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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.) Copyright 2011 Pearson Canada Inc. 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. Copyright 2011 Pearson 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. Copyright 2011 Pearson Canada Inc. 6 - 14 3RT urms M Figure 6-16 Distribution of Molecular Speeds – the effect of mass and temperature Copyright © 2011 Pearson Canada Inc. 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. Copyright © 2011 Pearson Canada Inc. 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 Copyright © 2011 Pearson Canada Inc. 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! Copyright 2011 Pearson Canada Inc. 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. Copyright 2011 Pearson Canada Inc. 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 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 23 of 41 Gas Nonideality • Class discussion of evidence for.