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STOICHIOMETRY

STOICHIOMETRY. Mass relationships between reactants and products in a chemical reaction. STOICHIOMETRY. Stoichiometry : looks at what mass of products (in grams) is produced when you start with a certain mass of reactant

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STOICHIOMETRY

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  1. STOICHIOMETRY Mass relationships between reactants and products in a chemical reaction

  2. STOICHIOMETRY • Stoichiometry: looks at what mass of products (in grams) is produced when you start with a certain mass of reactant • Tells you how much you make when you start with a certain amount of stuff.

  3. EXAMPLE • Let’s say you start with the following recipe: • 2 eggs + 1 cup of flour + 2 tubs of frosting  1 cake + 3 cupcakes • Write this down and answer the following questions:

  4. QUESTIONS • If you start with 6 eggs, how many cupcakes would you make? • If you started with 4 cups of flour, how many cakes could you make? • If you made 6 cupcakes, how many tubs of frosting did you need? • If you started with 1 ½ cups of flour, how many cupcakes could you make?

  5. ANSWERS • 9 Cupcakes • 4 Cakes • 4 Tubs of frosting • 4 ½ Cupcakes Question: What method did you use to answer these questions?

  6. STOICHIOMETRY • Answer: You used the ratio between one ingredient and what you made. • How does this relate? • Chemistry is like cooking. Chemical reactions occur in exact ratios. As a result, we can also predict how much stuff we make.

  7. EXAMPLE • Write the balanced equation for the following: • Sodium and chlorine mix to form sodium chloride.

  8. BALANCED EQUATION • 2Na + Cl2 2NaCl • In this reaction, you have a ratio of reactants and products. • They are always the same. • ANSWER THE FOLLOWING QUESTIONS?

  9. QUESTIONS • If you start with 4 Na, how much NaCl do you make? • If you start with 4 Cl2, how much NaCl do you make? • If you made 5 NaCl, how much Cl2 did you start with?

  10. ANSWERS • 4 NaCl • 8 NaCl • 2.5 Cl2 • In each case you take the ratio from the chemical reaction • IMPORTANT NOTE: WHAT ARE THE NUMBERS A RATIO OF???

  11. MOLAR RATIO • The numbers in front of the compounds are ALWAYS RATIOS OF MOLES.

  12. USING DIMENSIONAL ANALYSIS • To solve for the amounts of reactants and products, we use the balanced chemical equation • Just like every other dimensional analysis problem, you start by writing what you start with and cancel out the stinking units.

  13. EXAMPLE • Let’s use the previous reaction: • 2Na + Cl2 2 NaCl • If you start with 3.75 moles of Cl2, how many moles of NaCl can you make? • Start with what you know: • 3.75 moles of Cl2

  14. ANSWER • YOU USE THE RATIO OF MOLES AS THE CONVERSION FACTOR • 3.75 moles Cl2 | 2 moles of NaCl = 1 mole of Cl2 • 7.50 moles of NaCl • IMPORTANT NOTE: Not only do you have to put the units in dimensional analysis, you also have to put the name of the compound.

  15. TRY THESE • H2 + N2 NH3 • For the above reaction, balance the reaction. Then, if you start with 2.5 moles of H2, how many moles of NH3 do you make? • Zn + HCl  ZnCl2 + H2 • For the above reaction, balance the reaction. If you start with 5.0 moles of Zn, how many moles of HCl do you need to complete the reaction?

  16. ANSWERS • 1.7 moles of NH3 • 10 moles of HCl • NOTE: In each case the units and name of the compound were included. • NOTE: In #2, you can also find the amount of other reactants from a starting reactant.

  17. MOLECULES  MOLES MASS • Review: • How do you convert from molecules to moles? • How do you convert from mass to moles?

  18. MOLECULES  MOLES MASS • Molecules Moles 1mole = 6.02x1023 molecules • Mass Moles #grams = 1 mole (periodic table)

  19. HOW DOES THIS WORK FOR US? • Reminder: Chemical reactions are ratios of moles. • To use molecules or mass, YOU MUST FIRST CONVERT TO MOLES

  20. EXAMPLE • You begin with the following reaction: MgBr2 + 2NaOH  2NaBr + Mg(OH)2 • If you start with 1.2 x 1024 molecules of NaOH, how many moles of Mg(OH)2 will you produce?

  21. STEP 1 • Convert molecules to moles (ALL CHEMICAL REACTIONS ARE RATIOS OF MOLES ONLY!!!) • 1.2 x 1024 molecules | 1 mole __ = 6.02x1023 molecules • 2.0 moles

  22. STEP 2 • MgBr2 + 2NaOH  2NaBr + Mg(OH)2 • 2.0moles NaOH | 1 moles Mg(OH)2 = 2 moles NaOH • 1 mole of Mg(OH)2 • YOU CAN SET THIS UP IN ONE DIMENSIOAL ANALYSIS

  23. EXAMPLE #2 • You begin with the following reaction: 2H2 + O2 2H2O • If you start with 96.0g of O2, how many moles of H2O do you produce?

  24. STEP 1 • Convert mass to moles first (CHEMICAL REACTIONS ARE ALWAYS RATIOS OF MOLES!!!) • 96 g O2 | 1 mole O2 | 2 moles H2O | 32 g O2 | 1 mole O2 • = 6 moles of H2O • NOTICE: EVERY NUMBER HAS A UNIT AND THE NAME OF THE COMPOUND

  25. TRY THIS • For the following balance the equation and then answer the question: • AgNO3 + CaBr2 AgBr + Ca(NO3)2 • If you start with 5.00 g of CaBr2, how many moles of AgBr do you make?

  26. ANSWER • 5.00g CaBr2 | 1 mole CaBr2 | 2 moles AgBr | 200g of CaBr2| 1 mole CaBr2 • = .0500 moles of AgBr • NOTE: AGAIN, YOU CANCEL OUT THE UNITS AND THE NAME OF THE COMPOUD!!!

  27. MASS TO MASS • The final goal of stoichiometry: • Predict what mass of substance is produced or used by having a balanced chemical reaction

  28. SO FAR . . . • We’ve looked at going from a starting number of moles and calculating the number of moles we produce • We’ve looked at going from a starting mass or number of molecules and calculating the number of moles we produce • NOW: We are going to start with a mass and calculate what mass we produce

  29. MASS TO MASS • REMINDER: A CHEMICAL REACTION SHOWS A RATIO OF MOLES!!!! • To go from mass to mass, we must use the following format: • Mass  Moles  Moles  Mass molar mass chemical reaction molar mass

  30. EXAMPLE • Given the following chemical reaction: NaOH + HCl  NaCl + H2O • If you start with 345g of HCl, how many grams of NaCl do you produce?

  31. STEP 1 • Find the number of moles of HCl 345g HCl | 1 mole of HCl_ | 36.46 g of HCl = 9.46 moles of HCl

  32. STEP 2 • Find the number of moles of NaCl produced: 9.46 moles of HCl | 1 mole of NaCl | 1 mole of HCl = 9.46 moles of NaCl

  33. STEP 3 • Find the mass of NaCl 9.46 moles of NaCl | 58.44g of NaCl | 1 mole of NaCl = 553g of NaCl • NOTE: In each conversion, the name of the compound was included.

  34. ONE DIMENSIONAL ANALYSIS SETUP • 345g HCl | 1 mole of HCl | 1 mole of NaCl | 58.44g of NaCl | 36.46g of HCl | 1 mole of HCl | 1 mole of NaCl = 553g of NaCl

  35. TRY THIS • Balance the following reaction: P + O2 PO5 • If you produce 155g of PO5, what mass of O2 did you start with (in grams)?

  36. ANSWER • 2P + 5O2 2PO5 155g PO5 | 1mole PO5 | 5moles O2 | 32g O2 | 111g PO5 | 2moles PO5 | 1mole O2 = 112 g O2

  37. CONCEPT MAP Molecule/Atoms Molecule/Atoms 1 mole = 6.02x1023 Moles Balanced chemical equation Moles molar mass (#g =1 mole) Mass Mass density (#g = 1mL) Volume Volume

  38. TRY THIS • Translate and balance: • Iron (III) chloride combines with bromine to form iron (III) bromide and chlorine. • If you start with 5.66x1023 molecules of bromine, what volume of iron (III) bromide do you produce? The density of iron (III) bromide is 4.50g/mL.

  39. ANSWER 41.2 mL of FeBr3

  40. TRY THIS • Translate and balance: • Aluminum combines with hydrobromic acid to form aluminum bromide and hydrogen gas. • If you start with 22.4mL of aluminum what volume of aluminum bromide do you produce? The density of aluminum is 7.87g/mL. The density of aluminum bromide is 2.67g/mL.

  41. ANSWER 653 mL of AlBr3

  42. TRY THIS • Translate and balance: • Sulfuric acid decomposes to form sulfur dioxide, oxygen and hydrogen. • If you start with 125 mL of sulfuric acid, how many molecules of oxygen do you produce? The density of sulfuric acid is 1.84g/mL.

  43. ANSWER 1.41 X 1024 molecules O2

  44. PERCENT YIELD • In theory, you should always produce a certain mass of product if you start with a specific mass of reactant. • In reality, things aren’t perfect: • You could measure inaccurately • Some product could be lost during the reaction • The reaction doesn’t go to completion

  45. PERCENT YIELD • As a result . . . • YOU ALWAYS PRODUCE LESS PRODUCT THAN IS PREDICTED • There is a way to measure how much product that you did produce: PERCENT YIELD

  46. CALCULATING PERCENT YIELD • To calculate the percent yield, you first need to calculate the theoretical amount of product that should be produced • Just like how we have been doing. • You then have to measure the actual amount or product made (this value will be give)

  47. CALCULATING PERCENT YIELD • Then you use the following formula: Actual amount of product x 100 Theoretical amount of product • The closer the number is to 100%, the closer you are to the theoretical value

  48. EXAMPLE • You start with the following reaction: NaOH + HCl  NaCl + H2O • If you start with 75.0g of HCl and produce 35.0g of H2O, what is your percent yield for H2O?

  49. STEP 1 • Calculate the theoretical yield (the amount of H2O you should produce) 75.0gHCl | 1mole HCl | 1 mole H2O | 18.02g H2O | 36.34g HCl | 1mole HCl | 1mole H2O = 37.1g of H2O

  50. STEP 2 • Use the amount of product measured to calculate %yield • Theoretical yield = 37.1g • Actual yield = 35.0 g • %yield = 35.0g X 100 37.1g • = 94.3%

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