Stoichiometry “The Mathematics of Chemical Reactions” By: Ms. Buroker

What you’ll Learn …. You should be able to…. 1.) write mole ratios from balanced chemical equations. 2.) calculate the number of moles and the mass of a reactant or product when given the number of moles or the mass of another reactant or product. 3.) identify the limiting reactant in a chemical reaction. 4.) determine the percent yield of a chemical reaction.

Okay, so what in the world is stoichiometry? Technically, stoichiometry is the study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction. Stoichiometry allows a chemist or scientist to know how much of an element or reactant to use, how much product is expected to come out of the reaction, and how to express different amounts of a substance in many units of measurement.

A Little Bit of Review Before We Get Started … So let's say I ask you to hand me 5 moles of Li. You look at me and wonder how you're supposed to measure out 5 moles. Before completely giving up you remember that you can use a simple conversion equation to find a simpler measuring unit to give you 5 moles of Li.

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?

Cookies and Chemistry…Huh!?!? Just like chocolate chip cookies have recipes, chemists have recipes as well Instead of calling them recipes, we call them reaction equations Furthermore, instead of using cups and teaspoons, we use moles Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients

Chemistry Recepies Looking at a reaction tells us how much of something you need to react with something else to get a product (like the cookie recipe) Be sure you have a balanced reaction before you start! Example: 2 Na + Cl 2  2 NaCl This reaction tells us that by mixing 2 moles of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride What if we wanted 4 moles of NaCl? 10 moles? 50 moles?

Mole Ratios Within a chemical equation, coefficients not only represent the individual number of atoms or molecules but also the number of moles! Example: Fe (s) + O 2(g)  Fe 2 O 3(s) 4 32 What we can then say is, 4 moles of Fe react with 3 moles of O 2 to produce 2 moles of Fe 2 O 3

Mole Ratio: a ratio between the number of moles of any two substances in a balanced chemical equation. Note: Think of the mole ratio as your bridge between reactants, between products, and between reactants and products

Determine …. All possible mole ratios for the following balanced chemical equations: 4Al (s) + 3O 2(g)  2Al 2 O 3(s) 2HgO (s)  2Hg (l) + O 2(g) 4 mol Al/ 3 mol O 2 3mol O 2 / 2mol Al 2 O 3 4 mol Al/ 2 mol Al 2 O 3 2 mol Al 2 O 3 / 4 mol Al 3 mol O 2 / 4 mol Al 2 mol Al 2 O 3 / 3 mol O 2 2 mol HgO/ 2 mol Hg2 mol Hg/ 1 mol O 2 2 mol HgO/ 1 mol O 2 1 mol O 2 / 2 mol HgO 2 mol Hg/ 2 mol HgO1 mol O 2 / 2 mol Hg

Practice Write the balanced reaction for hydrogen gas reacting with oxygen gas. 2 H 2 + O 2  2 H 2 O –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?

Mole- to- Mole Conversions What happens when I want to go from moles of one substance to moles of another substance? Or If I know how much I have of one thing and I want to use that information to make predictions about something else in the reaction? Moles of Known X Moles of unknown Moles of known = Moles of unknown Mole Ratio

Mole- to- Mole Conversions Let’s look at the following reaction involving the reaction of sodium metal with chlorine gas to produce sodium chloride: 2Na (s) + Cl 2(g)  2NaCl (s) 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? Remember the formula!! Moles of known x mole ratio = moles of unknown 2.6 mol Cl 2 x 2 mol NaCl = 5.2 mol NaCl 1mol Cl 2

Let’s Try Another One! One disadvantage of burning propane (C 3 H 8 ) is that carbon dioxide (CO 2 ) is one of the products. The released CO 2 increases the growing concentration of CO 2 in the atmosphere. How many moles of carbon dioxide are produced when 10.0 moles of propane are burned in excess oxygen in a gas grill? C 3 H 8(g) + 5O 2(g)  3CO 2(g) + 4H 2 O (g) 10.0 mol C 3 H 8 x 3 mol CO 2 = 30.0 mol CO 2 1 mol C 3 H 8

Mole- to- Mass Conversion Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to produce sodium chloride? 2 Na + Cl 2  2 NaCl 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 ?5 moles 5 mol Na x 1 mol Cl 2 x 70.9 g Cl 2 = g Cl 2 2 mol Na 1 mol Cl 2

What if I Wanted to go in Reverse? Mass- to- Moles 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 number of moles of ethane (C 2 H 6 ) needed to produce 10.0 g of water – 2 C 2 H O 2  4 CO H g H 2 O 1 mol H 2 O 2 mol C 2 H g H 2 O 6 mol H 2 0 = mol C 2 H 6

Let’s Practice! Calculate how many moles of oxygen are required to make 10.0 g of aluminum oxide. 4Al (s) + 3O 2(g)  2Al 2 O 3(s) ? 10.0g 10.0g Al 2 O 3 X 1 mol Al 2 O g Al 2 O 3 X 3 mol O 2 2 mol Al 2 O 3 = mol O 2

Mass- to- 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

Mass-Mass Conversion Ex. Calculate how many grams of ammonia are produced when you react 2.00g of nitrogen with excess hydrogen. N H 2  2 NH g N 2 1 mol N 2 2 mol NH g NH g N 2 1 mol N 2 1 mol NH 3 = 2.4 g NH 3

How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen? 3Ca (s) + N 2(g)  Ca 3 N 2(s) ? 2.00g 2.00g Ca 1 mol Ca g Ca X = mol Ca MOLE RATIO!!! mol Ca x 1 mol Ca 3 N 2 3 mol Ca = mol Ca 3 N 2 X g 1 mol = 2.48 g Ca 3 N 2

Stoichiometry with Gases According to gas laws, when gases are at STP (standard temperature and pressure), 1 mole of a gas will occupy 22.4L. 1.) Standard Temperature is 25°C or 273K 2.) Standard Pressure is 1atm 3.) This allows us to take volumes of gases that are at STP and turn it into moles.

Let’s take a look at an example! If I had the following reaction: How many Liters of ammonia could I make (at STP) if I began with 25g of N 2 gas? N 2(g) + 3H 2(g)  2NH 3(g)

Why do reactions stop? The answer is … limiting reagents! If you put 10 boys and 6 girls in a room and asked them to pair up: one boy to one girl, who will be left without a partner? The boys … there will be four boys left with no partner. The girls are the limiting factor, in other words, they control how many pairs will be formed.

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?

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.

How do I Find The Limiting Reagent? Convert from grams of both reactants into grams of product … whichever one makes the least amount is your limiting reagent.

Finding Excess Reagent Step 1: Take the limiting reagent and convert from grams of the limiting reagent into grams of the excess reagent. Step 2: Subtract grams of excess reagent actually used from how much was available.

Percent Yield Percent yield is often found to give one a picture of the accuracy of their work. Remember!!! Stoichiometry gives you the theoretical yield, or what you should get in a perfect world! Your actual yield is what you actually get back from the experiment. Percent yield= (actual yield/theoretical yield) x 100