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Physical Science Walk-In Tutoring Center
Physical Sciences Building Room 122
Tuesdays 5:30-8:30 pm
Wednesdays 5:30-8:30 pm
Feel like you’re trapped in a flask or like Newton’s apple has just hit you on the head?
Chemistry, Physics, and Earth Science tutors are available to help!
FREE, no-appointment, walk-in tutoring is available for:
CHEM 100, 112, & 226; PHYS 100, 103/104, & 204;
ESCI 100, GEOL 120, & METR 110
Funding is provided by the Provost.
Heating
Curves
Measuring
DH
DH and
Reactions
• Qualitative- phase changes
• Quantitative- Heat of fusion/vaporization
• Constant Pressure Calorimetry (“coffee
cup”)
• Constant Volume Calorimetry (bomb)
• Hess’ Law (this week’s lab)
• Standard Heats of Formation
Thermal Energy and Phase Changes
Which line segment represents a point at which
you would use the heat capacity of the liquid to
calculate heat exchange?
1.
2.
3.
4.
5.
A->B
B->C
D->E
F->G
H->J
48%
16%
16%
13%
7%
1
2
3
4
5
Enthalpy Changes and Chemical
Reactions
 DH = energy needed to break bonds – energy
released forming bonds
 Calorimetry is used to measure enthalpy changes
 Constant Pressure gives DH
 Constant Volume gives DE
Hess’ Law- The Rules
 Enthalpy is a state function.
 If a reaction can be written as the sum of two or more
reactions, the DH for the net reaction is the sum of the
DH's for the individual steps.
 If a reaction is reversed, the sign of DH is reversed.
 If the coefficients of the reactants/products are
multiplied by a number, then the DH must be
multiplied by that number, as well.
Hess’ Law
 If you can add the reactions, you can add the DH’s.
Hess’s Law
Hess’ Law- Lab This Week
Mg(s) + ½ O2(g) 
MgO(s)
DH = ???
Constant Pressure Calorimetry (“Coffee Cup”)
Constant Volume Calorimetry (“Bomb”)
N2H4 + 3 O2  2 NO2 + 2 H2O
Ereleased = Eabsorbed by water + Eabsorbed by calorimeter
Ewater =
Ecalorimeter =
Total E =
DH = energy/moles =
0.500 g N2H4
600 g water
420 J/oC
Calculating Reaction Enthalpies
 Use values that have been determined from experiment
 Two types of data that can be used
 Standard enthalpies of formation
 Bond enthalpies
Standard Enthalpy of Formation
 Standard conditions: pure form, 1 bar pressure, usually
at 298K (25°C)
 Heat of formation for an element in its most stable
form is zero
 Standard heat of formation is given in Joules per mole
 Results in fractional coefficients on occasion



D
H
D
H
D
H


r xn
f
f
prod
ru
ea
ct
ct
s ants
Table 5-2, p. 195
Using Standard Enthalpies of Formation
 What is the DHrxn for the detonation of nitroglycerin?
 How much energy is released when 10g is detonated?
1
2
C
H
(
NO
)
(
l
)

3
N
(
g
)

(
g
)

6
CO
(
g
)

5
H
O
(
g
)
3
5
3
3
2 O
2
2
2
2

Bond Enthalpy
 Remember that bond energy is the amount of energy
required to break a bond in a gas phase molecule
 Can only use bond enthalpies for reactions in which
everything is in the gas phase
 When calculating DH using bond enthalpies, assume
all bonds are broken in the reactants (DH= +) and
formed in the products (DH= -)
D
H

broke

form


rxn
Bond Enthalpy
D
H

brok

fo


rxn
 Formation of water

2
H

O

2
H
O
2
2
2

Chemical Reactions and Enthalpy
Change
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
What does this mean?
How much energy is released by a 468-g tank of propane?
DH = -2045 kJ