Cal = 1-kcal = 1000-cal

• Joule - 1-cal = 4.184 J = 1-kg*m2/sec2

Dr. S. M. Condren

Law of Conservation of Energy

• energy can neither be created nor destroyed

• the total amount of energy in the universe is a constant

• energy can be transformed from one form to another

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First Law of Thermodynamics

• the amount of heat transferred into a system plus the amount of work done on the system must result in a corresponding increase of internal energy in the system

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Thermochemistry Terminology

system => that part of the universe under investigation

surroundings => the rest of the universe

universe = system + surroundings

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Thermodynamic System

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Energy Transfer

Energy is always transferred from the hotter to the cooler sample

Heat – the energy that flows into or out of a system because of a difference in temperature between the thermodynamic system and its surroundings

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Thermochemistry Terminology

state properties => properties which depend only on the initial and final states

=> properties which are path independent

non-state properties => properties which are path dependent

state properties => E

non-state properties => q & w

Dr. S. M. Condren

Thermochemistry Terminology

exothermic - reaction that gives off energyendothermic - reaction that absorbs energychemical energy - energy associated with a

chemical reactionthermochemistry - the quantitative study of

the heat changes accompanying chemical reactions

thermodynamics - the study of energy and its transformations

Dr. S. M. Condren

2 H2 H2(g)2(g) + O + O2(g)2(g) --> 2 H --> 2 H22OO(g)(g) + heat and light + heat and light

Energy & Chemistry

This can be set up to provide This can be set up to provide ELECTRIC ENERGY ELECTRIC ENERGY in a in a fuel cell..

Oxidation:Oxidation: 2 H2 H22 ---> 4 H+ + 4 e- ---> 4 H+ + 4 e-

Reduction: Reduction: 4 e- + O2 + 2 H4 e- + O2 + 2 H22O ---> 4 OH-O ---> 4 OH-

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Energy & Chemistry

Dr. S. M. Condren

Enthalpy• heat at constant pressure

qp = H = Hproducts - Hreactants

Exothermic ReactionH = (Hproducts - Hreactants) < 0

H2O(l) -----> H2O(s) H < 0

Endothermic ReactionH = (Hproducts - Hreactants) > 0

H2O(l) -----> H2O(g) H > 0

Dr. S. M. Condren

Enthalpy

H = E + PV

H = E + PV

E = H – PV

Where text uses U for internal energy

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Pressure-Volume Work

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First Law of Thermodynamics

heat => q

internal energy => E

internal energy change =>E

work => w

E = q - w

(Engineering convention)

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Specific Heat

• the amount of heat necessary to raise the temperature of 1 gram of the substance 1oC

• independent of mass

• substance dependent

• s.h.

• Specific Heat of Water = 4.184 J/goC

Dr. S. M. Condren

Heat

q = m * s.h. * t

where q => heat, J

m => mass, g

s.h. => specific heat, J/g*oC

t = change in temperature, oC

Dr. S. M. Condren

Molar Heat Capacity

• the heat necessary to raise the temperature of one mole of substance by 1oC

• substance dependent

• C

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Heat Capacity

• the heat necessary to raise the temperature 1oC

• mass dependent

• substance dependent

• C

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Heat Capacity

C = m X s.h.

where C => heat capacity, J/oC

m => mass, g

s.h. => specific heat, J/goC

Dr. S. M. Condren

Plotted are graphs of heat absorbed versus temperature for two systems. Which system has the larger heat capacity?

A, B

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Heat Transfer

qlost = - qgained

(m X s.h. X t)lost = - (m X s.h. X t)gained

Dr. S. M. Condren

EXAMPLE If 100. g of iron at 100.0oC is placed in 200. g of water at 20.0oC in an insulated container, what will the temperature, oC, of the iron and water when both are at the same temperature? The specific heat of iron is 0.106 cal/goC.

(100.g*0.106cal/goC*(Tf - 100.)oC) = qlost

- qgained = (200.g*1.00cal/goC*(Tf - 20.0)oC)

10.6(Tf - 100.oC) = - 200.(Tf - 20.0oC)

10.6Tf - 1060oC = - 200.Tf + 4000oC

(10.6 + 200.)Tf = (1060 + 4000)oC

Tf = (5060/211.)oC = 24.0oC

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EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC?

q = Hice

+ Hfusion

+ Hwater

+ boil.

+ steam

q = Hice + Hfusion + Hwater + boil. + steam

Dr. S. M. Condren

Heat Transfer

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*((0.0 – (-15.0))oC))

Mass of the ice specific heat of iceTemperature change

{

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*15.0oC)

+ (10.0g*333J/g)

Mass of ice Heat of fusion

Melting of ice occurs at aconstant temperature

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*15.0oC)

+ (10.0g*333J/g)

+ (10.0g*4.18J/goC*((100.0-0.00)oC))

Mass of water Specific heat of liquid water

Temperature change of the liquid water

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*15.0oC)

+ (10.0g*333J/g)

+ (10.0g*4.18J/goC*100.0oC)

+ (10.0g*2260J/g)

Mass of water Heat of vaporization

Boiling of water occurs at aconstant temperature

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*15.0oC)

+ (10.0g*333J/g)

+ (10.0g*4.18J/goC*100.0oC)

+ (10.0g*2260J/g)

+ (10.0g*2.03J/goC*((127.0-100.0)oC))

Mass of steam Specific heatof steam

Temperature change for the steam

Dr. S. M. Condren

EXAMPLE: How much heat is required to heat 10.0 g of ice at -15.0oC to steam at 127.0oC? q = Hice + Hfusion + Hwater + boil. + steam

q = (10.0g*2.09J/goC*15.0oC)

+ (10.0g*333J/g)

+ (10.0g*4.18J/goC*100.0oC)

+ (10.0g*2260J/g)

+ (10.0g*2.03J/goC*27.0oC)

q = (314 )J+ 3.33X103 + 4.18X103 + 2.26X104 + 548

= 30.96 kJ

Dr. S. M. Condren

Spreadsheet of Previous Problem

10 2.09 15 313.5 314 31 x1010 333 1 3330 3330 333 x1010 4.18 100 4180 4180 418 x1010 2260 1 22600 22600 2260 x1010 2.03 27 548.1 548 54 x10

3096 x10

30.96 x10̂ 3

Dr. S. M. Condren

Bomb CalorimeterParr

calorimeter

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EXAMPLE

A 1.000g sample of a particular compound produced 11.0 kJ of heat. The temperature of the calorimeter and 3000 g of water was raised 0.629oC. How much heat is gained by the calorimeter?

heat gained = - heat lostheatcalorimeter + heatwater = heatreaction

heatcalorimeter = heatreaction - heatwater

Dr. S. M. Condren

EXAMPLE

A 1.000g sample of a particular compound produced 11.0 kJ of heat. The temperature of the calorimeter and 3000 g of water was raised 0.629oC. How much heat is gained by the calorimeter?

heatcalorimeter = heatreaction - heatwater

heat = 11.0 kJ - ((3.00kg)(0.629oC)(4.184kJ/kgoC))

= 3.1 kJ

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Example

What is the mass of water equivalent of the heat absorbed by the calorimeter?

#g = (3.1 kJ/0.629oC)(1.00kg*oC/4.184kJ)

= 6.5 x 102 g

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Example

A 1.000 g sample of ethanol was burned in the sealed bomb calorimeter described above. The temperature of the water rose from 24.284oC to 26.225oC. Determine the heat for the reaction.

m = (3000 + "647")g H2Oq = m X s.h. X t = (3647g)(4.184J/goC)(1.941oC) = 29.61 kJ

Dr. S. M. Condren

When graphite is burned to yield CO2, 394 kJ of energy are released per mole of C atoms burned. When C60 is burned to yield CO2 approximately 435 kJ of energy is released per mole of carbon atoms burned. Would the buckyball-to-graphite conversion be exothermic or endothermic?

exothermic, endothermic

Dr. S. M. Condren

Laws of Thermochemistry

1. The magnitude of is directly proportional to the amount of reactant or product.

s --> l H => heat of fusion

l --> g H => heat of vaporization

Dr. S. M. Condren

Laws of Thermochemistry

2. H for a reaction is equal in magnitude but opposite in sign to H for the reverse reaction.

H2O(l) -----> H2O(s) H < 0

H2O(s) -----> H2O(l) H > 0

Dr. S. M. Condren

Laws of Thermochemistry

3. The value of H for the reaction is the same whether it occurs directly or in a series of steps.

Hoverall = H1 + H2 + H3 + · · ·

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Hess' Law

• a relation stating that the heat flow in a reaction which is the sum of a series of reactions is equal to the sum of the heat flows in those reactions

Dr. S. M. Condren

EXAMPLECH4(g) + 2 O2(g) -----> CO2(g) + 2 H2O(l)

CH4(g) -----> C(s) + 2 H2(g) H1

2 O2(g) -----> 2 O2(g) H2

C(s) + O2(g) -----> CO2(g) H3

2 H2(g) + O2(g) -----> 2 H2O(l) H4

---------------------------------------------

CH4(g) + 2 O2(g) -----> CO2(g) + 2 H2O(l)

Hoverall = H1 + H2 + H3 + H4

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Standard Enthalpy of Formation

the enthalpy associated with the formation of a substance from its constituent elements under standard state conditions

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Calculation of Ho

Ho = c*Hfoproducts - c*Hf

oreactants

Dr. S. M. Condren

Example What is the value of Hrx for the reaction:

2 C6H6(l) + 15 O2(g) --> 12 CO2(g) + 6 H2O(g)

from Appendix J Text

C6H6(l) Hfo = + 49.0 kJ/mol

O2(g) Hfo = 0

CO2(g) Hfo = - 393.5

H2O(g) Hfo = - 241.8

Hrx c*Hfoproduct - c*Hf

oreactants

Dr. S. M. Condren

ExampleWhat is the value of Hrx for the reaction:

2 C6H6(l) + 15 O2(g) --> 12 CO2(g) + 6 H2O(g)

from Appendix J TextC6H6(l) Hf

o = + 49.0 kJ/mol; O2(g) Hfo = 0

CO2(g) Hfo = - 393.5; H2O(g) Hf

o = - 241.8 Hrx c*Hf

oproduct - c*Hf

oreactants Hrx - 393.5) + 6(- 241.8)product

- 2(+ 49.0 ) + 15(0)reactants kJ/mol

= - 6.271 x 103 kJ

Dr. S. M. Condren

ExampleWhat is the value of Hrx for the reaction:

Fe2O3(s) + 2 Al(s) --> 2 Fe(l) + Al2O3(s)

from Appendix J Text Fe2O3(s) Hf

o = -825.5 kJ/mol; Al(s) Hfo = 0 kJ/mol Al2O3(s)

Hfo = - 1675.7 kJ/mol; Fe(l) Hf

o = +12.4 kJ/mol

Hrx c*Hfoproduct - c*Hf

oreactants

Hrx - 1675.7) + 2(- 12.4)product

- 1(-825.5 ) + 2(0)reactants kJ/mol

= - 2.4764 x 103 kJ

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Thermite Reaction on Saturday

~150g Fe2O3 ~1 mol Fe2O3 ~2.5x106J ~2.5MJ

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Fossil Fuels

natural gascoal petroleum

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Energy Sources

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Based on 1998 Data