Chemistry and Heat
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Transcript of Chemistry and Heat
Chemistry and Heat
That’s hot stuff.
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
Energy is the capacity to do work.
Forms include:
• Kinetic energy
• Gravitational potential energy
• Elastic potential energy
• Electrical energy
• Chemical potential energy
• Heat
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
Exothermic process
System
Surroundings
An exothermic process releases energy
Energy
Endothermic process
System
Surroundings
Energy
An endothermic process absorbs energy
If you add heat to matter, it may…
a)
b)
c)
d)
If you add heat to matter, it may…
a) warm up.
b) melt
c) boil
d) expand (tough to calculate, don’t bother)
Let’s try to warm up a cup of cold coffee.
Step 1: Add heat.
Let’s try to warm up a cup of cold coffee.
Step 1: Add heat.
Well, that was easy.
Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
Let’s try to warm up a cup of cold coffee.
What if you add half as much heat?
a)
b)
c)
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)
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)
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
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)
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)
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)
q=mCT
• q – heat, in Joules
• m –mass, in grams
• C –specific heat capacity, in J/goC
• T—change in temperature (Tfinal-Tinitial)
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
How much heat?
• How much heat does it take to raise 50.g water from 15oC to 80.oC?
• q=mCT
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)
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)
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)
What is the change in temperature?
• If you add 1550 J to 12 g water, how much will it heat up?
T =q/mC
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 )
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
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 …
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
Calorimetry
• --the measurement of heat.
Calorimetry
• --the measurement of heat.
• If one thing gains heat…
Calorimetry
• --the measurement of heat.
• If one thing gains heat…
…something else lost it.
• 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?
Step 1
• How much heat did the water gain?
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
Step 2
• How much heat did the metal lose?
Step 2
• How much heat did the metal lose?
• Heat lost = - heat gained
• qlost=-qgained
Step 3
• What is the specific heat capacity of the metal?
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
• 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
Thermochemistry
• 2H2(g)+O2(g) 2H2O(g)+ 443,000 J
• (read it.)
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.
Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds releases energy
• Breaking bonds requires energy
Chemical Energy
• Chemical energy (enthalpy) is stored in bonds.
• Forming bonds is exothermic
• Breaking bonds is endothermic
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
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!
Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
Thermochemistry
2H2 +O22H2O
Breaking these bonds requires energy
Forming these bonds releases a lot more energy
The mass to heat problem
gkJ
g
1 mol kJ
mol
The heat of reaction, Hrxn
How about some stoichiometry?
• The oxidation of carbon releases 394 kJ/mol.
• How much heat is produced from the oxidation of 15 g C?
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
Special reactions:• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution
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
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
Special Heats
Hformation of CO2 (g) =-393.5 kJ/mol
?
2Na(s)+C(s)+1½O2(g)Na2CO3(s)+1131 kJ
?
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.)
Special Heats
The Heat of Combustion
• Hcombustion of N2 (g) =90.4 kJ/mol
says
• N2(g)+O2(g) + 90.4 kJ 2NO(g)
Thermochemistry
N2 +O22NO
Breaking these bonds requires more energy…
…than forming these bonds releases
H=
+ 90.4 kJ/mol
Special Heats
Hcombustion of H2 (g) =-286 kJ/mol
?
CH3OH+1½O2(g)CO2(g)+H2O(l) +726 kJ
?
Special Heats
The Heat of Fusion
• Hfusion of Fe =13.8 kJ/mol
says
• Fe(s)+ 13.8 kJ Fe(l) at 1536oC
Special Heats
Hfusion of C6H6 =9.87 kJ/mol
?
CH3OH(s) + 3160 J CH3OH(l) at -98oC
?
Special Heats
The Heat of Vaporization
• Hvaporization of CS2 =28 kJ/mol
says
• CS2(l)+ 28 kJ CS2(g) at 46.3oC
Special Heats
Hvaporization of CH4 =8.2 kJ/mol
?
O2(l) + 6820 J O2(g) at -183oC
?
Special Heats
The Heat of Solution
• Hsolution of (NH2)2CO =-14.0 kJ/mol
says
• (NH2)2CO(s) (NH2)2CO (aq) + 14.0 kJ
Special Heats
Hsolution of NH3 (g) = -34 kJ/mol
?
H2SO4(l) H2SO4 (aq) + 909 kJ
?
Special Heats
• Formation
• Combustion
• Fusion
• Vaporization
• Dissolution
Hess’s Law
• If:
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
Hess’s Law
• If
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
Hess’s Law
• If
• 2C + O22CO+ 221 kJ and
• 2CO + O22CO2 + 566 kJ
…then
• 2C + 2O22CO2 + 787 kJ
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
Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
Hess’s Law is usually used with heats of formation
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
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
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
Hess’s Law
• CH4 + 2O2 CO2 + 2 H2O
• Hrxn= Hform,products-Hform,reactants
Hess’s Law is usually used with heats of formation
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
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
Heating and cooling curve— naphthalenenaphthalene in a water bath
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Notice the rate of cooling (from 75oC)
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