QUESTION: Consider the reaction: N2 + 3 H2  2 NH3 What is the theoretical yield of NH3 from a mixture of 2.00 mol N2 and 3.00 mol H2? A. 0.500 mol, B. 1.00 mol, C. 2.00 mol, D. 4.00 mol One reactant will run out first. This reactant is called the “limiting reactant.” Theoretical Yield = maximum amount of product that can be obtained. This is the amount produced when the limiting reactant is completely consumed. Consider the reaction: N2 + 3 H2  2 NH3 What is the theoretical yield of NH3 from a mixture of 2.00 mol N2 and 3.00 mol H2? A. 0.500 mol, B. 1.00 mol, 2.00 mol, B. 4.00 mol PAUSE CLICK In this problem we’re given the amounts of two reactants, but we’re not told how much of either one is actually consumed in the reaction. In general, as a reaction progresses, one of the reactants will run out first. The amount of product produced by the time one reactant is used up is called the theoretical yield. The theoretical yield is the maximum amount of product that we can get if the reaction were to occur. because once one of the reactants is completely consumed, we won’t be able to generate any more product. CLICK CONTINUED ON NEXT SLIDE

QUESTION: Consider the reaction: N2 + 3 H2  2 NH3 What is the theoretical yield of NH3 from a mixture of 2.00 mol N2 and 3.00 mol H2? A. 0.500 mol, B. 1.00 mol, C. 2.00 mol, D. 4.00 mol General strategy: calculate yield assuming each reactant is completely used up. Lower yield is the theoretical yield; higher yield is impossible. Reactant with lower yield is the “limiting reactant.” 2 mol NH3 3.00 mol H2 The general strategy for solving this type of problem is to calculate the product yield assuming each reactant is completely used up. The lower yield is the theoretical yield and the higher yield is impossible. So let’s calculate how much product we get if 3.00 moles of H2 are consumed. By now you should know how to do this calculation. We start with 3.00 mol H2 CLICK then multiply it by a conversion factor based on the coefficients in the balanced equation. CLICK CLICK The coefficient of ammonia is 2 HIGHLIGHT coefficient and the coefficient of hydrogen is 3 We find that the amount of ammonia produced from 3.00 moles of H2 is 2.00 moles. SHOW CANCELLATION OF UNITS Similarly, we can calculate the amount of ammonia that would be produced from 2.00 moles of nitrogen. 2.00 moles of nitrogen CLICK times a conversion factor based on the coefficients in the balanced equation CLICK HIGHLIGHT coefficient of NH3 The coefficient of nitrogen is 1. CLICK CALLOUT “implied 1” pointing to left of N2 in balanced equation We find that the amount of ammonia produced from 2.00 moles of N2 is 4.00 moles. SHOW CANCELLATION OF UNITS CLICK The lower of the two yields is 2.00 moles. HIGHLIGHT 2.00 mol NH3 and “Lower yield is the theoretical yield” The correct answer is C. The theoretical yield of ammonia is 2.00 moles. By the time we get this much ammonia, the hydrogen is completely used up. If the hydrogen is completely used up, then we can’t get any more. We say that, in this case, hydrogen is the limiting reactant. It sets a limit on the amount of product that we can get. CLICK PAUSE END RECORDING x = 2.00 mol NH3 3 mol H2 2 mol NH3 2.00 mol N2 x = 4.00 mol NH3 1 mol N2

Video ID: 4-9-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08