Chapter 4 - Stoichiometry PDF

Larry Brown Tom Holme www.cengage.com/chemistry/brown Chapter 4 Stoichiometry Technological University of the Philippines - Taguig Campus Basic Arts and Sciences Department Chemistry Section Chapter Ob...

Larry Brown Tom Holme www.cengage.com/chemistry/brown Chapter 4 Stoichiometry Technological University of the Philippines - Taguig Campus Basic Arts and Sciences Department Chemistry Section Chapter Objectives Describe the chemical composition of gasoline. Write balanced chemical equations for the combustion of fuels. Calculate the amount of product expected from a chemical reaction, given the amounts of reactants used. Calculate the amounts of reactants needed in a chemical reaction to produce a specified amount of product. 2 Chapter Objectives Identify a limiting reagent and calculate the amount of product formed from a nonstoichiometric mixture of reactants. Calculate the percentage yield of a chemical reaction. Identify at least two common additives in gasoline and explain why they are used. 3 Gasoline and Other Fuels Gasoline is a very complex mixture of compounds, but contains predominantly alkanes. Alkanes are hydrocarbons where the carbon atoms are linked together with single bonds. Hydrocarbons are compounds composed only of hydrogen and carbon. 4 Alkanes Alkanes have the general formula CnH2n+2where n is an integer. The five smallest straight chain alkanes. 5 Alkanes The next five straight chain alkanes 6 Isomers Isomers are compounds that have the same chemical formula but are connected differently. Three isomers of pentane, C5H12. One straight chain Two branched chains 7 Octane: Complete Combustion Octane is used as a simplified model for gasoline. Complete combustion of octane with excess oxygen produces carbon dioxide and water. The stoichiometric ratio between octane and oxygen is 2:25. The stoichiometric ratio between carbon dioxide and water is 16:18. 8 Octane: Incomplete Combustion Incomplete combustion occurs when the amount of oxygen is limited. The products are carbon monoxide and water. The stoichiometric ratio between octane and oxygen is 2:17. The stoichiometric ratio between carbon monoxide and water is 16:18. 9 Octane: Combustion Complete and incomplete combustion both occur with the relative amounts of each determined by: Ratio of fuel to oxygen Engine temperature Engine tuning Engineers help control these factors to maximize fuel efficiency. 10 Fundamentals of Stoichiometry Stoichiometry is a term used to describe quantitative relationships in chemistry. “How much?” of a product is produced or reactant is consumed. Balanced chemical equation needed. Conversion between mass or volume to number of moles frequently needed. 11 Ratios from a Balanced Chemical Equation Mole ratios are obtained from the coefficients in the balanced chemical reaction. 1 mol CH : 2 mol O : 1 mol CO : 2 mol H O 4 2 2 2 These ratios can be used in solving problems: 1 mol CH4 2 mol H 2 O or 2 mol O2 1 mol CH4 12 Example Problem 4.1 In the combustion of methane, how many moles of O2 are required if 6.75 mol of CH4 is to be completely consumed? 13 Ratios from a Balanced Chemical Equation This flow diagram illustrates the various steps involved in solving a typical reaction stoichiometry problem. No different than unit conversion Usually more than one conversion is necessary Write all quantities with their complete units 14 Example Problem 4.2 How many grams of water can be produced if sufficient hydrogen reacts with 26.0 g of oxygen? 15 Ratios from a Balanced Chemical Equation Solution to Problem 4.2 using the stoichiometry problem flow diagram, Figure 4.3. 16 Example Problem 4.3 If we have 153 g of S8 and an excess of phosphorus, what mass of P4S3can be produced in the reaction shown? 8P4 + 3S8 → 8P4S3 =350.625 g of P4S3 17 Limiting Reactants In many chemical reactions, one reactant is often exhausted before the other reactants. This reactant is the limiting reactant. Limiting reactant is determined using stoichiometry. The limiting reactant limits the quantity of product produced. 18 Limiting Reactants Reaction between 6 H2 and 2H2 (g) + O2 (g) ⎯⎯→2H2O(g) 6 O2will produce 6 H2O. 6 H can produce 6 H O. 2 2 6 O can produce 12 H O. 2 2 H is limiting reactant. 2 3 O left over. 2 19 Limiting Reactants In many cases, we manipulate the amounts of reactants to ensure that one specific compound is the limiting reactant. For example, a more expensive or scarce reagent is usually chosen to be the limiting reagent. Other times, it is best to have a stoichiometric mixture (equal ratio of moles) to prevent waste. For example, rocket fuel is designed so that no mass is left over, which would add unnecessary weight to the rocket. 20 Example Problem 4.4 A solution of hydrochloric acid contains 5.22 g of HCl. When it is allowed to react with 3.25 g of solid K2CO3, the products are KCl, CO2, and H2O. Which reactant is in excess? 21 Example Problem 4.5 If 28.2 g of P4 is allowed to react with 18.3 g of S8, which is the limiting reactant? 8P4 + 3S8 → 8P4S3 22 Example Problem 4.6 If 45.0 kg of methanol is allowed to react with 70.0 kg of isobutene, what is the maximum mass (theoretical yield) of MTBE that can be obtained? CH3OH + (CH3 )2 C=CH2 → (CH3 )3 COCH3 Methanol Isobutene MTBE 23 Example Problem 4.7 The solid fuel rockets of the space shuttle are based on the following reaction between ammonium perchlorate and aluminum: 3NH4ClO4 (s) + 3Al(s) → Al2O3 (s) + AlCl3 (g) + 3NO(g) + 6H2O(g) If either reactant is in excess, unnecessary mass will be added to the shuttle, so a stoichiometric mixture is desired. What mass of each reactant should be used for every kilogram of the fuel mixture? 24 Theoretical Yield The maximum mass of a product that can be obtained in a reaction is determined by the limiting reactant. Determine which reactant is the limiting reactant. Calculate the mass of product that can be made from the limiting reactant. This mass is the theoretical yield. In stoichiometric mixtures, however, both reactants are consumed completely, so either could be considered the limiting reactant. 25 Theoretical and Percent Yields Many factors determine the amount of desired product actually produced in a reaction. Temperature of the reaction The possibility of side reactions Further reaction of the product Time 26 Theoretical and Percentage Yields  actual yield  Percentage Yield =    100% theoretical yield  Reaction efficiency is measured with percentage yield. The mass of product obtained is the actual yield. The ideal mass of product obtained from calculation is the theoretical yield. 27 Example Problem 4.8 In a laboratory experiment, a student heats 42.0 g of NaHCO3 and determines that 22.3 g of Na2CO3 is formed. What is the percentage yield of this reaction? 2NaHCO3 (s) ⎯⎯→heat Na CO (s) + CO (g) + H O(g) 2 3 2 2 28 Solution Stoichiometry For reactions occurring in solution, the concentration and volume of reactants and products are often used instead of mass to solve solution stoichiometry problems. n = number of moles; M = mol/L; V = L 29 Example Problem 4.9 If 750.0 mL of 0.806 M NaClO is mixed with excess ammonia, how many moles of hydrazine can be formed? NaClO(aq) + 2NH3 (aq) → N 2 H 4 (aq) + NaCl(aq) + H2O(l ) If the final volume of the resulting solution is 1.25 L, what will be the molarity of hydrazine? 30 Solution Stoichiometry A titration is a common laboratory technique that uses solution stoichiometry. A solution-phase reaction is carried out under controlled conditions so that the amount of one reactant can be determined with high precision. An indicator is a dye added to a titration to indicate when the reaction is complete. 31 Solution Stoichiometry 1. A solution of one of the reactants (A) is added to a burette. 2. The burette is positioned above a flask containing the second reactant (B). 3. The burette is used to add A to the flask in a controlled manner; volume is determined from initial and final burette readings. 4. The reaction is complete when the indicator changes color. 32 Example Problem 4.10 If 24.75 mL of 0.503 M NaOH solution is used to titrate a 15.00 mL sample of sulfuric acid, H2SO4, what is the concentration of the acid? 33 Alternative Fuels and Fuel Additives Fuel additives are added to gasoline to improve engine performance, reduce undesirable engine emissions, and reduce dependence on imported petroleum products. Some additives, oxygenates , increase the oxygen content of gasoline and gasoline containing them is called an oxygenated fuel. Ensure more complete combustion by reducing emitted carbon monoxide, hydrocarbons, and soot. Gasoline containing at least 2% oxygen by weight is call reformulated gasoline (RFG), which is mandatory in some areas with severe pollution. 34 Alternative Fuels and Fuel Additives Additives improve engine performance by improving the octane rating. Higher octane rating delivers better performance and has lower “knocking”. Knock is the result of premature cylinder ignition when gasoline-air mixture is compressed. 35 Alternative Fuels and Fuel Additives Tetraethyl lead, used until the 1970s, increased octane rating. Poisoned the surfaces of catalytic converters. Discontinued due to the toxicity of lead. 36 Alternative Fuels and Fuel Additives MTBE, methyl tert-butyl ether, is an oxygenate. As much as 15% MTBE can be used in gasoline. Possible health concerns have led to it being banned in some areas. 37 Alternative Fuels and Fuel Additives Ethanol is another oxygenate. Produced from crops such as corn, barley, and wheat. Gasoline containing 10% ethanol can be burned in modern automobiles. Gasoline containing 85% ethanol can be burned in specially designed engines. At this concentration, it is considered an alternative fuel rather than an oxygenate. 38