STOICHIOMETRYUSING THE CHEMICAL EQUATION LIKE A RECIPE

Stoichiometry

What is It? Quantitative relationships In chemical reactions Based on law of Conservation of Mass

Why/How is it Useful?

Straight from Wikipedia

Stoichiometry (pron.: /ˌstɔɪkiˈɒmɨtri/) is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of positive integers. For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3:

N2 + 3H2 → 2NH3

Stoichiometry is a lot like making tacos. Taco Recipe

What if I need to make tacos for 50 people?

1o shells 1 lb ground beef 1 pkg of taco seasoning ¾ c water ½ cup salsa 1 cup cheddar cheese 1 cup of lettuceMakes 10 tacos.

You know how much product you need (tacos)

You have to calculate how much of each ingredient you need.

That’s stoichiometry!!!!

Equations are a chemist’s recipe. Equations tell chemists

what amounts of reactants to mix and what amounts of products to expect.

If you know the quantity of one substance in a reaction you can calculate the quantity of

any other substance consumed or created in the reaction

Quantity meaning the amount of a substance in grams, liters, molecules, or moles.

Balanced Chemical Equations The coefficients in a balanced

chemical equation shows how many moles/particles of one reactant are needed to react with other reactants and how many moles/particles of product will be formed.

Based on Law of Conservation of Mass

4Fe + 3O2 2Fe2O3

4Fe + 3O2 2Fe2O3

Write the relationships we can derive from this equation.

C3H8 + 5O2 3CO2 + 4H2O

The combustion of propane provides energy for heating homes, cooking food, and soldering metal parts. Interpret the equation for the combustion of propane in terms of particles, moles, and mass.

Show that the law of conservation of mass is observed

C3H8 + 5O2 3CO2 + 4H2O

The calculation of quantities in chemical reactions is called stoichiometry.

Imagine you are in charge of manu-facturing for Rugged Rider Bicycle Company.

The business plan for Rugged Rider requires the production of 128 custom-made bikes each day.

You are responsible for insuring that there are enough parts at the start of each day.

An Analogy for Stoichiometry Assume that the major

components of the bike are the frame (F), the seat (S), the wheels (W), the handlebars (H), and the pedals (P).

The finished bike has a “formula” of FSW2HP2.

The balanced equation for the production of 1 bike is.F +S+2W+H+2P FSW2HP2

Now in a 5 day workweek, Rugged Riders is scheduled to make 640 bikes. How many wheels should be in the plant on Monday morning to make these bikes?

What do we know? Number of bikes = 640 bikes 1 FSW2HP2=2W (balanced eqn)

What is unknown? # of wheels = ? wheels

The connection between wheels and bikes is 2 wheels per bike. We can use this information as a conversion factor to do the calculation.

640 FSW2HP2

1 FSW2HP2

2 W= 1280

Wheels

• We can make the same kinds of connections from a chemical rxn eqn.

N2(g) + 3H2(g) 2NH3(g)• The key is the “coefficient ratio”.

The coefficients of the balanced chemical equation indicate the numbers of moles of reactants and products in a chemical reaction.

1 mole of N2 reacts with 3 moles of H2 to produce 2 moles of NH3. N2 and H2 will always react to

form ammonia in this 1:3:2 ratio of moles.

So if you started with 10 moles of N2 it would take 30 moles of H2 and would produce 20 moles of NH3

Using the coefficients, from the balanced equation as ratios to make connections between reactants and products, is the most important information that an equation provides. Using this information, you can

calculate the amounts of the reactants involved and the amount of product you might expect.

Mole Ratios

Shows the mole-to-mole ratio

between two of the substances

in a balanced equation

Written from the coefficients of

any two substances in the

equation

This is a very important

conversion and the key to

solving stoichiometry problems.

2Al + 3Br2 2AlBr3

4 Fe + 3 O2 2 Fe2O3

3 H2(g) + N2(g) 2 NH3(g)

A mol ratio for H2 and N2

A mol ratio for NH3 and H2

Stoichiometric Calculations How much product can be obtained

from a given reaction? How much reactant is needed to

produce this much product?

Review of Mole Conversions  

Three mole equalities

1 mol = 6.02 x 1023 particles1 mol = (molar mass) (from periodic table)

1 mol = 22.4 L for a gas at STP

Review practice  How many moles are in 5.0 x 1025

molecules of CO2?

What is the mass in grams of 0.250 moles of CO2?

How many moles of CO2 gas are in 100.0 L at STP?

Types of Stoichiometry Calculations Mole - Mole Calculations

Moles of Known Moles of Unknown Mole-Mass Calculations

Moles of Known Mass of Unknown Mass-Mole Calculations

Mass of Known Moles of Unknown Mass-Mass Calculations

Mass of Known Mass of Unknown

MOLE – MOLE EXAMPLE The following reaction shows the

synthesis of aluminum oxide.

3O2(g) + 4Al(s) 2Al2O3(s)• If you only had 1.8 mols of Al how much

product could you make?

3O2(g) + 4Al(s) 2Al2O3(s)

Solve for the unknown:

1.8 mol Al4 mol Al

2 mol Al2O3 = 0.90mol Al2O3

3O2(g) + 4Al(s) 2Al2O3(s)

Mole Ratio

3O2(g) + 4Al(s) 2Al2O3(s)

If you wanted to produce 24 moles of product how many moles of each reactant would you need?

Solve for the unknowns:

24 mol Al2O32 mol Al2O3

4 mol Al= 48 mol Al

3O2(g) + 4Al(s) 2Al2O3(s)

24 mol Al2O32 mol Al2O3

3 mol O2 = 36 mol O2

Practice Problem How many moles of hydrogen will

be produced if 0.44 mol of CaH2 reacts according to the following equation?

CaH2 + 2H2O Ca(OH)2 + 2H2

Iron will react with oxygen to produce Iron III oxide. How many moles of Iron (III) oxide will be produced if 0.18 mol of Iron reacts?

4 4 Fe + 3 O2 2 Fe2O3

4 Fe + 3 O2 2 Fe2O3

How many moles of Fe2O3 are

produced when 6.0 moles O2

react?

31

How many moles of Fe are needed to react with 12.0 moles of O2?

MASS – MASS CALCULATIONS No lab balance measures moles

directly, generally mass is the unit of choice.

From the mass of 1 reactant or product, the mass of any other reactant or product in a given chemical equation can be calculated, provided you have a balanced equation.

As in mole-mole calculations, the unknown can be either a reactant or a product.

Acetylene gas (C2H2) is produced by adding water to calcium

carbide (CaC2).

How many grams of C2H2 are produced by adding water to 5.00 g CaC2?

CaC2 + 2H2O C2H2 + Ca(OH)2

Example # 1 Mass-Mass

CaC2 + 2H2O C2H2 + Ca(OH)2

Step 1: “Get to Moles!” in this case that can be done by using the Molar Mass of your given compound.

5.0 g CaC2

64.0 g CaC2

1 mol CaC2 = .07813 mol CaC2

Step 2: Now we are ready for the KEY step…converting from mols of our given to mols of unknown using the mole ratio.

.07813 mol CaC2 1 mol CaC2

1 mol C2H2 = .07813 mol C2H2

Step 3: Since we are asked for mass of our unknown in this problem, we need to use our molar mass of our unknown and convert our newly calculated mols into grams.

.07813 mol C2H2 1 mol C2H2

26.0 g C2H2 = 2.03 g C2H2

Summary of 3 Steps of Stoichiometry Problems

1. Get to Moles of Given 2. Mole Ratio to calculate moles of unknown3. Get to wanted final unit

The double replacement reaction between Lead II nitrate and Potassium Iodide produces a bright yellow precipitate that can be used as

a color additive in paint. How many grams of potassium iodide would we need to completely

react 25.3 g of lead (II) nitrate?

mass A mols A mols B mass B

Pb(NO3)2 + 2 KI PbI2 + 2 KNO3Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

25.3 g Pb(NO3)2331.2g Pb(NO3)2

1mol Pb(NO3)2

= 25.4 g KI

1mol Pb(NO3)2

2mol KI

1mol KI

166 g KI

Practice Problems What mass of Barium chloride is

needed to react completely with 46.8 g of Sodium phosphate according to the following equation?

BaCl2 + Na3PO4 Ba3(PO4)2 + NaCl

Another problemUse the equation to determine what mass of FeS

must react to form 326g of FeCl2.

FeS + 2HCl H2S + FeCl2

Moles of Known Mass of Unknown

Find moles of unknown using the mole ratio. Convert moles of unknown to mass of unknown.

Calculate the mass of carbon dioxide produced by the decomposition of 2.5 moles of copper(II) carbonate.

  

CuCO3 CuO + CO2

   

Mass to Moles Calculations Mass of known moles of unknown Convert mass of known to moles of

known. Use mole ratio to find moles of

unknown. Calculate the moles of water

produced by the reaction of 20 grams of hydrogen gas with excess oxygen gas.

 

A balanced reaction equation indicates the relative numbers of moles of reactants and products.

We can expand our stoichiometric calculations to include any unit of measure that is related to the mole.

The given quantity can be expressed in numbers of particles, units of mass, or volumes of gases at STP.

The problems can include mass-volume, volume-volume, and particle-mass calculations.

In any of these problems1. the given quantity is first converted

to moles.2. Then the mole ratio from the

balanced equation is used to convert from the moles of given to the number of moles of the unknown

3. Then the moles of the unknown are converted to the units that the problem requests.

The next slide summarizes these steps for all typical stoichiometric problems

Moles Given

Particles G

Mass GVolume G

Stoichiometry Roadmap

Moles UnknownCoefficient mol U

Coefficient mol G

Balanced Equation

Mass UVolume U

Particles UCoefficient part U

Coefficient part G

Mol

ar m

ass

1 m

ole

Mol

ar m

ass

1 m

ole

1 m

ole

1 m

ole

22.4

L

22.4

L

How many molecules of O2 are produced when a sample of 29.2 g

of H2O is decomposed by electrolysis according to this

balanced equation:2H2O 2H2 + O2

The last step in the production of nitric acid is the reaction of NO2 with H2O.How many liters of NO2 must react

with water to produce 5.00x1022 molecules of NO?

3NO2+H2O2HNO3+NO

molecules A mols A mols B volume B

5.0x1022 mol-ecules NO

6.02x1023 mol-ecules NO

1 mol NO

1 mol NO

3 mol NO2

= 5.58 L NO2

1 mol NO2

22.4 L NO2

Aspirin can be made from a chemical reaction between the reactants salicylic acid and acetic anhydride. The products of the reaction are acetylsalicylic acid (aspirin) and acetic acid (vinegar). Our factory makes 125,000 100-

count bottles of Bayer Aspirin/day. Each bottle contains 100 tablets, and each tablet contains 325mg of aspirin. How much in kgs + 10% for

production problems, of each reactant must we have in order to meet production?

C7H6O3 + C4H6O3 C9H8O4 + HC2H3O2Salicylic

acidAcetic

anhydride aspirin vinegar

22,549.4 mols

aspirin 1 mol asp

1 mol C7H6O3

1 mol C7H6O3

136.10g C7H6O3

1000 g

1 kg = 3068.97 kg salicylic acid + (306.897 g)

Salicylic Acid:

= 3380 kg of salicylic acid

22,549.4 mols

aspirin 1 mol asp

1 mol C4H6O3

1 mol C4H6O3

102.06g C4H6O3

1000 g

1 kg = 2301.39 kg Acetic anhydride

+ 230.139 kg

Acetic Anhydride:

= 2530 kg Acetic anhydride

Example

4 Fe + 3 O2 2 Fe2O3

How many grams of O2 are needed to

produce 0.400 mol of Fe2O3?

52

Calculating Mass of A Substance

Balance equation

Convert starting amount to moles

Use coefficients to write a mol-mol ratio

Convert from moles of known to moles of

unknown

Convert moles of unknown to grams 53

Calculation

The reaction between H2 and O2

produces 13.1 g of water. How many grams of O2 reacted?

Write the equation

H2 (g) + O2 (g) H2O (g)

Balance the equation

2 H2 (g) + O2 (g) 2 H2O (g)54

2 H2 (g) + O2 (g) 2 H2O (g)

? g 13.1 g

55

Points to Remember

1. Read an equation in moles2. Convert given amount to moles3. Use mole ratio to find unknown moles4. Convert moles to grams

moles (given) moles (unknown)

grams (given) grams (unknown)

56

How many O2 molecules will react with

505 grams of Na to form Na2O?

4 Na + O2 2 Na2O

57

Acetylene gas C2H2 burns in the

oxyactylene torch for welding. How

many grams of C2H2 are burned if the

reaction produces 75.0 g of CO2?

2 C2H2 + 5 O2 4 CO2 + 2

H2O

58

Mass of Known Mass of Unknown

Moles of Unknown

Convert gram

s to moles

Convert moles known to moles of unknown Use Mole Ratio from Coefficients

Convert m

oles to grams

Moles of Known

Limiting Reactants

The reactant that is not completely used up in the reaction The limiting reactant is not present in

sufficient quantity to react with all other reactants.

The reaction stops when the limiting reactant is completely consumed.

Any remaining reactants are considered "excess reactants".

The amount of product formed is determined by the "limiting reactant"

Steps in solving a limiting reactant problem 1. Write a balanced equation for

the reaction. 2. Convert both reactant

quantities to moles. 3. Determine the moles of product

that could be formed by each reactant.

4. The least amount in step #3 identifies the limiting reactant.

5. Use that number of moles of product to determine the mass produced.

An example of a limiting reactant problem

What mass of water can be produced by 4 grams of H2 reacting with 16 grams of O2?

Remember the following things about limiting reactants: The reaction will stop when the

reactants are used up. If one reactant is used up before

the other, the reaction stops then.

The first reactant used up is the limiting reactant, use it for the calculation.

The other reactant is the excess reactant.

Step 1. Write a balanced equation for the reaction.

2 H2 + O2 2 H2O

Step 2. Convert both reactant quantities to moles.

Step 3. Using the mole ratio from the equation, determine the moles of water that could be formed by each reactant.

Step 4. Oxygen produces the least amount of water.

16 grams of oxygen cannot produce as much water as 4 grams of hydrogen. In other words, 16 grams of oxygen will be used up in the reaction before 4 grams of hydrogen.

Oxygen is the "limiting" reactant. Use oxygen for the calculation of

product amount.

Step 5.Complete the problem by converting moles of H2O to mass of H2O.

Analogy

Suppose you are preparing cheese sandwiches. Each sandwich requires 2 pieces of bread and 1 slice of cheese. If you have 4 slices of cheese and 10 pieces of bread, how many cheese sandwiches can you make?

68

Cheese Sandwich Products

Sandwich 1 + + =

Sandwich 2

+ + =

69

How many sandwiches can you make?

____ slices of bread

+ ____ slices of cheese

= ____ sandwiches

What is left over? ________________

What is the limiting reactant?

70

How many sandwiches can you make?

__10__ slices of bread

+ __4__ slices of cheese

= __4__ sandwiches

What is left over? _2 slices of bread

What is the limiting reactant? cheese

71

Hints for LR Problems

1. For each reactant amount given, calculate the

moles (or grams) of a product it could produce.

2.The reactant that produces the smaller amount of product is the limiting reactant.

3. The number of moles of product produced by the limiting reactant is ALL the product possible. There is no more limiting reactant left.

72

Example

25 grams of hydrogen and 25 grams of oxygen react to produce water. How much water is produced?

Percent Yield

Theoretical yield: Maximum amount of product that can be obtained from a given amount of reactant.

A reaction rarely produces maximum amount. Things do not go perfectly.

From the calculation (equation)

Actual Yield: The amount actually produced.

Percent Yield In all of the calculations we have done thus far

the amount of product we have calculated has been determined in perfect lab conditions. This means everything goes exactly right. Everything is ideal. 

This yield of product is called Theoretical Yield Theoretical yield is obtained from stoichiometric

calculations.

  

However, in real lab situations, conditions don’t always work out perfectly.

When an experiment is carried out it is nice to be able to determine how efficiently a reaction has proceeded.

This can be done by determining the actual yield produced in the experiment and then using an equation called percent yield.

Percent yield is the ratio of the actual amount of product to the theoretical amount of product.

Equation

Percent Yield = Actual Yield/Theoretical Yield x 100

  

Example

You produced 2.5 grams of magnesium oxide in a reaction of magnesium with excess oxygen. Calculate the percent yield for this reaction.

Percent Yield

You prepared cookie dough to make 5 dozen cookies. The phone rings while a sheet of 12 cookies is baking. You talk too long and the cookies burn. You throw them out (or give them to your dog.) The rest of the cookies are okay.

How many cookies could you have made (theoretical yield)?How many cookies did you actually make to eat? (Actual yield)

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Vocabulary

Actual yield is the amount of product actually recovered from an experiment

Theoretical (possible) yield is the maximum amount of product that could be produced

from the reactant. Calculated Yield.

Percent Yield is the actual yield compared to the maximum (theoretical yield) possible.

80

Percent Yield Calculation

What is the percent yield of cookies?

Percent Yield = Actual Yield (g) recovered X 100Possible Yield (g)

% cookie yield = 48 cookies x 100 = 80% yield

60 cookies 81