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READING for Thursday: 7.3 – 7.4 HOMEWORK – DUE TUESDAY 10/20/15
Heat of Reaction for Heats of FormationJust like in the enthalpy diagrams, but with less diagram
N2O(g) + 2 O2(g) N2O5(s)
2 5( ),43.1
sf N OH
kJ
mol2 ( ),82.05
gf N OH
kJ
mol
formation, products43.1
1 43.11
H
2 52 5
N O N O
kJ mol kJ
mol
formation, reactants82.05 0
1 2 82.051 1
H
2 22 2
N O O N O O
kJ kJ mol mol kJ
mol mol
843.1( ) ( ) 12. 5 22 05 .rxnH kJ kJ kJ
2( ),0
gf OH kJ
mol
formation, products formation, reactantsrxnH H H
Heat CapacityWhen a system absorbs heat, its temperature
increasesThe increase in temperature is directly proportional
to the amount of heat absorbedThe proportionality constant is called the heat
capacity, Cunits of C are J/°C or J/K
q = C x DT
The specific heat capacity is the amount of heat energy required to raise the temperature of one gram of a substance 1°C units are J/(g∙°C)
The molar heat capacity is the amount of heat energy required to raise the temperature of one mole of a substance 1°C
Specific Heat Capacity
q Cm T q Cm T q m C T
oCcal
g oCJ
g
q m C T heat (J or cal)
mass (g)
specific heat ( or )
change in temp. (oC or K)
DT = Tfinal - Tinitial
oCcal
g oCJ
g
q m C T
Specific Heat Capacity
The amount of energy (J or cal) that is required to raise 1 g of a substance by 1 degree Celsius.
Specific heat for water = or1.00
oC cal
g
4.184oC
J
g
Specific heat for silver = or0.0566
oC cal
g
0.237oC
J
g
Specific Heat Capacity
A SMALLER specific heat means a LARGER temperature change given the same amount of energy.
Specific Heat
Temperature change when 51000 calories are added to 1000.0 grams of each:
~900 oC
oCg
cal
11.00
oCg
al c
1 0.0566
o cal
cal
g C 1 510001.00 1
o cal
cal
g C
1 510000.0566 1
o cal
cal g
g C
1 51000 11.00 1 1000.0
o cal
c
g C
gal
1 51000 10.0566 1 1000.0
g1 o
c
C
al1.00
a c l51000
g
11 1000.0
g1 o
a
C
l c0.0566
a c l51000
g
11 1000.0
g1 o
c
C
al1.00
a c l51000
g
11 1000.0
oC51
g1 o
a
C
l c0.0566
a c l51000
g
11 1000.0
oC29.0 10
75.0 mL of ethanol at 25.0 oC is poured into a perfectly insulating coffee cup calorimeter containing 50.0 g of water at 33.4 oC. What is the final temperature of the ethanol? C(water) = 4.184 J/goC, C(ethanol) = 2.42 J/goC, d(ethanol) = 0.789g/mL
What mass of propane would need to be burned to raise 2500 grams of water from 25.0 oC to 100.0 oC? (MMpropane = 44.11 g/mol)
2500 2 H Og 4.184
25001 1
22
H O H O
g
J g
C4.184 75.0
250011 1
22
H O H O
J
C
g
Cg
3
4.184 75.0 12500
11 1 1 10
2
2
H O H O
J C kJg
g C
J
3
4.184 75.0 1 12500
1 11 1 1 10
2
2
H O H O
J C kJg
kJg C
kJ
J
3
14.184 75.0 1 12500
1 1 2202.01 1 1 10
2
3 82
C H O
H O
HJ C k
Jg
kJg C
m
JJ
olkJ
k
3
1 44.114.184 75.0 1 12500
1 1 2202.0 11 1 1 10
3 8 3 8
32
82
H O H
C H C H
HO C
J C k
Jg
kJg C
mol gkJ
kJ mol J
3
1 44.114.184 75.0 1 12500 16
1 1 2202.0 11 1 1 10
2
3 8 3 83 8
3 82
C H C H C H O H
H CH O
mol gkJJ C kJ
g
kJ molg
kJg C J
1 kJ absorbed by the water = 1 kJ released by burning propane
+1 kJwater = –1 kJpropane
4.184
1 12 H O
J
g
C
Enthalpy Calculations1.00 grams of solid KClO3 (122.55 g/mol) is dropped into 25.00 grams of water at 25.0 oC. If the heat of solution for KClO3 is +41.38 kJ/mol, what is the final temperature of the water?
What is the system? Dissolving of KClO3
What is the surrounding? Water!!
What would we measure? Temperature of WATER!!
What would we observe? Temperature of water decreasing!
Did the water gain or lose energy? Water LOST energy!!
Where did it go? Into the process of dissolving the KClO3(s)
1 41.381.00 0.3377
122.55 1
3
33 3
KClO KClO
KClO KClO
mol kJ g kJ
g mol
31
1 104.184
0.3377 25.00 25.01 1
final finalT T
o2
2
H O 21.8 C
H O
kJoJ
o
J kJ g C
g C
Enthalpy Calculations1.00 grams of solid KClO3 (122.55 g/mol) is dropped into 25.00 grams of water at 25.0 oC. If the heat of solution for KClO3 is +41.38 kJ/mol, what is the final temperature of the water?
What is the system? Dissolving of KClO3
What is the surrounding? Water!!
What would we measure? Temperature of WATER!!
What would we observe? Temperature of water decreasing!
Did the water gain or lose energy? Water LOST energy!!
Where did it go? Into the process of dissolving the KClO3(s)
3 1 1.001 41.381 1 1 1 103.228
4.184 25.00 1 1 122.1 55 1T
3 3
3 3
2
2
H O KCl
O KClO
KClO KH CO lO
oo mol g kJ kJ
m
g C JC
J g kJ kJ ol g
3.228 25.0final finalT T T o21.8 Co oC C
Enthalpy CalculationsWhen 2.10 grams of solid KOH (56.11 g/mol) is dropped into 55.00 grams of water at 22.5 oC, the water raises to 31.9 oC. What is the heat of solution for KOH in kJ/mol?
4.18455.00 9.4 2163
1 1
2
2
H O J H O
oo
J g C
g C
31
1 101
56.11
2163 57.8
12.10
KOH KOH
KOH KOH
kJJ
mol g
J kJ
mol g
3
1 1 56.11 1 57.8
1 2.10 11 10
55.004.184 9.4
1 1 11 1
2
2
H O
H
KOH
KOH KOH KOHO
o
o
g J C
J
J kJ g kJ
mol g olg mJC
What is the system? Dissolving of KOH
What is the surrounding? Water!!
What would we measure? Temperature of WATER!!
What would we observe? Temperature of water increasing!
Did the water gain or lose energy? Water GAINED energy!!
Where did it go? From the process of dissolving the KOH(s)