Chapter 3 OWL Deadline 27-September All material through Chapter 3 is fair game for Exam #1 on 24-September. Lab the week of 10-September: Synthesis of Alum Stoichiometry and Equations The Mole Stoichiometry and Compounds Chemical Reactions Limiting Reactants % and Theoretical Yield
What is a mole? A standard unit of quantity. The number of atoms present in exactly 12 grams of 12C. Avogadro’s number is the constant for 1 mole NA = 6.02 x 1023
Stoichiometry & Compounds In terms of compounds, stoichiometry refers to the ratio of elements within a molecule or substance (including polyatomic ions and ionic compounds) Any unique compound will have its formula The formula will have a specific ratio of atoms These ratios can be expressed in weight (grams for example) or quantity (moles usually) H2O C3H8 PO4-3 H2C2O42 H2O Na2CO3
Empirical Formula Mass Formula Mass (weight) The sum of the atomic masses of all atoms in a formula unit of the compound, whether molecular or not. Empirical Formula Mass Molecular Formula Mass (another term for molecular mass) The mass of some other unit containing multiple atoms Molar Mass (weight) The sum of the masses of all of the atoms or molecules in a mole of a compound, ordinarily expressed in grams/mole. Copyright © Cengage Learning. All rights reserved.
Copyright © Cengage Learning. All rights reserved. Ca(OH)2 1 Ca 1(40.08) = 40.08 amu 2 O 2(16.00) = 32.00 amu 2 H 2(1.008) = 2.016 amu 3 significant figures 74.1 amu Total 74.095 Calculate the formula weight of the following compound from its formula. Report your answers to three significant figures. Calcium hydroxide, Ca(OH)2 Copyright © Cengage Learning. All rights reserved.
Copyright © Cengage Learning. All rights reserved. What is the mass in amu of the nitrous acid molecule, HNO2? Copyright © Cengage Learning. All rights reserved.
A sample of nitric acid, HNO3, contains 0. 253 mol HNO3 A sample of nitric acid, HNO3, contains 0.253 mol HNO3. How many grams is this? First, find the molar mass of HNO3: 1 H 1(1.008) = 1.008 1 N 1(14.01) = 14.01 3 O 3(16.00) = 48.00 63.018 (2 decimal places) 63.02 g/mol Copyright © Cengage Learning. All rights reserved.
Copyright © Cengage Learning. All rights reserved. Next, using the molar mass, find the mass of 0.253 mole: = 15.94406 g Copyright © Cengage Learning. All rights reserved.
Remember, moles (mol) are really used for counting and deriving data from those counts. How many grams does 0.45 mol of sulfur trioxide weigh?
Percent Composition Often used in chemical analysis and identifying empirical or molecular formulas A weight ratio of the elements in a compound Write the name of, identify the stoichiometry involved in, and determine the % composition of each element in MgCO3
Empirical/Molecular Formulas and % Composition Chemists may discover a compound and, if it is new to them, have no idea what the chemical formulas are Empirical Molecular You can use a step-by-step approach to determine the Empirical and Molecular formulas even if you have limited data. Determine % composition (wt %) of each element. Calculate the mole ratio for each element. Round up to whole numbers Use the molecular weight and the empirical formula weight to calculate the molecular formula We estimate, pretty closely, the molecular weight from techniques such as mass spectrometry
You’ve discovered what you think is a new compound: 2.00 grams of it were found Chemical analysis shows that it has Carbon and Hydrogen What kind of compound is it? Chemical analysis shows that of the sample that you have, 0.151 grams of Hydrogen. Mass spectrometry shows that the molecular weight is 26.02 grams/mole What compound is it?
Figure 3.6: Combustion method for determining the percentages of carbon and hydrogen in a compound
Calculate % Composition Calculate Stoichiometry (Mole Ratios) Empirical Formula Calculate Ratio of Molecular Weight to Empirical Formula Weight Use ratio from #3 to decipher molecular formula from empirical formula
Stoichiometry and Reactions Chemical reactions take place with defined ratios of reactants and products These defined ratios are based on the stoichiometry of the compounds or atoms in the reaction, and the stoichiometry related to the reaction itself. Stoichiometric coefficients
Let’s identify Products (and name them) Reactants (and name them) Stoichiometry of compounds in this reaction Stoichiometric coefficients Phases Reaction Direction (arrow) What important law is adhered to by this reaction (and all the reactions we will study in CHEM 111)
BaCl2 (aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq) Let’s identify Products (and name them) Reactants (and name them) Stoichiometry of compounds in this reaction Stoichiometric coefficients Phases Reaction Direction (arrow) What important law is adhered to by this reaction (and all the reactions we will study in CHEM 111)
Balancing Equations Remember, mass MUST be conserved! All of the atoms of reactants must appear, in the same mole quantities, in the products This can be extrapolated to weights later on with reaction stoichiometry
Practice Balancing Equations Methane gas reacts with oxygen gas to produce carbon dioxide gas and liquid water. Write the reaction Balance elements that appear in only one compound first Remember that compound stoichiometry (subscripts) can not change Balance elements that appear in other compounds second Round coefficients up to the lowest whole numbers Check answer
Aqueous acetic acid reacts with aqueous sodium hydroxide to produce aqueous sodium acetate and water. This reaction is in “equilibrium”.
Reaction Stoichiometry Once we have a balanced reaction, we can evaluate how much of one substance can form (or cause to be formed) another substance. Reaction stoichiometry is almost ALWAYS studied using moles (stoichiometric coefficients and subscripts)
Propane is combusted in a cooking grill using atmospheric oxygen Propane is combusted in a cooking grill using atmospheric oxygen. The products are carbon dioxide and water. How much water (mol and grams) will form if 10 grams of propane are consumed?
In the Haber Process, 2. 00 grams of hydrogen are consumed In the Haber Process, 2.00 grams of hydrogen are consumed. How much nitrogen is consumed in with it?
Limiting Reactants
Often, we have reactants where: There is an excess of one (compared to the others) There is a limiting amount of another (compared to the others. The Limiting reactant controls how much product can be produced If in the reaction above, we have 4 moles of butane and 20 moles of oxygen (O2), which is limiting and which is excess?
Molecular view of 2H2 + O2 2H2O reaction with H2 as the limiting reactant If we have one gram of hydrogen gas (H2), how many moles of O2 will be consumed and how many grams of water will be formed?
Theoretical and % Yield Rarely do reactions produce 100% of the product they should Impurities Some reactants/products produce other things Spillage The theoretical yield is what you would get, ideally. Calculated from the balanced equation and limiting reactant The % yield (percent yield) is what you actually produce, relative to the theoretical yield
If we have 1. 00 grams of hydrogen gas (H2), and 2 If we have 1.00 grams of hydrogen gas (H2), and 2.00 grams of O2 reacting to form hydrogen peroxide (H2O2 (g)), answer the following: -Write the balanced chemical reaction -Identify the limiting reactant -Calculate the theoretical yield -Calculate the percent yield if 1.5 grams of hydrogen peroxide is formed.