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Bell Work
“We’re Cookin’ Now”
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? 2 eggs
How much butter is needed for the amount of chocolate chips used?
1 cup
How many eggs would we need to make 9 dozen cookies? 6 eggs
How much brown sugar would I need if I was cutting the recipe in half?
1/2 cup
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
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?
To reduce waste
To predict amounts of products
To make money
How does it work?
Using balanced equations we can calculate the amounts of substances in the chemical reaction.
Since stoichiometry is used to predict amounts where there are no error, we can expect that the reaction in the lab won’t create the same amount of product predicted.
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”)
Hydrogen gas reacts with oxygen gas to form liquid water.
Now, we can get a bunch of relationships: molecules, atoms, moles, and masses
The most important relationship is the mole relationship!
We call that the MOLE RATIO
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
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.
Examples of 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 in anyway tell you HOW MUCH is reacting.
Example of a mole ratio problem:
N2 + 3H2 2NH3
Write the molar ratios for N2 and H2.
Write the molar ratios for NH3 and H2.
Stoichiometry has 5 basic steps:
Step 1: Make sure the equation is balanced
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 (you may also use a proportion to solve the problem.)
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 + H2 Fe + H2O
How many moles of iron are made from 25 moles of Fe2O3?
Moles to Moles
Given: 25 moles Fe2O3
Find: ? Moles Fe Ratio: What mole ratio will we use?
25 moles Fe2O3 2mol Fe = 50 mol Fe
1mol Fe2O3
Examples of a mole-mole stoichiometric problem:
2H2 + O2 2H2O
1. How many moles of H2O are
produced when 5 moles of oxygen are used?
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?
Examples of a mole-mole stoichiometric problem continued:
2H2 + O2 2H2O
3. How many moles of hydrogen gas must be used, given the data in problem #2.
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 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.
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 interest– 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%.