Presentation on theme: "Limiting Reactants and Percent Yields"— Presentation transcript:
1Limiting Reactants and Percent Yields Objectives:1. Define limiting reactant.2. Explain how the quantities of products are determined in a chemical reaction.3. Explain how to calculate the percent yield of a reaction.
2Limiting Reactant:The limiting reactant in a chemical equation is defined as the reactant that limits the amount of product that can be produced. The unused reactant is left in excess after the reaction.
3Determining limiting reactants using ratios of coefficients Consider the following reactionN2 + 3 H2 g 2NH3If 10 moles of N2 react with 10 moles of H2 , how many moles of NH3 will be produced? What is the limiting reactant?1st step is to find the limiting reactant.divide the moles given by the coefficient listed in the formula10/1 N2 = 1010/3 H2 = 3.33the reactant with the smallest quantity is the limiting reactant2nd step - solve the equation using mol - mol10 mol H2 x 2 mol NH3 /3 mol H2 = 6.7 mol NH3At the end of the reaction there is an excess of N2 remaining
4Percent Yield Percent Yield: The experimental (actual) yield or amount of product obtained in a chemical reaction rarely matches the theoretical (predicted ) yield calculated through stoichiometric calculations. This could be due to a number of different situations including:testing conditionsside reactionserrors in recovering the products
5Calculation for percent yield Percent Yield = (actual yield /theoretical yield) x 100%Consider the following reactionN2 + 3 H2 g 2NH310 mol H2 x 2 mol NH3 /3 mol H2 = 6.7 mol NH3In the above problem 112.9g of NH3 are obtained from the reaction. What is the percent yield from the experiment?1st step - find the theoretical yieldmol - mass problem6.7 mol NH3 x 17g NH3 / 1mol NH3 = 113.9g NH32nd step - plug into formula(112.9g / 113g) x 100% = 99%If done correctly the value should ALWAYS be less than 100%