1 / 39

Stoichiometry

Stoichiometry. Stoichiometry. Allows you to calculate quantitative relationships of reactants and products What does that mean? Quantitative – numbers Reactants produce products Allows you to calculate how much reactant you need to make a certain amount of product, etc. . Stoichiometry.

judd
Download Presentation

Stoichiometry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Stoichiometry

  2. Stoichiometry • Allows you to calculate quantitative relationships of reactants and products • What does that mean? • Quantitative – numbers • Reactants produce products • Allows you to calculate how much reactant you need to make a certain amount of product, etc.

  3. Stoichiometry • Allows you to calculate how much reactant you need to make a certain amount of product • Why is this important? • Chemical reactions produce everything from medicine to energy to candy. You need to know how much you’re making so you can do what you need to do!

  4. Chocolate Chip Cookies!! 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips  Makes 3 dozen How many eggs are needed to make 3 dozen cookies? How much butter is needed for the amount of chocolate chips used? How many eggs would we need to make 9 dozen cookies? How much brown sugar would I need if I had 1 ½ cups white sugar?

  5. Chemistry Recipes • A reaction tells us how much (moles) of something you need to react with something else to get a product • Be sure you have a balancedequation before you start!!! 2 Na + Cl2  2 NaCl • 2 moles of Na reacts with 1 mole of Cl2 to get (yield) 2 moles of NaCl What if we wanted 4 moles of NaCl? 10 moles? 50 moles?

  6. Practice Write the balanced reaction for hydrogen gas reacting with oxygen gas. 2 H2 + O2 2 H2O • How many moles of reactants are needed? • What if we wanted 4 moles of water? • What if we had 3 moles of oxygen, how much hydrogen would we need to react and how much water would we get? • What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced?

  7. Steps • Balanced equation • Write down what you are given • Moles! • Mole ratio (coefficients of balanced equation) • Ending units

  8. Set these up like unit conversions!

  9. Mole Ratios • They’re just like conversions • Keep your units with your number • Keep your compound with your units • If it’s 2.6 mol Na, write mol Na • If it’s 5 mol Cl2, write 5 mol Cl2 • Decide what you goes on the top and what you goes on the bottom to end up with the units you want • What you want goes on the top; what you want to cancel goes on the bottom

  10. Mole Ratios • The mole ratios can be used to calculate the moles of one chemical from the given amount of a different chemical • How many moles of chlorine are needed to react with 5 moles of sodium (without any sodium left over)? 2 Na + Cl2 2 NaCl 5 moles Na 1 mol Cl2 2 mol Na = 2.5 moles Cl2

  11. Mole-Mole Conversions • How many moles of sodium chloride will be produced if you react 2.6 moles of chlorine gas with an excess (more than you need) of sodium metal? 2 Na + Cl2 2 NaCl 2.6 moles Cl2 2 mol NaCl 1mol Cl2 = 5.2 moles NaCl

  12. Mole-Mass Conversions • Most of the time in chemistry, the amounts are given in grams instead of moles • We still go through moles and use the mole ratio, but now we also use molar mass to get to grams • Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to produce sodium chloride? 2 Na + Cl2 2 NaCl 5.00 moles Na 1 mol Cl2 70.90g Cl2 2 mol Na 1 mol Cl2 = 177g Cl2

  13. Practice • Calculate the mass in grams of Iodine required to react completely with 0.50 moles of aluminum.

  14. Mass-Mole • We can also start with mass and convert to moles of product or another reactant • We use molar mass and the mole ratio to get to moles of the compound of interest • Calculate the moles of ethane (C2H6) needed to produce 10.0 g of water in a combustion reaction • 2 C2H6 + 7 O2 4 CO2 + 6 H20 10.0 g H2O 1 mol H2O 2 mol C2H6 18.0 g H2O 6 mol H20 = 0.185 mol C2H6

  15. Practice • Calculate how many moles of oxygen are required to make 10.0 g of aluminum oxide

  16. Mass-Mass Conversions • Most often we are given a starting mass and want to find out the mass of a product we will get (called theoretical yield) or how much of another reactant we need to completely react with it (no leftover ingredients!) • Now we must go from grams to moles, mole ratio, and back to grams of compound we are interested in

  17. Mass-Mass Conversion • Calculate how many grams of ammonia (NH3) are produced when you react 2.00g of nitrogen with excess hydrogen. • N2 + 3 H2  2 NH3 2.00g N2 1 mol N2 2 mol NH3 17.06g NH3 28.02g N2 1 mol N2 1 mol NH3 = 2.4 g NH3

  18. Practice • Partner up! Try it! • How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen?

  19. Groups of 3 • Check your answers; try to find your errors • Use the 5 steps as a guide • Problems on the back • #1: 177 g Cl2 • #2: 190 g I2 • #3: 0.185 mol C2H6 • #4: 0.147 mol O2 • #5: 2.43 g NH3 • #6: 2.47 g Ca3N2

  20. Any questions over stoichiometry so far? • What are some main points that we learned about stoichiometry? • Stoichiometry lets us figure out amounts of chemicals involved in reactions • Must go through moles! • Need balanced equation!

  21. Limiting Reactant: Cookies 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips Makes 3 dozen If we had the specified amount of all ingredients listed, could we make 4 dozen cookies? What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies? What if we only had one egg, could we make 3 dozen cookies?

  22. Limiting Reactant: Cookies 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips Makes 3 dozen What if we had 5 lbs of flour? How many cookies could we make? What if we had 16 oz of salt? How many cookies could we make?

  23. Limiting Reactant • Most of the time in chemistry we have more of one reactant than we need to completely use up other reactant. • That reactant is said to be in excess (there is too much). • The other reactant limits how much product we get. Once it runs out, the reaction s. This is called the limiting reactant.

  24. Limiting Reagent The reactant that is completely consumed by the reaction The number of bicycles that can be assembled is limited by whichever part runs out first. In the inventory shown in this figure, wheels are that part.

  25. A molecular view of a limiting reactant situation for the ammonia synthesis. To make 4 molecules of NH3 requires 2 molecules of N2 and 6 molecules of H2. If we start with 4 molecules of N2 and 6 molecules of H2, H2 is the limiting reactant.

  26. Limiting Reactant • To find the correct answer, we have to try all of the reactants. We have to calculate how much of a product we can get from eachof the reactants to determine which reactant is the limiting one. • The lower amount of a product is the correct answer. • The reactant that makes the least amount of product is the limiting reactant. Once you determine the limiting reactant, you should ALWAYS start with it! • Be sure to pick a product! You can’t compare to see which is greater and which is lower unless the product is the same!

  27. LimitingReactant Limiting Reactant: Example • 10.0g of aluminum reacts with 35.0 grams of chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced? 2 Al + 3 Cl2 2 AlCl3 • Start with Al: • Now Cl2: 10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3 27.0 g Al 2 mol Al 1 mol AlCl3 = 49.4g AlCl3 35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3 71.0 g Cl2 3 mol Cl2 1 mol AlCl3 = 43.9g AlCl3

  28. LR Example Continued • We get 49.4g of aluminum chloride from the given amount of aluminum, but only 43.9g of aluminum chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction comes to a complete .

  29. Limiting Reactant Practice • 15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting and how much product is made.

  30. Bell work 3/7 • Grab a nuts and bolts follow up from my desk. It’s a half sheet. Complete it.

  31. Agenda • Bell work • Finding amount of excess, % yield, % composition • In class: Limiting reagents, excess, % yield, % composition • HW: Homework: Limiting reagents, excess, % yield, % composition

  32. Finding the Amount of Excess • By calculating the amount of the excess reactant needed to completely react with the limiting reactant, we can subtract that amount from the given amount to find the amount of excess. • Can we find the amount of excess potassium in the previous problem?

  33. Finding Excess Practice • 15.0 g of potassium reacts with 15.0 g of iodine. 2 K + I2 2 KI • We found that iodine is the limiting reactant, and 19.6 g of potassium iodide are produced. • Now we want to know how much K will be left over. 15.0 g I2 1 mol I2 2 mol K 39.1 g K 254 g I2 1 mol I2 1 mol K = 4.62 g K USED! 15.0 g K – 4.62 g K = 10.38 g K EXCESS Note that we started with the limiting reactant! Once you determine the LR, you should only start with it! Given amount of excess reactant Amount of excess reactant actually used

  34. More Practice • 30.0 g of sodium reacts with 30.0 g of oxygen (typo on yours; sorry!). Calculate which reactant is limiting, how many grams of product are made, and how many grams of excess reactant remain.

  35. Limiting Reactant: Recap • You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT. • Convert ALL of the reactants to the SAME product (pick any product you choose.) • The lowest answer is the correct answer. • The reactant that gave you the lowest answer is the LIMITING REACTANT. • The other reactant(s) are in EXCESS. • To find the amount of excess, subtract the amount used from the given amount. • If you have to find more than one product, be sure to start with the limiting reactant. You don’t have to determine which is the LR over and over again!

  36. %, actual, and theoretical yield • Write this somewhere on your notes or on a separate sheet of paper • Theoretical yield: • if everything goes correctly, this is the amount of product you will make • Actual yield: • the amount you actually make • % yield: actual x 100% theoretical

  37. 19.6 g KI • From the previous problem, what is the theoretical yield of potassium iodide? • If only 18 grams of KI were produced, what is the percent yield? • What mass of KI would result in 72% yield? actual 18 g KI x 100% x 100% = 91.8% theoretical 19.6 g KI x x 100% = 72% 14.1 g KI = x 19.6 g KI

  38. Percent Composition of Compounds • Percent composition consists of the mass percent of each element in a compound: Mass of element . x 100 % composition = Molar mass of compound

  39. Homework • Before you leave, I need your exit ticket! • In Class: Limiting reagents, excess, % yield, % composition

More Related