# Chapter 12 Stoichiometry

Chapter 12 Stoichiometry SCSh5.e: Solve scientific problems by substituting quantitative values, using dimensional analysis and/or simple algebraic formulas as appropriate. SC2.d: Identify and solve different types of stoichiometry problems, specifically relating mass to moles and mass to mass. SC2.e: Demonstrate the conceptual principle of limiting reactants. Online Resources: Stoichiometry Practice Problems: http://mailer.fsu.edu/~rlight/stoich/ Click Stoichiomentry: http://misterguch.brinkster.net/explains2.html 1 Stoichiometry A balanced chemical equation provides the same kind of quantitative information that a recipe does.

Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or product is formed in a reaction. 2 Stoichiometry Stoichiometry is the calculation of quantities in chemical reactions. When you know the quantity of one substance in a reaction, you can calculate the quantity of another substance consumed or created in the reaction. A quantity can be grams, moles, liters, molecules, atoms, ions, formula units or particles. 3 Stoichiometry

A balanced equation indicates the number and type of each atom, molecules, and/or moles that makes up each reactant and each product A balanced chemical equation obeys the law of conservation of mass The total number of grams of reactants DOES equal the total number of grams of product Assuming standard temperature and pressure, a balanced equation also tells you about the volume of gases. Mass and atoms are conserved in every chemical reaction 4 Stoichiometry Mole ratio is a conversion factor derived from

coefficients of a balanced chemical equation interpreted in terms of moles. In chemical calculations, mole ratios are used to convert between moles of reactants and moles of product, or moles of products and moles of reactants or between moles of two products, or two reactants In the mole ratio you MUST use the COEFFICIENTS of the BALANCED chemical reaction 5 Mole to Mole Conversion In order to do stoichiometry conversions you MUST have a balanced chemical reaction. Use our basic dimensional analysis set up

Unit getting rid of on bottom, unit going to on top NOT just the units, it is ALSO the chemical formulas Example 1: If you decompose 6.50 moles of ammonia (NH3) how many moles of each product do you produce? Skeleton equation: NH3 N2 + H2 Balanced equation: 2NH3 1 N2 + 3 H2 6 Mole to Mole Conversion Example 1: If you decompose 6.50 moles of ammonia (NH3) how many moles of each product do you produce? Balanced equation: 2NH3 1 N2 + 3 H2 1 2 6.50 3

=3.25 2 2 3 3 2 6.50 3 =9.7 5 2 2 3 7 Mole to Mole Conversion 3 Na2S + 2AlP 2Na3P + Al2S3 Example 2: 72.50 mol of Na2S would produce how many moles of Na3P? Example 3: 14.45 mol Al2S3 was produce by how many moles of AlP?

8 Things to remember You MUST have a balanced chemical equation to do ANY mole to mole conversions. The coefficients in the balanced chemical reaction are used in the mole ratio ONLY Mole ratios are the ONLY place that you can switch substances. Can do this [], you can only go from gram to mole, volume to mole, particles to mole The starting amount (# given in the problem) is only

written ONE time, and never in a conversion fraction. Any time you have a reactant in excess it does NOT affect calculations (you can ignore it) 9 New Mole Map 10 Terminology Theoretical yield is the maximum amount of product that will form during a reaction. Any time you are calculating the amount of product produced you are calculating theoretical yield. Actual yield is the amount of product that actually forms when the reaction is carried out in

a laboratory. 11 Stoichiometry Calculations 1. the first step is to convert the give substance measurement to moles. (if not starting with moles) 2. Next use the mole ratio to switch between substances 3. Finally convert to the desired substance to the correct unit for the final answer. Use your mole map to help determine the number of fractions needed to do conversion. 12 Mass to Mass Conversion

2NH3 1 N2 + 3 H2 Example 1: 48.38 g NH3 would produce how many grams of nitrogen. Use map to lay out our path 3 bridges = 3 fractions Example 2: 48.38 g NH3 would produce how many grams of hydrogen 13 Mass to Volume Conversion 2NH3 1 N2 + 3 H2 Example 3: 48.38 g NH3 would produce how many liters of nitrogen. 1 2

22.4 2 31.80 2 2 3 1 2 Example 4: If you have 68.92 g NH3 what would the theoretical yield of hydrogen be in molecules? 1 3 3 2 6.02 1023 2 68.92 3 1 2 2 17.04 3

3 3.659 10 2 4 2 Things to remember UNLESS it is a mole ratio mole always has a 1 in front of it. The numbers for the mole ratio come from the BALANCED chemical equation. Use your mole map to see how many steps it will take There are only 4 valid possible options. 1 mole = (molar mass) g

1 mole = 22.4 L 1 mole = 6.02 x 1023 particles __ mole X = ____ mole Y (___ come from equation) 15 Terminology Part II Limiting reagent is the reagent that determines the amount of product that can be formed by a reaction. Excess reagent is the reactant that is not completely used up in a reaction. To determine the limiting reactant convert one reactant into grams of second reactant Compare the two values If you wont have enough of the second reactant it is the limiting

If you have enough of the second reactant it is the excess reagent 16 Limiting Reactant example 1 If starting with 6.25 g Na3P and 5.88 g CaF2 what is the limiting reactant? __ Na3P + __ CaF2 __ NaF + __ Ca3P2 Balanced: 2 Na3P + 3 CaF2 6 NaF + 1 Ca3P2 1 3 6 .25 3 99.94 3 3 2 2 3

78.08 2 1 2 . This says we NEED 7.32 g CaF2 but we only have 5.88 g CaF2 Since we dont have enough of it CaF2 is the limiting And Na3P is the excess reagent 17 Limiting Reactant example 2 If starting with 25.0 grams of each reactant determine the limiting reagent : C3H8 + 5O2 3CO2 + 4H2O 2 5.0 3 8

1 3 8 44.11 3 8 32.00 2 5 2 1 2 1 3 8 90.7 2 We have 25.0 grams of oxygen we need 90.7 g oxygen Oxygen is the limiting reagent because we dont have enough of it. The excess reagent is the C3H8

18 Limiting Reactant example 3 If starting with 2.50g S8 and 3.54 g O2 determine the limiting reagent : S8 + 12 O2 8SO3 2 .50 8 1 8 256.56 8 32.00 2 12 2 1 2

1 8 3.74 2 We have 3.54 grams of oxygen we need 3.74 g oxygen Oxygen is the limiting reagent because we dont have enough of it. The S8 is the excess reagent 19 Limiting Reactant (Reagent) Just by looking at the starting masses it is IMPOSSIBLE to determine the limiting reactant Just by looking at the coefficients it is IMPOSSIBLE to determine the limiting reactant You can only determine limiting reactant IF you

are comparing the same COMPOUND and same UNIT. 20 Terminology Part III Excess reagent is the reactant that is not completely used up in a reaction. To determine the excess remaining covert the limiting reactant to excess reactant and subtract that number from the stating amount of excess reactant. 21 Amount of Excess Remaining If starting with 6.25 g Na3P and 5.88 g CaF2. How much excess

remains after reaction? 2 Na3P + 3 CaF2 6 NaF + 1 Ca3P2 Recall that the limiting in this reaction was the CaF 2 5 .88 2 1 2 78.08 2 We used 5.02 grams of Na3P 2 3 3 2 99.94 3

1 3 5.023 Excess remaining = Starting amount amount used Excess Remaining = 6.25 g Na3P 5.02 g Na3P = 1.23 g Na3P Limiting and Excess Example If starting with 15.0g C2H8 and 75.0 g O2 2 C2H6 + 7 O2 4 CO2 + 6 H2O a) determine the limiting reagent b) the grams of excess reagent left after the reaction: 1 2 6 32.00 2 7 2 15.0 2 6

30.08 2 6 1 2 2 2 6 We have 75.0 grams of oxygen 55.9 2 we need 55.85g oxygen We have more than we need so Oxygen is the excess. a) So The limiting reagent is the C2H6 b) excess remaining: 75.0 O2 55.9 g O2 = 19.1 g O2 23 Terminology Part IV Percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent.

The percent yield is a measurement of the efficiency of a reaction carried out in the laboratory. Unless you are actually conducting the experiment the actual yield would have to be give to you in the problem. 24 Percent Yield What is the theoretical yield of P2O5 if you have 5.25 g O2? What is the percent yield if you actually produced 8.34 g P2O5? 4P + 5O2 2P2O5 1 2 2 2 5 5 .25 2 32.00 2

5 2 141.94 2 5 1 2 5 9.31 2 5 Theoretical yield of P2O5 is 9.31g Percent yield = Percent yield = 25 Everything combined 2Na3PO4 + 3CaCl2 Ca3(PO4)2 + 6NaCl If you have 2.38 g Na3PO4 and 6.98 g CaCl2

a. determine the limiting reagent b. Calculate the theoretical yield of NaCl (you MUST use the limiting as your starting point) c. Calculate the mass of excess reagent left after the reaction d. If you actually produced 1.56 g NaCl what is the percent yield for this reaction? 26 Everything combined 2Na3PO4 + 3CaCl2 Ca3(PO4)2 + 6NaCl If you have 2.38 g Na3PO4 and 6.98 g CaCl2 a. determine the limiting reagent Turn grams of one reactant into grams of the other 1 3 4 2 .38 3 4

163.94 3 4 32 2 3 4 110.98 2 1 2 2.42 2 We have 6.98 g CaCl2 and we need 2.42 g CaCl2 so it is the excess reagent Na3PO4 is the limiting reagent 27

Everything combined 2Na3PO4 + 3CaCl2 Ca3(PO4)2 + 6NaCl If you have 2.38 g Na3PO4 and 6.98 g CaCl2 b. Calculate the theoretical yield of NaCl (you MUST use the limiting as your starting point) 1 3 4 2 .38 3 4 163.94 3 4 6 2 3 4 58.44 1

2.55 28 Everything combined 2Na3PO4 + 3CaCl2 Ca3(PO4)2 + 6NaCl If you have 2.38 g Na3PO4 and 6.98 g CaCl2 c) Calculate the mass of excess reagent left after the reaction 1 3 4 2 .38 3 4 163.94 3 4 32 2 3 4

110.98 2 1 2 2.42 2 Excess remaining 6.98 CaCl2 2.42 g CaCl2 = 4.56 g CaCl2 29 Everything combined 2Na3PO4 + 3CaCl2 Ca3(PO4)2 + 6NaCl If you have 2.38 g Na3PO4 and 6.98 g CaCl2 d) If you actually produced 1.56 g NaCl what is the percent yield for this reaction?

61.2 % NaCl 30

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