2 Let’s make some Cookies! When baking cookies, a recipe is usually used, telling the exact amount of each ingredient.If you need more, you can double or triple the amountThus, a recipe is much like a balanced equation.
3 Stoichiometry is… Greek for “measuring elements” Pronounced “stoy kee ahm uh tree”Defined as: calculations of the quantities in chemical reactions, based on a balanced equation.There are 3 ways to interpret a balanced chemical equation
4 #1. In terms of Particles An Element is made of atoms A Molecular compound (made of only nonmetals) is made up of molecules (Don’t forget the diatomic elements)Ionic Compounds (made of a metal and nonmetal parts) are made of formula units
5 Example: 2H2 + O2 → 2H2O 2 formula units Al2O3 form 4 atoms Al and 3 Two molecules of hydrogen and one molecule of oxygen form two molecules of water.Another example: 2Al2O3 ® 4Al + 3O22formula unitsAl2O3form4atomsAland3moleculesO2Now read this: 2Na + 2H2O ® 2NaOH + H2
6 Remember: A balanced equation is a Molar Ratio #2. In terms of MolesThe coefficients tell us how many moles of each substance2Al2O3 ® 4Al + 3O22Na + 2H2O ® 2NaOH + H2Remember: A balanced equation is a Molar Ratio
7 #3. In terms of Mass 2H2 + O2 ® 2H2O The Law of Conservation of Mass appliesWe can check mass by using moles.2H2 + O2 ® 2H2O2.02 g H22 moles H2=4.04 g H21 mole H2+32.00 g O21 mole O2=32.00 g O21 mole O236.04 g H2 + O236.04 g H2 + O2reactants
8 In terms of Mass (for products) 2H2 + O2 ® 2H2O18.02 g H2O=36.04 g H2O2 moles H2O1 mole H2O36.04 g H2 + O2=36.04 g H2O36.04 grams reactant = grams productThe mass of the reactants must equal the mass of the products.
9 Practice:Show that the following equation follows the Law of Conservation of Mass (show the atoms balance, and the mass on both sides is equal)2Al2O3 ® 4Al + 3O2
10 Practice p 3711).Interpret these equations in terms of particles, moles and mass. Show that the law of conservation is observed.-N2(g)+3H2(g)--> 2 NH3(g)-HCl(aq)+KOH(aq)-->KCl(aq)+H2O(l)-2Mg(s)+O2(g)-->2MgO(s)2).Balance each equation and interpret in terms of particles, moles and mass. Also show that the law of conservation is observed.__Na(s)+__H20(l)-->__NaOH(aq)+__H2(g)__Zn(s)+__HNO3(aq)-->Zn(NO3)2(aq)+__N2O(g)+__H2O(l)
11 Practice p 372 3). Determine all possible mole ratios 4Al+3O2-->2Al2O33Fe+4H2O-->Fe3O4+4H22HgO-->2H2+O24). Balance the following equations and determine the possible mole ratios-ZnO+HCl-->ZnCl2+H2OC4H10+oxygen-->carbon dioxide+water
12 Mole to Mole conversions 2Al2O3 ® 4Al + 3O2each time we use 2 moles of Al2O3 we will also make 3 moles of O22 moles Al2O33 mole O2or3 mole O22 moles Al2O3These are the two possible conversion factors to use in the solution of the problem.
13 Mole to Mole conversions How many moles of O2 are produced when 3.34 moles of Al2O3 decompose?2Al2O3 ® 4Al + 3O23 mol O23.34 mol Al2O3=5.01 mol O22 mol Al2O3Conversion factor from balanced equationIf you know the amount of ANY chemical in the reaction, you can find the amount of ALL the other chemicals!
14 Practice:2C2H2 + 5 O2 ® 4CO2 + 2 H2OIf 3.84 moles of C2H2 are burned, how many moles of O2 are needed?(9.6 mol)How many moles of C2H2 are needed to produce 8.95 mole of H2O?(8.95 mol)If 2.47 moles of C2H2 are burned, how many moles of CO2 are formed?(4.94 mol)
16 Practice p 375 11. Methane and sulfur react to produce CS2. __CH4+__S8-->__CS2+__H2Sa.Balance the equation.b. Caculate the moles of CS2 produced when 1.5 moles S8 is used.c. How many moles of H2S are produced?12. Sulfuric acid, H2SO4, is formed when sulfur dioxide SO2 reacts with oxygen and water.Write the balanced chemical equation for the reaction.b. How many moles of H2SO4 are produced from 12.5 moles of SO2?c. How many moles of O2 are needed?
17 Steps to Calculate Stoichiometric Problems Correctly balance the equation.Convert the given amount into moles.Set up mole ratios.Use mole ratios to calculate moles of desired chemical.Convert moles back into final unit.
18 Mass-Mass Problem: 4Al + 3O2 2Al2O3 12.3 g Al2O3 are formed 6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?4Al O2 2Al2O36.50 g Al1 mol Al2 mol Al2O3g Al2O3=? g Al2O326.98 g Al4 mol Al1 mol Al2O312.3 g Al2O3are formed(6.50 x 1 x 2 x ) ÷ (26.98 x 4 x 1) =
19 Another example:If 10.1 g of Fe are added to a solution of Copper (II) Sulfate, how many grams of solid copper would form?2Fe + 3CuSO4 ® Fe2(SO4)3 + 3CuAnswer = 17.2 g Cu
20 Practice p 37613). 2.5 moles of NaCl are decomposed by passing electricity. How much Cl2 (in g) is obtained?14). TiO2+C+2Cl2--> TiCl4+CO2What mass of Cl2 gas is needed to react with 1.25 mol of TiO2?b. What mass of C is needed to react with 1.25 mol of TiO2?c. What is the mass of all the products formed by reaction with 1.25 mol of TiO2?
21 Practice p 37715). In the reaction, 2NaN3-->2Na+3N2, determine the mass of N2 produced from 100 g of NaN3?16). In the formation of acid rain, SO2 reacts with oxygen and water to form sulfuric acid H2SO4. Write the balanced equation for the reaction. If 2.5 g of SO2 reacts with excess oxygen and water, how much H2SO4, in grams, is produced?
22 Volume-Volume Calculations: How many liters of CH4 at STP are required to completely react with 17.5 L of O2 ?CH4 + 2O2 ® CO2 + 2H2O22.4 L O21 mol O21 mol CH41 mol CH422.4 L CH41 mol O222.4 L CH417.5 L O222.4 L O22 mol O21 mol CH4= 8.75 L CH4Notice anything relating these two steps?
23 Avogadro told us:Equal volumes of gas, at the same temperature and pressure contain the same number of particles.Moles are numbers of particlesYou can treat reactions as if they happen liters at a time, as long as you keep the temperature and pressure the same.1 mole = 22.4 STP
24 Shortcut for Volume-Volume? How many liters of CH4 at STP are required to completely react with 17.5 L of O2?CH4 + 2O2 ® CO2 + 2H2O1 L CH417.5 L O2= 8.75 L CH42 L O2Note: This only works for Volume-Volume problems.
25 “Limiting” ReagentIf you are given one dozen loaves of bread, a gallon of mustard, and three pieces of salami, how many salami sandwiches can you make?The limiting reagent is the reactant you run out of first.The excess reagent is the one you have left over.The limiting reagent determines how much product you can make
27 How do you find out which is limited? The chemical that makes the least amount of product is the “limiting reagent”.You can recognize limiting reagent problems because they will give you 2 amounts of chemicalDo two stoichiometry problems, one for each reagent you are given.
28 Cu is the Limiting Reagent, since it produced less product. If 10.6 g of copper reacts with g sulfur, how many grams of the product (copper (I) sulfide) will be formed?2Cu + S ® Cu2SCu is the Limiting Reagent, since it produced less product.1 mol Cu1 mol Cu2Sg Cu2S10.6 g Cu63.55g Cu2 mol Cu1 mol Cu2S= 13.3 g Cu2S= 13.3 g Cu2S1 mol S1 mol Cu2Sg Cu2S3.83 g S32.06g S1 mol S1 mol Cu2S= 19.0 g Cu2S
29 How much excess reagent will remain? Another example:If 10.3 g of aluminum are reacted with 51.7 g of CuSO4 how much copper (grams) will be produced?2Al + 3CuSO4 → 3Cu + Al2(SO4)3the CuSO4 is limited, so Cu = 20.6 gHow much excess reagent will remain?Excess = 4.47 grams
30 Practice p 383 23. 6Na+Fe2O3-->3Na2O+2Fe If 100g of Na and 100g of Fe2O3 are used in this reaction, determine the following- limiting reactant, reactant in excess, mass of solid iron produced, mass of excess reactant that remains after reaction is completed.24. A plant has 88g of CO2 and 64g of H2O to produce glucose, C6H12O6, by photosynthesis.Write the balanced equation for it.Determine the limiting reagent.Determine the excess reagent.Determine the mass in excess.Determine the mass of glucose produced.
31 The Concept of:A little different type of yield than you had in Driver’s Education class.
32 What is Yield?Yield is the amount of product made in a chemical reaction.There are three types:1. Actual yield- what you actually get in the lab when the chemicals are mixed2. Theoretical yield- what the balanced equation tells should be made3. Percent yield = Actual Theoreticalx 100
33 Example:6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate.2Al + 3 CuSO4 ® Al2(SO4)3 + 3CuWhat is the actual yield?What is the theoretical yield?What is the percent yield?= 6.78 g Cu= 13.8 g Cu= 49.1 %
34 Percent yield tells us how “efficient” a reaction is. Details on YieldPercent yield tells us how “efficient” a reaction is.Percent yield can not be bigger than 100 %.Theoretical yield will always be larger than actual yield!Why? Due to impure reactants; competing side reactions; loss of product in filtering or transferring between containers; measuring
35 Practice p 38728.Al(OH)3 +3HCl-->AlCl3+3H2O, If 14g of Al(OH)3 is present, determine the theoretical yield of AlCl3 produced when the tablet reacts with HCl.29. Zn+I2-->ZnI2 (a).Determine the theoretical yield if mol of zinc is used. (b).Determine the percent yield is 515.6g of product is recovered.30. When copper wire is placed into a silver nitrate AgNO3 solution, silver crystals and Cu(NO3)2 solution form.Write the balanced chemical equation for the reaction.If a 20g sample of copper is used, determine the theoretical yield of silver.If 60g of silver is recovered from the reaction, determine the percent yield of the reaction.
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