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Dr. S. M. Condren
Chapter 6
Principles of Reactivity:
Energy and
Chemical Reactions
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 or its chemical bonding
Dr. S. M. Condren
Kinetic Energy
Dr. S. M. Condren
Chemical Potential Energy
Dr. S. M. Condren
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
Dr. S. M. Condren
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
Dr. S. M. Condren
Thermochemistry Terminology
system => that part of the universe under investigation
surroundings => the rest of the universe
universe = system + surroundings
Dr. S. M. Condren
System and Surroundings
• SYSTEM–The object under
study
• SURROUNDINGS–Everything outside
the system
Dr. S. M. Condren
Announcement
Learn@UW will be unavailable
on Thursday July 19
from 5:00am until 12:00 noon.
Dr. S. M. Condren
2 H2 H2(g)2(g) + O + O2(g)2(g) --> -->
2 H2 H22OO(g)(g) + heat and light + heat and light
This can be set up to provide This can be set up to provide ELECTRIC ENERGY in a ELECTRIC ENERGY in a fuel cell..Oxidation:Oxidation: 2 H2 H22 ---> 4 H ---> 4 H++ + 4 e + 4 e--
Reduction: Reduction: 4 e4 e-- + O + O22 + 2 H + 2 H22O ---> 4 OHO ---> 4 OH--
Energy & Chemistry
H2/O2 Fuel CellEnergy, page 288
Dr. S. M. Condren
ENERGY is the capacity to do work or is the capacity to do work or transfer heat.transfer heat.
HEAT is the form of energy that flows is the form of energy that flows between 2 objects because of their between 2 objects because of their difference in temperature.difference in temperature.
Other forms of energy —Other forms of energy —
• lightlight
• electricalelectrical
• kinetic and potentialkinetic and potential
Energy & Chemistry
Dr. S. M. Condren
• Positive and negative particles (ions) attract one another.
• Two atoms can bond
• As the particles attract they have a lower potential energy NaCl — composed
of Na+ and Cl- ions.
http://mrsec.wisc.edu/Edetc/pmk/NaCl_alt.html
Potential Energy in the Atomic Scale
Dr. S. M. Condren
PE + KE = Internal energy (E or U)PE + KE = Internal energy (E or U)
Int. E of a chemical system Int. E of a chemical system depends ondepends on–number of particlesnumber of particles
–type of particlestype of particles
–temperaturetemperature
Internal Energy (E)
Dr. S. M. Condren
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
Dr. S. M. Condren
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
Exothermic Reaction
First-Aid Hotpacks, containing either calcium chloride or magnesium sulfate, plus water
Dr. S. M. Condren
Endothermic Reaction
First-aid cold packs, containing ammonium nitrate and water in separate inner pouches
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
Dr. S. M. Condren
First Law of Thermodynamics
heat => q
internal energy => E
internal energy change =>E
work => w = - P*V
E = q + w
Dr. S. M. Condren
Specific Heat-Specific Heat Capacity
• 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,
(always tf – ti)
Dr. S. M. Condren
Molar Heat Capacity
• the heat necessary to raise the temperature of one mole of substance by 1oC
• substance dependent
• C
Dr. S. M. Condren
Heat Capacity
• the heat necessary to raise the temperature 1oC
• mass dependent
• substance dependent
• C
Dr. S. M. Condren
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
Dr. S. M. Condren
Heat Transfer
qlost = - qgained
(m X s.h. X t)lost = - (m X s.h. X t)gained
Dr. S. M. Condren
Heat Transfer
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
Dr. S. M. Condren
Melting of Ice
http://mrsec.wisc.edu/Edetc/pmk/ice.html
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 = 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
Coffee Cup Calorimeter
Dr. S. M. Condren
Bomb Calorimeter
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?
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 - ((3.000kg)(0.629oC)(4.184kJ/kgoC))
= 3.10 kJ
Dr. S. M. Condren
Example
What is the mass of water equivalent of the heat absorbed by the calorimeter?
#g = (3.10 kJ/0.629oC)(1.00kg*oC/4.184kJ)
= 6.47 x 102 g
Dr. S. M. Condren
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 + · · ·
Dr. S. M. Condren
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
Dr. S. M. Condren
Standard Enthalpy of Formation
the enthalpy associated with the formation of a substance from its constituent elements under standard state conditions
Dr. S. M. Condren
Calculation of Ho
Ho = c*Hfo
products - c*Hfo
reactants
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 L Text
C6H6(l) Hfo = + 48.95 kJ/mol
O2(g) Hfo = 0
CO2(g) Hfo = - 393.509
H2O(g) Hfo = - 241.83
Hrx c*Hfoproducts - 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 L TextC6H6(l) Hf
o = + 48.95 kJ/mol; O2(g) Hfo = 0
CO2(g) Hfo = - 393.509; H2O(g) Hf
o = - 241.83 Hrx c*Hf
oproducts - c*Hforeactants
Hrx 1mol)- 393.509kJ/mol)
+ (6mol)(- 241.83kJ/mol)products
- 2mol)(+ 48.95kJ/mol)
+ (15mol)(0kJ/mol)reactants
= - 6.271 x 103 kJ
Dr. S. M. Condren
Fossil Fuels
natural gascoal petroleum
Dr. S. M. Condren
Based on 1998 Data
