Download Lecture 5 (Slides Microsoft 97-2003) September 12

Gases and Limiting Reagents
Class Example: The Haber process produces ammonia, NH3,
for fertilizer manufacturing
N2(g) + 3H2(g) → 2NH3(g)
(a) Find the limiting reagent when 2.22 L of N2(g) at STP is
reacted with 2.22 L of H2(g) at 1150 mm Hg at 19.5 oC.
(b) If 4.00 L of both N2(g) and H2(g) at STP are reacted what
volume of NH3(g) would be produced at STP?
Gases and Limiting Reagents – NH3(g)
• Class example – Part (b): 4.00 L of N2(g) at STP
and 4.00 L of H2(g) at STP contain the same
number of moles of gas. Why? Since H2(g) is
consumed three times as quickly as N2(g) in
the reaction
• N2(g) + 3H2(g) → 2NH3(g)
• necessarily H2(g) is the limiting reagent. If the
reaction goes to completion the number of
moles of 2NH3(g) formed would be two thirds
the initial number of moles of H2(g).
Gases and Limiting Reagents – NH3(g)
Gases and Limiting Reagents – NH3(g)
Gases and Limiting Reagents – NH3(g)
Moles N2(g)
Moles H2(g)
Moles NH3(g)
Intial
0.176
0.176
0.000
Changes
-0.0587
-0.176
0.117
Final
0.117
0.000
0.117
Mass N2(g)
Mass H2(g)
Mass NH3(g)
Initial
4.93
0.355
0.000
Changes
-1.65
-0.355
2.00
Final
3.28
0.000
2.00
Gas Stoichiometry – Text Example
• Stoichiometric calculations can involve gases,
solids and solutions. A simple example from
the text involves the decomposition of sodium
azide – a very rapid reaction used in
automobile air bags. The very rapid
production of nitrogen gas quickly inflates the
air bags in the car. (Subsequent reactions
produce silicate glass.)
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6-7
Stoichiometry and Dalton’s Law
• Dalton’s Law can be useful when a reaction
produces a mixture of gases. It cane also be
needed when a gas produced by a chemical
reaction is collected over water. In such cases
the gas collected will contain water vapor.
• Then: PWet Gas = PDry Gas + Pwater Vapor
• The vapor pressure of water increases with T.
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Canada Inc.
6-9
Copyright  2011 Pearson
Canada Inc.
6 - 10
Kinetic Theory of Matter
• Observations:
• All gases exert pressure on the walls of their
container.
• The pressure exerted by a fixed amount of gas
in a rigid container increases steadily as the
gas temperature rises (Charles’s Law).
• At a given T equal amounts (moles) of
different gases with the same T and V exert
the same pressure.
Kinetic Theory of Matter
• The observations on the previous slide can be
explained if we assume that:
• Gas molecules are in rapid translational
motion.
• Gas molecules move more quickly as the T
increases. (Average kinetic energy α Temp.)
• At a given T the average kinetic energy of He
and Ar atoms (for example) are the same. (He
atoms move faster? Why?) Molecules?
Kinetic Theory of Gases
• Absolute zero (0 Kelvin): molecules cease to
rotate or translate (zero kinetic energy).
• Finite temperatures ( > 0 Kelvin): increase T
and gas molecules translate (and rotate) with
higher and higher average energies. At a given
T a distribution of molecular velocities (and,
correspondingly, kinetic energies!) is seen.
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6 - 14
3RT
urms
M
Figure 6-16
Distribution
of Molecular Speeds – the effect of mass and
temperature
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General Chemistry: Chapter 6
Slide 15 of 41
6-8 Gas Properties Relating to the
Kinetic-Molecular Theory
•Diffusion
–Net rate is proportional to
molecular speed.
•Effusion
–A related phenomenon.
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General Chemistry: Chapter 6
Slide 16 of 41
Graham’s Law
rate of effusion of A (u rms )A
3RT/M A
MB



rate of effusion of B (u rms )B
3RT/MB
MA
• Only for gases at low pressure (natural escape, not a jet).
• Tiny orifice (no collisions)
• Does not apply to diffusion.
Ratio used can be:
 Rate of effusion (as above)
 Molecular speeds
 Effusion times
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 Distances traveled by molecules
 Amounts of gas effused.
General Chemistry: Chapter 6
Slide 17 of 41
Graham’s Law Demonstration
• Graham’s Law tells us that light gaseous
molecules move more quickly (higher average
velocity) than heavy molecules. This is
sometimes demonstrated (inadvertently) in
chemistry labs when bottles of concentrated
HCl(aq) and NH3(aq) are left often in close
proximity. The following reaction cane be seen
• HCl(g) + NH3(g) → NH4Cl(s) Phase change!
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6 - 19
Gas Diffusion
• In the previous slide the solid ammonium
chloride, NH4Cl(s), forms closer to the opening
of the HCl(aq) bottle than the NH3(aq) bottle.
This tells us that the lighter ammonia
molecules diffuse through the atmosphere
faster than the heavier hydrogen chloride
molecules.
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6 - 21
Copyright  2011 Pearson
Canada Inc.
6 - 22
6-7 Kinetic Molecular Theory of Gases
• Particles are point masses in constant,
random, straight line motion.
• Particles are separated by great
distances.
• Collisions are rapid and elastic.
• No force between particles.
• Total energy remains constant.
Figure 6-14
Visualizing Molecular Motion
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General Chemistry: Chapter 6
Slide 23 of 41
Gas Nonideality
• Class discussion of evidence for.