Stoichiometry and Terminology

Stoichiometry C7H16(l) + 11 O2(g) -> 7 CO2(g) + 8 H2O(g) Stoichiometric coefficients Stoichiometric ratios Stoichiometric quantity

General Chemical Reaction Equation cC + dD = aA + bB

Examples (E9.2, DMH) In the combustion of heptane, CO2 is produced. Assume that you want to produce 500 kg of dry ice per hour, and that 50% of the CO2 can be converted into dry ice, as shown in the figure. How many kilograms of heptane must be burned per hour? Ans. 3325 kg C7H16

Examples (E9.3, DMH) A limestone analyses (wt%) CaCO3 92.89% MgCO3 5.41% Inert 1.70% By heating the limestone you recover oxides known as lime. How many pounds of calcium oxide can be made from 1 ton of this lime-stone? How many pounds of CO2 can be recovered per pound of limestone? How many pounds of limestone are needed to make 1 ton of lime? Ans. (a) 1041 lb CaO, (b) 0.437 lb CO2/lb limestone, (c) 3550 limestone/ton lime

Terminology 8.2.1 Extent of Reaction 8.2.2 Limiting and Excess Reactants 8.2.3 Conversion and degree of completion 8.2.4 Selectivity 8.2.5 Yield

Extent of Reaction (Moles Reacting)

Example (E9.4, DMH) Determine the extent of reaction for the following chemical reaction N2 + 3H2 -> 2NH3 given the following analysis of feed and product: Feed Product N2 100 g H2 50 g NH3 5 g 90 g Also, determine the g and g mol of N2 and H2 in the product.

Limiting and Excess Reactants The reactant with the smallest maximum extent of reaction is the limiting reactant.

Example (E9.5, DMH) If you feed 10 grams of N2 gas and 10 grams of H2 gas into a reactor: What is the maximum number of grams of NH3 that can be produced? What is the limiting reactant? What is the excess reactant?

Conversion and Degree of Completion Conversion (or % Conversion) : fraction (or percentage) of the feed (or some key material in the feed) that is converted into products. Degree of completion : percentage or fraction of the limiting reactant converted into products.

Selectivity

Example (E9.7, DMH) Selectivity in the Production of Nanotubes A carbon nanotube may consist of a single wall tube or a number of concentric tubes. A single wall tube ma be produced as unaligned structures or bundles of ropes packed together in an orderly manner. The structure of the nanotubes influences its properties, such as conductance. Some kinds are conductors and some semiconductors. In nanotechnology, numerous methods (arc-discharge, laser vaporization, chemical vapor deposition, and so on) exist to produce nanotubes. For example, large amounts of single wall carbon nanotubes can be produced by the catalytic decomposition of ethane over Co and Fe catalysts supported on silica C2H6 -> 2C + 3H2 (a) -> C2H4 + H2 (b) If you collect 3 gmol of H2 and 0.50 gmol of C2H4, what is the selectivity of C relative to C2H4?

Yield

Notes High selectivity or yield is desired. Why doesn’t the actual yield in a reaction equal the theoretical yield predicted from the chemical reaction equation? impurities among the reactants leaks to the environment side reactions reversible reactions

Example (E9.6, DMH) Yields in the Reaction of Glucose to Produce Ethanol Yeasts are living organisms that consume sugars and produce a variety of products. For example, yeasts are used to convert malt to beer and corn to ethanol. The growth of S. cerevisiae (a specific type of yeast) on glucose (a sugar) under anaerobic conditions (in the absence of oxygen) preceeds by the following overall reaction to produce biomass, glycerol, and ethanol C6H12O6(glucose) + 0.118NH3 -> 0.59CH1.74N0.2O0.45(biomass) + 0.43C3H8O3(glycerol) + 1.54CO2 + 1.3C2H5OH(ethanol) + 0.03H2O Calculate the theoretical yield of biomass in g of biomass per g of glucose. Also, calculate the yield of ethanol in g of ethanol per g of glucose.

Example (E9.6, DMH) Calculation of Various Terms Pertaining to Reactions Semenov (Some Problems in Chemical Kinetics and Reactivity, Princeton Univ. Press (1959), Vol II, pp.39-42) described some of the chemistry of allyl chlorides. The two reactions of interest fro this example are Cl2(g) + C3H6(g) -> C3H5Cl(g) + HCl(g) (a) Cl2(g) + C3H6(g) -> C3H6Cl2(g) (b) C3H6 is propylene (propene) (MW = 42.08) C3H5Cl is allyl chloride (3-chloropropene) (MW=76.53) C3H6Cl2 is propylene chloride (1,2-dichloropropane) (MW=112.99) The species recovered after the reaction takes place for some time are listed in Table

Example (E9.6, DMH) Calculation of Various Terms Pertaining to Reactions Species gmol Cl2 141.0 C3H6 651.0 C3H5Cl 4.6 C3H6Cl2 24.5 HCl 4.6 Based on the product distribution assuming that no allyl chlorides were present in the feed, calculate the following: How much Cl2 and C3H6 were fed to the reactor in g mol? What was the limiting reactant? What was the excess reactant? What was the fraction conversion of C3H6 to C3H5Cl? What was the selectivity of C3H5Cl relative to C3H6Cl2? What was the yield of C3H5Cl expressed in g of C3H5Cl to the g of C3H6 fed to the reactor? What was the extent of reaction of the first and second reactions? In the application of green chemistry, you would like to identify classes of chemical reactions that thave the potential for process improvement, particularly waste reduction. In this example the waste if HCl(g). The Cl2 is not considered to be a waste because it is recycled. What is the mole efficiency, i.e., the fraction of an element in the entering reactants that emerges in the exiting products, for chlorine?