Chemistry II Stoichiometry Chapter 3. Stoichiometery: Calculations with Chemical Formulas and Equations.

Chemistry II

Stoichiometry

Chapter 3

Stoichiometery: Calculations with

Chemical Formulas and Equations

1. ) What are the advantages of writing chemical formulas over writing chemical names?

•abbreviation•important quantitative information•Structural information

2.)What two things is stoichiometry dependent on?

1.)Our understanding of atomic masses and 2.the Law of Conservation of mass

3.) How are chemical reactions represented?

By chemical equations

4.) In a chemical reaction the starting substances are called reactants and the substances produced are called products.

5.) The number written before the formula is called the coefficient and the number after the symbol used to indicate the number of atoms of that element in the formula is called the subscript.

6.) When balancing equations which number can we change?

We change the coefficient but not the subscript. Changing the coefficient changes the number of particles. Changing the subscripts changes the identity of the substance.

7.) How do we indicate physical state of reactants and products?

By using the following symbols:

Gas (g) solid (s)

Liquid (l) aqueous (aq)

8.) How do we indicate that the addition of heat is necessary?

By placing the delta symbol, Δ, above the arrow.

Balance this equation:Na(s) + H2O(l) → NaOH(aq) + H2(g)

Balance this equation:2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Balance the following:

Fe + O2 → Fe2O3

C2H4 + O2 → CO2 + H2O

Balance the following:

4Fe + 3O2 → 2Fe2O3

C2H4 + 3O2 → 2CO2 + 2H2O

Section 2: Some Simple Patterns

In this section we will review the five basic classifications of chemical reactions. Not all chemical reactions will fit neatly into one of the five classes. But, by recognizing typical patterns you will be more likely to predict products.

9.) What are the five common types of reactions?

Combination (synthesis)DecompositionSingle replacementDouble replacementcombustion

10.) What is a combination reaction?

In a combination reaction, two or more substances react to form a single product.

A + B → AB2Mg + O2→ 2 MgO

11.) What is produced by a direct combination reaction between a metal and a nonmetal?

An ionic solid

2 Li + Cl2 → 2LiCl

12.)What is a decomposition reaction?

A reaction in which one substance produces two or more other substances.

CaCO3 → CaO + CO2

2HgO → 2 Hg + O2

13.)What is a single replacement reaction?

An uncombined element displaces an element that is part of a compound. The reactants in a single replacement reaction are always one compound and one element.

13.)What is a single replacement reaction? contd

A + BX → AX + BMg + 2HCl → MgCl2 + H2

In a single replacement reaction, metals replace metals and nonmetals replace nonmetals.

The activity series can be used to determine if a single replacement reaction will happen or not.

14.)Describe a double replacement reaction.

In a double replacement reactions, atoms or ions from two different compounds replace each other.

14.Describe a double replacement reaction. contd

A double replacement reaction will have two compounds as reactants and two compounds as products.

AX + BY → AY + BXCaCO3 + 2HCl → CaCl2 + H2CO3

Although most chemical reactions will fit into one of three categories, there are many that do not. Combustion reactions often do not fit into one of these categories.

15.) What are combustion reactions?

Rapid reactions that produce flames. Combustion reactions include O2 as a reactant.

When hydrocarbons and oxygen containing derivatives combust, they form CO2 and H2O as products.

Write balanced equations for the following reactions:

The combination reaction that occurs when lithium metal and fluorine gas react.


The combination reaction that occurs when lithium metal and fluorine gas react.

2Li + F2 → 2LiF


The decomposition reaction that occurs when solid barium carbonate is heated. (Two products form: a solid and a gas.)


The decomposition reaction that occurs when solid barium carbonate is heated. (Two products form: a solid and a gas.)

BaCO3 → BaO + CO2


Solid mercury(II) sulfide decomposes into its component elements when heated.


Solid mercury(II) sulfide decomposes into its component elements when heated.

HgS → Hg + S


The surface of aluminum metal undergoes a combination reaction with oxygen in the air.


The surface of aluminum metal undergoes a combination reaction with oxygen in the air.

4Al + 3O2 → 2Al2O3


methanol, CH3OH(l) , is burned in air.


methanol, CH3OH(l) , is burned in air.

2CH3OH + 3O2 → 2CO2 + 4H2O

16.)What is formula weight?

The sum of the atomic weights of each atom in its chemical formula.

Formula weights are given in a.m.u.’s

Calculate the formula weight of sucrose C12H22O11

Calculate the formula weight of sucrose C12H22O11

342.0 amu

Calculate the formula weight of Ca(NO3)2

Calculate the formula weight of Ca(NO3)2

164.1 amu

17.)What is molecular weight?

If the chemical formula is a molecule (covalently bonded) we call the formula weight “molecular weight.”

18.) What are formula units?

Formula units represent the chemical formula of ionic compounds.

We speak of molecules for molecular compounds and formula units for ionic compounds.

19.) What is percent composition?

The percent by mass contributed by each element in a substance.

20.) How do you calculate the % composition ?

% element= (# atoms)( atomic weight) x 100

formula weight of compound

Calculate the % composition of C12H22O11

%C = (12)(12 amu) X 100 = 42.1%

342.0 amu

% H = (22)(1 amu) x 100 = 6.4%

342.0 amu

%O = (11)(16.0 amu) x 100 = 51.5%

342.0 amu

Calculate the percentage of nitrogen in Ca(NO3)2

Calculate the percentage of nitrogen in Ca(NO3)2

17.1 %

Section 4The mole

# atomsmass mass Ratio of

of H of O masses

1 1 161:16

Section 4The mole

# atomsmass mass Ratio of

of H of O masses

1 1 16 1:16

2 2 32 1:16

Section 4The mole

# atoms mass mass Ratio of of H of O masses

1 1 16 1:162 2 32 1:16

10 10 160 1:16

As long as the # of atoms are the same then the ratio of masses is the same as the reported atomic masses.

21.) Who was Amedeo Avogadro?

An Italian Chemist who determined that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

22.)What is the mole?

A mole is the amount of matter that contains as many objects as the number of atoms in exactly 12.0 grams of carbon-12.

23.)What is Avogadro’s number?

6.02 x 1023

1 mole of carbon 12 has 6.02 x 1023 carbon 12 atoms.

What is Avogadro’s number?

6.02 x 1023

1 mole of H2O has 6.02 x 1023 water molecules.

What is Avogadro’s number?

6.02 x 1023

1 mole of NaCl has 6.02 x 1023 formula units

Without using a calculator, arrange the following samples in order of increasing numbers of carbon atoms:

12 g 12C, 1 mol C2H2, 9x1023molecules of CO2

12 g 12C < 9x1023 CO2< 1 mole C2H2

Calculate the number of H atoms in 0.350 mol of C6H12O6

Calculate the number of H atoms in 0.350 mol of C6H12O6

(0.350 mole C6H12O6 )(12 mole H/ 1 C6H12O6 )

(6.02 x 1023 H atoms/ 1 mole H atoms)= 2.53 x 1024 mole H

How many oxygen atoms are in 0.25 mole Ca(NO3)2?

How many oxygen atoms are in 0.25 mole Ca(NO3)2?

9.0 x 10 23

How many oxygen atom are in 1.50 mole of sodium carbonate?

How many oxygen atom are in 1.50 mole of sodium carbonate?

2.71 x 1024

Molar Mass A mole of any two objects will be the same

number, but will have different masses. When we are dealing with a particular isotope

of an element we use the mass of the isotope for molar mass. ( 1 mole of Carbon-12 is 12g/mol.) Otherwise, we use the atomic weight listed on the periodic table for carbon 12.0107 g/mole.

24What is molar mass?

The mass in grams of 1 mole of a substance. The molar mass in g/mole is numerically equal to its formula weight.

A single formula unit of NaCl would be 58 amu( formula mass.)

A mole of NaCl formula units would be 58 gram ( molar mass)

What is the mass in grams of 1.000 moles of glucose C6H12O6?

What is the mass in grams of 1.000 moles of glucose C6H12O6?

(6) (12) + (12) (1) + (6)(16) = 180 g/mole

Calculate the number of moles of glucose (C6H12O6) in 5.380 g C6H12O6

Calculate the number of moles of glucose (C6H12O6) in 5.380 g C6H12O6

5.380 g (1 mole C6H12O6 / 180 g) = 0.02989 mole

The mole provides the bridge between mass, number and volume.

diagram

25.) What is the volume of any gas at STP?

1 mole of any gas will occupy a volume of 22.4L

Calculate the mass in grams of 0.433 mole of calcium nitrate.

Calculate the mass in grams of 0.433 mole of calcium nitrate.

71.1 g Ca(NO3)2

What is the mass in grams of 6.33 mol of NaHCO3

What is the mass in grams of 6.33 mol of NaHCO3

532 g

What is the mass in grams of 3.0 x 10-5 mole of sulfuric acid?

What is the mass in grams of 3.0 x 10-5 mole of sulfuric acid?

2.9 x 10-3 g

How many glucose molecules are in 5.23 g of C6H12O6

How many glucose molecules are in 5.23 g of C6H12O6

1.75 x 10 22 molecules

How many oxygen atoms are in this sample?

How many oxygen atoms are in this sample?

1.05 x 10 23 atoms O

Molecular Formula from Empirical Formula

If we know the molecular weight we can determine the molecular formula.

28.) How do the substances in the molecular formula compare to the empirical formula.

The molecular formula subscripts are always a whole number multiple of the empirical formula subscripts.

Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4. The experimentally determined molecular weight of this substance is 121 amu. What is the molecular formula of mesitylene.

C3H4 = 36 + 4 = 40

x 3

120

C9H12

Combustion Analysis

The empirical formula of a compound is based on experiments that give the number of moles of each element in a sample of the compound.

29.) What is combustion analysis?

A method used to determine the empirical formula for compounds containing only hydrogen and carbon.

All of the carbon is converted to carbon dioxide and all of the hydrogen is converted to water.

29.) What is combustion analysis?contd

The amount of CO2 and H2O produced is determined by mass increase.

If a third element is present, its mass can be determined by subtracting the masses of carbon and hydrogen from the original mass.

Isopropyl alcohol, a substance sold as rubbing alcohol is composed of C,H,and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561 g of CO2 and 0.306 g of H2O. Determine the empirical formula of isopropyl alcohol.

C3H8O

Caproic acid, which is responsible for the foul odor of dirty socks, is composed of C,H and O atoms. Combustion of a 0.225 g sample of this compound produces 0.512 g of CO2 and 0.209 g of H2O. A.) What is the empirical formula of caproic acid? B.) Caproic acid has a molar mass of 116 g/mol. What is its molecular formula?

A.) C3H6O

B.) C6H12O2

Section 6: Quantitative Information from Balanced Equations

The coefficients in a balanced chemical equation can be interpreted both as the relative number of molecules ( or formula units) involved in the reaction and as the relative numbers of moles.

The coefficients and the associated substances in a balanced equation are called stoichiometrically equivalent quantities.

Section 6: Quantitative Information from Balanced Equations

2H2 + O2→ 2H2O

2 mol H2≈ 1 mol O2≈ 2H2O

Because the substances in a balanced chemical equation are stoichiometrically equivalent, we can predict the amount of any other substance in the equation that will be needed or will be produced.

30.) Predict the moles of H2O that will be produced from 1.57 mol of O2?

3.14 mole H2O

31.) To predict the amount of any other substance that will be needed or will be produced follow these steps: 1.) Convert known value into moles. 2.) Convert from moles of your known to

moles of your unknown by using the coefficient in your balanced equation

3.) Convert from moles of unknown to the value requested.

32.) Calculate the mass of CO2

produced when 1.00 gram of C4H10 is burned.

3.03 g CO2

How many grams of water are produced in the oxidation of 1.00 gram of glucose, C6H12O6?

.6 g C6H12O6

The decomposition of KClO3 is commonly used to prepare small amounts of O2 in the laboratory:2KClO3(s) → 2KCl (s) + 3O2(g)

How many grams of O2 can be prepared from 4.5 g of KClO3?

1.8 g O2

Solid lithium hydroxide is used in space vehicles to remove the carbon dioxide to form solid carbonate and liquid water. How many grams of carbon dioxide can be absorbed by 1.00 g of lithium hydroxide.

.919 g carbon dioxide

Propane, C3H8, is a common fuel used for cooking and home heating. What mass of O2 is consumed in the combustion of 1.00 g propane?

3.64 g

Section 7: Limiting Reactants

In chemical reactions when one reactant is totally consumed the reaction stops.

33.) What is the limiting reactant?

The reactant that is completely consumed in a reaction.

34.) Which reactant determines the amount of product that can be formed ?

Limiting reactant

35.) What do we call the other reactants?

The reactants in excess

The most important commercial process for converting N2 from the air into nitrogen containing compounds is based on the reaction of N2 and H2 to form ammonia (NH3) :How many mole of NH3 can be formed from 3.0 mole of N2 and 6.0 mole of H2

N2 + 3H2 → 2NH3

4 mole of NH3

Consider the reaction 2Al + 3Cl2→ 2AlCl3

A mixture of 1.50 mole of Al and 3.00 mole of Cl2 is allowed to react. (a) Which is the limiting reactant? (b) how many moles of AlCl3 are formed? ( c) How many moles of the excess reactant remain at the end of the reaction?

a.) Al

b.) 1.5 mole of AlCl3

c.) .75 mole Cl2 remain.

Consider the following reaction that occurs in a fuel cell:

2 H2 + O2 → 2H2O

This reaction, done properly, produces energy in the form of electricity and water. Suppose a fuel cell is set up with 150 g of hydrogen gas and 1500 grams of oxygen gas. ( each measurement is given with two significant figures). How many grams of water can be formed?

1400 grams of water.

When working with limiting reactant problems, you will need to pick one product, determine the number of moles of that product that will be produced by each reactant. The reactant that gives the smallest amount number is the limiting reactant and determines the amount of product that can be formed.

A strip of zinc metal with a mass of 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate, causing the following reaction to occur:

Zn + 2AgNO3 → 2Ag + Zn(NO3)2


a.) Which reactant is the limiting reactant?

b) How many grams of Ag will form?

c.) How many grams of Zn(NO3)2 will form?

d) How many grams of the excess reactant will be left at the end of the reaction?


a.) AgNO3

b) 1.6 g Ag

c.) 1.39 g of Zn(NO3)3

d)1.53 g Zn left over.

36. What is the theoretical yield?

The quantity of product that is calculated to form when all of the limiting reactant reacts.

37.) What is the actual yield?

The amount of product that actually forms.

38.) What is the percent yield?

A comparison of the amounts of product that formed to the amount you expect to form.

% yield = Actual x 100

Theorectical