Notes #14, Ch. 9 Stoichiometry and Theoretical Yield.

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Presentation transcript:

Notes #14, Ch. 9 Stoichiometry and Theoretical Yield

What is Stoichiometry? It is a way to calculate how much of a chemical is consumed or produced in a chemical reaction using Dimensional Analysis.

Propanol (rubbing alcohol) C 3 H 7 OH, burns in air. Write and balance the reaction: 2C 3 H 7 OH + 9O 2  6CO 2 + 8H 2 O What are the Equivalents? 2 mols C 3 H 7 OH  9 mols O 2  6 mols CO 2  8 mols H 2 O This reaction will be used in the next few examples.

Example 1: How many moles of water are produced from the combustion of 20.0 g propanol? The arrow: 20.0 g C 3 H 7 OH  ? mols H 2 O Chemistry Tip of the Year: “When in doubt, go to moles!” Why? So you can use the mole ratios in the reaction as equivalents.

Solution: So, 1.33 mols of H 2 O is produced

Example 2: How many grams of oxygen is needed to react with 12.0 grams of propanol? 12.0 g C 3 H 7 OH  ? g O 2

Solution: So, 28.8 grams of oxygen is needed for the 12.0 grams of propanol to burn

Example 3: If 25.3 g of carbon dioxide is produced, how many grams of propanol was burned? 25.3g CO 2  ? g C 3 H 7 OH

Solution: So 11.5 grams of propanol was burned to produce the 25.3 grams of CO 2.

The Theoretical Yield Concept What if the reacts aren’t present in the exact ratio needed?

What theoretical yield is all about: Ordinarily, reactants are not present in the exact ratio needed to complete a reaction. One reactant is used up, or “limits the reaction.” The other reactant has some left over, unused.

Steps for calculating theoretical yield: 1. Calculate the yield as if the first reactant was limiting how much product is made. 2. Calculate the yield as if the second reactant was limiting the amount of product. 3. The Lesser of these two yields is the theoretical yield (T.Y.), or how much product you could ideally make. 4. The reactant in the D.A. which leads to the T.Y. is used up and is your “limiting reactant.”

Hot Dog Example: 8 buns + 10 wieners  ? hot dogs?

Step 1. 8 buns  ? hot dogs

Step wieners  ? hot dogs

Step 3: TY & LR Possible Yield from step 1 was 8 hot dogs Possible Yield from step 2 was 10 hot dogs You can make lesser yield, 8 hot dogs. This is the Theoretical Yield The buns are the limiting reactant. There are extra wienies.

Skateboard Factory Example: You inherit the remnants of a skateboard store which went under. You have:  167 wheels  93 wheel “trucks”  71 boards How many skateboards can you make?

So, 41 skateboards is the theoretical yield. The limiting reactant is the wheels.

Chemistry Example: 2Ag + I 2  2AgI If you start with 1.00 g of each reactant, how much silver iodide could you make?

2Ag + I 2  2AgI Step g Ag  ? g AgI Possible Yield: 2.18 g AgI

2Ag + I 2  2AgI Step g I 2  ? g AgI Possible Yield: 1.85 g AgI

Step 3: So, the theoretical Yield is 1.85 g AgI, lesser of the two. Limiting Reactant is iodine, it’s all used up. There’s left over silver. The actual yield is always a little less, due to unwanted side reactions and loss.

Percent Yield Example: If you do the reaction in lab, mixing 1.0 grams of each reactant, and you get 1.50 g AgI in your filter paper, what is the actual percent yield?