STOICHIOMETRY Mass relationships between reactants and products in a chemical reaction.

STOICHIOMETRY

Mass relationships between reactants and products in a chemical reaction

STOICHIOMETRYStoichiometry: looks at what

mass of products (in grams) is produced when you start with a certain mass of reactant

Tells you how much you make when you start with a certain amount of stuff.

EXAMPLELet’s say you start with the

following recipe:

2 eggs + 1 cup of flour + 2 tubs of frosting 1 cake + 3 cupcakes

Write this down and answer the following questions:

QUESTIONS1. If you start with 6 eggs, how many

cupcakes would you make?2. If you started with 4 cups of flour, how

many cakes could you make?3. If you made 6 cupcakes, how many

tubs of frosting did you need?4. If you started with 1 ½ cups of flour,

how many cupcakes could you make?

ANSWERS1. 9 Cupcakes2. 4 Cakes3. 4 Tubs of frosting4. 4 ½ Cupcakes

Question: What method did you use to answer these questions?

STOICHIOMETRY Answer: You used the ratio between

one ingredient and what you made.

How does this relate? Chemistry is like cooking. Chemical

reactions occur in exact ratios. As a result, we can also predict how much stuff we make.

EXAMPLEWrite the balanced equation for

the following:

Sodium and chlorine mix to form sodium chloride.

BALANCED EQUATION2Na + Cl2 2NaCl

In this reaction, you have a ratio of reactants and products.

They are always the same.ANSWER THE FOLLOWING

QUESTIONS?

QUESTIONS If you start with 4 Na, how much NaCl

do you make?

If you start with 4 Cl2, how much NaCl do you make?

If you made 5 NaCl, how much Cl2 did you start with?

ANSWERS1. 4 NaCl2. 8 NaCl3. 2.5 Cl2

In each case you take the ratio from the chemical reaction

IMPORTANT NOTE: WHAT ARE THE NUMBERS A RATIO OF???

MOLAR RATIO

The numbers in front of the compounds are ALWAYS RATIOS OF MOLES.

USING DIMENSIONAL ANALYSIS

To solve for the amounts of reactants and products, we use the balanced chemical equation

Just like every other dimensional analysis problem, you start by writing what you start with and cancel out the stinking units.

EXAMPLE Let’s use the previous reaction:

2Na + Cl2 2 NaCl

If you start with 3.75 moles of Cl2, how many moles of NaCl can you make?

Start with what you know: 3.75 moles of Cl2

ANSWER YOU USE THE RATIO OF MOLES AS

THE CONVERSION FACTOR 3.75 moles Cl2 | 2 moles of NaCl =

1 mole of Cl2

7.50 moles of NaCl IMPORTANT NOTE: Not only do you

have to put the units in dimensional analysis, you also have to put the name of the compound.

TRY THESE1. H2 + N2 NH3

For the above reaction, balance the reaction. Then, if you start with 2.5 moles of H2, how many moles of NH3 do you make?

2. Zn + HCl ZnCl2 + H2 For the above reaction, balance the

reaction. If you start with 5.0 moles of Zn, how many moles of HCl do you need to complete the reaction?

ANSWERS1. 1.7 moles of NH3

2. 10 moles of HCl

NOTE: In each case the units and name of the compound were included.

NOTE: In #2, you can also find the amount of other reactants from a starting reactant.

MOLECULES MOLES MASS

Review:

How do you convert from molecules to moles?

How do you convert from mass to moles?

MOLECULES MOLES MASS

Molecules Moles

1mole = 6.02x1023 molecules

Mass Moles

#grams = 1 mole (periodic table)

HOW DOES THIS WORK FOR US?

Reminder: Chemical reactions are ratios of moles.

To use molecules or mass, YOU MUST FIRST CONVERT TO MOLES

EXAMPLEYou begin with the following

reaction:MgBr2 + 2NaOH 2NaBr + Mg(OH)2

If you start with 1.2 x 1024 molecules of NaOH, how many moles of Mg(OH)2 will you produce?

STEP 1 Convert molecules to moles (ALL

CHEMICAL REACTIONS ARE RATIOS OF MOLES ONLY!!!)

1.2 x 1024 molecules | 1 mole __ =

6.02x1023 molecules

2.0 moles

STEP 2 MgBr2 + 2NaOH 2NaBr + Mg(OH)2

2.0moles NaOH | 1 moles Mg(OH)2 =2 moles NaOH

1 mole of Mg(OH)2

YOU CAN SET THIS UP IN ONE DIMENSIOAL ANALYSIS

EXAMPLE #2You begin with the following

reaction:2H2 + O2 2H2O

If you start with 96.0g of O2, how many moles of H2O do you produce?

STEP 1 Convert mass to moles first

(CHEMICAL REACTIONS ARE ALWAYS RATIOS OF MOLES!!!)

96 g O2 | 1 mole O2 | 2 moles H2O | 32 g O2 | 1 mole O2

= 6 moles of H2O

NOTICE: EVERY NUMBER HAS A UNIT AND THE NAME OF THE COMPOUND

TRY THISFor the following balance the

equation and then answer the question:

AgNO3 + CaBr2 AgBr + Ca(NO3)2

If you start with 5.00 g of CaBr2, how many moles of AgBr do you make?

ANSWER 5.00g CaBr2 | 1 mole CaBr2 | 2 moles AgBr

| 200g of CaBr2| 1 mole CaBr2

= .0500 moles of AgBr

NOTE: AGAIN, YOU CANCEL OUT THE UNITS AND THE NAME OF THE COMPOUD!!!

MASS TO MASS The final goal of stoichiometry:

Predict what mass of substance is produced or used by having a balanced chemical reaction

SO FAR . . . We’ve looked at going from a starting

number of moles and calculating the number of moles we produce

We’ve looked at going from a starting mass or number of molecules and calculating the number of moles we produce

NOW: We are going to start with a mass and calculate what mass we produce

MASS TO MASS REMINDER: A CHEMICAL REACTION

SHOWS A RATIO OF MOLES!!!! To go from mass to mass, we must use

the following format: Mass Moles Moles Mass

molar mass chemical reaction molar mass

EXAMPLEGiven the following chemical reaction:

NaOH + HCl NaCl + H2O

If you start with 345g of HCl, how many grams of NaCl do you produce?

STEP 1 Find the number of moles of HCl345g HCl | 1 mole of HCl_

| 36.46 g of HCl

= 9.46 moles of HCl

STEP 2 Find the number of moles of NaCl

produced:9.46 moles of HCl | 1 mole of NaCl

| 1 mole of HCl

= 9.46 moles of NaCl

STEP 3 Find the mass of NaCl9.46 moles of NaCl | 58.44g of NaCl

| 1 mole of NaCl= 553g of NaCl

NOTE: In each conversion, the name of the compound was included.

TRY THISBalance the following reaction:

P + O2 PO5

If you produce 155g of PO5, what mass of O2 did you start with (in grams)?

CONCEPT MAPMolecule/Atoms Molecule/Atoms

1 mole = 6.02x1023

Moles Balanced chemical equation Moles molar mass (#g =1 mole)

Mass Mass density (#g = 1mL)

Volume Volume

TRY THIS Translate and balance:

Iron (III) chloride combines with bromine to form iron (III) bromide and chlorine.

If you start with 5.66x1023 molecules of bromine, what volume of iron (III) bromide do you produce? The density of iron (III) bromide is 4.50g/mL.

ANSWER

41.2 mL of FeBr3


Aluminum combines with hydrobromic acid to form aluminum bromide and hydrogen gas.

If you start with 22.4mL of aluminum what volume of aluminum bromide do you produce? The density of aluminum is 7.87g/mL. The density of aluminum bromide is 2.67g/mL.

ANSWER

653 mL of AlBr3


Sulfuric acid decomposes to form sulfur dioxide, oxygen and hydrogen.

If you start with 125 mL of sulfuric acid, how many molecules of oxygen do you produce? The density of sulfuric acid is 1.84g/mL.

ANSWER

1.41 X 1024 molecules O2

PERCENT YIELD In theory, you should always produce a

certain mass of product if you start with a specific mass of reactant.

In reality, things aren’t perfect: You could measure inaccurately Some product could be lost during the

reaction The reaction doesn’t go to completion

PERCENT YIELD As a result . . .

YOU ALWAYS PRODUCE LESS PRODUCT THAN IS PREDICTED

There is a way to measure how much product that you did produce:

PERCENT YIELD

CALCULATING PERCENT YIELD

To calculate the percent yield, you first need to calculate the theoretical amount of product that should be produced Just like how we have been doing.

You then have to measure the actual amount or product made (this value will be give)

CALCULATING PERCENT YIELD

Then you use the following formula:

Actual amount of product x 100Theoretical amount of product

The closer the number is to 100%, the closer you are to the theoretical value

EXAMPLE You start with the following reaction:

NaOH + HCl NaCl + H2O

If you start with 75.0g of HCl and produce 35.0g of H2O, what is your percent yield for H2O?

STEP 2 Use the amount of product measured to

calculate %yield Theoretical yield = 37.1g Actual yield = 35.0 g

%yield = 35.0g X 100 37.1g

= 94.3%94.3%

TRY THE FOLLOWINGUsing the following unbalanced

equation, solve the problem:FeCl3 + NaBr FeBr3 + NaCl

If you start with 225g of NaBr and produce 200g of FeBr3, what is the percent yield?

SOLUTION %YIELD = 200g FeBr3 x 100

216g FeBr3

= 92.6%

LIMITING REACTANT If you start with the mass of 2

reactants, how do you determine the mass of the product?

EXAMPLE:2H2 + O2 2H2O

If you have 5.00g of H2 and 65.0g of O2, which reactant runs out first?

WHAT DOES IT MEAN? In this example:

5.00g of H2 would produce 44.5g of H2O

65.0g of O2 would produce 73.1g of H2O Therefore, you only produce 44.5g of

H2O. At this point the H2 runs out and you can’t make any more water.

Since the H2 produces less water than the O2, the H2 will run out before the O2

DEFINITIONS Since the H2 limits how much water is produced,

we call this reactant the:

LIMITING REACTANT: a reactant that is totally consumed during a chemical reaction. Determines the amount of product.

Since we will have extra O2 left over from the reaction, we call this reactant the: EXCESS REACTANT: a reactant that is

leftover when a chemical reaction stops.

LET’S PRACTICEZinc combines with hydrochloric

acid to from zinc chloride and hydrogen gas. If you start with 125g of zinc and 125g of HCl, how much hydrogen gas do you produce? What is the limiting reactant? What is the excess reactant?

WHAT ABOUT THE EXCESS REACTANT?

When you do a reaction, sometimes it is necessary to determine how much excess reactant is remaining.

To calculate this, you use the amount of limiting reactant to calculate the amount of excess reactant you actually used

Let’s look at an earlier example

EXAMPLE In this case, H2 was the limiting reactant

and O2 was the excess reactant. We know that all of the H2 was used up

We have to calculate how much O2 was used up

Using this, we can calculate how much O2 is remaining

ANSWER You started with 65.0g of O2

You used 39.6g of O2

65.0g – 39.6g = 25.4g

Therefore, you have 25.4g of O2 remaining at the end of the reaction.

SUMMARY1. Translate and balance reaction2. Figure out what you need to solve for

(grams of reactant to grams of product)3. Solve for each reactant4. Determine limiting reactant5. Use the limiting reactant to solve for

how much excess reactant you use6. Calculate how much excess reactant is

remaining

LET’S PRACTICE Aluminum combines with chlorine to form

aluminum chloride. If 55.0g of aluminum reacts with 155g of chlorine, find the following:

the limiting reactant the mass of product mass of excess reactant If 170.0g of aluminum chloride is actually

produced, what is the percent yield?

STOICHIOMETRY Mass relationships between reactants and products in a chemical reaction.

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