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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
0
