Continuing Stoichiometry: Chemical Quantities in Reactions Limiting Reactants Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Limiting Reactant A limiting reactant in a chemical reaction is the substance that Is used up first. Stops the reaction.

Example of Everyday Limiting Reactant How many peanut butter sandwiches could be made from 8 slices bread and 1 jar of peanut butter? With 8 slices of bread, only 4 sandwiches could be made. The bread is the limiting item. Copyright © 2008 by Pearson Education, Inc. publishing as Benjamin Cummings

Example of Everyday Limiting Reactant How many peanut butter sandwiches could be made from 8 slices bread and 1 tablespoon of peanut butter? With 1 tablespoon of peanut butter, only 1 sandwich could be made. The peanut butter is the limiting item. Copyright © 2008 by Pearson Education, Inc. publishing as Benjamin Cummings

Example Limiting Reactant Calculation: A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen.  Which is the limiting reactant and how much excess reactant remains after the reaction has stopped? First, we need to create a balanced equation for the reaction: 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)

Calculate how many moles of each? 

The EXCESS The reactant that produces the lesser amount of product in this case is the oxygen, which is thus the "limiting reactant." Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (ammonia) actually did react with the limiting reactant (oxygen).

Limiting Reactants Example Problem 1 When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form? H2(g) + Cl(g)  2HCl (g) 4.00 mol 2.00 mol ??? mol Calculate the moles of product from each reactant, H2 and Cl2. The limiting reactant is the one that produces the smaller amount of product.

Limiting Reactants Using Moles How much HCl can be formed from the H2 ? 4.00 mol H2 x 2 mol HCl = 8.00 mol HCl 1 mol H2 HCl from Cl2 2.00 mol Cl2 x 2 mol HCl = 4.00 mol HCl 1 mol Cl2 (smaller number) The limiting reactant is Cl2 because it is used up first. Thus Cl2 produces the smaller number of moles of HCl.

Limiting Reactants Using Mass Example Problem 3 If 192 grams Ca mixed with 56 grams N2, which is the limiting reactant? 3Ca(s) + N2(g)  Ca3N2(s) Moles of Ca3H2 from Ca 192 grams Ca /40 = 4.80 mol Ca x 1 mol Ca3N2 = 1.60 mol Ca3N2 3 mol Ca Moles of Ca3H2 from N2 56 grams N2/ 28 grams = 2.00 mol N2 x 1 molCa3N2 = 2.00 mol Ca3N2 All Ca is used up when 1.60 mol Ca3N2 forms. Thus, Ca is the limiting reactant.

Example Problem 3 Calculate the mass of water produced when 8.00 g H2 and 24.0 g O2 react? Which is the limiting reactant? 2H2(g) + O2(g) 2H2O(l)

Limiting Reactants Using Mass Example Problem 4 What is the limiting reactant when Silver metal reacts with sulfur to form silver sulfide. If you have 50 grams of Silver and 10 g of Sulfur