Download - Chemistry and Heat
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Chemistry and Heat
That’s hot stuff.
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Energy is the capacity to do work.
Energy is measured in Joules
1 Joule of energy can raise 1 N of weight exactly 1 meter
1 J=1N•m
(PS: your diet offers ~5-10 million J/day
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Energy is the capacity to do work.
Forms include:
• Kinetic energy
• Gravitational potential energy
• Elastic potential energy
• Electrical energy
• Chemical potential energy
• Heat
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Energy is the capacity to do work.
Forms include:
• Kinetic energy
• Gravitational potential energy
• Elastic potential energy
• Electrical energy
• Chemical potential energy
• Heat …of the greatest interest to a
chemist
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Exothermic process
System
Surroundings
An exothermic process releases energy
Energy
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Endothermic process
System
Surroundings
Energy
An endothermic process absorbs energy
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If you add heat to matter, it may…
a)
b)
c)
d)
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If you add heat to matter, it may…
a) warm up.
b) melt
c) boil
d) expand (tough to calculate, don’t bother)
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Let’s try to warm up a cup of cold coffee.
Step 1: Add heat.
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Let’s try to warm up a cup of cold coffee.
Step 1: Add heat.
Well, that was easy.
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Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
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Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
a)
b)
c)
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Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
a) Raise the temperature only half as much.
b)
c)
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Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
a) Raise the temperature only half as much.
b) Use half as much coffee (and cup)
c)
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Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
a) Raise the temperature only half as much.
b) Use half as much coffee (and cup)
c) Use a different substance
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The effect of heat, q!
• When something warms up:
The heat, q, depends on:
• The mass of the sample (m)
• The change in temperature (T)
• The nature of the sample (C)
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The effect of heat (q)
• When something warms up:
The heat, q, depends on:
• The mass of the sample (m)
• The change in temperature (T)
• The nature of the sample (C)
C is the specific heat capacity for a given substance. Its units are (J/goC)
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If you add heat to a sample, it may…
a) warm up. q=mCT
b) melt
c) boil
d) expand (tough to calculate, don’t bother)
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q=mCT
• q – heat, in Joules
• m –mass, in grams
• C –specific heat capacity, in J/goC
• T—change in temperature (Tfinal-Tinitial)
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Cwater=4.184 J/goC
• Cwater =4.2 J/goC
• Cethanol =2.4 J/goC
• Cice =2.1 J/goC
• CAl =.90 J/goC
• CFe =.46 J/goC
• Cglass =.50 J/goC
• CAg =.24 J/goC
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How much heat?
• How much heat does it take to raise 50.g water from 15oC to 80.oC?
• q=mCT
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How much heat?
• How much heat does it take to raise 50.g water from 15oC to 80.oC?
• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC)
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How much heat?
• How much heat does it take to raise 50.g water from 15oC to 80.oC?
• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC) = 50.g x 4.18 J/goC x (65oC)
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How much heat?
• How much heat does it take to raise 50.g water from 15oC to 80.oC?
• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC) = 50.g x 4.18 J/goC x (65oC)
=14000 J (14 kJ)
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What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC
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What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC1550 J / (12 g x 4.18 J/goC )
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What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC1550 J / (12 g x 4.18 J/goC )
= 31oC
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What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC1550 J / (12 g x 4.18 J/goC )
= 31oC
If the temperature starts at 25oC, it will heat up to …
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What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC1550 J / (12 g x 4.18 J/goC )
= 31oC
If the temperature starts at 25oC, it will heat up to 56oC
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Calorimetry
• --the measurement of heat.
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Calorimetry
• --the measurement of heat.
• If one thing gains heat…
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Calorimetry
• --the measurement of heat.
• If one thing gains heat…
…something else lost it.
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• If 75 g of a metal at 96oC is placed in 58 g of water at 21oC and the final temperature reaches 35oC, what is the specific heat capacity of the metal?
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Step 1
• How much heat did the water gain?
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Step 1
• How much heat did the water gain?
q=mCT
Mass of water, in grams
Specific heat of water, 4.18 J/goC
Change in the temperature of water, in oC
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Step 2
• How much heat did the metal lose?
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Step 2
• How much heat did the metal lose?
• Heat lost = - heat gained
• qlost=-qgained
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Step 3
• What is the specific heat capacity of the metal?
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Step 3
• What is the specific heat capacity of the metal?
C=q/mT
Mass of metal, in grams
Specific heat of metal, in J/goC
Change in the temperature of metal, in oC
Heat lost by metal
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• If 75 g of a metal at 96oC is placed in 58 g of water at 21oC and the final temperature reaches 35oC, what is the specific heat capacity of the metal?
.74 J/goC
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Thermochemistry
• 2H2(g)+O2(g) 2H2O(g)+ 443,000 J
• (read it.)
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Thermochemistry
• 2H2(g)+O2(g) 2H2O(g)+ 443,000 J
• Two moles of hydrogen gas reacts with one mole of oxygen gas to form two moles of water vapor, releasing 443 kJ of heat.
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Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
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Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds releases energy
• Breaking bonds requires energy
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Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds is exothermic
• Breaking bonds is endothermic
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Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Exothermic reactions have a negative change in enthalpy
• Endothermic reactions have a positive change in enthalpy
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Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Exothermic reactions have a negative change in enthalpy
• Endothermic reactions have a positive change in enthalpy
The substances gain energy!
The substances lose energy!
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Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
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Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
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Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
Forming these bonds releases a lot more energy
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The mass to heat problem
gkJ
g
1 mol kJ
mol
The heat of reaction, Hrxn
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How about some stoichiometry?
• The oxidation of carbon releases 394 kJ/mol.
• How much heat is produced from the oxidation of 15 g C?
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How about some stoichiometry?
• The oxidation of carbon releases 394 kJ/mol.
• How much heat is produced from the oxidation of 15 g C?
490 kJ
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Special reactions:• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution
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Special reactions:• Formation —forming of 1 mole of a
compound from its elements in their normal state
• Combustion —burning 1 mole of a substance in oxygen
• Fusion —freezing 1 mole of a substance at its melting point
• Vaporization —boiling 1 mole of a substance at its boiling point
• Dissolution —dissolving 1 mole of a substance in water
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Special Heats
The Heat of Formation
• Hformation of Mg(OH)2 (s) =-925 kJ/mol
says
• Mg(s)+O2(g)+H2(g)Mg(OH)2(s)+ 925 kJ
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Special Heats
Hformation of CO2 (g) =-393.5 kJ/mol
?
2Na(s)+C(s)+1½O2(g)Na2CO3(s)+1131 kJ
?
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Special Heats
Hformation of a pure element in its standard state is 0 kJ.
(Ex. It takes no energy to make oxygen gas from oxygen gas.)
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Special Heats
The Heat of Combustion
• Hcombustion of N2 (g) =90.4 kJ/mol
says
• N2(g)+O2(g) + 90.4 kJ 2NO(g)
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Thermochemistry
N2 +O22NO
Breaking these bonds requires more energy…
…than forming these bonds releases
H=
+ 90.4 kJ/mol
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Special Heats
Hcombustion of H2 (g) =-286 kJ/mol
?
CH3OH+1½O2(g)CO2(g)+H2O(l) +726 kJ
?
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Special Heats
The Heat of Fusion
• Hfusion of Fe =13.8 kJ/mol
says
• Fe(s)+ 13.8 kJ Fe(l) at 1536oC
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Special Heats
Hfusion of C6H6 =9.87 kJ/mol
?
CH3OH(s) + 3160 J CH3OH(l) at -98oC
?
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Special Heats
The Heat of Vaporization
• Hvaporization of CS2 =28 kJ/mol
says
• CS2(l)+ 28 kJ CS2(g) at 46.3oC
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Special Heats
Hvaporization of CH4 =8.2 kJ/mol
?
O2(l) + 6820 J O2(g) at -183oC
?
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Special Heats
The Heat of Solution
• Hsolution of (NH2)2CO =-14.0 kJ/mol
says
• (NH2)2CO(s) (NH2)2CO (aq) + 14.0 kJ
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Special Heats
Hsolution of NH3 (g) = -34 kJ/mol
?
H2SO4(l) H2SO4 (aq) + 909 kJ
?
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Special Heats
• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution
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Hess’s Law
• If:
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
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Hess’s Law
• If
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
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Hess’s Law
• If
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
…then
• 2C + 2O22CO2 + 787 kJ
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Hess’s Law
• If
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
…then
• 2C + 2O22CO2 + 787 kJ
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• CH4 + 74.86 kJ C + 2 H2
• C + O2 CO2 +393.5 kJ
• 2H2 + O2 2 H2O + 571.6 kJ
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• CH4 + 74.86 kJ C + 2 H2
• C + O2 CO2 +393.5 kJ
• 2H2 + O2 2 H2O + 571.6 kJ
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• CH4 + 74.86 kJ C + 2 H2
• C + O2 CO2 +393.5 kJ
• 2H2 + O2 2 H2O + 571.6 kJ
• CH4 + 2O2 CO2 + 2 H2O + 890.2 kJ
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• Hrxn= Hform,products-Hform,reactants
Hess’s Law is usually used with heats of formation
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Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• Hrxn= Hform,products-Hform,reactants
=(Hform,CO2+2Hform,H2O)-Hform,CH4
=(-393.5 kJ + 2x-285.8 kJ)-(-74.86 kJ)
= -890.2 kJHess’s Law is usually used with heats of formation
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Hess’s Law
• Using table 11.6, What is Hrxn for…
1) 2H2O2 2H2O + O2
2) CaCO3 CaO + CO2
3) 2H2S + 3O22H2O + 2SO2
Hess’s Law is usually used with heats of formation
![Page 81: Chemistry and Heat](https://reader036.vdocument.in/reader036/viewer/2022070405/56814013550346895dab58ec/html5/thumbnails/81.jpg)
Heating and cooling curve— naphthalenenaphthalene in a water bath
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What is going on?
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Notice the rate of cooling (from 75oC)
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