Chapter 3 Stoichiometry STOICHIOMETRY: the Chemical Arithmetic Used to Relate the Amount of Products and Reactants to Each Other

STOICHIOMETRY : The chemical arithmetic used to relate the amount of products and reactants to each other 1st Write Chemical Equation

2nd Balance Equation

3rd Interpret Equation 1st Write Chemical Equation

REACTANTS PRODUCTS

A2 + B 2 A2B 2nd Balance Equation

A balanced chemical equation has the same type and number of ______in the reactants as in the products.

A2 + B 2 A2B = A = = B = 3rd Interpret Equation

2 A2 + B 2 2 A2B

Two units of A2 _

React with One unit of B2 __

Forming Two units of A2B__ Magnesium reacts with “air” forming Magnesium Oxide

1st Write Chemical Equation → Mg(s) + O 2(g) MgO(s) 2nd Balance Equation

→→→ 2 Mg + O 2 2 MgO

2 = Mg = 2 2 = O = 2 3rd Interpret Equation

→→→ 2 Mg + O 2 2 MgO Some Balanced Chemical Reactions Combination & Decomposition reactions

In a combination reaction two or more substances form a single compound. → __N12(g) + __H3 2(g) __NH 2 3(g)

In a decomposition reaction, a single compound forms two or more new substances. 2 → __KClO 3(s) __KCl(s)2 + __O 3 2(g) CHEMICAL REACTIONS (YOU SHOULD KNOW)

1. COMBUSTION (Of a Hydrocarbon)

2. NEUTRALIZATION (Acid + Base)

3. ACID + ACTIVE METAL

4. FORMATION Reaction CHEMICAL REACTIONS (YOU SHOULD KNOW)

1. COMBUSTION (Of a Hydrocarbon) The combustion of a Hydrocarbon

produces CO 2 and H 2O : →→→ CH 4 + O 2 CO 2 + H 2O Combustion of Butane

→→→ C4H10 + 6 ½ O2 4 CO 2 + 5 H 2O

Is This Reaction Balanced ?

With Fractions ? CHEMICAL REACTIONS (YOU SHOULD KNOW)

2. NEUTRALIZATION (ACID+ BASE)

→→→ HCl (aq) + NaOH H2O + A SALT In this case the salt is NaCl Sodium Chloride CHEMICAL REACTIONS (YOU SHOULD KNOW)

3. ACID + ACTIVE METAL →→→ HCl (aq) + Zn (s) H2 (g) + A SALT

In this case the salt is CHEMICAL REACTIONS (YOU SHOULD KNOW)

4. FORMATION Reaction →→→ Zn (s) + Cl 2 (g) ZnCl 2 (s) Reactants are in their “natural” state Which of the following are Balanced ?

→→→ C4H10 + 6 ½ O2 4 CO 2 + 5 H 2O

→ 16 4 C 4H10 + 26 O 2 CO 2 + 20 H 2O Uniquely balanced equation

→→→ C4H10 + 6 ½ O2 4 CO 2 + 5 H 2O

→ 16 4 C 4H10 + 26 O 2 CO 2 + 20 H 2O The ONLY Uniquely balanced equation is: → 8 2 C4H10 + 13 O2 CO 2 + 10 H2O WEIGHTS formula.

(MOLECULAR) The FORMULAThe WEIGHT of a substance is the sum of the atomic atomic the is of sum the substance weights of each atom in its chemical in chemical of each its weights atom Calculate the Formula Weight of

Hydrogen Sulfide Nickel II Carbonate 1st you must know Magnesium Acetate the formula Ammonium Sulfate Potassium Phosphate Iron III Oxide Diphosphorus pentasulfide Calculate the Formula Weight of

H2S NiCO 3 Mg(C 2H3O2)2 (NH 4)2SO 4 K3PO 4 Fe 2O3 P2S5 Avogadro’s Number & The Mole

Avogadro’s number is chosen so that 1 mole of 12 C atoms has a mass of exactly 12 grams.

1 mole 12 C atoms = 6.02x10 23 12 C atoms

6.02x10 23 12 C atoms = 12 g Mascots

Lamar University CARDINALS University of Texas LONGHORNS LSU TIGERS Rice OWLS Oregon DUCKS Nederland BULLDOGS Biology Department Mascot Chemistry Department Mascot Rabbits (& other animals)

If you had a mole of rabbits, how many rabbits would you have ? How many rabbit ears would you have? How many rabbit feet? Interpreting Chemical Formulas

If you had a mole of water , how many molecules of water would you have ?

How many Hydrogen atoms would you have?

How many Oxygen atoms? • The mole is just a conversion factor that allows us to accurately work with atoms and molecules. • One mole of any substance contains 6.02 x 10 23 units of that substance. Grams, Moles & Avogadro

Conversion Factors Use Molecular Weight

To Convert Grams to Moles or To Convert Moles to Grams Grams to Moles

3.2 Grams of Oxygen = ? Moles

1 Mole ? Grams x = ? Moles ? Grams

1 Mole 2.3 Grams x = 0.10 Moles 32 Grams Moles to Grams

1.5 Moles of Methane = ? Grams ? Grams ? Moles x = ? Grams 1 Mole 16 Grams 1.5 Moles x = 24 Grams 1 Mole Use AVOGARDRO’S Number

• For NUMBER of Atoms • Or NUMBER of Molecules

• 32 Grams of Oxygen = ? Molecules • 32 Grams of Oxygen = ? Atoms • 1 Molecule of Oxygen = ? Grams Interpretation of Chemical Reactions Using Stoichiometry

Example 1 Nitrogen monoxide reacts with oxygen to produce Nitrogen dioxide

1st Write the reaction 2nd balanced reaction 3rd Interpret Reaction How many moles of oxygen gas are required to react completely with 2.0 mole NO?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 0.5 mol O 2

(b) 1.0 mol O 2

(d) 2.0 mol O 2 How many moles of oxygen gas are required to react completely with 1.0 mole NO?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 0.5 mol O 2

(b) 1.0 mol O 2

(d) 2.0 mol O 2

(e) 2.5 mol O 2 How many moles of oxygen gas are required to react completely with 2.50 moles NO?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 0.5 mol O 2

(b) 1.00 mol O 2

(d) 1.50 mol O 2

(e) 2.00mol O 2 How many moles of oxygen gas are required to react completely with 10 moles NO?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 5 mol O 2

(b) 10 mol O 2

(d) 20 mol O 2

(e) 25 mol O 2 If 6.0 moles of NO are reacted with 3.0 mole O 2, how many moles NO 2 are produced?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 2.0 mol NO 2

(b) 6.0 mol NO 2

(d) 16.0 mol NO 2

(e) 32.0 mol NO 2 If 10.0 moles of NO are reacted with 5.0 mole

O2, how many moles NO 2 are produced?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 2.0 mol NO 2

(b) 6.0 mol NO 2

(d) 16.0 mol NO 2

(e) 32.0 mol NO 2 If 10.0 moles of NO are reacted with 6.0 moles

O2, how many moles NO 2 are produced?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a) 2.0 mol NO 2 (b) 6.0 mol NO 2 (c) 10.0 mol NO 2 (d) 16.0 mol NO 2 (e) 32.0 mol NO 2 • Chemical Reactions do not always go the way we expect them to

• Using stoichiometry we can calculate the theoretical (Maximum) amount of product formed in a reaction. .

If the actual amount of product formed in a reaction is less than the theoretical amount we can calculate a percentage yield .

Actual product yield % yield = ×100% Theoretica l product yield The Limiting Reactant

A reaction stops when one reactant is totally consumed.

This is the limiting reactant .

The other reactants are excess reactants . The Limiting Reactant

• How many bikes can be made from 10 frames and 16 wheels ? 1 frame + 2 wheels →→→ 1 bike

What is the limiting “reagent” ? Excess “reagent” ? If 10.0 moles of NO are reacted with 6.0 moles

O2, how many moles NO 2 are produced?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

(a)What LIMITS the reaction?

(b)What is in excess ? If 10.0 moles of NO are reacted with 6.0

moles O 2, how many moles of the excess reagent remain?

2 NO ( g) + O 2 (g) 2 NO 2 (g)

• 1.0 mol O 2

• 5.0 mol O 2 • 4.0 mol NO • 8.0 mol NO Example 2: Hydrogen reacts with Nitrogen to form Ammonia

1st Write Reaction Next Balance Equation Interpret Balanced Equation Interpretation of a Chemical Reaction

→→→ 3 H 2 + 1 N 2 2 NH 3 Three moles One mole Two moles of Hydrogen of Nitrogen of Ammonia ======Mole Ratio MUST Always be 3 : 1 : 2 →→→ 3 H 2 + 1 N 2 2 NH 3

→→→ 3/3 H 2 + 1/3 N 2 2/3 NH 3 →→→ 3/2 H 2 + 1/2 N 2 1 NH 3 →→→ 3/4 H 2 + 1/4 N 2 2/4 NH 3 →→→ 3/8 H 2 + 1/8 N 2 2/8 NH 3 All are 3 : 1 : 2 How many moles of Nitrogen are needed to react with 3 moles of Hydrogen ?

→ 3 H 2 + 1 N 2 2 NH 3

(a) ½ (b) 1 (c)1 ½ (d) 2 (e) 2 ½ (f) 3 How many moles of Nitrogen are needed to react with 1 ½ moles of Hydrogen ?

→ 3 H 2 + 1 N 2 2 NH 3

(a) ½ (b) 1 (c)1 ½ (d) 2 (e) 2 ½ (f) 3 Chemical Reactions are Interpreted on the

Mole basis but chemicals are weighed

in the laboratory in Grams Use Molecular Weight

To Convert Grams to Moles or To Convert Moles to Grams Units, Units, Units

1 moles ___Grams x = ___moles ? Grams Units, Units, Units

? Grams ___moles x = ___Grams 1 moles How many moles of Nitrogen are needed to react with Three grams of Hydrogen ?

3 g x 1 mole = 1 ½ moles of H 2 2 g

→ 3 H 2 + 1 N 2 2 NH 3 (a) ½ (b) 1 (c)1 ½ (d) 2 (e) 2 ½ (f) 3 →→→ 3 H 2 + 1 N 2 2 NH 3

How many moles of Ammonia ( NH 3 ) are produced from

(a) 3 grams of H 2 and ½ mole of N 2?

1 mole = (1 mole)x(17 g/mole) grams of NH 3

(b) 3 grams of H 2 and 28 grams of N 2?

1 mole NH 3 with 14 g of Nitrogen in excess →→→ 3 H 2 + 1 N 2 2 NH 3

With a 50 % Yield, How many moles of NH 3 are produced from

(a) 3 grams of H 2 and ½ mole of N 2?

½ mole = (½ mole)x(17 g/mole) grams of NH 3

(b) 3 grams of H 2 and 28 grams of N 2?

½ mole NH 3 with 14 g of Nitrogen in excess DESCRIBING COMPOUNDS

Percent Composition Empirical Formulas Molecular Formulas Two Approaches to Chemical Formulas

1. Given Formula Determine Percent Composition of Each Element in Compound

2. Given Percent Composition of Each Element in Compound Determine Formula A student obtains the following data

Mass of crucible and cover = 28.288 gms Mass crucible, cover & K = 28.709 gms Mass crucible, cover & = 28.793 gms potassium oxide

Determine the EMPIRICAL formula First determine % by Weight

Weight K = 28.709 – 28.288 = 0.421 Weight O = 28.793 – 28.709 = 0.084 Total = 0.505

0.421 % by Wt K = x 100 = 83.36633 0.505 83.4 % by wt K and 16.6 % by wt O

Choose any total weight 100 grams is convenient Construct the following work sheet Element % Wt moles ratio K 83.4 83.4 g 83.4 g / 39 = 2.14 = 2 O 16.6 16.6 g 16.6 g / 16 = 1.04 = 1 Therefore formula is