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Understanding Stoichiometry in Chemical Reactions

Learn about stoichiometry, mole ratios, and limiting reactants in chemical reactions to calculate yields accurately. Explore realistic conditions and critical calculations to master the concepts.

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Understanding Stoichiometry in Chemical Reactions

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  1. Chapter 7 Quantitative Relationships in Chemical Reactions

  2. Setting the Stage – Limiting Reactants and Energy • One possible result of the increase in CO2 as a result of global warming is an increase in vegetation. • However, what is found is that some vegetation is increasing, (converting CO2 to carbohydrates) but not all. • Photosynthesis often requires more than CO2 and sunlight – increasing one reactant does not always result in the increase in product, unless the other reactants are increased). Malone and Dolter - Basic Concepts of Chemistry 9e

  3. Setting a Goal - Part AMass Relationships in Chemical Reactions • You will gain command over the aspects of stoichiometric calculation involving masses, moles, and molecules under both ideal and realistic conditions. Malone and Dolter - Basic Concepts of Chemistry 9e

  4. Objective for Section 7-1 • Perform stoichiometry calculations using mole ratios from balanced equations. Malone and Dolter - Basic Concepts of Chemistry 9e

  5. 7-1 Stoichiometry • Stoichiometry is the name for quantitative relationships between reactants and products of a reaction • The balanced chemical equation gives us the relationships in moles. • Consider: N2 + 3H2 2NH3 • Multiple mol-mol conversion factors or mole ratios can be derived from balanced equations (see Table 7-1). Malone and Dolter - Basic Concepts of Chemistry 9e

  6. The Mole Ratio • One mol of N2 produces 2 mol of NH3 • One mol of N2 reacts with 3 mol of H2 Malone and Dolter - Basic Concepts of Chemistry 9e

  7. The Mole Ratio • 3 mol of H2 produce 2 mol of NH3 • Inverse can also be used as a conversion factor Malone and Dolter - Basic Concepts of Chemistry 9e

  8. Example • How many moles of NH3 can be produced from 33.6 g of N2? • Convert grams of N2 to mol of N2, then convert mol of N2 to mol of NH3 gmol conversion Stoichiometric interconversion Malone and Dolter - Basic Concepts of Chemistry 9e

  9. Stoichiometry - Procedure 1. Write down what is given and what is requested in the problem. 2. (a) If a mass is given, use the molar mass to convert mass to moles of what is given.(b) If a number of molecules is given, use Avogadro’s number to convert to moles of what is given. Malone and Dolter - Basic Concepts of Chemistry 9e

  10. Stoichiometry Procedure 3. Using the correct mole ratio from the balanced equation, convert moles of what is given to moles of what is requested. Malone and Dolter - Basic Concepts of Chemistry 9e

  11. Stoichiometry Procedure 4. (a)If a mass is required, convert moles of what is requested to mass of what is requested.(b) If a number of molecules is requested, use Avogadro’s number to convert to numbers of molecules of what is requested. 5. The procedure is summarized on next slide. Malone and Dolter - Basic Concepts of Chemistry 9e

  12. Stoichiometry Malone and Dolter - Basic Concepts of Chemistry 9e

  13. Objective for Section 7-2 • Given the masses of two different reactants, determine the limiting reactant and calculate the yield of product. Malone and Dolter - Basic Concepts of Chemistry 9e

  14. 7-2 Limiting Reactant • If specific amounts of each reactant are mixed, the reactant that produces the least amount of product is called the limiting reactant. • Think of hot dogs and buns: • hot dogs are sold in packs of ten • hot dog buns are sold in packs of twelve • how many hot dog-hot dog bun combinations can you make with one pack of hot dogs and one pack of hot dog buns? Malone and Dolter - Basic Concepts of Chemistry 9e

  15. Think of Hot Dogs and Hot Dog Buns Malone and Dolter - Basic Concepts of Chemistry 9e

  16. Limiting Reactant Considerations • When reactants are mixed in exactly the mass ratio determined from the balanced equation, the mixture is said to be a stoichiometric mixture. • Example:4.0 g H2 + 32.0 g O2 36.0 g H2O • Other mass ratios require calculations to determine the limiting reactant. Malone and Dolter - Basic Concepts of Chemistry 9e

  17. Limiting Reactant Procedure • Convert amount of each reactant to the number of moles of product using mole ratios. • The limiting reactant is the one that produces the smallest amount of product. Malone and Dolter - Basic Concepts of Chemistry 9e

  18. Limiting Reactant Example • 2 CH3OH(l) + 3 O2 (g)  2 CO2 (g) + 4 H2O(g) • Mix 40.0 g of methanol with 46.0 g of O2 - what is the mass of CO2 produced? • Methanol, CH3OH, has a molar mass of 32.04 g • O2 has a molar mass of 32.00 g Malone and Dolter - Basic Concepts of Chemistry 9e

  19. Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  20. Limiting Reactant Example (2) • Determine the limiting reactant when SO2 (400 g), O2 (175 g) and H2O (125 g) are mixed and reacted to produce H2SO4. • Hence determine the number of moles and mass of sulfuric acid produced by this reaction. Malone and Dolter - Basic Concepts of Chemistry 9e

  21. Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  22. Objective for Section 7-3 • Calculate the percent yield of a reaction from the measured actual yield and the calculated theoretical yield. Malone and Dolter - Basic Concepts of Chemistry 9e

  23. 7-3 Percent Yield • The actual yield is the amount of product obtained when the reaction is run. • The theoretical yield is the calculated amount of product that would be obtained if all of the limiting reactant was converted to a given product. • The percent yield is the actual yield in grams or moles divided by the theoretical yield in grams or moles times 100%. Malone and Dolter - Basic Concepts of Chemistry 9e

  24. Incomplete Conversion • In some cases, a reverse reaction occurs whereby reactants are reformed from products. • This limits the percent of reactants that are converted to products. • Such reactions are known as reversible reactions. Malone and Dolter - Basic Concepts of Chemistry 9e

  25. Incomplete Conversion • The reaction between N2 and H2 to produce NH3 is a reaction which is reversible. This has severe consequences for the commercial production of ammonia. Malone and Dolter - Basic Concepts of Chemistry 9e

  26. Example Calculation • For the conversion of N2 and H2 to NH3 • 4.70 g H2 react with N2 • 12.5 g of NH3 is formed • The theoretical yield is 26.5 g of NH3 in this reaction. The percent yield is 47.5% (commercially, the yield is only ca 28%). Malone and Dolter - Basic Concepts of Chemistry 9e

  27. Factors that Result in Lower % Yield • Losses are inevitable and occur through: • Incomplete reaction • Competing reactions • Necessary experimental manipulation, such as crystallization, distillation, filtration and solvent extraction.  Malone and Dolter - Basic Concepts of Chemistry 9e

  28. Summary Chart for Part A Malone and Dolter - Basic Concepts of Chemistry 9e

  29. Setting a Goal – Part BEnergy Relationships in Chemical Reactions • You will extend your knowledge of stoichiometry to include amounts of heat involved in chemical reactions. Malone and Dolter - Basic Concepts of Chemistry 9e

  30. Objective for Section 7-4 • Given the change in enthalpy for a reaction, calculate the amount of heat gained or released by a given mass of reactant. Malone and Dolter - Basic Concepts of Chemistry 9e

  31. 7-4 Heat Energy in Chemical Reactions • Heat can be a reactant or a product in a reaction. • Endothermic reactions are those that require heat to occur; hence heat is a reactant. • Exothermic reactions produce heat; hence heat is a product. • A balanced chemical reaction that includes heat energy is referred to as a thermochemical equation. Malone and Dolter - Basic Concepts of Chemistry 9e

  32. Heat in Chemical Reactions • A thermochemical reaction can be expressed in one of two ways • 2 H2 (g) + O2 (g) 2 H2O (l) H = -572 kJ • note that the heat evolved is per 2 mol of H2 or 2 mol of H2O produced • H is termed the heat of reaction or enthalpy • H is negative for exothermic reactions and positive for endothermic reactions Malone and Dolter - Basic Concepts of Chemistry 9e

  33. Heat in Chemical Reactions • The second way to indicate heat in a chemical reaction is as a reactant or product. • 2 H2 (g) + O2 (g) 2 H2O(l) + 572 kJ(an exothermic reaction) • N2 (g) + O2 (g) + 181 kJ  2 NO(g) (an endothermic reaction) • Heat energy can be treated quantitatively in a manner similar to the amount of reactant or product. Malone and Dolter - Basic Concepts of Chemistry 9e

  34. Example 1 – Converting Heat to Mass Acetylene combusts according to the thermochemical equation 2 C2H2 (g) + 5O2 (g)  4 CO2 (g) + 2 H2O(l)H = -2602 kJ • If 550 kJ is evolved in the combustion of C2H2, what is the mass of CO2 formed? Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  35. Example 2 – Converting Mass to Heat • CO2 is formed from its elements according to the thermochemical equation C(s) + O2(g) CO2(g) DH = -393.5 kJ • How much heat is generated from 50.0 g of C? Solution Malone and Dolter - Basic Concepts of Chemistry 9e

  36. Summary Chart for Part B Malone and Dolter - Basic Concepts of Chemistry 9e

  37. Further Worked Example The complete combustion of 1 mole of octane (C8H18), 1 mole of methanol, and 1 mole of methane (CH4) evolves 5480 kJ, 1750kJ and 890 kJ of heat, respectively. Calculate the heat evolved per gram by each of these fuels, and hence determine which is the most efficient fuel, in terms heat evolved per unit mass. Malone and Dolter - Basic Concepts of Chemistry 9e

  38. Malone and Dolter - Basic Concepts of Chemistry 9e

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