Chemistry 141 Lecture
Dr. Ben Tovrog
Chapter 6 Supplemental Problems  Answers
1.
Calculate Δ E for a system undergoing an endothermic process in which 15.6 kJ of heat flows and
where 1.4 kJ of work is done on the system.
Δ
E = q
+
w
Where q = +15.6 kJ, since ht process is endothermic, and w = +1.4 kJ, since work is done on the system.
Δ E = 15.6 kJ
+ 1.4 kJ = 17.0 kJ.
The system has gained 17.0 kJ.
2. Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external
pressure of 15 atm.
For a gas at constant pressure,
w = P Δ
V.
In this case P = 15 atm, and
V = 64  46 = 18 L.
w =
 15 atm * 18 L =
270 L atm.
(Note that since the gas expands, it does work on its surroundings.
Energy flows out of the gas, so w is a negative quantity).
3.
A balloon is being inflated to its full extent by heating the air inside it.
In the final stages of this
process, the volume of the balloon changes from 4.00 x 10
6
L to 4.50 x 10
6
L by the addition of 1.3 x 10
8
J of energy as heat.
Assuming that the balloon expands against a constant pressure of 1.0 atm, calculate
E for the process (To convert between L atm and J use 1 L atm = 101.3 J)
Δ E = q + w.
q = +1.3 x 10
8
J.
w = P Δ V.
P = 1 atm and
Δ V = V initial – V final = 4.5 x 10
6
L  4.00 x 10
6
L = 5.0 x 10
5
L.
w = 1.0 atm * 5.0 x 10
5
L =  5.0 x 10
5
L atm. (the negative sign is because the gas is expanding and
doing work on the surroundings).
Convert to J
w = 5.0 x 10
5
L atm * (101.3 J / L atm) = 5.1 x 10
7
J.
Δ E = q + w = (+1.3 x 10
8
J) + (5.1 x 10
7
J) = 8 x 10
7
J.
Since more energy is added through heating than the gas expends doing work, there is a net increase in
the internal energy of the gas in the balloon.
Hence
Δ E is positive.
4.
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 Summer '08
 Mulfod
 Chemistry, Hrxn, C Ethanol, Hforxn

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