ADVANCED CHEMISTRYChapter 3 Stoichiometry

WHAT IS STOICHIOMETRY? Antoine Lavoisier observed that the total

mass before a reaction is equal to the total mass after a reaction.

This observation was termed the Law of Conservation of Mass

WHAT IS STOICHIOMETRY? Stoichiometry is the study of the

quantitative nature of chemical reactions.

It is used to balance chemical equations and determine molar mass and percent yield of products in chemical reactions.

TOPICS IN STOICHIOMETRY Balancing Chemical Equations Chemical Reactivity – types of reactions Weight, Mass and Mole Calculations Formula and Balanced Equation Analysis Limiting Reactants – Conclude w/Lab

BALANCING CHEMICAL EQUATIONS

RECAP All forward equations are written with

reactants of the left an products on the right

Example: 2H2 + O2 2H2O

Coefficient means 2 moleculesof H2, 4 atomstotal

Subscript means2 atoms of H

4 atoms ofH in product

BALANCING CHEMICAL EQUATIONS

Atoms are neither created or destroyed…so all equations must be balanced!

RULES FOR BALANCING EQUATIONS Cannot change subscripts Can change coefficients

BALANCING CHEMICAL EQUATIONS

Examples – Balance this equation

Na(s) + H2O(l) NaOH(aq) + H2(g)

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

Patterns of Chemical Reactivitytypes of reactions

We can use the periodic table to make predictions of chemical reactivity

Elements in the same group react with other compounds in a similar way

Patterns of Chemical Reactivitytypes of reactions

ExampleAlkali metals (group 1A elements) react with

(OH)-, hydroxide, in similar way

2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

Patterns of Chemical Reactivitytypes of reactions

Combustion reactions – are rapid reactions that produce a flame Most of these reactions involve

hydrocarbon compounds and oxygen as reactants and carbon dioxide and water are products

Example:C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)

Combination and Decomposition Reactions

Combination reactions – occur when two or more substances react to form one product

Decomposition reactions – occur when one substance undergoes a reaction to produce two or more substances

Patterns of Chemical Reactivitytypes of reactions

For example..

Combination reactions A + B C

Patterns of Chemical Reactivitytypes of reactions

Examples

C(s) + O2(g) CO2(g)

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

Decomposition reactionsC A + B

Patterns of Chemical Reactivitytypes of reactions

Examples

CaCO3(s) CaO(s) + CO2(g)

2NaN3(s) 2Na(s) + 3N2(g)

Sections 3.3 – 3.5 Section 3.3

Formula and Molecular Weights Average Atomic mass Percent Composition

Section 3.4 The Mole Molar Mass The Mole, Interconverting Masses, Moles and # of Particles

Section 3.5 Empirical Formula from Analysis Determining formulas through experimentation

The Atomic Mass Scale The atomic mass scale gives the mass of each

element relative to the mass of 12C.

The amu is defined by assigning a mass of 12 amu as the mass of a 12C atom.

The atomic mass unit scale allows us to use a chemical formula to determine the percentage composition by mass of a compound.

1 amu = 1.66054 x 10-24 g and 1g = 6.022 x 1023 amu

Section 3.3

Average Atomic mass

Weights and Mass Questions:

Why is the term weight the proper tem to use?

What’s the difference between Formula Weights and Molecular Weights?

Empirical and Molecular Weights

Mass % Occasionally, we must calculate the percentage

composition of a compound For example, in order to verify the purity of the compound,

we may wish to compare the calculated composition of a

substance with that found experimentally.

The Mole

Section 3.4

Just as a baker uses "dozen" to mean twelve, a chemist uses "mole" (abbreviated  mol) to mean 6.022 x 1023.

Further, just as the word dozen can apply to any collection of twelve objects, the word mole can apply to any collection of 6.022 x 1023 objects, whether they be atoms, molecules, or ions.

Conversions Conversions between mass, Moles

and Numbers of particles

Practice problem #1 Calculate the number of C atoms in

0.350 mol of C6H12O6

Practice problem #2 Calculate the mass, in grams, of

0.765 moles of Sodium Nitrate.

Empirical Formulas from Analysis

What’s the difference between an Empirical Formula and a Molecular Formula?

Section 3.5

Practice problem #3 A 5.325 g sample of methyl benzoate, a

compound used in the manufacture of perfumes, is found to contain 3.758 g of carbon, 0.316 g of hydrogen, and 1.251 g of oxygen. What is the empirical formula of the substance?