The Math of Chemical Reactions Unit 8 - Stoichiometry The Math of Chemical Reactions

Recall… In our last unit, we talked about how to convert between moles & mass and moles & atoms/molecules for one compound. In this unit, we will be applying these skills to convert between moles and mass of two different compounds.

What is Stoichiometry? The study of the mathematical relationships between the substances in balanced chemical reactions Allows for calculation of amounts of reactants or products in a chemical reaction i.e. How much of one thing can you make if you start with a certain amount of another thing?

Stoichiometry in Chemistry 2 H2 + O2  2 H2O Coefficients in a balanced chemical equation indicate the relative amounts of each substance involved in the reaction One way to describe the ratios involved in the reaction above would be “two molecules of hydrogen gas plus one molecule of oxygen gas yields two molecules of water”

Mole Ratios However, since these are only ratios, using the same equation, 2 H2 + O2  2 H2O It is equally true that, “two moles of hydrogen plus one mole of oxygen yields two moles of water” These mole relationships are called “mole ratios”

Mole Ratio N2 (g) + 3H2 (g)  2NH3 (g) Mole ratio – conversion factor which relates moles of one compound to moles of another compound in a balanced equation Coefficients from balanced equation are used to write mole ratios N2 (g) + 3H2 (g)  2NH3 (g)

Mole Ratios Example #1: What is the mole to mole ratio of oxygen to water in the following equation: 2 H2 + O2  2 H2O

___H2SO4 + ___ NaOH  ___Na2SO4 + ___H2O Mole Ratios Example #2: Balance the equation and then determine the mole ratio of sodium hydroxide to sodium sulfate in the following equation: ___H2SO4 + ___ NaOH  ___Na2SO4 + ___H2O

Stoichiometry Flowchart

Warm Up: 1. Balance the equation: ___Na + ___Cl2  ___NaCl 2. What is the molar ratio between Na and Cl2? 3. What is the molar ratio between Cl2 and NaCl?

Stoichiometric Calculations Mole to Mole Calculations – converting from moles of one compound to moles of a different compound Write and balance the equation if not already done Write the mole ratio between the two mentioned compounds using the coefficients from the balanced equation. This will serve as your conversion factor. Use dimensional analysis & the mole ratio to convert from moles of compound #1 to moles of compound #2

Mole to Mole Calculations Ex #1: In the equation below, if 0.5 moles of magnesium hydroxide react, how many moles of water would be produced? 2 H3PO4 + 3 Mg(OH)2  Mg3(PO4)2 + 6 H2O

Mole to Mole Calculations Ex #2: In the equation below, if 0.246 moles of H3PO4 react, how many moles of water would be produced? 2 H3PO4 + 3 Mg(OH)2  Mg3(PO4)2 + 6 H2O

Mole Conversions WS – Prob. 1 Aluminum metal and hydrogen chloride react to form aluminum chloride and hydrogen gas.   How many moles of aluminum metal are needed to produce 3.33 moles of aluminum chloride?

Mole Conversions WS - Answers 1. a. 3.33 mol Al b. 1.64 mol HCl 2. a. 4.65 mol NaBr b. 1.55 mol Al(OH)3 3. a. 3.5 x 10-4 mol CH4 b. 7.0 x 10-4 mol O2

Warm-Up: Balance the following equation: __N2 + __H2  __NH3 If you have 4.5 moles of hydrogen gas, how many moles of ammonia (NH3) would you produce?

General Tips for Stoic. Problems Always convert the given value to moles as your first step! Once you get to moles, you can convert to moles of the unknown Moles of unknown can then be converted to whatever final units the problem asks for (grams, liters, milliliters, molecules, etc.)

Mass to Mass Calculations Convert from grams of given compound to moles of given compound (using molar mass) Convert from moles of given to moles of unknown (using the mole ratio) Convert from moles of unknown to grams of unknown (using molar mass)

Mass to Mass Calculations Master Formula for Mass to Mass Calculations

Mass to Mass Calculations Ex #1: In the unbalanced equation below, how many grams of carbon dioxide will be produced by the reaction of 108 grams of C5H12? ___ C5H12 + ___ O2  ___ CO2 + ___ H2O

Mass to Mass Calculations Ex #2: In the balanced equation below, how many grams of C5H12 would be needed to completely react with 11.5 grams of oxygen? C5H12 + 8 O2  5 CO2 + 6 H2O

Answers for Mass-Mass WS 15.2 g KCl A. 10.7 g H2 B. 60.7 g NH3 A. 4.2 g AgCl B. 4.6 g BaCl2

Percent Yield Theoretical yield – amount of product that should be formed when the limiting reactant (the reactant that runs out first) is completely used up Actual yield – the actual amount of product produced when the reaction is carried out experimentally Percent yield =

Percent Yield Example #1: If a student made 1.72 g of NaCl in the lab when they should have made 2.00 g, what is the percent yield?

Percent Yield Example #2: N2 + 3 H2  2 NH3 A student calculates that for the reaction above, 5.0 grams of NH3 should be produced. After carrying out the reaction, the student made 4.5 g of NH3. What is the percent yield of the reaction?

Example #3 ___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate? (b) If a student did this reaction and produced 2.13 g of zinc phosphate, what is their % yield?

Example #3 ___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate?

Example #3 ___ZnCl2 + ___(NH4)3PO4  ___Zn3 (PO4)2 + ___NH4Cl (a) How many grams of zinc phosphate will be produced by the reaction of 5.00g of ammonium phosphate? (b) If a student did this reaction and produced 2.13 g of ammonium phosphate, what is their % yield?

Answers to Percent Yield WS 1. a. 33.1 g b. 56% 2. a. 8.2 g b. 76% 3. 42%

0.290 mole H2O 1.68 g H2O a. 1.62 g H3PO4 b. 93% (Good) Answers to HW #5 0.290 mole H2O 1.68 g H2O a. 1.62 g H3PO4 b. 93% (Good)

Limiting Reactant The reactant that is used up first in a chemical reaction Determines how much product can be formed by a reaction The reactant with fewer moles present at the beginning of the reaction

Excess Reactant Reactant present in a quantity that is more than sufficient to react with the limiting reactant Any reactant that remains after all the limiting reactant is used up The reactant present with more moles at the beginning of the reaction

Limiting Reactant Balloon Demo What was the limiting reactant for the first balloon? _____________________________ MMbaking soda= 84.01 g/mol NaHCO3 MMvinegar= 60.05 g/mol CH3COOH Moles baking soda: Moles vinegar:

Limiting Reactant Balloon Demo What was the limiting reactant for the second balloon? _____________________________ MMbaking soda= 84.01 g/mol NaHCO3 MMvinegar= 60.05 g/mol CH3COOH Moles baking soda: Moles vinegar:

Balloon Demo Questions Why were the third and fourth balloons the same size even though more baking soda was added to the fourth balloon? Limiting reactant 3rd balloon: Limiting reactant 4th balloon:

Stoichiometry Flowchart