Starter A sandwich consists of two slices of bread, 3 slices of meat, and one slice of cheese. For each of the following amounts, determine the number of sandwiches that can be made and what is left over: 6 bread, 10 meat, 4 cheese slices 10 bread, 6 meat, 8 cheese slices 25 bread, 40 meat, 12 cheese slices

Starter 6 bread, 10 meat, 4 cheese slices 3 sandwiches 0 bread, 1 meat, 1 cheese 10 bread, 6 meat, 8 cheese slices 2 sandwiches 6 bread, 0 meat, 6 cheese 25 bread, 40 meat, 12 cheese slices 12 sandwiches 1 bread, 4 meat, 0 cheese

Ch. 9 Stoichiometry 9.3 Limiting Reactant

Why is there a limiting reactant? a reaction rarely has exactly the right amount of each reactant usually have some left over limiting reactant reactant that limits the amount of product created always completely used up excess reactant reactant not completely used up

When do you have to find a LR? whenever two amounts of reactants are given in a problem when only one amount of reactant is given in a problem, then the other is assumed to be in excess

Finding Limiting Reactant using reactants Figure out how much you need of B if you use up all of A Convert grams A to grams B using stoichiometry You may start with either reactant Determine whether you will have enough If you don’t have enough of B, then B is LR If you don’t have enough of A, then A is LR

Finding Limiting Reactant using products Convert each of the reactant amounts into an amount of product (doesn’t matter which product) Compare product amounts and find lowest amount. Whichever reactant led to lowest product amount is LR

Example 1 The reaction begins with 2.51 g of HF and 4.56 g of SiO2. What is the limiting reactant and the excess reactant? How much excess reactant will be left over? Write the balanced chemical equation SiO2(s) + 4HF(g)  SiF4(g) + 2H2O(l)

Example 1 Find the number of moles available of each reactant:

Example 1 If we use up all of the HF, how much SiO2 will we need to go with it? Do we have enough SiO2? 0.0759 mol available > 0.0313 mol needed YES- there will be some left over Limiting Reactant : HF

How much of the product can be formed? Start conversion with amount of limiting reactant. Convert to amount of product using stoichiometry

Example 1 How many grams of water could be formed? Convert grams of HF to moles. Convert moles of HF to moles of water. Convert moles to grams using molar mass.

Example 2 A reaction was done with 36.8 g C6H6 and 41.0 g of O2. Write the balanced chemical equation 2C6H6 + 15O2  12CO2 + 6H2O What is the limiting reactant and how much of each product can be produced?

Example 2 Reactant Method = 113 g O2 Product Method = 25.5 g H2O 36.8 g C6H6 1 mol C6H6 15 mol O2 32 g O2 78.12 g C6H6 2 mol C6H6 1 mol O2 = 113 g O2 Because 113 g O2 is greater than what we have available, O2 is LR Product Method 36.8 g C6H6 1 mol C6H6 6 mol H2O 18.02 g H2O 78.12 g C6H6 2 mol C6H6 1 mol H2O = 25.5 g H2O 41.0 g O2 1 mol O2 6 mol H2O 18.02 g H2O 32 g O2 15 mol O2 1 mol H2O = 9.24 g H2O Because 9.24 is less than 25.5, O2 is the LR

Example 2 Once you find the LR, you can then calculate the amount of each product formed… Amount of H2O 41.0 g O2 1 mol O2 6 mol H2O 18.02 g H2O 32 g O2 15 mol O2 1 mol H2O = 9.24 g H2O Amount of CO2 41.0 g O2 1 mol O2 12 mol CO2 44.01 g CO2 32 g O2 15 mol O2 1 mol CO2 = 45.11 g CO2

Example 3 If the reaction below begins with 51.03 grams of Fe and 37.5 grams of oxygen, what is the limiting reactant? 4Fe(s) + 3O2(g)  2Fe2O3

Example 3 How many grams of oxygen will be left over after the reaction? How many grams of iron (III) oxide can be formed?