Introduction to STOICHIOMETRY Mr Shumway Chemistry 1 STOICHIOMETRY

Introduction to STOICHIOMETRY Mr. Shumway Chemistry 1

STOICHIOMETRY �"Stoichiometry" is derived from the Greek words στοιχεῖον (stoicheion, meaning element]) and μέτρον (metron, meaning measure. �That’s why it’s a funny looking word

STOICHIOMETRY �Nevertheless, Stoichiometry is the science that deals with measuring the various elements in chemical reactions

STOICHIOMETRY �Write down all that you can about this reaction below N 2(g) + 3 H 2(g) 2 NH 3(g)

STOICHIOMETRY –INTERPRETING CHEMICAL EQUATIONS �What do you look at when looking at a chemical equations? There is a lot of information in a chemical equation… Particles Moles Mass Volume

STOICHIOMETRY N 2(g) + 3 H 2(g) 2 NH 3(g) Particles How many particles are there for each reactant? How many particles are there in the product? There is 1 molecule of Nitrogen reacting with 3 molecules of hydrogen gas Producing 2 molecules of ammonia What is the mole ratio for this reaction? The mole ratio for this reaction is 1: 3: 2

STOICHIOMETRY N 2(g) + 3 H 2(g) 2 NH 3(g) H H H N N N H H H H N H

STOICHIOMETRY N 2(g) + 3 H 2(g) 2 NH 3(g) Moles How many moles are there for each reactant? How many moles are there in the product? There is 1 mol of Nitrogen reacting with 3 mol of hydrogen gas Producing 2 mol of ammonia What is the mole ratio for this reaction? The mole ratio for this reaction is 1: 3: 2

STOICHIOMETRY N 2(g) + 3 H 2(g) 2 NH 3(g) Mass – Law of conservation of mass What is the mass of each reactant? What is the mass of each product? Ex ▪ 1 mol of N 2 = 28. 0 grams of N 2 ▪ 1 mol of H 2 = 2. 0 grams of H 2 , therefore, 3 mol of H 2 = 6. 0 grams of H 2 ▪ 1 mol of NH 3= 17 grams, therefore, 2 mol of NH 3 = 34. 0 grams ▪ The mass of the reactants = the mass of the products 28. 0 + 6. 0 = 34. 0

STOICHIOMETRY N 2(g) + 3 H 2(g) 2 NH 3(g) Volume 1 mol of a gas at STP = 22. 4 L Therefore, what are the volumes of each gas from the reactants? What is the volume of the product gas that is formed? Ex ▪ 22. 4 L of N 2 reacts with 67. 2 L of H 2 (3 mol x 22. 4 L) ▪ Forming 44. 8 L of ammonia

STOICHIOMETRY Let’s Look at this problem 2 H 2 S(g) + 3 O 2(g) 2 SO 2(g) + 2 H 2 O(g)

STOICHIOMETRY –INDIVIDUAL WORK �Balance the following equation and write what down all the information that you can Number of particles Number of moles of reactants and products Mass of reactants and products Volumes of gases at STP (if applicable) �C 2 H 2(g) + O 2(g) CO 2(g) + H 2 O(g) �Na(s) + H 2 O(l) Na. OH(aq) + H 2(g) �CO(g) +O 2(g) CO 2(g)

DRILL �What do the coefficients tell you in a chemical reaction? What can the coefficients tell you?

DRILL �What do the coefficients tell you in a chemical reaction? The coefficients tell you the number of molecules/atoms there are in a chemical reaction. What can the coefficients tell you? ▪ They can tell you the mole ratios between each of the reactants and products

Mole to Mole Ratios STOICHIOMETRY Mr. Shumway Chemistry 1

STOICHIOMETRY We can use the information from the chemical equation and use them to help us calculate useful information Mole – Mole Calculations Mass – Mass Calculations Mass-mole and mole mass conversions

STOICHIOMETRY � Mole – Mole Calculations Knowing the amount of moles of reactants, we can calculate the number of moles of product ▪ Using the mole ratios of the chemical equation

STOICHIOMETRY � Mole – Mole Calculations How many moles of ammonia are produced when 0. 60 mol of nitrogen reacts with hydrogen? N 2(g) + 3 H 2(g) 2 NH 3(g) This is what we want This is what we have

STOICHIOMETRY How many moles of ammonia are produced when 0. 60 mol of nitrogen reacts with hydrogen? 1 N 2(g) + 3 H 2(g) 2 NH 3(g) What is the ratio/relationship between these molecules?

STOICHIOMETRY How many moles of ammonia are produced when 0. 60 mol of nitrogen reacts with hydrogen? N 2(g) + 3 H 2(g) 2 NH 3(g) 0. 60 mol of Nitrogen Write down what you have first X 2 mol of NH 3(g) 1 mol of N 2(g) = 1. 2 mol NH 3 Because we have the number of moles, we need a conversion factor that has mol of Nitrogen in it “A mole ratio”

STOICHIOMETRY Let’s look at this problem on the board �Calculate the number of moles of reactants required to make 7. 24 mol of Ammonia N 2(g) + 3 H 2(g) 2 NH 3(g)

STOICHIOMETRY - Individual Work The formation of aluminum oxide from its constituent elements is represented by this equation 1. How many moles of aluminum are needed to form 2. 3 mol of Al 2 O 3? 2. How many moles of oxygen are required to react completely with 0. 84 mol of Al? 3. Calculate the number of moles of Al 2 O 3 formed when 17. 2 mol of O 2 reacts with aluminum.

Drill �Design a step-by-step method that will help you make conversions between moles to moles in a chemical reaction.

Mole to Mass Calculations STOICHIOMETRY Mr. Shumway Chemistry 1

STOICHIOMETRY �Review You can calculate the number of moles of product through the mole ratios of the chemical equation. ▪ Following through with step by step dimensional analysis �If we can calculate the number of moles, we can also calculate the mass of the product and/or the reactants.

STOICHIOMETRY Steps to Follow A + X AX It is important that if you are given grams to first convert to moles because you can use the mole ratio from the chemical equation 2. From moles of X to moles of AX (using the mole ratio) 3. Convert moles of AX to grams of AX 1.

STOICHIOMETRY �Calculate the number of grams of NH 3 produced by the reaction of 5. 40 g of hydrogen with nitrogen N 2(g) + 3 H 2(g) 2 NH 3(g)

STOICHIOMETRY �Step 1: calculate the number of moles by the mass given N 2(g) + 3 H 2(g) 2 NH 3(g) 5. 40 g of H 2 gas Write down what you have first X 1 mol of H 2(g) 2. 0 g of H 2(g) = 2. 7 mol H 2 Because we have the grams of Hydrogen, we need a conversion factor that has grams/mol with the proper molecules. Using the molar mass of the molecule.

STOICHIOMETRY �Step 2: Convert from moles to moles N 2(g) + 3 H 2(g) 2 NH 3(g) 2. 70 mol of H 2 gas Write down what you got from the last problem X 2 mol of NH 3(g) 3 mol of H 2(g) = 1. 8 mol NH 3(g) Because we have the moles of Hydrogen, we need a conversion factor that will give us the number of moles of ammonia gas, using the mole ratio in the chemical reaction. Essentially, we’re converting from moles given to moles desired

STOICHIOMETRY �Step 3: Convert from moles to grams N 2(g) + 3 H 2(g) 2 NH 3(g) 1. 80 mol of NH 3(g) Write down what you got from the last problem X 17. 0 g of NH 3(g) 1 mol of NH 3(g) = 30. 6 g NH 3(g) Because we have the moles of ammonia, we need a conversion factor that will give us the grams of ammonia gas, using the molar mass of the molecule.

STOICHIOMETRY Step 1 • Grams of “A” • Volume of Gas “A” Step 2 • Moles of “A” Step 3 • Moles of “B” Answer • Grams of “B” • Volume of Gas “B”

STOICHIOMETRY –TEACHER The combustion of acetylene gas is represented by this equation. 2 C 2 H 2(g) + 5 O 2(g) 4 CO 2(g) + 2 H 2 O(g) How many grams of oxygen are required to burn 13. 0 g of C 2 H 2? 2. How many grams of CO 2 and grams of H 2 O are produced when 13. 0 g of C 2 H 2 reacts with the oxygen required to burn 13. 0 g of C 2 H 2? 1.

STOICHIOMETRY –INDIVIDUAL WORK Acetylene gas, C 2 H 2, is produced by adding water to calcium carbide, Ca. C 2(s) + 2 H 2 O(l) C 2 H 2 + Ca(OH)2(aq) How many grams of acetylene are produced by adding water to 5. 00 g of Ca. C 2? 2. How many moles of Ca. C 2 are needed to react completely with 98. 0 g of H 2 O? 1.

STOICHIOMETRY Step 1 Alternate Step 1 Step 2 • Atoms/particles of “A” • Grams of “A” • Volume of Gas “A” • Moles of “A” Step 3 Answer Alternate • Moles of “B” • Grams of “B” • Volume of Gas “B” • Atoms/particles of “B”

DRILL �Construct a flow chart/map of how to solve the problems you’ve encountered thus far.

Review �Get into your table groups. �Each group will have a different problem to work on, you will have 20 min to solve the problem. Everyone in the group MUST KNOW HOW TO SOLVE THE PROBLEM �One random person will be chosen to come to the board and do the problem and explain their work, this will determine the groups grade.

Drill �What is a limiting reagent that you described in your mini lab?

Limiting Reagent STOICHIOMETRY Mr. Shumway Chemistry 1

STOICHIOMETRY –LIMITING REAGENT �Limiting reagent Limits or determines the amount of product that can be formed in a reaction �Excess reagent Is more than enough to react with a limiting reagent

STOICHIOMETRY –LIMITING REAGENT Ex. Problem Sodium Chloride is prepared by the reaction of sodium metal with chlorine gas. What will occur when 6. 70 mol of Na reacts with 3. 20 mol of Cl 2? What is the limiting reagent? How many moles of Na. Cl are produced? How much excess reagent remains unreacted?

STOICHIOMETRY –LIMITING REAGENT �Step 1 - What do we know? We know/have ▪ 6. 70 mol of Na ▪ 3. 20 mol of Cl 2 �Step 2 – What do we want to know? We want to calculate three things ▪ What is the limiting reagent? ▪ How many moles of Na. Cl are produced ▪ How much excess reagent remains unreactedd

STOICHIOMETRY –LIMITING REAGENT �Step 3 – Write down a BALANCED chemical reaction �Step 4 - convert what you know into moles if not already

STOICHIOMETRY –LIMITING REAGENT �Step 5 – Look at the mole ratio and calculate the amount of substance needed for a complete reaction Use this information to determine the limiting reagent and excess reagent and label them.

STOICHIOMETRY –LIMITING REAGENT Start with what we know Required amount of Chlorine 1 mol of Cl 2 = 3. 35 mol of Cl 2 2 mol of Na Mole Ratio 6. 70 mol of Na x Knowing that we have 6. 70 mol of Na and knowing the mole ratio between sodium and chlorine, we can calculate the minimum amount of chlorine we need for this reaction to take place Using the mole ratio, we can see that 3. 35 mol of Cl 2 is needed to react with 6. 70 mol of Na Because we do not have enough Cl 2, Cl 2 is the limiting reagent. How much Chlorine did the problem say we had? Can someone explain why? 3. 20 mol of Cl 2!

STOICHIOMETRY –LIMITING REAGENT Because Cl 2 is the limiting reagent, the amount of product is determined by the amount of Cl 2 we have. Therefore, the number of moles of Cl 2 is going to be used to calculate the maximum amount of product that can be formed. Mole Ratio Total amount of product formed HOW MUCH EXCESS DO WE HAVE? ( Na? )

STOICHIOMETRY –LIMITING REAGENT The amount of excess reagent remaining is the difference between the given amount (in the beginning of the problem, a value of 6. 70 mol of Na) and the amount of sodium needed to react with the limiting reagent. Therefore, we need to now calculate how much Na would be used up in the reaction with Cl 2 Mole Ratio Amount of Na used up in the reaction 6. 70 mol Na – 6. 40 mol Na = 0. 30 mol Na in excess

STOICHIOMETRY: L. R. TEACHER DEMO Copper reacts with sulfur to form copper (I) sulfide. What is the maximum number of grams of Cu 2 S that can be formed when 1. 87 mol of Cu reacts with 2. 19 mol of S?

STOICHIOMETRY: L. R. INDIVIDUAL WORK Hydrogen gas can be produced in the laboratory by the reaction of magnesium metal with hydrochloric acid. How many grams of hydrogen can be produced when 1. 45 mol of HCl is added to 2. 31 mol of Mg? Assuming STP, what is the volume of this hydrogen?

STOICHIOMETRY –LIMITING REAGENT Write down a systematic approach to dealing with Limiting reagent type problems. How would you solve this type of problem, step-by step?

Drill �What is a limiting reagent? How can you know which reactant is the limiting reagent? �What is an excess reagents? How can you know which reactant is the excess reagent?

Percent Yield STOICHIOMETRY Mr. Shumway Chemistry 1

Percent Yield Theoretical Yield When an equation is used to calculate the amount of product that will form during a reaction, the value is theoretical. Actual Yield The amount of product that forms when the reaction is carried out in the laboratory Percent Yield Is the ratio of the actual yield to theoretical yield. It measure the efficiency of the reaction

Percent Yield

Percent Yield What is the percent yield of this reaction if 24. 8 g of Ca. CO 3 is heated to give 13. 1 g of Ca. O? Looking at this problem, let’s identify the important information What do we want? We have the actual yield, now we need theoretical. And we have all we need to solve for theoretical yield. This problem tells us the actual yield, 13. 1 g of Ca. O

Percent Yield Given Converting to moles Mole ratio, looking using molar mass of at the coefficients Ca. CO 3 24. 8 g Ca. CO 3 This is our theoretical yield. Now we can plug this into our equation Converting to grams using molar mass of Ca. O

Percent Yield

Percent Yield – Individual Work What is the percent yield if 3. 74 g of copper is produced when 1. 87 g of aluminum is reacted with an excess of copper (II) sulfate?

Energy Changes in Chemical Reactions STOICHIOMETRY Mr. Shumway Chemistry 1

Energy Changes in Chemical Reactions Exothermic Reactions Releases energy in the form of heat in a chemical reaction Endothermic Reactions Energy is absorbed in a chemical reaction Thermochemical Equation An equation that includes the amount of heat produced or absorbed by a chemical reaction.

Energy Changes in Chemical Reactions Exothermic Reaction “Releases Energy” Or gives off energy Where would the energy be in the chemical reaction? The products or reactants? The Energy would be in the products

Energy Changes in Chemical Reactions Endothermic Reaction “Absorbs Energy” Or requires energy Where would the energy be in the chemical reaction? The products or reactants? This equation is called a thermochemical equation The Energy would be in the reactants

Energy Changes in Chemical Reactions We can use thermochemical equation to help us calculate the energy required for a chemical equation. Think of this as another conversion factor to add

Energy Changes in Chemical Reactions Using the equation just given, calculate the energy required to decompose 5. 20 mol of Ca. CO 3 (in Kilojoules) 1 5. 20 mol Ca. CO 3 = 915 KJ

Energy Changes in Chemical Reactions Enthalpy Is the amount of heat that a substance has at a given temperature and pressure. Is symbolized by “H” If an exothermic reaction is taking place then the reaction has a “-ΔH” If an endothermic reaction is taking place then the reaction has a “+ΔH”

Energy Changes in Chemical Reactions Standard Heat of Formation is where the “ΔH” for a reaction in which one mole of a compound is formed from its elements of that compound.

Energy Changes in Chemical Reactions – Teacher Demo The air pollutant sulfur trioxide reacts with water in the atmosphere to produce sulfuric acid and heat. How much heat is released when 583 g of SO 3(g) reacts with water?

Energy Changes in Chemical Reactions – Individual Work Carbon dioxide can be decomposed into carbon monoxide and oxygen by the absorption of heat. How many molecules of CO 2(g) can be decomposed by the addition of 22. 2 KJ of heat energy?