1. 5.58
A 2.5-L flask at 15oC contains a mixture of 3 gases, N2, He, and Ne, at partial pressures of 0.32 atm for N2, 0.15 atm for He, and 0.42 atm for Ne. (a) Calculate the total pressure of the mixture. (b) Calculate the volume in liters at STP occupied by He and Ne if the N2 is removed selectively.
(a) = atm
(b) 1.4 L
12/3/2018
S.A. McFarland©2006

2. 5.62
A sample of Zn metal is allowed to react completely with an excess of HCl:
Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)
The H2 gas produced is collected over water at 25oC. The volume of the gas is 7.8 L, and the pressure is atm. Calculate the amount of Zn metal in grams consumed in the reaction. (Vapor pressure of water at 25oC is 23.8 mmHg.)
19.8 g Zn
12/3/2018
S.A. McFarland©2006

3. 5.64
A sample of ammonia (NH3) gas is completely decomposed to nitrogen and hydrogen gases over heated iron wool. If the total pressure is 866 mmHg, calculate the partial pressures of N2 and H2.
2NH3 -> N2 + 3H2
1 mol N2: 3mol H2 so XN2 is 0.25 and XH2 is 0.75
PN2 = XN2 x PT = 217 mmHg
PH2 = XH2 x PT = 650 mmHg
12/3/2018
S.A. McFarland©2006

4. 5.72
The temperature of the stratosphere is -23oC. Calculate the root-mean-square speed of N2 molecules in this region.
urms = sqrt(3RT/M)
urms = sqrt (3x8.314x250K/ )
= 471 m/s
*Remember: R is in J/mol.K and M is kg/mol
because 1 J= 1 kg m2/s2
12/3/2018
S.A. McFarland©2006

5. 5.80
At 27oC 10.0 moles of a gas in a 1.50-L container exert a pressure of 130 atm. Is this an ideal gas?
PV = nRT
P = 164 atm
No. An ideal gas under these conditions would
exert a pressure of 164 atm.
12/3/2018
S.A. McFarland©2006

6. Energy relationships in chemical reactions
Chapter 6

7. Energy is the capacity to do work
Radiant energy is earth’s primary energy source and comes from the sun
Thermal energy is the energy associated with the random motion of atoms and molecules
Chemical energy is the energy stored within the bonds of chemical substances
Nuclear energy is the energy stored within the collection of neutrons and protons in the atom
Electrical energy is the energy associated with the flow of electrons
Potential energy is the energy available by virtue of an object’s position
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S.A. McFarland©2006

8. Energy changes in chemical reactions
Heat is the transfer of thermal energy between two bodies that are at different temperatures.
Temperature is a measure of the thermal energy.
Temperature = Thermal energy
400C
900C
greater thermal energy
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S.A. McFarland©2006

9. Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
400C
900C
greater thermal energy
larger volume, greater mass
more H2O molecules, more molecular motion
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S.A. McFarland©2006

10. Thermochemistry is the study of heat change in chemical reactions.
The system is the specific part of the universe that is of interest in the study.
SURROUNDINGS
SYSTEM
open
closed
isolated
Exchange:
mass & energy
energy
nothing
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S.A. McFarland©2006

11. 2H2 (g) + O2 (g) 2H2O (l) + energy
Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.
2H2 (g) + O2 (g) H2O (l) + energy
H2O (g) H2O (l) + energy
Endothermic process is any process in which heat has to be supplied to the system from the surroundings.
energy + H2O (s) H2O (l)
energy + 2HgO (s) Hg (l) + O2 (g)
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S.A. McFarland©2006

12. 12/3/2018
S.A. McFarland©2006

13. Thermochemistry is part of a broader subject called thermodynamics.
Thermodynamics is the study of interconversion of heat and other kinds of energy.
changes in the state of a system
composition
energy
temperature
pressure
volume
Chapter 18 gives all of the details about thermodynamics.
For now, we’re concerned with only the 1st law of thermodynamics.
12/3/2018
S.A. McFarland©2006