Unit 6 –Stoichiometry and

Reaction Types

Cartoon courtesy of NearingZero.net

I. Types of Chemical Reactions

A. Combustion

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

• The burning of any substance in O2 to produce heat

A + O2 B

Na(s)+ O2(g)

C3H8(g)+ O2(g) 5 3 4

A. Combustion

• Products:– contain oxygen– hydrocarbons form CO2 + H2O

CO2(g)+ H2O(g)

Na2O(s) 4 2

B. Synthesis

• The combination of 2 or more substances to form a compound

• Only one product

A + B AB

B. Synthesis

H2(g) + Cl2(g) 2 HCl(g)

Al(s)+ Cl2(g) AlCl3(s)2 3 2

B. Synthesis

• Products:– ionic - cancel charges– covalent - hard to tell

C. Decomposition

• a compound breaks down into 2 or more simpler substances

• only one reactant

AB A + B

C. Decomposition

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

KBr(l) K(s) + Br2(l) 2 2

C. Decomposition

• Products:– binary - break into elements– others - hard to tell

NI3(s)

D. Single Replacement

• one element replaces another in a compound– metal replaces metal (+)– nonmetal replaces nonmetal (-)

A + BC B + AC

D. Single ReplacementCu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)

D. Single Replacement• Products:

– metal metal (+)– nonmetal nonmetal (-)– free element must be more active (check activity

series)

Br2(l)+ NaCl(aq) N.R.

Aluminum + Ferric oxide

Reaction Type: _____________________________

AB + CD AD +

CB

E. Double Replacement• ions in two compounds “change

partners”• cation of one compound combines

with anion of the other

E. Double ReplacementPb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)

E. Double Replacement

• Products:– switch negative ions

2 2

Plumbous nitrate + Sodium iodide

Reaction Type: _____________________________

Cobaltous chloride + Sodium hydroxide

Reaction Type: _____________________________

IQ 1#1: Calculate the number of atoms in 47.5 g carbon.

#2: Calculate the mass of 5.1 x 1023 molecules of water.

47.5 g C12.01 g C1 mol C

1 mol C

6.022 x 1023 atoms C

= 2.38 x 1024 atoms C

= 15 g H2O or 1.5 x 101 g H2O

1 mol H2O18.02 g H2O

6.022 x 1023 molec H2O

1 mol H2O5.1 x 1023 molec H2O

Stoichiometry

Sherlock Holmes, in Sir Arthur Conan Doyle’s A Study in Scarlet

“In solving a problem of this sort, the grand thing is to be able to reason backward. This is a very useful accomplishment, and a very easy one, but people do not practice it much.”

Stoichiometry - The study of quantities of materials consumed and produced in chemical reactions.

FlowchartAtoms or Molecules

Moles

Mass (grams)

Divide by 6.02 X 1023

Multiply by 6.02 X 1023 Multiply by

atomic/molar mass from periodic tableDivide by

atomic/molar mass from periodic table

molar mass Avogadro’s number Grams Moles particles

Everything must go through Moles!!!

Calculations

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 chipsMakes 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?

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

Cookies and Chemistry…Huh!?!?

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

• Instead of calling them recipes, we call them reaction equations

• Furthermore, instead of using cups and teaspoons, we use moles

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

Chemistry Recipes

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

• Be sure you have a balanced reaction before you start!

• Example: 2 Na + Cl2 2 NaCl• This reaction tells us that by mixing 2 moles

of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride

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

Practice

• Write the balanced reaction for hydrogen gas reacting with oxygen gas.

2 H2 + O2 2 H2O

– How many moles of reactants are needed?– What if we wanted 4 moles of water?– What if we had 3 moles of oxygen, how much

hydrogen would we need to react and how much water would we get?

– What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced?

Solving a Stoichiometry Problem

1. Balance the equation.2. Convert masses to moles.3. Determine which reactant is limiting.4. Use moles of limiting reactant and mole

ratios to find moles of desired product.5. Convert from moles to grams.

Working a Stoichiometry Problem

6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.

1. Identify reactants and products and write the balanced equation.

Al + O2 Al2O3

b. What are the reactants?

a. Every reaction needs a yield sign!

c. What are the products?

d. What are the balanced coefficients?

4 3 2

Mole Ratios

• These mole ratios can be used to calculate the moles of one chemical from the given amount of a different chemical

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

2 Na + Cl2 2 NaCl

5 moles Na 1 mol Cl2

2 mol Na= 2.5 moles Cl2

Mole-Mole Conversions• How many moles of sodium chloride

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

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.

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

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!)

• Now we must go from grams to moles, mole ratio, and back to grams of compound we are interested in

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 NH32.00g N2 1 mol N2 2 mol NH3 17.04g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

IQ21. Determine the mass of Iron III oxide formed when 25.1 g

of iron reacts with oxygen.

Balanced equations: ______________________________

2. What mass of oxygen is needed to react with 6.8 x 1023 atoms of iron? (Use the above equations).

159.70 g Fe2O3

1 mol Fe2O3

2 mol Fe2O3

4 mol Fe1 mol Fe

55.85 g Fe

25.1 g Fe

4 Fe + 3 O2 → 2 Fe2O3

= 35.9 g Fe2O3

6.8 x 1023 atoms Fe

6.022 x 1023 atoms Fe

1 mol Fe

4 mol Fe

3 mol O2 32.00 g O2

1 mol O2

= 27 g O2

Example 26.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?

4 Al + 3 O2 2Al2O3

=6.50 g Al

? g Al2O3

1 mol Al

26.98 g Al 4 mol Al

2 mol Al2O3

1 mol Al2O3

101.96 g Al2O3

6.50 x 2 x 101.96 ÷ 26.98 ÷ 4 =

12.3 g Al2O3

Practice

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

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 chipsMakes 3 dozen

If we had the specified amount of all 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

LimitingLimitingReactantReactant

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 stop .

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.

Finding the Amount of Excess

• By calculating the amount of the excess reactant needed to completely react with the limiting reactant, we can subtract that amount from the given amount to find the amount of excess.

• Can we find the amount of excess potassium in the previous problem?

Finding Excess Practice• 15.0 g of potassium reacts with 15.0 g of

iodine. 2 K + I2 2 KI

• We found that Iodine is the limiting reactant, and 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: Recap1. 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 amount of excess, subtract the amount

used from the given amount.7. If you have to find more than one product, be sure

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