STOICHIOMETRY

STOICHIOMETRY

INTRODUCTION to STOICHIOMETRY Chapter 9 The elements lithium and oxygen react explosively to form lithium oxide. How many moles of lithium oxide will form if 2 moles of lithium react and oxygen is in excess? When sodium azide (NaN3) is activated

in a car airbag, sodium and nitrogen gas are produced. If 0.500 moles of NaN3 react, what mass, in grams, of nitrogen would result? 1 mol Li2O 21.0g N2 Composition stoichiometry describes the quantitative (mass/mol) relationships among elements in compounds. Ex: NH3 hydrogen 1:3 ratio between nitrogen and

Reaction stoichiometry allows us to determine the amount of substance that is consumed or produced by a reaction. For example, in a reaction that forms ammonia (NH3), exactly one mole of nitrogen (N2) reacts with three moles of hydrogen (H2) to produce two moles of NH3: N2 + 3H2 2NH3 Reaction stoichiometry describes the 1:3:2 ratios of molecules of nitrogen, hydrogen and ammonia. The quantitative aspects of chemical equations are crucially important. How much oil will you buy to produce enough heat for the winter?

How much gas is needed to get you to New York City? How much sulfur does Union Carbide have to mine to produce 1 million tons of sulfuric acid? How many Calories do you need for lunch in order to run 5 miles? Chemical reactions are everywhere. We now need to develop methods for answering questions of this type. Mole Ratio A conversion factor that relates the amount in moles of any two substances

involved in a chemical reaction. The coefficients in a balanced equation indicate the mole ratio. Example: A reaction produces 10.0 moles of O2. Predict how many moles of KClO3 will react. Balance the equation- Find the mole ratio and multiply:

Molar Mass The mass, in grams, of 1 mole of a substance. Conversion factors: mass in grams/1 mole or 1 mole/ mass in grams Home Fun Pg 277 q 1, 2, 3b Ch 9-2

Ideal Stoichiometric Calculations A chemical equation for a reaction is essentially a recipe. You intend to make 40 fruit salads for a large dinner party according to the following recipe: 1 apple + 1 orange + 10 grapes = 1 fruit salad You go to the store to purchase starting materials, and come home with the 10

pounds of apples, 10 pounds of oranges, and 10 pounds of grapes. Are these materials sufficient to produce 40 fruit salads? We have a problem at the outset: the equation is expressed in terms of numbers of apples, oranges, and grapes; however, we know only the total amounts of each fruit by mass. We need more information, specifically, the average mass of an apple, an orange, and a grape.

Suppose that we can weigh a typical apple, orange, and grape on a simple kitchen scale. Suppose further that the apples, oranges, and grapes are of unusually uniform size, so that the weight of one apple faithfully represents the weight of any apple. 1 apple-weight = 0.25 lb/apple 1 orange-weight = 0.33 lb/orange 1 grape-weight = 0.021 lb/grape These unit weights enable us to convert grocery store masses into

numbers of apples, oranges, and grapes: Number of apples = 10 lbs/(0.25 lb/apple) = 40 apples Number of oranges = 10 lbs/(0.33 lb/orange) = 30 oranges Number of grapes = 10 lb/(0. 021 lb/grape) = 476 grapes Now we are in a position to apply the recipe, which tells us that for each apple used, we must use 1 oranges and 10 grapes. To use 40 apples would require 40

oranges and 400 grapes. We have more than enough grapes to use up all of the apples, but we do not have enough oranges. We can conclude that the number of fruit salads will be limited by the number of oranges that we have. The recipe is expressed in numbers of units of each ingredient, not in terms of their masses. Similarly a chemical equation is expressed in terms of numbers of atoms, molecules, or formula units, not in terms of their masses.

The amounts of our fruit salad ingredients are expressed as masses when we buy them at the store. Similarly, when we buy chemical substances, the amounts are expressed as masses. To apply the recipe, we must convert the masses to numbers of things using the mass per thing. Then we can figure out how many fruit salads it is possible to make with the given amounts of ingredients. We must do the same thing with a chemical equation.

One of the ingredients is found to limit the number of fruit salads that can be made. In many cases, one of the reactants limits the amount of product that can be formed in a chemical reaction. This will be the reactant that is used up first; it is called the limiting reactant. Time to apply the recipe analogy to chemical reactions. Mass mass (given) (find) moles

moles (given) (find) by by molar mass mass x by mole ratio x molar

What mass of pure silicon can be obtained by reduction of SiCl4 with 10.0 g of magnesium metal at elevated temperature? Assume that a large supply of SiCl4 is on hand. (This reaction is used in the process of extracting silicon from naturally occurring ores. Silicon is used to make semiconductors.) 1. Balance the equation. SiCl4(l) + 2Mg(l) -> Si(s) + 2MgCl2(s) 2. Convert the given mass of Mg to moles Mg. Moles Mg = 10.0 g Mg/(24.305 g Mg/mole) = 0.4114 moles Mg 3. Use the mole ratios in the equation to relate moles Mg (reactant) to moles Si (product). Moles Si = 0.4114 moles Mg x (1 mole Si/2 moles Mg) = 0.2057 moles Si

Convert moles Si to grams of Si. g Si = 0.2057 moles Si x (28.086 g Si/mole) = 5.78 g Si Mass reactant moles reactant moles product mass product. moles moles reactant moles product mass product. reactant moles reactant moles product mass product. moles moles reactant moles product mass product. product moles reactant moles product mass product. moles reactant moles product mass product. mass moles reactant moles product mass product. product. moles reactant moles product mass product. How much pure silicon can be produced by reaction of 38.0 g of Mg and 26.7 g of SiCl4(l)? Determine the number of moles of each reactant: moles Mg = 38.0 g Mg/(24.305 g/mole Mg) = 1.563 moles Mg moles SiCl4 = 26.7 g SiCl4/(169.90 g/mole SiCl4) = 0.1572 moles SiCl4 The chemical equation indicates that we require 2 moles of Mg for each mole of SiCl4 . What is our

limiting reactant? Try this: How many moles of sodium will react with water to produce 4.0 mol of hydrogen in the following reaction? 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g) ans: 8.0 mol Na How many moles of lithium chloride will be formed by the reaction of chlorine with 0.046 mol of lithium bromide in the following reaction? 2LiBr(aq)+ Cl2(g) -> 2LiCl(aq)+ Br2(l)

ans: 0.046 mol LiCl Limiting Reactants and Percent Yields Limiting reactants are the reactants that are completely used up in a reaction. These limit the amounts of the other reactants that can combine and the amount of product that can form. Excess reactants are not used up in a

chemical reaction. Try this: SiO2 + 4HF SiF4 + 2H2O If two mol of HF are exposed to 4.5 mol of SiO2, what is the limiting reactant? The given amount of either reactant can be used. The calculated amount is then compared to the given amount. If there is more of the second reactant than needed, it is in excess. If there is not enough of the second reactant to completely react the first, then it is

limiting. Percent Yield Although we can write perfectly balanced equations to represent perfect reactions, the reactions themselves are often not perfect. A reaction does not always produce the quantity of products that the balanced equation seems to guarantee. This happens not because the equation is wrong but because reactions in the

real world seldom produce perfect results. Reasons for imperfect reactions Reactants or products leak out, especially when they are gases. The reactants are not 100% pure. Some product is lost when it is purified. The products decompose back into reactants

The products react to form different substances. Some of the reactants react in ways other than the one shown in the equation. These are called side reactions. The reaction occurs very slowly. This is especially true of reactions involving organic substances. Percent Yield Theoretical yield the maximum amount of product that can be formed from a given amount of reactant. Ex: The cookie recipe said that for each batch of cookies 24 cookies could be

made. Actual yield- The amount of product that actually IS formed from a given amount of reactant. How many cookies did you make? The PERCENT YIELD Measures the efficiency of a reaction. The ratio of actual yield to theoretical yield, multiplied by 100. (actual yield/theoretical yield) x 100

Try this: If the actual yield of a reaction is 22 g and the theoretical yield is 25 g, calculate the percent yield. Answer: 88% 6.0 mol of N2 are mixed with 12.0 mol of H2 according to the following equation: N2(g) + 3H2(g) 2NH3(g) Which chemical is in

excess and what is the excess amount in moles? Theoretically, how many moles of NH3 will be produced? If the percent yield of NH3 is 80%, how many moles of NH3 are actually produced? N2; 2.0 mol 8.0 mol

6.4 mol Dichlorine monoxide, Cl2O is sometimes used as a powerful chlorinating agent in research. It can be produced by passing chlorine gas over heated mercury(II) oxide according to the following equation: HgO + Cl2 HgCl What 2 + Cl2Ois the percent yield, if the quantity of reactants is

sufficient to produce 0.86 g of Cl2O but only 0.71 g is obtained? 83% yield Acetylene, C2H2 , can be used as an industrial starting material for the production of many organic compounds. Sometimes, it is first brominated to

form 1,1,2,2tetrabromoethane, CHBr2CHBr2 , which can then be reacted in many different ways to make other substances. The equation for the bromination of acetylene follows:: C2H2 + 2Br2 CHBr2CHBr2 If 72.0 g of C2H2 reacts with excess bromine and 729 g of the product is recovered, what is the percent yield of

the reaction? 76.3% yield Home fun Pg 294 q 1-4 Pg 297 q 22, 24, 26-28, 30

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