Bhavana Challa - Lab #4

Introduction Stoichiometry is defined as the process of “keeping track” of the amounts of reacts and products that are involved within a chemical reaction. Often using stoichiometry involves calculations using the balanced equation of a chemical reaction. Balanced equation provides a ratio for the reactants and products which are essential for stoichiometric calculations. In most cases the reaction does not follow the exact ratios shown in the equation thus identifying a limiting reagent condition. The purpose of this experiment is to apply the process of stoichiometry to a chemical reaction to demonstrate the effect of limiting and excess reagents. The experiment will be using the chemical reaction that occurs between CaCl 2 and NaOH. In this reaction 1 mole of aqueous CaCl 2 and 2 moles of aqueous NaOH are combined to form 1 mole of solid Ca(OH) 2 and 2 moles of aqueous NaCl. To understand the limiting reagent and excess reagent concepts we can think of them in terms of assembling a car. For example, let consider having 50 car bodies and 160 tires. Having the 50 car bodies we can determine that we can make 50 cars but because we only have 160 tires, and each car body needs four tires we can determine that we can only make 40 cars. Even though we have enough car bodies to make 50 cars we don’t have enough tires and therefore we can only make 40 cars. In this scenario the tires are the limiting reagent since there are not enough for all the car bodies. On the other hand, the car bodies are the excess reagent because some remain unassembled. This is similar in a chemical equation except moles of the reactants are used instead of car bodies and tires. To solve a problem using these stoichiometric calculations there are two separate calculations that would need to be made. One using the CaCl 2 reactant and the other using the NaOH reactant. Using dimensional analysis and given the moles of each reactant we can set up molar ratio calculations. Using 2.0 moles of CaCl 2 and 3.0 moles of NaOH as an example for the CaCl 2 we can set up 2.0 moleCaCl 2 × 1 molCa ( OH ) 2 1 molCaCl 2 = 2.0 molCa ( OH ) 2 and for the NaOH we can set up 3.0 mole NaOh× 1 molCa ( OH ) 2 2 mol NaOH = 1.5 molCa ( OH ) 2 . In this scenario NaOH is the limiting reagent because it produces less of the product than CaCl 2 , making the CaCl 2 the excess reagent. The value of product that could be formed by the limiting reagent is the theoretical yield. In this experiment the amount of NaOH will initially be kept constant while the amount of CaCl 2 is gradually changes meaning NaOH will initially be excess. However, at some point the NaOH will become the limiting reagent. There are two methods that can be used to determining the limiting and excess reagents of a chemical reaction: stoichiometric calculations and experimental testing. In order to use both an experiment will need to take place. This experiment will be done by reacting different amounts of CaCl 2 and NaOH solution. Both solutions are clear solution but after the reaction occurs a white precipitate of Ca(OH) 2 and a clear solution of NaCl is formed. Often within the aqueous NaCl there may be excess CaCl 2 or NaOH. With this method we will be given the molarity and volume of each solution of the reactants and will have to determine the number of moles using the molarity formula. Simply, to determine the number of moles we would multiply the volume of the solution in liters with the molarity of the solution. Once this the number of moles of each solution are identified we can use the stoichiometric calculation method. For example, we are using 0.5 L of 0.75M CaCl 2 and 0.5L of 1.0 NaOH solutions. We would have