The Wisdom of GallagherWhy are there Interstate Highways in Hawaii?

Why are there floatation devices under plane seats instead of parachutes?

Why do we drive on parkways and park on driveways?

Why do hot dogs come ten to a package and hot dog buns only eight?

Limiting Reactants

Percentage Yield

Hot Dogs in the NewsTakeru Kobayashi of Japan downed 44½ hot dogs in 12 minutes.

Source: CNN.com

WHAT IF…

One hot dog = one hot dog + one bun.

Mr. Kobayashi didn’t do his math correctly. He bought 5 packs of hot dogs (10 per package) and 5 packs of hot dog buns (8 per package). How many hot dogs (according to the official formula) could he have eaten?

Hot Dogs in the News

Source: CNN.com

WHAT IF…

One hot dog = one hot dog + one bun.

Mr. Kobayashi didn’t do his math correctly. He bought 5 packs of hot dogs (10 per package) and 5 packs of hot dog buns (8 per package). How many hot dogs (according to the official formula) could he have eaten?

5 hot dog packs x 10 hot dogs

1 hot dog pack

50 hot dogs=

5 bun packs 8 buns

1 bun pack

40 buns=x

40 possible hot dogs

• 2.25 cups flour• 8 Tbsp butter• 0.5 cups shortening• 0.75 cups sugar• 0.75 cups brown sugar• 1 tsp salt• 1 tsp baking soda• 1 tsp vanilla• 0.5 cups Egg Beaters• 12 oz. Chocolate chips

For 1 batch:In my pantry, I have:• 5.5 cups of flour• 16 Tbsp of butter• lots of everything else

How many batches of cookies can I make?

Let’s Revisit the Cookies (again)…

• 2.25 cups flour• 8 Tbsp butter• 0.5 cups shortening• 0.75 cups sugar• 0.75 cups brown sugar• 1 tsp salt• 1 tsp baking soda• 1 tsp vanilla• 0.5 cups Egg Beaters• 12 oz. Chocolate chips

For 1 batch: How many batches of cookies can I make?

Let’s Revisit the Cookies (again)…

5.5 c flour x2.25 c flour

1 batch cookies=

2.4 batches

16 Tbsp butter x1 batch cookies

8 Tbsp butter=

2.0 batches

EXCESS

LIMITING

Now I Want to Bake a Cake!But do I have all the ingredients I need?

How much flour do I have left after baking all those cookies?

5.5 c flour x2.25 c flour

1 batch cookies=

16 Tbsp butter x1 batch

8 Tbsp butter

=

2.4 batches of cookies

2.0 batches of cookies

GONE!

SOME FLOUR LEFT OVER…

2.0 batches x 2.25 cups flour

1 batch cookies=

4.5 cups flour used

5.5 cups – 4.5 cups = 1.0 cups left

• In the laboratory, a reaction is rarely carried out with exactly the required amounts of each reactant. In most cases, one or more of the reactants is present in excess, that is, there is more than the exact amount required to react.

• Once one of the reactants is used up, no more product can be formed. The substance that is completely used up first in a reaction is the limiting reactant.

• The limiting reactant (reagent) is the reactant that limits the amounts of the other reactants that can combine and the amount of product that can form in a chemical reaction.

• The excess reactant is the substance that is not used up completely in a reaction.

Limiting Reactants in Chemistry5.0 moles of chlorine gas react with 5.0 moles of sodium to produce sodium chloride.

Which reagent is the limiting reactant? How much of the excess reactant is left

over?

Cl2 (g) + Na NaCl2 2

2 givens = 2 equations!5.0 mol Cl2 x1 mol Cl2

2 mol NaCl = 10. mol NaCl

5.0 mol Na x 2 mol NaCl2 mol Na

= 5.0 mol NaCl

EXCESS

LIMITING

5.0 mol Na x2 mol Na1 mol Cl2 =

2.5 mol Cl2

5.0 mol Cl2 given

2.5 mol Cl2 used

2.5 mol Cl2 left

Practice Problems1. 3 CuSO4 + 2 Al Al2(SO4)3 3 Cu+

20.0 g CuSO4

20.0 g Al

x

x

26.98 g Al

1 mol Al

1 mol CuSO4

159.61 g CuSO4

3 mol Cu

3 mol Cu

3 mol CuSO4

2 mol Al

x

x =

= 0.125 mol Cu

1.11 mol Cu

20.0 g CuSO4 x159.61 g CuSO4

1 mol CuSO4 x 26.98 g Al

1 mol Al

=

2.25 g Al

USED20.0 g Al – 2.25 g Al = 17.8 g Al EXCESS

3 mol CuSO4

2 mol Al x

In this reaction, 20.0 g of CuSO4 reacts with 20.0 g of Al. Which reactant is limiting?How much of the excess reactant is left over?

Practice Problems2. 2 H2 (g) + O2 (g) 2 H2O

5.0 g H2 x1 mol H2

2.02 g H2

2 mol H2O

2 mol H2

x = 2.5 mol H2O

5.0 g O2 x

32.00 g O2

1 mol O2 2 mol H2Ox = 0.31 mol H2O

1 mol O2

In this reaction, 5.0 g of H2 reacts with 5.0 g O2. Which reactant is limiting? How many grams of H2O are produced?

0.31 mol H2O

smallest numberof moles

x18.02 g H2O

1 mol H2O= g H2O5.6

On Perfection“Perfection never exists in reality, but only in our

dreams.”

- Dr. Rudolf Dreikurs“Perfection is our goal, excellence will be

tolerated.”- J. Yahl

Johnny took a quiz yesterday. He missed 4 questions and earned 63 points out of 70.

- Was he perfect?- What was his possible score?- What was his actual percent score?

Get Real!

Get Real!Mrs. Wilson ran a reaction in her lab yesterday. She predicted that 183 grams of product would be formed. The reaction only yielded 162 grams of product.

- Was her reaction perfect?

- What was the percent yield?

162 grams

183 gramsx 100 = 88.5 %

162 grams = actual yield183 grams = theoretical yield

Percentage Yield • The theoretical yield is the maximum amount of product

that can be produced from a given amount of reactant.• In most chemical reactions, the amount of product

obtained is less than the theoretical yield.• The measured amount of product obtained from a reaction

is called the actual yield of the product.• The percent yield is the ratio of the actual yield to the

theoretical yield, multiplied by 100.

100xYieldlTheoretica

YieldActualYieldPercent

Percentage Yield, continued

Sample Problem HChlorobenzene, C6H5Cl, is used in the production of many important chemicals, such as aspirin, dyes, and disinfectants. One industrial method of preparing chlorobenzene is to react benzene, C6H6, with chlorine, as represented by the following equation.

C6H6 (l) + Cl2(g) → C6H5Cl(l) + HCl(g)

When 36.8 g C6H6 react with an excess of Cl2, the actual yield of C6H5Cl is 38.8 g.What is the percentage yield of C6H5Cl?

Percentage Yield, continuedSample Problem H Solution

C6H6 (l) + Cl2(g) → C6H5Cl(l) + HCl(g)

Given: mass of C6H6 = 36.8 gmass of Cl2 = excessactual yield of C6H5Cl = 38.8 g

Unknown: percentage yield of C6H5ClSolution:Theoretical yield

6 6 6 5 6 56 6 6 5

6 6 6 6 6 5

mol C H mol C H Cl g C H Clg C H g C H Cl

g C H mol C H mol C H Cl

molar mass mol ratio molar mass

38.8 gpercentage yield 100

53.0 g73.2%

Percentage Yield, continuedSample Problem H Solution, continued

C6H6(l) + Cl2(g) → C6H5Cl(l) + HCl(g)Theoretical yield

6 6 6 5 6 56 6

6 6 6 6 6 5

6 5

1 mol C H 1 mol C H Cl 112.56 g C H Cl36.8 g C H

78.12 g C H 1 mol C H 1 mol C H Cl

53.0 g C H Cl

6 5

actual yieldpercentage yield C H Cl 100

theorectical yield

Percentage yield