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Gases and Limiting Reagents Class Example: The Haber process produces ammonia, NH 3, for fertilizer manufacturing N 2 (g) + 3H 2 (g) → 2NH 3 (g) (a)Find.

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Presentation on theme: "Gases and Limiting Reagents Class Example: The Haber process produces ammonia, NH 3, for fertilizer manufacturing N 2 (g) + 3H 2 (g) → 2NH 3 (g) (a)Find."— Presentation transcript:

1 Gases and Limiting Reagents Class Example: The Haber process produces ammonia, NH 3, for fertilizer manufacturing N 2 (g) + 3H 2 (g) → 2NH 3 (g) (a)Find the limiting reagent when 2.22 L of N 2 (g) at STP is reacted with 2.22 L of H 2 (g) at 1150 mm Hg at 19.5 o C. (b)If 4.00 L of both N 2 (g) and H 2 (g) at STP are reacted what volume of NH 3 (g) would be produced at STP?

2 Gases and Limiting Reagents – NH 3 (g) Class example – Part (b): 4.00 L of N 2 (g) at STP and 4.00 L of H 2 (g) at STP contain the same number of moles of gas. Why? Since H 2 (g) is consumed three times as quickly as N 2 (g) in the reaction N 2 (g) + 3H 2 (g) → 2NH 3 (g) necessarily H 2 (g) is the limiting reagent. If the reaction goes to completion the number of moles of 2NH 3 (g) formed would be two thirds the initial number of moles of H 2 (g).

3 Gases and Limiting Reagents – NH 3 (g)

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5 Moles N 2 (g)Moles H 2 (g)Moles NH 3 (g) Intial0.176 0.000 Changes-0.0587-0.1760.117 Final0.1170.0000.117 Mass N 2 (g)Mass H 2 (g)Mass NH 3 (g) Initial4.930.3550.000 Changes-1.65-0.3552.00 Final3.280.0002.00

6 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.)

7 Copyright  2011 Pearson Canada Inc. 6 - 7

8 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: P Wet Gas = P Dry Gas + P water Vapor The vapor pressure of water increases with T.

9 Copyright  2011 Pearson Canada Inc. 6 - 9

10 Copyright  2011 Pearson Canada Inc. 6 - 10

11 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.

12 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?

13 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.

14 Copyright  2011 Pearson Canada Inc. 6 - 14

15 Distribution of Molecular Speeds – the effect of mass and temperature Figure 6-16 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 15 of 41

16 6-8 Gas Properties Relating to the Kinetic-Molecular Theory Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 16 of 41 Diffusion – Net rate is proportional to molecular speed. Effusion – A related phenomenon.

17 Graham’s Law Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 17 of 41 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  Distances traveled by molecules  Amounts of gas effused.

18 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 NH 3 (aq) are left often in close proximity. The following reaction cane be seen HCl(g) + NH 3 (g) → NH 4 Cl(s) Phase change!

19 Copyright  2011 Pearson Canada Inc. 6 - 19

20 Gas Diffusion In the previous slide the solid ammonium chloride, NH 4 Cl(s), forms closer to the opening of the HCl(aq) bottle than the NH 3 (aq) bottle. This tells us that the lighter ammonia molecules diffuse through the atmosphere faster than the heavier hydrogen chloride molecules.

21 Copyright  2011 Pearson Canada Inc. 6 - 21

22 Copyright  2011 Pearson Canada Inc. 6 - 22

23 Visualizing Molecular Motion Figure 6-14 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6Slide 23 of 41 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.

24 Gas Nonideality Class discussion of evidence for.


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