Understanding Stoichiometry in Chemical Reactions
Stoichiometry is the concept of predicting the amounts of reactants and products in a chemical reaction, similar to following a recipe in cooking. It involves balancing chemical equations and determining the quantities of substances involved. By paying attention to coefficients, one can calculate how many moles of each substance are required and produced. Through examples like the reaction of ethanol with oxygen to form carbon dioxide and water, stoichiometry helps in understanding the quantitative aspects of chemical changes.
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Stoichiometry (Chapter 12) Dr. Walker
Stoichiometry (stoy-KEY-ah-muh-tree) Definition: predicting the amounts of reactants and/or products that will be involved in a reaction
Chocolate Chip Cookie Recipe Similar to using recipes to cook When cooking, recipes require specific amounts of each ingredient to make a set amount of food. Example: Chocolate Chip Cookies 2 cups flour teaspoon salt cup butter 1 cup brown sugar 1 cup shortening Makes 6 dozen cookies cup sugar 2 eggs 1 teaspoon vanilla extract 1 pkg chocolate chips 1 cup chopped nuts
Recipes = Chemical Formulas When cooking, recipes require specific amounts of each ingredient to make a set amount of food. Chemical reactions are the exact same thing!!! We need to know how much material can be made from existing amounts (i.e., how much food is in the pantry) OR how much we need to make a specific amount (how much of everything do I need to make 10 dozen cookies)
Chemical Reactions Chemical change involves a reorganization of the atoms in one or more substances. C2H5OH + 3O2 reactants When the equation is balanced it has quantitative significance: 2CO2 + 3H2O products 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water Pay attention to the coefficients!!!!!
Chemical Reactions Chemical change involves a reorganization of the atoms in one or more substances. C2H5OH + 3O2 reactants When the equation is balanced it has quantitative significance: 2CO2 + 3H2O products How many moles of ethanol could react with 9 moles of oxygen? If this happened, what products would we get?
Chemical Reactions C2H5OH + 3O2 reactants How many moles of oxygen could react with 9 moles of oxygen? If this happened, what products would we get? 2CO2 + 3H2O products If 1 mole of ethanol reacts with 3 moles of oxygen, then it would require 3 moles of ethanol to react with 9 moles of oxygen. If 1 mole of ethanol gives 2 moles CO2 and 3 moles H2O, 3 moles of ethanol would yield 3 x 2 = 6 moles CO2 and 3 x 3 = 9 moles H2O
Mole Ratios Using the coefficients we can write mole ratios. Definition: A mole ratio gives the relative amounts of reactants and products using a fraction The coefficients you used in the recipes are basically mole ratios. You re doing the same thing.
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of N2 to NH3
Using Mole Ratios Example: 1 N2 + 3 H2 2 NH3 Give the mole ratio of N2 to NH3 Use the coefficients 1 N2 to 2 NH3
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of H 2 to NH3
Using Mole Ratios Example: N2 + 3 H2 2 NH3 Give the mole ratio of H 2 to NH3 Use the coefficients 3 H2 to 2 NH3
Write all Mole-Mole Conversions Example: N2 + 3 H2 2 NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
Mole-Mole Conversions Example: N2 + 3 H2 2 NH3 6 moles ???? How many moles of ammonia could be produced from 6 moles of N2 and excess H2? What are we starting with? 6 moles of N2 Whenever you have excess, you have more than enough you won t use it in the problem!
Mole-Mole Conversions Example: N2 + 3 H2 6 moles ----------- 1 mole N2 2 NH3 ???? ----------- 2 moles NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2? Use the coefficients in a proportion
Mole-Mole Conversions Example: N2 + 3 H2 6 moles ----------- 1 mole N2 2 NH3 ???? ----------- 2 moles NH3 ??? = 12 moles NH3 How many moles of ammonia could be produced from 6 moles of N2 and excess H2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O ????? 1.2 moles C8H18 ----------------- 2 moles C8H18 ---------------- 18 moles H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
Another Mole-Mole Example 2 C8H18 + 25 O2 16 CO2 + 18 H2O ????? 1.2 moles C8H18 ----------------- 2 moles C8H18 ---------------- 18 moles H2O ???? = 10.8 moles H2O How many moles of water can be produced from 1.2 moles of octane and excess O2?
You try 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2 How many moles of hydrogen can be produced from 0.24 moles of aluminum? 2 Al2O3 4 Al + 3 O2 How many moles of aluminum can be made from 0.35 moles of Al2O3
You try 2 Al + 3 H2SO4 Al2(SO4)3 + 3 H2 0.24 moles Al ----------------- 2 moles Al ????? ???? = 0.36 moles H2 ---------------- 3 moles H2 How many moles of hydrogen can be produced from 0.24 moles of aluminum? 2 Al2O3 4 Al + 3 O2 0.35 moles Al2O3 ----------------- 2 moles Al2O3 ????? ???? = 0.70 moles Al ---------------- 4 moles Al How many moles of aluminum can be made from 0.35 moles of Al2O3
The Next Step Mass-Mass Stoichiometry When we measure materials for chemical reactions, the scales measure in grams, not moles Why can t we just multiply the original mass times the mole ratio?
The Next Step Mass-Mass Stoichiometry When we measure materials for chemical reactions, the scales measure in grams, not moles Why can t we just multiply the original mass times the mole ratio? We must account for the difference in molar masses between the two materials we are dealing with
Proportion Method Plug and chug Plug in the known numbers Solve for the unknown
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O How many grams of H2O could be produced from the combustion of 100 g of propane?
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O 100 g ???? How many grams of H2O could be produced from the combustion of 100 g of propane? Mass (start) = 100 g Coefficient (start) = 1 Molar Mass (start) = 44.11 g/mole Remember cross multiply, then divide for solving proportions (100 g x 4 moles x 18.02 g/mole) / (1 mole x 44.11 g/mole) = ??? Mass (finish) = unknown Coefficient (finish) = 4 Molar Mass (start) = 18.02 g/mole
Mass-Mass Example C3H8 + 5 O2 3 CO2 + 4 H2O 100 g ???? How many grams of H2O could be produced from the combustion of 100 g of propane? Grams H2O 100 g C3H8 1 mole C3H8 x 44.11 g/mole C3H8 4 mole H2O x 18.02 g/mole H2O Mass (start) = 100 g Coefficient (start) = 1 Molar Mass (start) = 44.11 g/mole Mass (finish) = 163.41 g (answer!!) Coefficient (finish) = 4 Molar Mass (start) = 18.02 g/mole
Some helpful hints Only use the materials provided in the problem The compounds not listed are irrelevant to the problem, even if they re in the reaction The coefficients and the molar masses are separate Do not count the coefficient in your molar mass!!! If you see the word excess , cross out that chemical you will not use it!!! The word excess just means you ve got plenty of material to react with the amounts that you have.
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe 10 g ???? How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe Grams Fe 10 g Na 2 mole Fe x 55.85 g/mole Fe 6 mole Na x 22.99 g/mole Na How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Another Example 6 Na + Fe2O3 3 Na2O + 2 Fe 10 g Na Grams Fe 2 mole Fe x 55.85 g/mole Fe 6 mole Na x 22.99 g/mole Na Grams Fe = 8.10 g (answer!!) How many grams of Fe can be produced by the reaction of 10 g Na and excess iron oxide?
Volume-Volume Stoichiometry It is difficult to measure gases with a mass, so they are usually measured with a volume Instead of using the molar mass, use 22.4 L 1 mole of any gas = 22.4 L volume (at 0oC and 1 atm pressure, known as STP)
Example N2 + 3 H2 2 NH3 How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?
Example N2 + 3 H2 2 NH3 Liters NH3 15 L 2 mole NH3 x 22.4 L/mole NH3 3 mole H2 x 22.4 L/mole H2 Answer = 10 liters NH3 How many liters of ammonia can be made from 15 liters of hydrogen and excess nitrogen?
Mixed Stoichiometry Sometimes, we need to start with one unit and finish with another. If you re converting between mass and moles, use the molar mass If you re converting between volume and moles, use 1 mole = 22.4 L If the conversion involves molecules, use Avogadro s number!
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) How many liters of oxygen are produced from the decomposition of 244 g KClO3?
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) How many liters of oxygen are produced from the decomposition of 244 g KClO3? Remember, since we re dealing with liters of oxygen, we ll use 22.4 L instead of the molar mass.
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) Liters O2 244 g KClO3 3 mole O2 x 22.4 L O2 2 mole KClO3 x 122.55 g/mole KClO3 How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
Mixed Example 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) Liters O2 244 g KClO3 3 mole O2 x 22.4 L O2 2 mole KClO3 x 122.55 g/mole KClO3 Liters O2 = 66.90 L (answer!!) How many liters of oxygen at STP are produced from the decomposition of 244 g KClO3?
Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?
Another Mixed Example 2 Ca (s) + O2 (g) 2 CaO X Grams Ca 4.48 L O2 2 mole Ca x 40.08 g/mole Ca 1 mole O2 x 22.4 L/mole O2 X = 16.03 g Ca How many grams of calcium are required to completely react with 4.48 liters of O2 at STP?
Other Possible Mixing Sometimes you may start with moles and go to mass or liters How will THAT work in the proportion? If you are dealing with MOLES for a particular compound, simply leave out the second number on the bottom.
Mole-Mass Example How many grams of iron could be made from 6 moles of carbon?
Mole-Mass Example Grams Fe 6 moles C 2 mole Fe x 55.85 g Fe 3 mole C Grams Fe = 223.4 g Fe Notice that since we re only dealing with moles, you don t need a second number on the bottom! How many grams of iron could be made from 6 moles of carbon?
Volume Mole Example How many moles of B5H9 are required to react with 100 L of O2?
Volume Mole Example Moles B5H9 100 L O2 2 moles B5H9 12 x 22.4 L Notice that since we re only dealing with moles, you don t need a second number on the bottom! Moles B5H9 = 0.74 moles How many moles of B5H9 are required to react with 100 L of O2?
Percent Yield Theoretical Yield The amount of material you can make from a reaction if it works perfectly .in theory Everything you have calculated so far has been a theoretical yield Yes, you ve been doing THIS already Actual Yield The amount of stuff you actually make in a reaction
Percent Yield Percent Yield Notice (again) that we multiply by 100 to turn a decimal to a percentage The percent yield should NEVER be > 100% If this occurs, you either made a math error or your sample is impure (wet).
Percent Yield Example (Easy) Some problems will give you a theoretical and percent yield A student produces 50 g of NaCl in a reaction yield where the theoretical yield is 60 g
