Presentation on theme: "Mathematical Relationships with Chemical Equations…Stoichiometry"— Presentation transcript:
1Mathematical Relationships with Chemical Equations…Stoichiometry The Mole IIMathematical Relationships with Chemical Equations…Stoichiometry
2Suppose…You have to make a number of BLTs (bacon, lettuce, tomato sandwiches).Each sandwich requires 2 slices of bread, 4 slices of bacon, 2 slices of tomato, 1 leaf of lettuce, and 1 tbsp of mayonnaise.How many sandwiches can you make with 24 slices of bacon? 14 pieces of bread? 7 slices of tomato?
31 molecule C3H8 + 5 molecules O2 Information Contained in a Balanced EquationViewed in Terms ofReactantsC3H8(g) + 5O2(g)Products3CO2(g) + 4H2O(g)molecules3 molecules CO2 + 4 molecules H2O1 molecule C3H8 + 5 molecules O2amount (mol)1 mol C3H8 + 5 mol O23 mol CO2 + 4 mol H2O44.09 amu C3H amu O2mass (amu)amu CO amu H2Omass (g)44.09 g C3H g O2g CO g H2Ototal mass (g)g
4StoichiometryThe relation between the quantities of substances that take part in a chemical reaction or form a compound“"Stoichiometry" is derived from the Greek words στοιχείον (stoikheion, meaning element) and μέτρον (metron, meaning measure.” (http://en.wikipedia.org/wiki/Stoichiometry)
5Info in a Chemical Equation 2C8H18(l) + 25O2 (g) 16CO2 (g) + 18H2O (g)2 mol C8H18 =25 mol O2 =16 mol CO2 =18 mol H2OSo…
6Summary of the mass-mole-number relationships in a chemical reaction. MASS(g)of compound AMASS(g)of compound BMM (g/mol) of compound AMM (g/mol) of compound BAMOUNT(mol)of compound Amolar ratio frombalanced equationAMOUNT(mol)of compound BAvogadro’s number(molecules/mol)Avogadro’s number(molecules/mol)MOLECULES(or formula units)of compound AMOLECULES(or formula units)of compound B
7Questions to Ask When Solving a Problem Are grams mentioned?If yes you will need to use a molar mass conversion.Are you comparing two different substances?If yes you will need a conversion using the coefficients in an equation.Are particles (atoms or molecules) involved?If yes you will need a conversion with Avogadro’s number.You may need any number of these conversions depending on the problem.
8Calculating Amounts of Reactants and Products PROBLEM:In a lifetime, the average American uses 1750 lb(794 g) of copper in coins, plumbing, and wiring. Copper is obtained from sulfide ores, such as chalcocite, or copper(I) sulfide, by a multistep process. After an initial grinding, the first step is to “roast” the ore (heat it strongly with oxygen gas) to form powdered copper(I) oxide and gaseous sulfur dioxide.How many moles of oxygen are required to roast 10.0 mol of copper(I) sulfide?(b) How many grams of sulfur dioxide are formed when 10.0 mol of copper(I) sulfide is roasted?(c) How many kilograms of oxygen are required to form 2.86 kg of copper(I) oxide?
9Calculating Amounts of Reactants and Products SOLUTION:2Cu2S(s) + 3O2(g) Cu2O(s) + 2SO2(g)(a) How many moles of oxygen are required to roast 10.0 mol of copper(I) sulfide?3mol O22mol Cu2S10.0mol Cu2S(a)= 15.0mol O2(b) How many grams of sulfur dioxide are formed when 10.0 mol of copper(I) sulfide is roasted?10.0mol Cu2S2mol SO22mol Cu2S64.07g SO21 mol SO2(b)= 641g SO2(c) How many kilograms of oxygen are required to form 2.86 kg of copper(I) oxide?2.86kg Cu2O103g Cu2O1kg Cu2O1 mol Cu2O143.10g Cu2O(c)3mol O22mol Cu2O32.00g O21 mol O21 kg O2103g O2= 0.959kg O2
10Now suppose…You have to make BLTs (bacon, lettuce, tomato sandwiches).Each sandwich requires 2 slices of bread, 4 slices of bacon, 2 slices of tomato, 1 leaf of lettuce, and 1 tbsp of mayonnaise.How many sandwiches can you make with 24 slices of bacon and 14 pieces of bread?
11Limiting ReactantsIn most chemical changes you do not have the exact amounts of all reactants.In this case, one reactant will run out first. This reactant will limit the amount of product and so is called the limiting reactant or limiting reagent.The reactant(s) left over is called the excess reactant.
12Calculating Amounts of Reactant and Product in a Limiting-Reactant Problem A fuel mixture used in the early days of rocketry is composed of two liquids, hydrazine (N2H4) and dinitrogen tetraoxide(N2O4), which ignite on contact to form nitrogen gas and water vapor. How many grams of nitrogen gas form when 1.00x102 g of N2H4 and 2.00x102 g of N2O4 are mixed?
13Calculating Amounts of Reactant and Product in a Limiting-Reactant Problem SOLUTION:2N2H4(l) + N2O4(l) N2(g) H2O(l)341 mol N2H432.05g N2H41.00x102g N2H43 mol N22mol N2H4= 4.68mol N22.00x102g N2O41 mol N2O492.02g N2O43 mol N21mol N2O4= 6.51mol N2N2H4 is the limiting reactant because it produces less product, N2, than does N2O4.1 mol N228.02g N24.68mol N2= 131g N2
14Excess Reactant How much reactant was left in the previous problem? 6.51 mol N2 possible but only 4.68 mol made= 1.83 mol N2 not made
15How many grams of aluminum carbonate are formed when 20 How many grams of aluminum carbonate are formed when 20.0 grams of sodium carbonate react with 25.0 grams of aluminum nitrate? How much of which reactant is left over?
16Percent YieldReactions do not always go to completion for many reasons. Percent yield is an expression of how much of the expected product actually formed.The theoretical amount generally comes from the stoichiometry of the reaction.
17Calculating Percent Yield PROBLEM:Silicon carbide (SiC) is an important ceramic material that is made by allowing sand (silicon dioxide, SiO2) to react with powdered carbon at high temperature. Carbon monoxide is also formed. When g of sand is processed, 51.4 g of SiC is recovered. What is the percent yield of SiC from this process?SOLUTION:SiO2(s) + 3C(s) SiC(s) + 2CO(g)100.0 g SiO21 mol SiO260.09 g SiO2= mol SiO2mol SiO2 = mol SiC = 1.66451.4 g66.73 g1.664 mol SiC40.10 g SiC1 mol SiCx 100=77.0%= g
18Calculating Amounts of Reactants and Products for a Reaction in Solution PROBLEM:Specialized cells in the stomach release HCl to aid digestion. If they release too much, the excess can be neutralized with antacids. A common antacid contains magnesium hydroxide, which reacts with the acid to form water and magnesium chloride solution. As a government chemist testing commercial antacids, you use 0.10M HCl to simulate the acid concentration in the stomach. How many liters of “stomach acid” react with a tablet containing 0.10g of magnesium hydroxide?
19Calculating Amounts of Reactants and Products for a Reaction in Solution Mg(OH)2(s) + 2HCl(aq) MgCl2(aq) + 2H2O(l)1 mol Mg(OH)258.33g Mg(OH)22 mol HCl1 mol Mg(OH)20.10g Mg(OH)2= 3.4x10-3 mol HCl1L0.10mol HCl3.4x10-3 mol HCl= 3.4x10-2 L HCl