Stoichiometry Chemistry IH: Chapter 9 Stoichiometry The method of measuring amounts of substances and relating them to each other.

StoichiometryStoichiometryChemistry IH: Chapter 9Chemistry IH: Chapter 9

Stoichiometry

The method of measuring amounts of

substances and relating them to each

other.

Chocolate Chip Cookies!!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 dozenHow 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?

How much brown sugar would I need if I had 1 ½ cups white sugar?

Cookies and Chemistry…Huh!?!?Cookies and Chemistry…Huh!?!?

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

Instead of calling them Instead of calling them recipes, we call them reaction recipes, we call them reaction equationsequations

Furthermore, instead of using Furthermore, instead of using cups and teaspoons, we use cups and teaspoons, we use molesmoles

Last, instead of eggs, butter, Last, instead of eggs, butter, sugar, etc. we use chemical sugar, etc. we use chemical compounds as ingredientscompounds as ingredients

Chemistry RecipesChemistry RecipesLooking at a reaction tells Looking at a reaction tells

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

Chemistry RecipesChemistry RecipesBe sure you have a balanced Be sure you have a balanced

reaction before you startreaction before you startExample: 2 Na + ClExample: 2 Na + Cl2 2 2 NaCl 2 NaClThis reaction tells us that by mixing This reaction tells us that by mixing

2 moles of Na with 1 mole of Cl we 2 moles of Na with 1 mole of Cl we will get 2 moles of sodium chloridewill get 2 moles of sodium chloride

What if we wanted 4 moles of What if we wanted 4 moles of NaCl? 10 moles? 50 moles?NaCl? 10 moles? 50 moles?

PracticePractice

Write the balanced equation for hydrogen gas Write the balanced equation for hydrogen gas reacting with oxygen gas.reacting with oxygen gas.

2 H2 H22 + O + O22 2 H 2 H22OO

How many moles of reactants needed?How many moles of reactants needed?

What if we wanted 4 moles of water?What if we wanted 4 moles of water?

What if we had 3 moles of OWhat if we had 3 moles of O22 , how , how

much Hmuch H22 would we need to react and would we need to react and

how much water would we get?how much water would we get?

What if we had 50 moles of HWhat if we had 50 moles of H22? ?

Mole RatiosMole Ratios Mole ratios come from the coefficients of the Mole ratios come from the coefficients of the

balanced equation.balanced equation.

Ex: 2 Na + ClEx: 2 Na + Cl22 2 NaCl 2 NaCl

2 mol Na 2 mol Na & & 1 mol Cl1 mol Cl22

1 mol Cl1 mol Cl22 2 mol Na2 mol Na

What are the 4 other mole ratios we can What are the 4 other mole ratios we can obtain from this equation?obtain from this equation?

2 mol Na & 2 mol NaCl 1 mol Cl2& 2 mol NaCl 2 mol NaCl 2 mol Na2 mol NaCl 1 mol Cl2

Mole RatiosMole Ratios They can be used to calculate the moles of They can be used to calculate the moles of

one chemical from the given amount of a one chemical from the given amount of a different chemical different chemical

Example: How many moles of chlorine are Example: How many moles of chlorine are needed to react with 5 moles of sodium needed to react with 5 moles of sodium (without any sodium left over)?(without any sodium left over)?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5 moles Na = moles Cl21 mol Cl2

2 mol Na

2.5

Mole-Mole ConversionsMole-Mole ConversionsHow many moles of sodium chloride will How many moles of sodium chloride will

be produced if you react 2.6 moles of be produced if you react 2.6 moles of chlorine gas with an excess (more than chlorine gas with an excess (more than you need) of sodium metal?you need) of sodium metal?

Mole-Mass ConversionsMole-Mass ConversionsMost of the time in chemistry, the amounts Most of the time in chemistry, the amounts

are given in grams instead of molesare given in grams instead of moles

We still go through moles and use the We still go through moles and use the mole ratiomole ratio

but now we also use molar mass to get to but now we also use molar mass to get to gramsgrams

Mole-Mass ConversionsMole-Mass ConversionsExample: How many grams of Example: How many grams of chlorine are required to react chlorine are required to react completely with 5.00 moles of completely with 5.00 moles of sodium to produce sodium sodium to produce sodium chloride?chloride?

22 Na + ( Na + (11)Cl)Cl22 2 2

NaClNaCl5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

Mass-MoleMass-MoleWe can also start with mass and convert to We can also start with mass and convert to

moles of product or another reactantmoles of product or another reactantWe use molar mass and the mole ratio to get We use molar mass and the mole ratio to get

to moles of the compound of interestto moles of the compound of interestEx: Calculate the number of moles of ethane Ex: Calculate the number of moles of ethane

(C(C22HH66) needed to produce 10.0 g of water) needed to produce 10.0 g of water

22 C C22HH66 + 7 O + 7 O22 4 CO 4 CO22 + + 66 H H220 0

10.0 g H2O

= mol C2H6 1 mol H2O 2 mol C2H

18.0 g H2O 6 mol H2O

0.185

PracticePractice

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

PracticePracticeCalculate how many moles of oxygen are Calculate how many moles of oxygen are

required to make 10.0 g of aluminum oxiderequired to make 10.0 g of aluminum oxide

Mass-Mass ConversionsMass-Mass Conversions

Most often we are given a starting mass Most often we are given a starting mass and want to find out the mass of a product and want to find out the mass of a product we will get we will get

OR how much of another reactant we OR how much of another reactant we need to completely react with it (no need to completely react with it (no leftover ingredients!)leftover ingredients!)

Now we must go from use 3 steps!Now we must go from use 3 steps!

Mass-Mass ConversionMass-Mass ConversionEx. Calculate how many grams of Ex. Calculate how many grams of

ammonia are produced when you react ammonia are produced when you react 2.00g of nitrogen with excess hydrogen.2.00g of nitrogen with excess hydrogen.

NN2 2 + 3 H+ 3 H2 2 2 NH 2 NH33

2.00g N2 1 mol N2 2 mol NH3 17.06g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

PracticePractice

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

Similar ConversionsSimilar Conversions

We have been using grams as our starting We have been using grams as our starting or ending amount.or ending amount.

It is possible to use the prior mole relation-It is possible to use the prior mole relation-ships we have learned in conversions.ships we have learned in conversions.

Avogadro’s numberAvogadro’s number

Molar VolumeMolar Volume

Avogadro’s numberAvogadro’s number

Ex: How many molecules of ClEx: How many molecules of Cl22 are are

needed to react fully with 4.0 g of Na in needed to react fully with 4.0 g of Na in the following reaction?the following reaction?

22 Na + ( Na + (11)Cl)Cl22 2 NaCl 2 NaCl

5.00 moles Na 1 mol Cl2 6.02 x 1023 molecules Cl2

2 mol Na 1 mol Cl2

= 3.00 x 10 24 molecules Cl2

Molar Volume CalculationsMolar Volume Calculations

If you want to produce 10.L of HIf you want to produce 10.L of H22, how , how

many moles of Na do you need to have?many moles of Na do you need to have?22Na + 2H(OH) Na + 2H(OH) 2NaOH +( 2NaOH +(11)H)H22

10.L H2 x _____ x _____ = __moles Na10.L H2 x _____ x _____ = __moles Na1 mol H2

22.4 L H2

2 mol Na 1 mol H2

Molar VolumeMolar Volume

Your client requires 5000g of ammonia. Your client requires 5000g of ammonia. What volume of nitrogen gas is required to What volume of nitrogen gas is required to produce this amount of product?produce this amount of product?

NN2 2 + 3 H+ 3 H2 2 2 NH 2 NH33

5000 g NH3 1 mol NH3 1 mol N2 22.4 L N2

1 17 g NH3 2 mol NH3 1 mol N2

Limiting Reactant: CookiesLimiting 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 all ingredients listed, could we make 4 dozen cookies?

What if we had 4 eggs and 3x 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 ReactantLimiting Reactant

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

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

The other reactant limits how much product The other reactant limits how much product we get. Once it runs out, the reaction we get. Once it runs out, the reaction

s. This is called the s. This is called the limiting reactantlimiting reactant..

Limiting ReactantLimiting Reactant To find the limiting reactant, must try To find the limiting reactant, must try allall of of

the reactants to calculate how much of the reactants to calculate how much of aa product we can get in each case. product we can get in each case.

The The lowerlower amount of amount of aa product is the correct product is the correct answer.answer.

This is the limiting reactant.This is the limiting reactant.

Be sure to use the same product when Be sure to use the same product when comparing reactants.comparing reactants.

Limiting Reactant: ExampleLimiting Reactant: Example 10.0g of aluminum reacts with 35.0 grams of 10.0g of aluminum reacts with 35.0 grams of

chlorine gas to produce aluminum chloride. chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, Which reactant is limiting, which is in excess, and how much product is produced?and how much product is produced?

2 Al + 3 Cl2 Al + 3 Cl22 2 AlCl 2 AlCl33 Start with Al:Start with Al:

Now ClNow Cl22::

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 ContinuedLR Example Continued

We get We get 49.4g49.4g of aluminum chloride from the given of aluminum chloride from the given amount of aluminum, but only amount of aluminum, but only 43.9g43.9g of aluminum of aluminum chloride from the given amount of chlorine. chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction the 35.0g of chlorine is used up, the reaction comes to a complete .comes to a complete .

Limiting Reactant PracticeLimiting Reactant Practice

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

2 K + I2 K + I22 2 KI 2 KI

Finding Excess PracticeFinding Excess Practice 15.0 g of potassium reacts with 15.0 g of iodine. 15.0 g of potassium reacts with 15.0 g of iodine.

2 K + I2 K + I22 2 KI 2 KI We found that Iodine is the limiting reactant, and We found that Iodine is the limiting reactant, and

19.6 g of potassium iodide are produced.19.6 g of potassium iodide are produced.

15.0 g I2 1 mol I2 2 mol K 39.1 g K

254 g I2 1 mol I2 1 mol K= 4.62 g K USED!

15.0 g K – 4.62 g K = 10.38 g K EXCESS

Given amount of excess reactant

Amount of excess reactant actually used

Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!

Limiting Reactant: RecapLimiting Reactant: Recap

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

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

3.3. The lowest answer is the correct answer.The lowest answer is the correct answer.4.4. The reactant that gave you the lowest answer is the The reactant that gave you the lowest answer is the

LIMITING REACTANT.LIMITING REACTANT.5.5. The other reactant(s) are in EXCESS.The other reactant(s) are in EXCESS.6.6. To find the amount of excess, subtract the amount used To find the amount of excess, subtract the amount used

from the given amount.from the given amount.7.7. If you have to find more than one product, be sure to If you have to find more than one product, be sure to

start with the limiting reactant. You don’t have to start with the limiting reactant. You don’t have to determine which is the LR over and over again!determine which is the LR over and over again!

Percent YieldPercent Yield

Theoretical Yield- the amount of product Theoretical Yield- the amount of product calculated (expected.) calculated (expected.)

Actual Yield- the amount of product Actual Yield- the amount of product obtained.obtained.

Percent Yield = Percent Yield = Actual YieldActual Yield x100x100

Theoretical YieldTheoretical Yield