Calculating Limiting and Excess Reagents SCH 3U

The Mole Ratio The quantities of substances that react in a chemical reaction are determined by the mole ratio given by the balanced chemical equation but, in the real world, it is less likely for the reactants to be in the mole ratios given by the equation – WHY??

The Answer In nature, compounds are found in random quantities In labs / industry, we might want to ensure that one reactant is completely used up

The Excess Reagent In any chemical reaction, one reactant is always present in EXCESS - in other words: the reagent that is not completely used up in the reaction there will be some left over at the end of the reaction in order to provide a reagent in excess, chemists add 10% to the mass required

Limiting Reactant To make a smiley face you need… 2 eyes + 1 head + 1 smile = 1 face How many smiles can we make with 18 eyes, 8 heads and 23 smiles?

The Limiting Reagent The other reactant is LIMITING - in other words: it is completely used up in the reaction the reagent in a chemical reaction that limits the amount of product that is able to form

Kitchen Stoichiometry – an analogy A recipe to make 30 candies requires the following: 2 cups water 5 cups sugar 1 cup strawberries Equation 2 c water + 5 c sugar + 1 c strawberries  30 candies

Questions: If you were given 10 cups of water, 10 cups of sugar and 10 cups of strawberries, which ingredient would limit the amount of candies being made? Which reagent is in excess?

Questions: How many candies can you make with the ingredients given in part a?

Questions: How much of the excess reagents will be left over?

Solving Limiting Reactant Problems If you see that a problem gives information for more than one reactant, you need to figure out which reactant you will run out of first (limiting reactant) before you can figure out how much produce is made.

Example: 25.54 of Mg reacts with 45.67g of oxygen gas. How much magnesium oxide will be produced? Steps: 1. Balance chemical equation 2. find the number of moles of each reactant 3. Divide each reactant by its coefficient 4. The smallest answer is the limiting reactant 5. Use the moles of the limiting reactant to do the rest of your calculations.

Your turn… Chlorine dioxide is a reactive oxidizing agent used to purify water. ClO2(g) + H2O(l)  HClO3(aq) + HCl(aq) A) If 71.00g of chlorine dioxide is mixed with 19.00g of water, what is the limiting reactant? B) What mass of HClO3 is expected in part a? C) how many formula units of HCl are expected in part a?

Question 2: Lithium nitride reacts with water to produce ammonia gas (NH3) and lithium hydroxide. If 4.5 g of lithium nitride and 5.8 g of water is available: How much (in grams) of the excess reagent will be left over?

Question 3: Nitrogen and hydrogen react to form ammonia. If 28.02 g of nitrogen gas is to be completely used up, what is a reasonable mass of hydrogen gas to use in the reaction?