2 Introduction Reaction stoichiometry involves the mass relationships between reactants and products in a chemical reaction.It is based on chemical equations similar to the ones studied in the last section.All reaction stoichiometry calculations start with a balanced equation.You will need to be familiar with gram/mole relationships as studied earlier this year.
3 The mole enables chemists to move from the microscopic world of atoms and molecules to the real world of grams .Stoichiometry problems are classified between the information given in the problem and the information you are expected to find, the unknown.The given and the unknown may be expressed in grams or moles.The masses in the reaction are usually expressed in grams.Definition of mole:mole of a substance = grams of substance/MW of substance
4 You will need to use:molar ratios in a balanced equation.molar masses of reactants and products.balancing equations.conversions between grams and moles.
5 Mole RatiosA mole ratio converts moles of one compound in a balanced chemical equation into moles of another compound.All stoichiometry problems use mole ratios.
6 ExampleReaction between magnesium and oxygen to form magnesium oxide. ( fireworks)2 Mg(s) + O2(g) MgO(s) (balanced)Mole Ratios:2 : 1 : 2
7 Stoichiometry (working with ratios) Ratios are found within a chemical equation.2HCl + Ba(OH)2 2H2O BaCl211coefficients give MOLAR RATIOS2 moles of HCl react with 1 mole of Ba(OH)2 to form 2 moles of H2O and 1 mole of BaCl2
8 Practice Problems 1) N2 + 3 H2 ---> 2 NH3 Write the mole ratios for N2 to H2 and NH3 to H2.
9 Review: Molar MassA substance’s molar mass (molecular weight) is the mass in grams of one mole of the compound.CO2 = grams per moleH2O = grams per moleCa(OH)2 = grams per mole
10 Review: Chemical Equations C2H5OH + 3O2 ® 2CO2 + 3H2Oreactantsproducts1 mole of ethanol reacts with 3 moles of oxygento produce 2 moles of carbon dioxide and 3 moles of water
14 Problem Type 1: Given and unknown quantities are in moles Amount in molesof known substanceAmount in molesof unknown substanceHow many moles of lithium hydroxide are required to react with 20 moles of CO2
15 20 mol CO2 x 2 mol LiOH / 1 mol CO2 = 40 mol LiOH Given: amount of CO2 = 20 molesUnknown: amount of LiOH in molesAmount of CO2 in molesAmount of LiOH in molesmol CO2 x mol LiOH / mol CO2 = mol LiOH20 mol CO2 x 2 mol LiOH / 1 mol CO2 = mol LiOHmole ratio
16 Mole – Mole Conversions Problem Type 1Mole – Mole ConversionsWhen N2O5 is heated, it decomposes:2N2O5(g) 4NO2(g) + O2(g)a. How many moles of NO2 can be produced from 4.3 moles of N2O5?2N2O5(g) 4NO2(g) + O2(g)4.3 mol? molUnits match
17 Mole – Mole Conversions When N2O5 is heated, it decomposes:2N2O5(g) 4NO2(g) + O2(g)a. How many moles of NO2 can be produced from 4.3 moles of N2O5?2N2O5(g) 4NO2(g) + O2(g)4.3 mol? molUnits match4.3 mol N2O5= moles NO28.6
18 2N2O5(g) 4NO2(g) + O2(g) 4.3 mol N2O5 = mole O2 2.2 b. How many moles of O2 can be produced from 4.3 moles of N2O5?2N2O5(g) 4NO2(g) + O2(g)4.3 mol? mol4.3 mol N2O5= mole O22.2
19 Problem Type 1: Given and unknown quantities are in moles Amount in molesof known substanceAmount in molesof unknown substanceHow many moles of ammonia are produced when 6 moles of hydrogen gasreact with an excess of nitrogen gas.
20 Given: amount of H2 = 6 moles Unknown: amount of NH3 in molesAmount of H2 in molesAmount of NH3 in moles(mol H2) x (mol NH3 / mol H2) = mol NH3(6 mol H2) x (2 mol NH3 / 3 mol H2) = 4 mol NH3Mole ratio
21 Problem Type 2: Given amount is in moles and unknown quantity is in grams Amount in molesof known substanceAmount in gramsof unknown substanceProblem Type 3: Given amount is in grams and unknown quantities are in molesAmount in gramsof known substanceAmount in molesof unknown substance
22 In plants when carbon dioxide reacts with water it Problem Type 2mole ↔ gramIn plants when carbon dioxide reacts with water itproduces glucose and oxygen:6CO2 + 6H2O(l) C6H12O6(s) + 6O2(g)How many grams of C6H12O6 is produced when 3.0 mol of waterreact with carbon dioxide?6CO2 + 6H2O C6H12O O23.0 mol? gramsUnits match3.0 mol H2O= g C6H12O690
23 In plants when carbon dioxide reacts with water it Problem Type 2mole ↔ gramIn plants when carbon dioxide reacts with water itproduces glucose and oxygen:6CO2 + 6H2O(l) C6H12O6(s) + 6O2(g)How many grams of CO2 is needed to react with 3.0 mol of water?6CO H2O C6H12O O2? grams3.0 molUnits match3.0 mol H2O= g CO2132
24 When magnesium burns in air, it combines with oxygen Problem Type 2mole ↔ gramWhen magnesium burns in air, it combines with oxygento form magnesium oxide according to the following equation:2Mg + O2(g) 2MgO(s)How many grams of MgO is produced from 2.0 mol of magnesium?2Mg O2 2MgO2.0 mol? gramsUnits match2.0 mol Mg= g MgO80
25 When N2O5 is heated, it decomposes: 2N2O5(g) 4NO2(g) + O2(g) Problem Type 3gram ↔ moleWhen N2O5 is heated, it decomposes:2N2O5(g) 4NO2(g) + O2(g)a. How many moles of N2O5 were used if 210g of NO2 were produced?2N2O5(g) 4NO2(g) + O2(g)? moles210gUnits match210 g NO2= moles N2O52.28
26 Nitric acid is produced from the catalytic oxidation of ammonia Problem Type 3gram ↔ moleNitric acid is produced from the catalytic oxidation of ammoniaNH3(g) + O2(g) NO(g) + H2O(g)a. How many moles of NO were produced from 824g of NH3?4NH3(g) + 5O2 4NO(g) + 6H2O(g)824g? molesUnits match824 g NH3= moles NO48.4
27 Steps Involved in Solving Mass-Mass Stoichiometry Problems Balance the chemical equation correctlyUsing the molar mass of the given substance, convert the mass given to moles.Determine the molar ratio.Using the molar mass of the unknown substance, convert the moles just calculated to mass.Amount in gramsof known substanceAmount in molesof known substanceAmount in molesof unknown substanceAmount in gramsof unknown substancemwmolarratiomw
28 When N2O5 is heated, it decomposes: Problem Type 4grams ↔ gramsWhen N2O5 is heated, it decomposes:How many grams of N2O5 are needed to produce 75.0 grams of O2?2N2O5(g) 4NO2(g) + O2(g)? grams75.0 g75.0 g O2= grams N2O5506
29 Stoichiometry Problem- Type 4 6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.1. Identify reactants and products and write the balanced equation.Al+O2Al2O3a. What are the reactants?b. What are the products?c. What are the balanced coefficients?
30 Working a Stoichiometry Problem 6.50 grams of aluminum reacts with an excess of oxygen.How many grams of aluminum oxide are formed?4 Al + 3 O2 2Al2O3=6.50 g Al1 mol Al2 mol Al2O3g Al2O3? g Al2O326.98 g Al4 mol Al1 mol Al2O36.50 x 2 x ÷ ÷ 4 =12.3 g Al2O3
31 Acetylene gas (C2H2) is produced by adding water to calcium carbide: How many grams of acetylene are produced by adding water to5.0 grams of CaC2?CaC2 + 2H2O C2H2 + Ca(OH)2 Balanced?5.0 g? grams5.0 g CaC2= grams C2H22.0
32 How many moles of CaC2 are needed to react completely with 49.0 g H2O: CaC2 + 2H20 C2H2(g) + Ca(OH)2How many moles of CaC2 are needed to react 49.0 g of H20?CaC2 + 2H20 C2H2(g) + Ca(OH)2? moles49.0gUnits match49 g H2O= moles CaC21.36
33 Stoichiometry Review Problem Suppose you want to figure out how many grams of ammonia you can produceif you react 60.0 g of hydrogen gas with excess nitrogen.Remember the balanced chemical equation gives the mole relationshipbetween reactants and products.N H NH31) Take the grams of hydrogen gas (recall that it is diatomic) and convert it to moles:2) Convert the moles of hydrogen gas to moles of ammonia, using the coefficients in the balancedchemical equation:3) Convert from moles of ammonia to grams of ammonia using the molecular massand solve the equation:
34 Calculating the Percent Yield The predicted amount of product – which we have been doing with ourstoichiometry problems has been for 100% yield, or thetheoretical yield of a reaction.The theoretical yield is the maximum amount of product that can be produces in a given reaction.When chemical reactions take place, they are almost never 100%complete. A reaction may not go to 100% due to not all the reactantsbecoming involved, impurities in the reactants, competing sidereactions, loss of product due to filtering, or just not gettingit all out of the vessel.The actual yield is how much product can be collected – and measured.The percent yield is a ratio given by:Percent Yield = Actual Yield x %Theoretical Yield
35 Calcium carbonate is decomposed by heating, as shown in the following equation: 1) What is the theoretical yield of this reaction if 24.8 g CaCO3 is heated to yield 13.1 g CaO?Handle like a typical stoichiometry problem: grams to gramsIgnore the 13.1 g CaO to solve. This is the actual yield.2) What is the percent yield?
36 Chlorobenzene is used in the production of many different chemicals, such as aspirin, dyes and disinfectants. One method of preparing chlorobenzene is to react benzene C6H6, with chlorine according to the following equation:C6H6 + Cl C6H5Cl + HCl1) When 36.8 g of C6H6 react with excess of Cl2, the actual yield of C6H5Cl is 38.8g?What is the theoretical yield?Handle like a typical stoichiometry problem: grams to grams2) What is the percent yield?
37 Standard Molar Volume 1 mole of a gas occupies 22.4 liters of volume Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.- Amedeo AvogadroAt STP (Standard Temperature and Pressure):1 mole of a gas occupies 22.4 liters of volume
38 Gas Stoichiometry #1If reactants and products are at the same conditions of temperature and pressure, then mole ratios of gases are also volume ratios.3 H2(g) + N2(g) 2NH3(g)3 moles H2 + 1 mole N2 2 moles NH33 liters H2 + 1 liter N2 2 liters NH3
39 Gas Stoichiometry #2How many liters of ammonia can be produced when 12 liters of hydrogen react with an excess of nitrogen?3 H2(g) + N2(g) 2NH3(g)12 L H22L NH3= L NH38.03L H2
40 Gas Stoichiometry #3How many liters of oxygen gas, at STP, can be collected from the complete decomposition of 50.0 grams of potassium chlorate?2 KClO3(s) 2 KCl(s) + 3 O2(g)50.0 g KClO31 mol KClO33 mol O222.4 L O2g KClO32 mol KClO31 mol O2= L O213.7