Chapter 3. Stoichiometry: Calculations with Chemical Formulas and Equations. Lecture Presentation

Transcription

1 Lecture Presentation Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations John D. Bookstaver St. Charles Community College Cottleville, MO

2 A. 1 Mg, 2 O, and 2 H B. 2 Mg, 2 O, and 2 H C. 6 Mg, 6 O, and 6 H D. 3 Mg, 6 O, and 6 H

3 Balancing Chemical Equation

4 Combustion of Magnesium Metal in Air

5 Decomposition of Sodium Azide

6 Combination and Decomposition Reactions

7 Propane Burning in the Air

8 Formula Weights 2012 Pearson Education, Inc.

9 Formula Weight (FW) A formula weight is the sum of the atomic weights for the atoms in a chemical formula. So, the formula weight of calcium chloride, CaCl 2, would be Ca: 1(40.08 amu) + Cl: 2( amu) amu Formula weights are generally reported for ionic compounds Pearson Education, Inc.

10 Molecular Weight (MW) A molecular weight is the sum of the atomic weights of the atoms in a molecule. For the molecule ethane, C 2 H 6, the molecular weight would be C: 2( amu) + H: 6( amu) amu 2012 Pearson Education, Inc.

11 Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: % Element = (number of atoms)(atomic weight) (FW of the compound) x 100% 2012 Pearson Education, Inc.

12 Practice Calculating Percentage Composition Calculate the percentage of carbon, hydrogen, and oxygen (by mass) in C 12 H 22 O 11. Solution Analyze We are given a chemical formula and asked to calculate the percentage by mass of each element.

13 Avogadro s Number 6.02 x mole of 12 C has a mass of g Pearson Education, Inc.

14 1 Mole Each of Substance

15 Practice Estimating Numbers of Atoms Without using a calculator, arrange these samples in order of increasing numbers of carbon atoms: 12 g 12 C, 1 mol C 2 H 2, molecules of CO 2. Solution

16 Practice Converting Moles to Number of Atoms Calculate the number of H atoms in mol of C 6 H 12 O 6. Solution Plan Avogadro s number provides the conversion factor between number of moles of C 6 H 12 O 6 and number of molecules of C 6 H 12 O 6 : 1 mol C 6 H 12 O 6 = molecules C 6 H 12 O 6. Once we know the number of molecules of C 6 H 12 O 6, we can use the chemical formula, which tells us that each molecule of C 6 H 12 O 6 contains 12 H atoms. Thus, we convert moles of C 6 H 12 O 6 to molecules of C 6 H 12 O 6 and then determine the number of atoms of H from the number of molecules of C 6 H 12 O 6 : Moles C 6 H 12 O 6 molecules C 6 H 12 O 6 atoms H

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18 Mole Relationships

19 Procedure for Interconverting Mass and Number of Formula Units

20 Practice Converting Grams to Moles Calculate the number of moles of glucose (C 6 H 12 O 6 ) in g of C 6 H 12 O 6. Solution

21 Practice Converting Moles to Grams Calculate the mass, in grams, of mol of calcium nitrate. Solution

22 Practice Calculating Numbers of Molecules and Atoms from Mass (a) How many glucose molecules are in 5.23 g of C 6 H 12 O 6? (b) How many oxygen atoms are in this sample? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.

23 How to Calculate an Empirical Formula from Percentage?

24 Practice Determining a Empirical Formula A molecule containing only nitrogen and oxygen contains (by mass) 36.8% N. a) How many grams of N would be found in a 100 g sample of the compound? How many grams of O would be found in the same sample? b) How many moles of N would be found in a 100 g sample of the compound? How many moles of O would be found in the same sample? c) What is the ratio of the number of moles of O to the number of moles of N? d) What is the empirical formula of the compound? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.

25 Practice Determining a Molecular Formula Mesitylene, a hydrocarbon found in crude oil, has an empirical formula of C 3 H 4 and an experimentally determined molecular weight of 121 amu. What is its molecular formula? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.

26 Combustion Analysis Compounds containing C, H, and O are routinely analyzed through combustion in a chamber like the one shown in Figure C is determined from the mass of CO 2 produced. H is determined from the mass of H 2 O produced. O is determined by difference after the C and H have been determined Pearson Education, Inc. Stoichiometry

27 Quantitative Information from Balanced Equations Stoichiometry

28 Rules for Balancing Chemical Equations Double check formulas NEVER change subscripts. ONLY use coefficients Save single atoms for last Start with the most complex component OR the odd numbered atoms Fractions can be used, but remember to multiply out. Coefficients should be integers Double check atom balance on reactant & product side PRACTICE! Stoichiometry

29 Sample Problem - Balancing Chemical Equations As 2 S 3 + O 2 As 2 O 3 + SO 2 Ca(OH) 2 (aq) + H 3 PO 4 (aq) H 2 O (l) + Ca 3 (PO 4 ) 2 (s) HClO 4 + P 4 O 10 H 3 PO 4 + Cl 2 O 7 Stoichiometry

30 Stoichiometric Calculations The stoichiometric coefficients in a balanced equation may be interpreted as: relative numbers of molecules involved in the reaction relative numbers of moles involved in the reaction Stoichiometry

31 Sample Problem A mol B mol C mol D mol Stoichiometry

32 Stoichiometric Calculations Starting with the mass of Substance A, you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it s a product) or used (if it s a reactant). Stoichiometry

33 Sample Problem Consider the following reaction: Na 2 SiO 3 (s) + HF ( aq) H 2 SiF 6 (aq) + NaF (aq) + H 2 O (l) Looks complicated? 1) How many mols of HF are needed to react with mols of the silicate Na 2 SiO 3? 2) How many grams of NaF forms when mols of HF reacts with the silicate Na 2 SiO 3? 3) Calculate the mass of silicate Na 2 SiO 3 that reacts with g HF. Stoichiometry

34 Practice Example C 6 H 12 O O 2 6 CO H 2 O Find the mass of water produced from 1.00 g glucose. Stoichiometry

35 LIMITING REACTANTS Stoichiometry

36 The Scenario Let s make some cheese sandwiches. 1 sandwich requires: 2 slices of bread (Bd) and 1 slice of cheese (Ch). The balanced equation: 2Bd + Ch Bd 2 Ch + Yummy! What we have: 10 slices of bread and 7 slices of cheese. How many sandwiches can we make? What limited you from making more sandwiches? So what is the limiting reactant/reagent? Which is the excess reactant? Stoichiometry

37 Limiting Reactants The limiting reactant in a chemical reaction is the reactant present in the smallest stoichiometric amount The one that gets used up first stops the reaction from proceeding. What is the limiting reagent in reaction below?.... H 2 would be the limiting reagent What is the reagent in excess? O 2 Stoichiometry

38 Problem-Solving Tips xa + yb product(s) x and y are the stoichiometric coefficients moles of A moles of B If (mol A / mol B) is > than x/y, then A is the excess reagent & B is the limiting reactant. If (mol A / mol B) is < than x/y, then B is the excess reagent & A is the limiting reactant. Stoichiometry

39 Sample Problem - To Find the Limiting Reactant NaOH (s) reacts with CO 2 (g) to produce sodium carbonate (s) and water. 1)Which is the limiting reactant when 1.85 mols NaOH and 1.00 mols CO 2 react? 2)How many mols of carbonate can be produced? 3)How many mols of excess reactant remain after the reaction is complete? The PLAN for qu 1: Write the balanced equation. Determine limiting reagent. The PLAN for qu 2: MUST use stoichiometry (mole ratio) based on the limiting reagent. Mols NaOH mols Na 2 CO 3 The PLAN for qu 3: Calculate the # mols excess reactant used Subtract initial mols of CO 2 from # mols CO 2 consumed in the reaction. Stoichiometry Ans: 1) NaOH 2) mols Na 2 CO 3 3) mols CO 2

40 Sample Problem 2.00 g of Zn metal reacts with 2.50 g silver nitrate to produce silver metal and zinc nitrate. 1)What reactant is limiting? 2)How many grams of Ag and zinc nitrate is formed? 3)What mass of the excess reagent is left after the reaction? Follow the similar steps as before but must now use Molar Mass (MM) to convert from g mols Stoichiometry

41 Theoretical Yield The theoretical yield is the maximum amount of..... product that can be made as calculated through the stoichiometry problem This is different from the actual yield, which is the amount one actually produces and measures by actually doing the experiment/reaction Percent yield: relates theoretical and actual yields Tells how much product was formed or recovered from the rxn. Actual Yield Percent Yield = x 100% Theoretical Yield Stoichiometry

42 The effect of side reactions on yield. A + B (reactants) C (main product) D (side products) Stoichiometry

43 Calculating % Yield Silicon carbide (SiC) is an important ceramic material that is made by allowing sand (silicon dioxide, SiO 2 ) to react with powdered carbon at high temperature. Carbon monoxide is also formed. When kg of sand is processed, 51.4 kg of SiC is recovered. What is the percent yield of SiC from this process? NOTE: Unless already stated or a quantity for a reactant is not given, you will have to determine the limiting reagent. Stoichiometry

44 Solution write balanced equation find moles of reactant & product find mass of product predicted actual yield/theoretical yield x 100 = percent yield Stoichiometry