Presentation on theme: "Limiting Reactants and Percent Yields"— Presentation transcript:
1 Limiting 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.
2 Limiting 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.
3 Determining 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
4 Percent 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
5 Calculation 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%