Week One Suggested Problems from Book
8.36. Which has more kinetic energy, a 1400 kg car moving at 115 km/h or a 12,000 kg
truck moving at 38 km/h?
8.38 Calculate the work done (in joules) by a chemical reaction if the volume increases
from 3.2 L to 3.4 L against a constant external pressure of 3.6 atm. What is the sign
of the energy change.
8.39 The addition of H2 to C=C double bonds is an important reaction used in the
preparation of margarine from vegetable oils. If 50.0 mL of H2 and 50.0 mL of
ethylene (C2H4) are allowed to react at 1.5 atm, the product ethane (C2H6) has a
volume of 50.0 mL. Calculate the amount of PV work done, and tell the direction
of the energy flow.
C2H4 (g) + H2 (g)  C2H6 (g)
ANSWERS NEXT PAGE
8.36 The formula for kinetic energy:
Ek = 1/2 mv2
To get the final answer in joules, the velocities must be converted from
km/h to m/s. This is because a joule = a kg * m2/s2. One hour = 3600 seconds.
Therefore, set up the following:
Car
Ek = ½ (1400 kg) (115 x 103 m/3600 s)2 = 7.1 x 105 J
Truck
Ek = ½ (12,000 kg) (38 x 103 m/3600 s)2 = 6.7 x 105 J
**The car has more kinetic energy.
8.38 w = -PV
Change in volume is (3.4 L – 3.2 L) = 0.2 L And since the system is expanding,
energy is leaving the system and work will be negative.
w = -(3.6 atm)(0.2 L) = -0.72 L*atm Convert to joules…
-0.72 L*atm (101 J/L*atm) = -73 J Again, the energy change is negative,
since the system is doing work, pushing the piston outward.
8.39 Calculating PV work, via w = -PV
Starting volume of the mixture is the combination of the ethylene and hydrogen.
50.0 mL + 50.0 mL = 100 mL is the starting volume
Final volume is the total volume of the ethane = 50.0 mL
The volume contracts/shrinks over the course of the reaction. The surroundings,
here, is doing work on the system, and the change in volume is defined as (final –
initial) = (50.0 mL – 100.0 mL) = -50.0 mL
Now, calculating the amount of PV work done:
w = -(1.5 atm)(-0.0500 L) = +0.075 L*atm
Convert to joules (0.075 L*atm)(101 J/L*atm) = +7.6 J
The sign of the work is positive. Energy is flowing into the system. The
surroundings is doing work on the system, as the system is growing
smaller over the course of the reaction.