Mole ratios and reaction stoichiometry lab report answers

Chemistry 151 College of the Canyons Week 8 – Reaction Stoichiometry Fall 2020 Name __________________________________ Date ____________________________ Section_________________________________ INTRODUCTION When a chemical reaction occurs, reactants are converted to products. The quantitative relationship between the reactants and products is termed stoichiometry. Because matter is not created or destroyed in a chemical reaction, the number of atoms of each element should be balanced on each side of the reaction. To do this, we use stoichiometric coefficients. Stoichiometric coefficients are the numbers placed in front of atoms, ions, or molecules in a chemical equation. Stoichiometric coefficients establish the mole ratio between reactants and products in a chemical reaction. For example, in this reaction: N2(g) + 3 H2(g) 2 NH3(g) The mole ratios between the molecules are: 1 mol N2 : 3 mol H2 : 2 mol NH3 The mole ratio means that 1 mol of N2 will react with 3 mol of H2 to form 2 mol of NH3. The mole ratio is used to compare the amount of reactants needed for the reaction to occur, the amount of products formed in the reaction, or to convert amounts of reactants to products. The stoichiometric coefficient and resulting mol:mol ratios of chemical reactions are essential to solving quantitative problems based on chemical reactions. Example #1– How many moles of NH3(g) can be produced from 2.62 moles of N2(g)? 2.62 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁2 ∗ 2 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁𝑁𝑁3 = 𝟓𝟓. 𝟐𝟐𝟐𝟐 𝒎𝒎𝒎𝒎𝒎𝒎 𝑵𝑵𝑵𝑵𝟑𝟑 1 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁2 While the mol:mol ratio provides information on different reactants or products in a chemical reaction, to calculate measurable data in the laboratory, the units must be in grams. From our prior experiments, we know that to convert from moles to grams, or vice versa, we must use the molar mass. For a general stoichiometric calculation, the scheme below can be used as a guide. In this example, ‘A’ represents the product or reactant we know something about and ‘B’ represents the product or reactant we want to know something about. grams A moles A moles B Scheme 8.1 – The Relationship between Stoichiometric Coefficients Page 1 grams B Chemistry 151 College of the Canyons Week 8 – Reaction Stoichiometry Fall 2020 Example #2 – How many grams of NH3 can be produced from the reaction of 1.32 grams of H2? 𝑁𝑁2 (𝑔𝑔) + 3𝑁𝑁2 (𝑔𝑔) → 2𝑁𝑁𝑁𝑁3 (𝑔𝑔) A B Grams H2 1.32 𝑔𝑔 𝑁𝑁2 ∗ Moles H2 Moles NH3 Grams NH3 1 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁2 2 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁𝑁𝑁3 17.04 𝑔𝑔 𝑁𝑁𝑁𝑁3 ∗ ∗ = 7.42 g NH3 2.02 𝑔𝑔 𝑁𝑁2 3 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁2 1 𝑚𝑚𝑚𝑚𝑚𝑚 𝑁𝑁𝑁𝑁3 In most chemical reactions, one of the reactants will run out first. For example, if you wanted to make peanut butter and jelly sandwiches, you may go to the store and buy a jar of peanut butter, a jar of jelly and a loaf of bread. To assemble the sandwiches (the product), you would use the following ‘reaction’: 2 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑆𝑆𝑚𝑚𝑆𝑆𝑆𝑆𝐵𝐵𝑆𝑆 + 1 𝑆𝑆𝑆𝑆𝑚𝑚𝑚𝑚𝑆𝑆 𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃𝑃𝑃𝑃𝑃 𝐵𝐵𝑃𝑃𝑃𝑃𝑃𝑃𝐵𝐵𝐵𝐵 + 1 𝑆𝑆𝑆𝑆𝑚𝑚𝑚𝑚𝑆𝑆 𝐽𝐽𝐵𝐵𝑚𝑚𝑚𝑚𝐽𝐽 → 1 𝑃𝑃𝐵𝐵𝐵𝐵𝑃𝑃𝑃𝑃𝑃𝑃 𝐵𝐵𝑃𝑃𝑃𝑃𝑃𝑃𝐵𝐵𝐵𝐵 & 𝐽𝐽𝐵𝐵𝑚𝑚𝑚𝑚𝐽𝐽 𝑆𝑆𝐵𝐵𝑃𝑃𝐵𝐵𝑆𝑆𝑆𝑆𝑆𝑆ℎ How could you predict how many sandwiches you could make? It’s unlikely that the bread, the peanut butter, and the jelly will run out at exactly the same time. The component that runs out first determines how many sandwiches you can make. If you run out of bread, you can no longer make sandwiches. The same is true in chemical reactions. In chemistry, the reactant that runs out first is called the limiting reactant. Because the limiting reactant determines the maximum amount of product that can form, the amount of product formed when all of the limiting reactant is converted to product is the theoretical yield. In our sandwich example, the bread is the limiting reactant. If a loaf consists of 36 slices of bread, the theoretical yield would be 18 peanut butter and jelly sandwiches. Any reactant that is not the limiting reactant is referred to as an excess reactant.