Friday 9-18 and Monday 9-21 Stoichiometry Mrs. Wilson

Objectives Calculate the # of moles or # g of a reactant needed or product formed in a chemical reaction process. Calculate the percent yield of a chemical reaction and judge the efficiency of the process. Homework: 1.10 Homework – Major Quiz Next Class (We will do a lot of review beforehand.) - FYI: Unit 1 Exam on Monday 9-28 and Tuesday 9-29

2 graham crackers + 1 marshmallow + 1 square chocolate = 1 s’more What is Stoichiometry? 2 graham crackers + 1 marshmallow + 1 square chocolate = 1 s’more 1. What quantities of each ingredient do you need to make 10 s’mores? 2. What quantities of each ingredient do you need to make 10 dozen s’mores? 3. What quantities of each ingredient do you need to make 10 moles of s’mores?

First Thing: Understand a Chemical Equation methane + oxygen  carbon dioxide + water   |------------ reactants-------------| |----------------- products---------------| CH4 (l) + 2 O2 (g)  CO2 (g) + 2 H2O (g) States: (s) Solid (g) Gas (l) Liquid (aq) Aqueous solution 1 mole 2 moles yields

Second Thing: What –Is- Stoichiometry? What does it mean?   The word stoichiometry derives from two Greek words: stoicheion (meaning “element") and metron (meaning “measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry, so be prepared for lots of calculator use.

What You Should Expect… The most common stoichiometric problem will present you with a certain amount of a reactant and then ask how much of a product can be formed. Here is a generic chemical equation: 2 A + B2 ---> 2 AB Here is a typically-worded problem: Given 20.0 grams of A and sufficient B, how many grams of AB can be produced?

Why do chemists use it? Chemists use stoichiometry to determine quantities of chemicals needed, and to predict the quantities produced for any chemical reaction. Without stoichiometry, we would not know the proportions in which chemicals react or how much they produce.

Mole ratios are obtained from balanced chemical equations. You will always use a mole ratio when working stoichiometry problems. A mole ratio is a conversion factor that relates the amounts of moles of any two substances involved in a chemical reaction. Mole ratios are obtained from balanced chemical equations. (and all the equations you’ll get – for now – will already be balanced.)

For example: 2 H2+ O2 → 2H2O What are the mole ratios in this reaction?   1) 2 mol H2 1 mol O2 (Both of these ratios are the same; 1 mol O2 2 mol H2 the only difference is which one is on top.) 2) 2 mol H2 2 mol H2O 2 mol H2O 2 mol H2 3.) What are the mole ratios that compare O2 and H2O? Write them down.

For example: 2 H2+ O2 → 2H2O What are the mole ratios in this reaction?   1) 2 mol H2 1 mol O2 (Both of these ratios are the same; 1 mol O2 2 mol H2 the only difference is which one is on top.) 2) 2 mol H2 2 mol H2O 2 mol H2O 2 mol H2 3.) 1 mol O2 2 mol H2O 2 mol H2O 1 mol O2

Example 1: What are some (give 3) mole ratios in this balanced reaction? 3 Ca + 2 H3PO4  Ca3(PO4)2 + 3 H2

At the heart of any stoichiometry problem…  You are given the amount of one substance and asked to find the number of moles of another substance. Write and balance the chemical equation. Convert everything to moles. Use mole ratios to solve for what you are trying to find. Convert everything into the required unit if needed.  

Percent Yield % Yield = actual yield x 100% experimental yield Side reactions, determinate errors, etc. can LOWER the amount of product that is really made. Theoretical yield is the mass of product you PREDICT, based on calculation. The actual yield is the amount of product you REALLY make. % Yield = a comparison of actual yield to theoretical yield. (The higher the percent yield, the more precise your experimental results are.) % Yield = actual yield x 100% experimental yield

There are two ways to solve these problems… 1. Dimensional Analysis 2. Use a Proportion – but Be Careful (Either way is an acceptable way of showing your work.)

0.480 moles calcium chloride Method 1: Dimensional Analysis How many moles of calcium chloride will be produced if you start out with 35.0 g of HCl? Ca(OH)2 + 2 HCl → CaCl2 + H2O 0.480 moles calcium chloride

0.480 moles calcium chloride Method 2: Use a Proportion – but Be Careful How many moles of calcium chloride will be produced if you start out with 35.0 g of HCl? Ca(OH)2 + 2 HCl → CaCl2 + H2O 0.480 moles calcium chloride

Stoichiometry Practice Problems Given chemical equation: Br2 + 2 NaI  2 NaBr + I2 1. Balance the above chemical equation. 2. How many moles of I2 are produced when 3.00 mol of Br2 completely reacts?  3.00 moles  

Given chemical equation: Br2 + 2 NaI  2 NaBr + I2 3. How many grams of sodium iodide (NaI) will completely react with 10.0 mol of bromine (Br2)? 3.00 x 103 grams

Given chemical equation: Br2 + 2 NaI  2 NaBr + I2 4. How many grams of iodine (I2) could be produced when 50.0 g of bromine (Br2) completely reacts ? (79.4 g) If 60.0 g of iodine is really produced from an experiment, calculate the percent yield. (75.6%)

Given chemical equation: Br2 + 2 NaI  2 NaBr + I2 5. How many grams of sodium bromide (NaBr) could be produced from 0.172 mol of bromine (Br2)? (35.4 g) What is the percent yield if 27.5 g of sodium bromide is recovered? (77.7%)

You Do: #6 - 10 We’ll have volunteers come up and draw out their work on the tablet or on the board (your choice)!

Given chemical equation: 2 H2 + O2  2 H2O 6. How many moles of H2O are produced when 2.50 moles of oxygen are used? 5.00 moles

Given chemical equation: 2 H2 + O2  2 H2O   7. If 3.6 moles of H2O are produced, how many grams of oxygen must be consumed? 57.6 g

How many grams of O2 can be produced by letting 12 How many grams of O2 can be produced by letting 12.00 moles of KClO3 react if given the following equation: 2 KClO3  2 KCl + 3 O2 576.0 g

Camels store the fat tristearin (C57H110O6) in the hump Camels store the fat tristearin (C57H110O6) in the hump. As well as being a source of energy, the fat is a source of water. How many moles of carbon dioxide are produced from 8.73 moles of tristearin? 2 C57H110O6(s) + 163 O2(g)  114 CO2(g) + 110 H2O(l) 498 moles

Exit Ticket What did you learn today about stoichiometry? Describe what you learned.