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STOICHIOMETRY

USING THE CHEMICAL EQUATION LIKE A RECIPE. STOICHIOMETRY. Stoichiometry. What is It? Quantitative relationships In chemical reactions Based on law of Conservation of Mass Why/How is it Useful? . Straight from Wikipedia.

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STOICHIOMETRY

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  1. USING THE CHEMICAL EQUATION LIKE A RECIPE STOICHIOMETRY

  2. Stoichiometry • What is It? • Quantitative relationships • In chemical reactions • Based on law of Conservation of Mass • Why/How is it Useful?

  3. Straight from Wikipedia • Stoichiometry (pron.: /ˌstɔɪkiˈɒmɨtri/) is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of positive integers. For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3: • N2 + 3H2 → 2NH3

  4. Stoichiometry is a lot like making tacos. Taco Recipe What if I need to make tacos for 50 people? • 1o shells • 1 lb ground beef • 1 pkg of taco seasoning • ¾ c water • ½ cup salsa • 1 cup cheddar cheese • 1 cup of lettuce Makes 10 tacos. • You know how much product you need (tacos) • You have to calculate how much of each ingredient you need. • That’s stoichiometry!!!!

  5. Equations are a chemist’s recipe. • Equations tell chemists • what amounts of reactants to mix and what amounts of products to expect. • If you know the quantity of one substance in a reaction • you can calculate the quantity of any other substance consumed or created in the reaction • Quantitymeaning the amount of a substance in grams, liters, molecules, or moles.

  6. Balanced Chemical Equations • The coefficients in a balanced chemical equation shows how many moles/particles of one reactant are needed to react with other reactants and how many moles/particles of product will be formed. • Based on Law of Conservation of Mass 4Fe + 3O2 2Fe2O3

  7. 4Fe + 3O2 2Fe2O3 • Write the relationships we can derive from this equation.

  8. C3H8 + 5O2 3CO2 + 4H2O • The combustion of propane provides energy for heating homes, cooking food, and soldering metal parts. Interpret the equation for the combustion of propane in terms of particles, moles, and mass. • Show that the law of conservation of mass is observed

  9. C3H8 + 5O2 3CO2 + 4H2O

  10. The calculation of quantities in chemical reactions is called stoichiometry. • Imagine you are in charge of manu-facturing for Rugged Rider Bicycle Company. • The business plan for Rugged Rider requires the production of 128 custom-made bikes each day. • You are responsible for insuring that there are enough parts at the start of each day.

  11. An Analogy for Stoichiometry • Assume that the major components of the bike are the frame (F), the seat (S), the wheels (W), the handlebars (H), and the pedals (P). • The finished bike has a “formula” of FSW2HP2. • The balanced equation for the production of 1 bike is. F +S+2W+H+2P FSW2HP2

  12. Now in a 5 day workweek, Rugged Riders is scheduled to make 640 bikes. How many wheels should be in the plant on Monday morning to make these bikes? • What do we know? • Number of bikes = 640 bikes • 1 FSW2HP2=2W (balanced eqn) • What is unknown? • # of wheels = ? wheels

  13. 1280 Wheels = • The connection between wheels and bikes is 2 wheels per bike. We can use this information as a conversion factor to do the calculation. 2 W 640 FSW2HP2 1 FSW2HP2 • We can make the same kinds of connections from a chemical rxn eqn. N2(g) + 3H2(g)  2NH3(g) • The key is the “coefficient ratio”.

  14. The coefficients of the balanced chemical equation indicate the numbers of moles of reactants and products in a chemical reaction. • 1 mole of N2 reacts with 3 moles of H2 to produce 2moles of NH3. • N2 and H2 will always react to form ammonia in this 1:3:2 ratio of moles. • So if you started with 10 moles of N2 it would take 30 moles of H2 and would produce 20 moles of NH3

  15. Using the coefficients, from the balanced equation as ratios to make connections between reactants and products, is the most important information that an equation provides. • Using this information, you can calculate the amounts of the reactants involved and the amount of product you might expect.

  16. Mole Ratios • Shows the mole-to-mole ratio between two of the substances in a balanced equation • Written from the coefficients of any two substances in the equation • This is a very important conversion and the key to solving stoichiometry problems.

  17. 2Al + 3Br2 2AlBr3

  18. 4 Fe + 3 O2 2 Fe2O3

  19. 3 H2(g) + N2(g) 2 NH3(g) A mol ratio for H2 and N2 A mol ratio for NH3 and H2

  20. Stoichiometric Calculations • How much product can be obtained from a given reaction? • How much reactant is needed to produce this much product?

  21. Review of Mole Conversions Three mole equalities 1 mol = 6.02 x 1023particles 1 mol = (molar mass) (from periodic table) 1 mol= 22.4 L for a gas at STP

  22. Review practice • How many moles are in 5.0 x 1025 molecules of CO2? • What is the mass in grams of 0.250 moles of CO2? • How many moles of CO2 gas are in 100.0 L at STP?

  23. Types of Stoichiometry Calculations • Mole - Mole Calculations • Moles of Known  Moles of Unknown • Mole-Mass Calculations • Moles of Known  Mass of Unknown • Mass-Mole Calculations • Mass of Known  Moles of Unknown • Mass-Mass Calculations • Mass of Known  Mass of Unknown

  24. MOLE – MOLE EXAMPLE • The following reaction shows the synthesis of aluminum oxide. 3O2(g) + 4Al(s)  2Al2O3(s) 3O2(g) + 4Al(s)  2Al2O3(s) • If you only had 1.8 mols of Al how much product could you make?

  25. Mole Ratio 3O2(g) + 4Al(s)  2Al2O3(s) • Solve for the unknown: 2 mol Al2O3 1.8 mol Al = 0.90mol Al2O3 4 mol Al

  26. If you wanted to produce 24 moles of product how many moles of each reactant would you need? 3O2(g) + 4Al(s)  2Al2O3(s)

  27. 3O2(g) + 4Al(s)  2Al2O3(s) • Solve for the unknowns: 4 mol Al 24 mol Al2O3 = 48 mol Al 2 mol Al2O3 3 mol O2 24 mol Al2O3 = 36 mol O2 2 mol Al2O3

  28. Practice Problem How many moles of hydrogen will be produced if 0.44 mol of CaH2 reacts according to the following equation? CaH2 + 2H2O  Ca(OH)2 + 2H2

  29. Iron will react with oxygen to produce Iron III oxide. How many moles of Iron (III) oxide will be produced if 0.18 mol of Iron reacts? 4 4 Fe + 3 O2 2 Fe2O3

  30. 4 Fe + 3 O22 Fe2O3 How many moles of Fe2O3 are produced when 6.0 moles O2 react?

  31. How many moles of Fe are needed to react with 12.0 moles of O2?

  32. MASS – MASS CALCULATIONS • No lab balance measures moles directly, generally mass is the unit of choice. • From the mass of 1 reactant or product, the mass of any other reactant or product in a given chemical equation can be calculated, provided you have a balanced equation. • As in mole-mole calculations, the unknown can be either a reactant or a product.

  33. Example # 1 Mass-Mass Acetylene gas (C2H2) is produced by adding water to calcium carbide (CaC2). CaC2 + 2H2O  C2H2 + Ca(OH)2 CaC2 + 2H2O  C2H2 + Ca(OH)2 How many grams of C2H2 are produced by adding water to 5.00 g CaC2?

  34. 1 mol CaC2 Step 1: “Get to Moles!” in this case that can be done by using the Molar Mass of your given compound. 5.0 g CaC2 = .07813 mol CaC2 64.0 g CaC2 Step 2: Now we are ready for the KEY step…converting from mols of our given to mols of unknown using the mole ratio. 1 mol C2H2 .07813 mol CaC2 = .07813 mol C2H2 1 mol CaC2

  35. Step 3: Since we are asked for mass of our unknown in this problem, we need to use our molar mass of our unknown and convert our newly calculated mols into grams. 26.0 g C2H2 .07813 mol C2H2 = 2.03 g C2H2 1 mol C2H2

  36. Summary of 3 Steps of Stoichiometry Problems • Get to Moles of Given • Mole Ratio to calculate moles of unknown • Get to wanted final unit

  37. The double replacement reaction between Lead II nitrate and Potassium Iodide produces a bright yellow precipitate that can be used as a color additive in paint. How many grams of potassium iodide would we need to completely react 25.3 g of lead (II) nitrate? mass A  mols A  mols B  mass B Pb(NO3)2 + 2 KI  PbI2 + 2 KNO3 Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

  38. 1mol Pb(NO3)2 2mol KI 331.2g Pb(NO3)2 1mol Pb(NO3)2 166 g KI 1mol KI 25.3 g Pb(NO3)2 = 25.4 g KI

  39. Practice Problems What mass of Barium chloride is needed to react completely with 46.8 g of Sodium phosphate according to the following equation? BaCl2 + Na3PO4  Ba3(PO4)2 + NaCl

  40. Another problem Use the equation to determine what mass of FeS must react to form 326g of FeCl2. FeS + 2HCl  H2S + FeCl2

  41. Moles of Known  Mass of Unknown • Find moles of unknown using the mole ratio. • Convert moles of unknown to mass of unknown. • Calculate the mass of carbon dioxide produced by the decomposition of 2.5 moles of copper(II) carbonate. CuCO3CuO + CO2

  42. Mass to Moles Calculations • Mass of known  moles of unknown • Convert mass of known to moles of known. • Use mole ratio to find moles of unknown. • Calculate the moles of water produced by the reaction of 20 grams of hydrogen gas with excess oxygen gas.

  43. A balanced reaction equation indicates the relative numbers of moles of reactants and products. • We can expand our stoichiometric calculations to include any unit of measure that is related to the mole. • The given quantity can be expressed in numbers of particles, units of mass, or volumes of gases at STP. • The problems can include mass-volume, volume-volume, and particle-mass calculations.

  44. In any of these problems • the given quantity is first converted to moles. • Then themole ratiofrom the balanced equation is used to convert from the moles of given to the number of moles of the unknown • Then the moles of the unknown are converted to the units that the problem requests. • The next slide summarizes these steps for all typical stoichiometric problems

  45. Stoichiometry Roadmap Coefficient mol U Moles Given Moles Unknown Coefficient mol G Balanced Equation 1 mole 22.4 L 1 mole Molar mass 22.4 L Molar mass 1 mole 1 mole Volume U Mass U Volume G Mass G Particles G Particles U Coefficient part U Coefficient part G

  46. How many molecules of O2 are produced when a sample of 29.2 g of H2O is decomposed by electrolysis according to this balanced equation: 2H2O  2H2 + O2

  47. The last step in the production of nitric acid is the reaction of NO2 with H2O. How many liters of NO2 must react with water to produce 5.00x1022 molecules of NO? 3NO2+H2O2HNO3+NO

  48. 1 mol NO 3 mol NO2 5.0x1022 mol-ecules NO molecules A mols A mols B volume B 1 mol NO 6.02x1023 mol-ecules NO 22.4 L NO2 1 mol NO2 = 5.58 L NO2

  49. C7H6O3 + C4H6O3 C9H8O4 + HC2H3O2 Salicylic acid Acetic anhydride aspirin vinegar Aspirin can be made from a chemical reaction between the reactants salicylic acid and acetic anhydride. The products of the reaction are acetylsalicylic acid (aspirin) and acetic acid (vinegar). Our factory makes 125,000 100-count bottles of Bayer Aspirin/day. Each bottle contains 100 tablets, and each tablet contains 325mg of aspirin. How much in kgs + 10% for production problems, of each reactant must we have in order to meet production?

  50. Salicylic Acid: 1 mol C7H6O3 136.10g C7H6O3 22,549.4 mols aspirin 1 mol C7H6O3 1 mol asp 1 kg = 3068.97 kg salicylic acid + (306.897 g) 1000 g = 3380 kg of salicylic acid

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