Chocolate Chip Cookies!!

1 cup butter

1/2 cup white sugar

1 cup packed brown sugar

1 teaspoon vanilla extract

2 eggs

2 1/2 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

2 cups semisweet chocolate chips

Makes 3 dozen

How many eggs are needed to make 3 dozen cookies?

How much butter is needed for the amount of chocolate chips used?

How many eggs would we need to make 9 dozen cookies?

2 Eggs

1 Cup

6 Eggs

Cookies and Chemistry…Huh!?!?

Just like chocolate chip cookies have recipes, chemists have recipes as well

Instead of calling them recipes, we call them chemical equations

Furthermore, instead of using cups and teaspoons, we use moles to measure amounts

Lastly, instead of eggs, butter, sugar, etc… we use chemical compounds as ingredients

Stoichiometry

Just what is stoichiometry?

The word stoichiometry is derived from two Greek words: stoicheion (meaning element) and metron (meaning measure).

Jeremias Benjamin Richter (1762-1807) was the first to lay down the

principles of stoichiometry. In 1792 he wrote:

“Stoichiometry is the science of measuring the quantitative

proportions or mass ratios in which chemical elements stand to one

another.”

This was verified by Antoine Lavoisier with the Law of Conservation of Mass

Why do we use stoichiometry?

Whenever chemists want to know how much of a desired product will be produced, they start with a balanced equation.

How does it work?

Then the chemist uses the equation to guide them in calculating the products.

Since stoichiometry is used to predict things in the “perfect world”, we can expect that the reaction in real life won’t create the maximum amount of product.

How do I use the chemical equation?

Looking at a chemical equation tells us how much of something you need to react with something else to get a product (like the cookie recipe)

The chemical equation is written like a mathematical expression. When balanced, the coefficients tell how many moles of each reactant or product is needed or expected.

The important thing is that the equation must be BALANCED!!!!

Example:

2H2 + O2 2H2O

One way to understand the information conveyed by a chemical equation is to convert the equation into an English sentence.

So: 2H2(g) + O2(g) 2H2O(l)

(the coefficients tell how many moles, atoms or molecules of each chemical are needed in the “recipe”)

2 “parts” hydrogen gas reacts with one “part” oxygen gas to form 2 “parts” liquid water.

Now, we can get a bunch of relationships – the “parts” could be: molecules, atoms, moles, or masses

The most important relationship for stoichiometry is the MOLE relationship!

What is a mole ratio?

Mole Ratios: The mole ratio is based on the

balanced chemical equation.You will use the coefficients to get the

mole ratio between two DIFFERENT substances

Molar Ratios

The expressing the ratio of moles in an equation using the coefficients

Ex. 2H2 + O2 2H2O

What is the molar ratio between H2 and O2?

2mol H2 or 1mol O2

1mol O2 2mol H2

What other mole ratios can we come up with?

Warning:

The coefficients of a reaction only give the ratio in which substances react.

They DO NOT directly tell you, HOW MUCH MASS is reacting.

You will need to convert from moles to mass using molar mass as the conversion factor.

How do you use a mole ratio?

The mole ratio is an equivalent which means that you can arrange the ratio in any way needed.

The mole ratio is the KEY to calculations based upon a chemical equation.

With it, you can calculate the amount of any other reactant in the equation and the maximum amount of product you can obtain.

Example of a mole ratio problem:

N2 + 3H2 2NH3

 Write the mole ratios for N2 and H2

Write the mole ratios for NH3 and H2

2

2 2

2

1 mol N 3 mol H or

3 mol H 1 mol N

3 2

2 3

2 mol NH 3 mol H or

3 mol H 2 mol NH

Stoichiometry has 5 basic steps:

Step 1: Write and balance the equation

Step 2: Write in all information given (make sure to identify what you are trying to find!)

Step 3: Convert everything to moles

Step 4: Use mole ratio to solve for what you are trying to find.

Step 5: Convert everything into the required unit if needed.

Moles to Moles

One way to change ion ore, Fe2O3, into metallic iron is to heat it together with hydrogen:

Fe2O3 + 3H2 2Fe + 3H2O

How many moles of iron are made from 25.0 moles of Fe2O3 reacting with excess hydrogen?

Moles to Moles

K: 25 moles Fe2O3

UK: ? moles Fe Use Dimensional Analysis :

25 moles Fe2O3 2mol Fe = 50. mol Fe

1 1mol Fe2O3

Examples of a mole-mole stoichiometric problem:

2H2 + O2 2H2O

 

1. How many moles of H2O are

produced when 5.00 moles of oxygen are burned in excess hydrogen gas?

Examples of a mole-mole stoichiometric problem continued:

2H2 + O2 2H2O

2. If 3.00 moles of H2O are produced, how many moles of oxygen must be consumed in a reaction with excess hydrogen gas?

Examples of a mole-mole stoichiometric problem continued:

2H2 + O2 2H2O

3. How many moles of hydrogen gas must be used, when reacting with 0.25 mol oxygen gas to create water.

Mole-Mass Conversions Most of the time in chemistry, the amounts are given in

grams instead of moles We STILL go through moles and use the mole ratio, but

now we also use molar mass (as a conversion factor) to get to grams

Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to produce sodium chloride?

2 Na + Cl2 2 NaCl

5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

Practice

Calculate the mass in grams of Iodine required to react completely with 0.50 moles of aluminum to make aluminum iodide.

2 Al + 3 I2 2 AlI3

Mass-Mole We can also start with mass and convert to moles of

product or another reactant We use molar mass and the mole ratio to get to

moles of the compound of interestEX: Calculate the number of moles of ethane (C2H6) needed

to produce 10.0 g of water

2 C2H6 + 7 O2 4 CO2 + 6 H20

10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185 mol C2H6

Practice

Calculate how many moles of oxygen are required to make 10.0 g of aluminum oxide

4 Al + 3 O2 2 Al2O3

Mass-Mass Conversions

Most often we are given a starting mass and want to find out the mass of a product we will get (called theoretical yield) or how much of another reactant we need to completely react with it (no leftover ingredients!)

1. We must go from grams to moles by using molar mass of the compound given

2. Go from moles of compound given to moles of compound needed using the mole ratio from the coefficients

3. Go back to grams of compound we are interested in by using molar mass of the compound you are looking for

Mass-Mass Conversion

Ex: Calculate how many grams of ammonia are produced when you react 2.00g of nitrogen with excess hydrogen.

N2 + 3 H2 2 NH3

Practice

How many grams of calcium nitride are produced when 2.00 g of calcium reacts with an excess of nitrogen?

3 Ca + N2 Ca3N2

Gas Stoichiometry (Volume-Volume)

1. Take volume of gas given.

2. Convert to moles using the fact that

22.4 L=1 mole

3. Change from moles of compound given to moles of new compound using molar ratio from the coefficients.

4. Change moles of new compound to volume using 22.4 L = 1 mole

Gases and Gases and StoichiometryStoichiometry

2 H2 H22OO2 2 (l) (l) 2 H 2 H22O (g) + OO (g) + O2 2 (g)(g)

3.5 L of H3.5 L of H22OO22 was decomposed in a flask. was decomposed in a flask.

What is the volume of OWhat is the volume of O22 made at STP? made at STP?

Bombardier beetle uses Bombardier beetle uses decomposition of decomposition of hydrogen peroxide to hydrogen peroxide to defend itself.defend itself.

Another Example

Find the number of liters of oxygen necessary for the combustion of 6.7 L of magnesium, assuming the reaction occurs at STP.

2 Mg + O2 2 MgO

LIMITING REACTANT

Limiting Reactant: Cookies1 cup butter

1/2 cup white sugar

1 cup packed brown sugar

1 teaspoon vanilla extract

2 eggs

2 1/2 cups all-purpose flour

1 teaspoon baking soda

1 teaspoon salt

2 cups semisweet chocolate chips

Makes 3 dozen

If we had the specified amount of ingredients listed, could we make 4 dozen cookies?

What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies?

What if we only had one egg, could we make 3 dozen cookies?

Limiting Reactant

Most of the time in chemistry we have more of one reactant than we need to completely use up other reactant.

That reactant is said to be in excess (there is too much).

The other reactant limits how much product we get. Once it runs out, the reaction s. This is called the limiting reactant.

Limiting Reactant

To find the correct answer, we have to try all of the reactants. We have to calculate how much of a product we can get from each of the reactants to determine which reactant is the limiting one.

The lower amount of a product is the correct answer. The reactant that makes the least amount of product is

the limiting reactant. Once you determine the limiting reactant, you should ALWAYS start with it!

Be sure to pick a product! You can’t compare to see which is greater and which is lower unless the product is the same!

Limiting Reactant: Example

10.0g of aluminum reacts with 35.0 grams of chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced?

2 Al + 3 Cl2 2 AlCl3 Start with Al:

Now Cl2:

10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3

27.0 g Al 2 mol Al 1 mol AlCl3

= 49.4g AlCl3

35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3

71.0 g Cl2 3 mol Cl2 1 mol AlCl3

= 43.9g AlCl3

LR Example Continued

We get 49.4g of aluminum chloride from the given amount of aluminum, but only 43.9g of aluminum chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction comes to a complete .

Limiting Reactant Practice

15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting and how much product is made.

2K + I2 2KI

Percent YieldPercent Yield

Theoretical Yield: The maximum quantity of product that can be obtained, according to the reaction stoichiometry, from a given quantity of reactant

Actual Yield: The quantity of product actuallyactually obtained from experimentation

•Percentage Yield = Actual Yield

Theoretical yield

X100%

What if I get an answer over 100%?

Percent yield can NEVER be over 100% This would conflict with the Law of

Conservation of Matter If you obtain a yield over 100% you either:

– A. Did your calculation incorrectly by putting theoretical yield over percent yield OR

– B. Made a mistake in the experiment

Percent yield problems:

We calculated that 19.6g of KI forms from 15.0g K and 15.0g of I2.

If we ran the experiment and only obtained 18.5g of KI what is the percent yield?

Limiting Reactant & Percent Yield: Recap

1. You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.

2. Convert ALL of the reactants to the SAME product (pick any product you choose.)

3. The lowest answer is the correct answer.4. The reactant that gave you the lowest answer is the

LIMITING REACTANT.5. The other reactant(s) are in EXCESS.6. To find the percent yield, divide the mass of what you

get from the experiment from mass of the product from the limiting reactant and multiply by 100%.