1 Limiting Reactants • Many reactions are carried out with a limited amount of one reactant and a plentiful amount of the other(s). • The reactant that is completely consumed in the reaction limits the amounts of products and is called the limiting reactant, or limiting reagent. • The limiting reactant is not necessarily the one present in smallest amount. 2 Limiting Reactant Analogy If we have 11 sandwiches, 16 cookies, and 10 oranges … … how many packaged meals can we make? 1 3 Limiting Reactant Analogy …totally consumed! 4 Molecular View of the Limiting Reactant Concept 1. Why is ethylene left over, when we started with more bromine than ethylene? (Hint: count the molecules.) 2. What mass of ethylene is left over after reaction is complete? (Hint: it’s an easy calculation; why?) When 28 g (1.0 mol) ethylene reacts with … … 128 g (0.80 mol) bromine, we get … … 150 g of 1,2dibromoethane, and leftover ethylene! 2 Recognizing and Solving Limiting Reactant Problems • We recognize limiting reactant problems by the fact that amounts of two (or more) reactants are given. • One way to solve them is to perform a normal stoichiometric calculation of the amount of product obtained, starting with each reactant. • The reactant that produces the smallest amount of product is the limiting reactant. 5 6 Magnesium nitride can be formed by the reaction of magnesium metal with nitrogen gas. a) How many grams of magnesium nitride can be made in the reaction of 35.00 g of magnesium and 15.00 g of nitrogen? b) How many grams of the excess reactant remain after the reaction? 3 7 Yields of Chemical Reactions ניצולת • The theoretical yield of a chemical reaction is the calculated quantity of product in the reaction. • The actual yield is the amount you actually get when you carry out the reaction. • Actual yield will be less than the theoretical yield, for many reasons … can you name some? actual yield Percent yield = ––––––––––––– × 100 theoretical yield 8 Actual yield of ZnS is less than the theoretical yield… 4 9 Solutions and Solution Stoichiometry • Solute: the substance being dissolved. מומס • Solvent: the substance doing the dissolving. ממס • Concentration of a solution: the quantity of a solute in a given quantity of solution (or solvent). – A concentrated solution contains a relatively large amount of solute vs. the solvent (or solution). – A dilute solution contains a relatively small concentration of solute vs. the solvent (or solution). 10 Molar Concentration Molarity (M), or molar concentration, is the amount of solute, in moles, per liter of solution: moles of solute Molarity = –––––––––––––– liters of solution • A solution that is 0.35 M sucrose contains 0.35 moles of sucrose in each liter of solution. • Keep in mind: molarity signifies moles of solute per liter of solution, not liters of solvent. 5 11 Preparing 0.01000 M KMnO4 Weigh 0.01000 mol (1.580 g) KMnO4. Dissolve in water. How much water? Doesn’t matter, as long as we don’t go over a liter. Add more water to reach the 1.000 liter mark. 12 You want to prepare a solution of 6.68 M NaOH. (a) How many moles of NaOH are required to prepare 0.500 L of 6.68 M NaOH? (b) How many liters of 6.68 M NaOH can we prepare with 2.35 kg NaOH? The label of a stock bottle of aqueous ammonia indicates that the solution is 28.0% NH3 by mass and has a density of 0.898 g/mL. Calculate the molarity of the solution. 6 13 Dilution of Solutions מיהול • Dilution is the process of preparing a more dilute solution by adding solvent to a more concentrated one. • Addition of solvent does not change the amount of solute in a solution but does change the solution concentration. • It is very common to prepare a concentrated stock solution of a solute, then dilute it to other concentrations as needed. Visualizing the Dilution of a Solution We start and end with the same amount of solute. 14 Addition of solvent has decreased the concentration. 7 15 Dilution Calculations • Moles of solute does not change on dilution. • Moles of solute = C × V – remains unchaged! ⇒ Cconc × Vconc = Cdil × Vdil OR… Cdil = Cconc × Vconc Dilution factor Vdil 16 How many milliliters of a 2.00 M CuSO4 stock solution are needed to prepare 0.250 L of 0.400 M CuSO4? 8 17 Solutions in Chemical Reactions • Molarity provides an additional tool in stoichiometric calculations based on chemical equations. • Molarity provides factors for converting between moles of solute (either reactant or product) and liters of solution. Adding to the previous stoichiometry scheme … 18 If substance A is a solution of known concentration … … we can start with molarity of A times volume (liters) of the solution of A to get here. If substance B is in solution, then … … we can go from moles of substance B to either volume of B or molarity of B. How? 9 19 אבן גיר תוססת כאשר מטפטפים עליה חומצה כלורית )כך מזהים התסיסה היא למעשה שחרור גז פחמן דו.(אותה בטיולים שנתיים .חמצני CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g) ? 3.25M HCl תמיסת225 mL יגיב עםCaCO3 כמה גרם 20 Summary of Concepts • The limiting reactant determines the amounts of products in a reaction. • The actual yield of the reaction is lower than the theoretical yield because of various factors. • The molarity of a solution is the number of moles of a solute per liter of solution. ⇒ Can be used as a conversion factor from moles to volume (or vice versa). 10
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