FSGs are Tuesdays 3:20-4:20 and Wednesdays

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1 CEM 115 toichiometry t Lecture 6 Prof. evian 1 Announcements FGs are Tuesdays 3:20-4:20 and Wednesdays 11:00-12:00 in Check out tudent opportunities link on course website - will be updated over the course of the semester with more opportunities for summer internships and scholarships as I receive informat If you haven t taken the 55-minute first term chemistry pre-test, please see me after class to schedule a time to take it by tomorrow worth 10 points on your grade for the course 2. evian 1

2 Agenda Balancing chemical equats When the units are s When the units are individual atoms toichiometry calculats Figuring out what the problem is asking Doing the calculats 3 What we have learned so far Moles are a counting group Mass and moles are two different measures of the quantity of matter Mass is what you measure in the lab (use a scale or mass balance) Moles are a count of how many particles (can t be measured directly) The two are connected because each particle has a mass which depends on what the particle is made of There are lots of ways to torture chemistry students with using mass to moles converss What is the molar mass of a compound? ow many moles are in a particular mass of compound? (and vice versa) What is the % composit by mass of the different elements in a particular compound? What is the empirical formula of a compound that has a particular set of % composit (by mass) of elements? What is the molecular formula if you know empirical formula and molar mass? Combinats of the above. evian 2

3 This was Clicker quest #5 from Lecture 5, but instead we will go over it as an example What kind of problem is this - what strategy would you use? erotonin is a compound that conducts nerve impulses in the brain. It contains 68.2 mass percent C, 6.86 mass percent, 15.9 mass percent N, and the rest. Its molar mass is 176 g/mol. Determine its molecular formula. A. Take the ratio of 68.2 : 6.86 : 15.9 : whatever part, convert to whole numbers to get molecular formula B. Divide each percentage (as grams) by 176 to get moles, then take ratio C. Assume 100 g so % s are grams, convert to moles, then take ratio of moles D. Assume 1 mol, calculate mass of each element in 179 g, then convert those masses to number of moles to get ratio of moles 14 Chemical Equats Represent a chemical change of matter Reactants (starting materials) on left Products (ending materials) on right Reactants Products What goes in must come out, just connected (bonded) differently 15. evian 3

4 Doing stoichiometry involves Balancing chemical equats to determine how many moles go in and come out 1. First determine what species to balance 2. Then balance the species on both sides (either s or atoms) a) Accounting method b) Pictures method Figuring out how much mass goes in and/or comes out Informat given Informat sought 16 General trategy for imple toichiometry Problems Mass of chemical A Molar mass Moles of chemical A Moles of chemical B Molar mass Chemical react toichiometric coefficients Mass of chemical B 17. evian 4

5 Balancing the Chemical Equat for the Format of Water Coefficients: 2, 1, (g) (g) (l) or Balancing the Chemical Equat for the Combust of Propane C C C C C 1. Balance the Carbons C 2. Balance the ydrogens 3. Balance the xygens 4. Is it balanced? Balanced C = 3 atoms in, chemical 3 out equat: = 8 atoms in, 8 out C C = 10 atoms in, 10 out. evian 5

6 Combust Reacts in General Flame-producing react of a chemical with oxygen to produce oxides 4 Fe (s) 3 2 (g) 2 Fe 2 3 (s) 2 Mg (s) 2 (g) 2 Mg (s) ydrocarbons (C and ) burn to produce C 2 and 2 (the oxides of C and ), if enough oxygen is present C 4 (g) 2 2 (g) C 2 (g) 2 2 (l) 2 C 2 6 (g) 7 2 (g) 4C 2 (g) 6(l) 2 rganic compounds (containing C,, N and ) burn to produce C 2, 2 and N 2, if enough oxygen is present 2 C 8 14 N 4 2 (s) 21 2 (g) 16 C 2 (g) 14 2 (l) 4 N 2 (g) When the balancing units are s, not atoms Another reason why it s important to know how to name chemicals: so you can identify the s to balance reacts that involve s Cu 4 Na 3 P 4 Cu 3 (P 4 ) 2 Na 2 4 Cu 2 Cu 2 Cu 2 - Na - - Na P - Cu 2 - P Na - Cu 2 Na Cu 2 - Na - - P - - Na - Na - - Na P Na - Na - Na Na Cu 4 2 Na 3 P 4 Cu 3 (P 4 ) 2 3 Na 2 4. evian 6

7 Balancing chemical equats tep 1: What species to balance? The important quest is: What are the smallest units (species) that both go in and come out? If the chemical react involves only s, chances are you should balance s instead of atoms If there are ic compounds involved, knowing how to name them will assist greatly in balancing, because the names are the s Practice: what species should be balanced? C C 2 2 Ca 4 AgN 3 Ag 2 4 Ca(N 3 ) 2 Mg 2 Mg Al Cu 4 Al 2 ( 4 ) 3 Cu 22 Balancing chemical equats tep 2: The balancing act Two methods 1. Accounting method 2. Pictures method Both methods work Whichever one you use just depends on your mathematical inclinat Method 2 is more time consuming 23. evian 7

8 Accounting method of balancing C C 2 2 Balancing units are C, and Balance the s last, since they are present in two different products 5 C in 5 C must come out, so C 2 has coefficient 5 10 in 10 must come out, so 2 has coefficient 5 Now, 5 C produced means (10 5) or 15 s come out, so 15 must go in The only way to get 15 to go in is to have the coefficient of 2 be 7.5 o far, we have C C Double all the coefficients to get 2 C C Pictures method of balancing Cu 4 Na 3 P 4 Cu 3 (P 4 ) 2 Na 2 4 Cu 2 Cu 2 Cu 2 - Na - - Na P - Cu 2 - P Na - Cu 2 Na Cu 2 - Na - - P - - Na - Na - - Na P Na - Na - Na Na Cu 4 2 Na 3 P 4 Cu 3 (P 4 ) 2 3 Na 2 4. evian 8

9 toichiometry Counting atoms Quantitative part of chemistry Foundat is conservat of matter Must use balanced chemical equats React coefficients (also called stoichiometric coefficients) tell you how many units of a chemical are required, compared to units of other chemicals in the react We can t measure units in the laboratory (we measure mass, volume, etc.) this is why it is important to know how to convert between lab measurements and moles Usually, units means moles 27 General trategy for imple toichiometry Problems Mass of chemical A Molar mass Moles of chemical A Moles of chemical B Molar mass Chemical react toichiometric coefficients Mass of chemical B 28. evian 9

10 Cooking is stoichiometry 2 cups flour 3 eggs 1 cup sugar Extending the idea Let s say the recipe makes 24 cookies What if you wanted to make 48 cookies? What if you wanted to make 12 cookies? What if you wanted to make 1.20 cookies? ow many cookies could you make if you only had 2 eggs instead of 3 (and enough of everything else)? 30. evian 10

11 imple toichiometry What mass of carbon dioxide gas is produced when 227 grams of propane (C 3 8 ) combust completely? Balanced chemical equat: C C tart 1 mol C 3 8 for every 3 mol C 2 End 227 g C mol C mol C g Cg 2 C 2 = g Cg 3 C mol C mol C 2? g C 2 g C g C 2 Types of toichiometry Problems imple stoichiometry: Mass of one chemical (reactant or product) is specified. Find out mass of another chemical required or produced in the react (assuming just enough of each reactant is present). Must use a balanced chemical equat. Limiting reagent: Masses of two different reactants are specified. ne of the reactants limits the react (it gets used up first). Figure out the maximum mass of a product that could be formed if all of the limiting reactant is used up. Must use a balanced chemical equat. Chemical analysis: Known and unknown chemicals or quantities given. Figure out unknown chemicals or quantities. 32. evian 11

12 Examples 1. Balance these chemical reacts a) C C 2 2 b) calcium hydroxide silver nitrate silver hydroxide calcium nitrate 2. When 2.06 g of nitrogen gas react with an excess of hydrogen gas, what mass of ammonia (N 3 3) would be formed? N N 3 Balancing with first figuring out chemical formulas calcium hydroxide silver nitrate silver hydroxide calcium nitrate The problem is figuring out the chemical formulas calcium hydroxide Made of Ca 2 s and s Must be present in a 1:2 ratio to make Ca() 2 silver nitrate Made of Ag s and N 3 s Must be present in a 1:1 ratio to make AgN 3 silver hydroxide Made of Ag s and s Must be present in a 1:1 ratio to make Ag calcium nitrate Made of Ca 2 s and N 3 s Must be present in a 1:2 ratio to make Ca(N 3 ) 2 Mistakes I often see: Incorrect charges on s Incorrect ratios Forgetting to write parentheses around polyatomic s Leaving charges written on the s in the formula. evian 12

13 Then balance the equat calcium hydroxide silver nitrate silver hydroxide calcium nitrate Ca() 2 AgN 3 Ag Ca(N 3 ) 2 Try balancing the names of the s, if the subscripts are confusing calcium hydroxide hydroxide silver nitrate silver hydroxide calcium nitrate nitrate silver nitrate silver hydroxide Ca() 2 2 AgN 3 2 Ag Ca(N 3 ) 2 The N N 3 react before after ere is what an N 3 molecule looks like: Draw the after picture. Assume that all of the molecules that can react, do.. evian 13

14 Let s look at the stoichiometry of N N 3 Before Change After N 2 2 N 3 Now let s look at the example problem N N grams of excess of ow many grams of this will be produced? N 2 2 N 3 Before Change 2.06 g N 2 1mol N g N g N 2 = mol N 2 excess mol N 2 you could figure out how much if you wanted to none 2mol N mol N = mol N mol N2 After none some g N mol N = 2.50 g N mol N3. evian 14

15 Concluss about solving stoichiometry problems Mass is what is conserved Converting from how much of one chemical to how much of another requires knowing how many moles Figure out what is given and what you re looking for Is the given informat in mass or moles? Is the answer sought in mass or moles? Which reactant is limiting? For complicated problems, it helps to set up a before-change-after table remember that quantities in this table must be in moles ee website for another worked example 43. evian 15