Limiting Reactant Videodisk 4 Unit 8 Lesson 2

Limiting Reactants Limiting Reactant - reactant you run out of first Reactant in Excess - reactant you have more than enough of To determine the limiting reactant, find the number of moles of each reactant and then divide by their respective coefficients Paperclip Demo Envelopes of paperclips Give students the formula of the molecule and have them build as many as possible of that molecule. Discuss the limiting reactant and the reactant in excess.

Example Problem What is the limiting reactant if you have 5.0g H2 and 10.0g O2? 2H2 + O2 → 2H2O Meaning for every 2 moles of H2 you need 1 mole of O2 5.0g H2 (1 mole H2/2.016g H2) = 2.5 moles of H2 10.0 g O2(1mole O2 /32.00g O2) = 0.313 moles O2

Continued: divide each by their coefficients O2 has the smaller value, so oxygen must be the limiting reactant

Example Problem 2Na + Cl2  2NaCl Identify the limiting reactant and the mass of sodium chloride produced when 1.7 g of sodium reacts with 2.6 L of chlorine gas at STP. 2Na + Cl2  2NaCl To identify the limiting reactant: 1.7 g Na x 1 mol Na/22.99 g Na = 0.074 mol Na Divide by Na coefficient: 0.074 mol Na/2 mol = 0.037 2.6 L Cl2 x 1 mol Cl2/22.4 L Cl2 = 0.12 mol Cl2 Divide by Cl2 coefficient: 0.12 mol Cl2/1 mol = 0.12

Na is limiting because less NaCl could be made from 1. 7 g of Na (0 Na is limiting because less NaCl could be made from 1.7 g of Na (0.037) than 2.6 L Cl2 (0.12) To determine NaCl produced: go back to mol Na and complete stoichiometry 0.074 mol Na x (2 mol NaCl/2mol Na) x 58.44 g NaCl/molNaCl = 4.3 g NaCl

Barium is limiting reactant Sample Problem When 22.6 g of barium and 4.2 g of nitrogen gas react, which is the limiting reactant and how many grams of barium nitride are produced? 3Ba + N2  Ba3N2 Barium is limiting reactant 24.2 g Ba3N2