Chemistry 114
First Hour Exam
Name:____________
(4 points)
Please show all work for partial credit
Note: This test is dedicated to ‘inflategate’
1. According to NFL regulations a football should be inflated to 13 psi above the ambient
pressure. This makes the total pressure in a football 27.7 psi (lbs/in2)
Convert 27.7 psi to:
A. (4 points) Atm
B. (4 points) Pascal
C.(4 points) bar
2. (12 points) One explanation of why the footballs in the Patriots - Colts Football game
could be under inflated was that they were filled to 27.7 psi in a room that was at 80oF
(27.7oC) but cooled to 50oF (10oC) on the playing field.
A. If a football is filled to 27.7 psi at 27.7oC, what is the pressure in the same football when
it is cooled to 10oC?
(If you did this in atm it is 1.78 atm)
(Some of the Patriots footballs were only filled to 25.7 psi)
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3A. If a football is filled to 27.7 psi at 80oC, and its volume is 4237 mL, how many moles
of air are in the football?
3B. If air is 80% nitrogen and 20% oxygen, how many moles of nitrogen in the above
football.
.275 mol x 80/100 part nitrogen = .22 mol nitrogen
3C. What is the partial pressure of oxygen in the above football?
27.7 psi x 20/100 parts oxygen = 5.54 psi oxygen
4. What is the density of the gas in a football when it is filled with CO2 to a pressure of 27.7
psi when the temperature is 80oC?
Or you could use PV=nRT and the volume from 3A to figure out n, and then multiply n by
the molar mass and divide by the volume. Longer, but it still gets an answer.
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5A.If the volume of a football is 4237 mL, how much work is involved in filling a football if
the external pressure is one atm? (Please give your answer in Joules)
w=-PÄV
=-4.237L x 1 atm = -4.237lAatm
-4.237lAatm x101.3J/lAatm = -429J
B. The specific heat capacity of air is 1J/gAoC. If a football contains 7.7 g of air, and the
temperature of the air in a football rises by 2oC as the football is filled, how much heat
energy was involved in filling the football?
Heat energy (q) = specific heat capacity ×mass×ÄT
= 1J/gAoC×7.7g×2oC
=15.4 J
Since the football got warmer, heat had to be lost from the system to warm
it so I called it q= -15.4J
C. What is the ÄU for the process of filling a football?
ÄU=w+q
-429J -15.4J
= -444.4J
However coming up with the correct sign for part B not simple, so I took both -444.4
and -429+15.4 or -413.6
Enough of footballs......
6A. Why do you use a ‘Bomb’ calorimeter to measure ÄU?
Bomb calorimeters measure heat evolved at a constant volume, and this is the
definition of ÄU
6B. What assumption do you make when you use a ‘coffee cup’ calorimeter to measure
ÄH?
One assumption is that we have adiabatic conditions. (No temperature exchange
with the surroundings.) What I was actually looking for was that the pressure did not
change during the experiment, so you were measuring q at a constant P, and hence were
measuring ÄH
6C. Is heat capacity an extensive or intensive property?
Heat capacity is extensive; it depends on the amount of material.
6D. Is specific heat capacity an extensive or intensive property?
When you divide heat capacity by the mass of the material to get specific heat
capacity, you have created a number that is independent of total amount, and hence an
intensive parameter.
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7. Determine the ÄHrxn for the combustion of acetylene with oxygen.
The unbalanced equation for this reaction is:
C2H2 +O2 6 H2O + CO2
A. (3 points) Balance the above reaction.
2C2H2 + 5O2 6 2H2O + 4CO2
B. (3 points) Give Lewis structures for all reactants and products in the reaction.
C. (6 points) Given the Bond Energies below, determine the Ährxn for the combustion
of acetylene with oxygen.
(All bond energies are in kJ/mol)
C-H
C=H
C-C
C=C
C/C
414
764
347
615
811
O-H
O=H
O-O
O=O
O/O
464
825
142
502
753
C-O 351
C=O 730
C/O 935
For Bond energies only its reactants - products
[(2A2(C-H) + 2(C/C) +5(O=O)] -[2A2(O-H) + 4A2(C=O)]
[(2A2A414)+(2A811)+(5A502)]-[(2A2A464)+(4A2A730)]
[1656+1622+2510]-[1856+5840]
=5788-7696
=-1908 kJ/mol
8. Classify the following as:
energy transfer functions or path functions (Transfer -t )
-orState functions (State-S)
heat
___t________
work ___t________
potential energy______S____________
distance traveled_____t___
(lots of different paths between points A and B)
q+w __S______________ (q+w=E!)
U + PV
__S_____________ (U+PV=H)
energy used to fill a football with air ____t___________
(Sorry, I couldn’t resist one last football reference)
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