# USING STOICHIOMETRY TO MAKE PREDICTIONS ABOUT REACTIONS Theme: Conservation Predictions % Yield Determining Limiting Reactant.

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USING STOICHIOMETRY TO MAKE PREDICTIONS ABOUT REACTIONS Theme: Conservation Predictions % Yield Determining Limiting Reactant

Recipe for ONE apple pie: 27 apple slices 3 cups flour 2/3 cup of sugar 1 teaspoon cinnamon 2 sticks butter You have in your pantry: 60 apple slices, 4 cups flour, 5 cup sugar, 2 tablespoons cinnamon, 4 sticks of butter. How many apple pies can you make?!? What is your limiting reactant?

Visual Representation of L. R.

Chemistry “Recipes” Recipe is balanced equation but chemists RARELY make the recipe as is. What do moles represent? What would be the limiting reactant in the following reaction if you started with 5 moles of aluminum and 12 moles of HCl?

Using the chemical recipe to predict product (without a limiting reactant) How much O 2 gas is generated from the complete decomposition of 5 g of H 2 O 2 ? 1. Determine balanced equation. 2. Determine moles of desired product as controlled by moles of given reactant. 1. Change moles of desired product to grams.

Practice Practice predicting outcomes of chemical reactions when given the reactant that controls the reaction.

% Yield The predicted or theoretical amount of product that can be produced in a chemical reaction is what is calculated by stoichiometry. The actual yield is what is actually produced in the reaction given uncontrollable sources of error. % Yield: How much of the theoretical yield did you actually get? (Actual yield/Theoretical Yield) X 100%

% Yield in a synthesis reaction. 0.5 g of Mg are reacted with unlimited oxygen according to the following balanced equation: What is the theoretical yield? If the actual yield of MgO is 0.73 g. What is the % yield? Why isn’t it 100%????

Finding the limiting reactant in a chemical “recipe” 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? 4 NH 3(g) + 5 O 2(g) 4 NO (g) + 6 H 2 O (g)

How to find the limiting reactant Take each reactant and figure out how many moles of the desired product would be produced. The reactant that produces FEWER MOLES of product is the limiting reactant and therefore controls the reaction!

Example worked out:

How to find out how much EXCESS there is of the other reactant: 1. Take the moles of limiting reactant and find out how many moles of product would be needed based on the “recipe”: 2. Change moles of excess reactant to grams. 3. Subtract grams needed from grams available to figure out how much would be left over.

Sample Problem worked out

Practice Limiting reactant problems

Our formal lab: We will react two aqueous solutions and determine which solution is the limiting reactant. We will predict how much product we can make (a solid precipitate) based on stoichimetry. We will use % yield to compare the actual product produced to the expected yield.

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