STOICHIOMETRY CHE 106 Chapter 3 3.7

Limiting Reagents Problems 3.7 In almost all chemical reactions – one reactant gets used up faster than the others. The leftover reactants are considered “excess” and the one we run out of first is called the limiting reagent.” Limiting Reagent:

Limiting Reagent Problems Pretend you have a job building tricycles. If you had 100 handle bars, 150 wheels, 25o pedals and 75 seats; how many tricycles could you build? What is the limiting reagent? 3.7

Limiting Reagent Problems 3.7

Limiting Reagent Problems 3.7 Given the equation: __ H 2 + __ O 2  __H 2 O If you have 10 moles of H2 and 7 moles of O 2 - what is the limiting reagent - how much of the excess will be leftover - what is the theoretical yield of H 2 O

Limiting Reagent 3.7 The first step to solving limiting reagent problems is to recognize that it is a limiting reagent problem rather than a regular stoichiometry problem. Regular stoichiometry problems will typically involve “excess” of one reactant. To spot a limiting reagent problem look for:

Limiting Reagent 3.7 There are several methods to solving limiting reagents problems. 1. Compare the reactants to each other – this must always be done in moles. 2. Compare the mole ratio of reactants in the balanced equation to the mole ratio of reactants available. 3. Calculate the theoretical yield of product produced by each reactant and choose the lesser amount.

Limiting Reagent 3.7 Example: A 50.6 g samples of magnsium hydroxide is reacted with 45.0 g of hydrochloric acid in a metathesis reaction to produce magnesium chloride and water. What is the theoretical yield of magnesium chloride? Balanced equation: * How can you tell this is a limiting reagent problem?*

Limiting Reagent 3.7 Method 1: Compare the moles needed of one of the reactants to the moles available. 1. Pick one of the reactants and calculate how many moles of the other would be needed to completely use up the reactant you picked. 2. Compare to the moles available.

Limiting Reagent Convert both to moles: 50.6g of Mg(OH) 2 : 45.0 g HCl: 2. Pick one to be limiting, and determine how much of the other you would need: 3. Compare what you need to what you have.

Limiting Reagent Use the limiting reagent to calculate the theoretical yield of the product MgCl 2 using the balanced equation. 5. You might have to do a percent yield. i.e. A chemist only obtained 55.4 g of MgCl 2, what is the percent yield?

Limiting Reagent 3.7 Method 2: Compare theoretical ratio of reactants to actual ratio of reactants. 1. Calculate moles of each reactant available. 2. Set up a proportion to determine what the actual mole ratio of reactant is and compare to the balanced equation mole ratio.

3.7 Method 3: Calculate the amount of your product – either in moles or grams, from the given amounts of each reactant. Whichever reactant gives you the lesser amount must be the limiting reactant g Mg(OH)2  moles or grams of MgCl g  moles or grams of MgCl 2

Limiting Reagent Problems 3.7 Given the reaction for the formation of B2H6, if 5.0g of LiH and 5.0g of BF3 were reacted how much of which reagent would be left over? Equation: LiH + BF 3  LiBF4 + B 2 H 6 We know: quantities (grams and moles) of starting materials; molar ratios Find: which reagent is completely consumed (limiting) and which is left over (excess)

Limiting Reagent Problems 3.7 Example: Equal weights 5.00 g of Zn and I 2 are mixed together to form ZnI 2. How much ZnI 2 is formed. How much of each reactant remains at the end of the reaction and which is the limiting reagent?

Limiting Reagent Problems 3.7 Given the metathesis reaction of 2.05 g of hydrogen sulfide with 1.84 g of sodium hydroxide, calculate the theoretical yield of sodium sulfide. What is the percent yield if the amt of sodium sulfide obtained was 3.65g?

Limiting Reagent Problems 3.7 Aluminum hydroxide reacts with sulfuric acid in a metathesis reaction. (a) write the balanced equation (b) which is the limiting reactant when moles of Al(OH) 3 and mol of H 2 SO 4 are allowed to react. (c) how many moles of Al 2 (SO 4 ) 3 can form under these conditions? (d) How many moles of the excess reactant remain after completion?

Limiting Reagent Problems 3.7 Example: One of the steps in the commerical process for converting ammonia to nitric acid is the conversion of NH3 to NO: 4 NH 3 (g) + 5 O 2 (g)  4 NO (g) + 6 H 2 O (g) In a certain experiment 2.00 g of NH 3 reacts with 2.50 g of O 2 (a) Which is the limiting reactant? (b) How many grams of NO and H 2 O form? (c) How many grams of the excess reactant remain after the limiting reagent is consumed. (d) show your calculations for (b) and (c) are consistent with the law of conservation of mass.