Chemistry 103

Lecture 14

Outline

I. Empirical/Molecular Formulas

II. Chemical Reactions - basic symbols - balancing - classificationIII. Stoichiometry

The molecular formula Is the true or actual number of the atoms in a

moleculeThe empirical formula Is the simplest whole number ratio of the atoms

(this is the formula for ionic compounds)

H2O2 HOmolecular formula empirical formula

Types of Formulas

Empirical & Molecular Formulas Ionic Compounds - Only need Empirical

formula.

Molecules - Need information on both to determine exact make-up of your system

Molecular Formula Empirical Formula

C6H6 CH

A molecular formula Is a multiple (or equal) of its empirical

formula Has a molar mass that is the empirical

mass multiplied by a whole numbermolar mass = a whole number empirical mass

Is obtained by multiplying the empirical formula by a whole number

Relating Molecular and Empirical Formulas

Determine the molecular formula of compound that has a molar mass of 78.11 g/mole and an empirical formula

of CH.

Finding the Molecular Formula

A compound is 24.27% C, 4.07% H, and 71.65% Cl. The molar mass is known to be 99.0 g. What are the empirical and molecular formulas?

Molecular Formula

Chemical Reactions

Physical Change

In a physical change,• The identity and composition

of the substance do not change

• The state can change or the material can be torn into smaller pieces

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Chemical Change

In a chemical change, • Reacting substances form

new substances with different compositions and properties

• A chemical reaction takes place

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Chemical Reaction

In a chemical reaction, Old bonds are broken and

new bonds are formed Atoms in the reactants are

rearranged to form one or more different substances

Fe and O2 form rust (Fe2O3)

Chemical Reaction

In a chemical reaction,

• A chemical change produces one or more new substances

• There is a change in the composition of one or more substances

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Evidence of a Chemical Reaction Changes that can be

seen are evidence of a chemical reaction.

Writing a Chemical Reaction

Chemists use a shorthand approach when writing the specifics of a chemical reaction. This approach is called the chemical equation.

Reactants -----> Products

Chemical Equations

A chemical equation,

• Gives the chemical formulas of the reactants on the left of an arrow and the products on the right

Reactants Product

C(s)

O2 (g)CO2 (g)

Symbols Used in Equations

Symbols used in chemical

equations show: The states of the reactants

The states of the products

The reaction conditions

Chemical Equations Are BalancedIn a balanced chemical reaction,

• Atoms are not gained or lost

Chemical Equations Are BalancedIn a balanced chemical reaction,

• The number of reactant atoms are equal to the number of product atoms

Chemical Equations•Chemical equations: symbolic descriptions of chemical reactions.•Two parts to an equation:

•reactants and products

H2 + O2 H2OA Chemical Equation must also be “balanced”.

2H2 + O2 --> 2H2O

Balanced Chemical Equations Chemical Equations must be balanced

There must be equal numbers of atoms of each element on both sides of the equation (both sides of the arrow) 1. Write the correct symbols and formulas for all of the

reactants and products. 2. Count the number of each type of atom on BOTH

sides of the equation. 3. Insert coefficients until there are the equal numbers

of each kind of atom on both sides of the equation.

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

H2(g) + N2(g) NH3 (g)

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

H2(g) + N2(g) NH3 (g)

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

H2(g) + N2(g) NH3 (g)

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

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

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

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

Example - making ammonia

Hydrogen gas and Nitrogen gas combine to make ammonia in the gaseous state.

BEFORE AFTER

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

Balancing Equations

Methane reacts with oxygen (combustion reaction) to form carbon dioxide and water.

Write a properly balanced chemical equation

The Numbers in Chemical Equations

More Practice:Balancing Reactions C2H6 + O2 CO2 + H2O

C3H6 + O2 CO2 + H2O

NH3 + O2 NO + H2O

Stoichiometry

Chemical Stoichiometry: using mass and quantity relationships among reactants and products in a chemical reaction to make predictions about how much product will be made.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

Four molecules NH3 react with five molecules O2

to produce

four molecules NO and six molecules H2O

Quantities in a Chemical Reaction

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

Four molecules NH3 react with five molecules O2

to produce

four molecules NO and six molecules H2O

and

Four mol NH3 react with five mol O2

to produce

four mol NO and six mol H2O

Quantities in a Chemical Reaction

We can read the equation in “moles” by placing the word “mole” or “mol” between each coefficient and formula.

4Fe(s) + 3O2(g) 2Fe2O3(s)

4 mol Fe + 3 mol O2 2 mol Fe2O3

Moles in Equations

Conservation of Mass

The Law of Conservation of Mass indicates:• No change in total mass occurs in a reaction• Mass of products is equal to mass of

reactants

In an ordinary chemical reaction, • Matter cannot be created or destroyed• The number of atoms of each element are equal • The mass of reactants equals the mass of products

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

2 mol H, 2 mol Cl = 2 mol H, 2 mol Cl

2(1.008) + 2(35.45) = 2(36.46) 72.92 g = 72.92 g

Law of Conservation of Mass

Conservation of Mass

2 mol Ag + 1 mol S = 1 mol Ag2S

2 (107.9 g) + 1(32.07 g) = 1 (247.9 g)

247.9 g reactants = 247.9 g product

A mole-mole factor is a ratio of the moles for two

substances in an equation.

4Fe(s) + 3O2(g) 2Fe2O3(s)

Fe and O2 4 mol Fe and 3 mol O2

3 mol O2 4 mol Fe

Fe and Fe2O3 4 mol Fe and 2 mol Fe2O3

2 mol Fe2O3 4 mol Fe

O2 and Fe2O3 3 mol O2 and 2 mol Fe2O3

2 mol Fe2O3 3 mol O2

Writing Mole-Mole Factors

How many moles of Fe2O3 can form from 6.0 mol O2?

4Fe(s) + 3O2(g) 2Fe2O3(s)

Relationship: 3 mol O2 = 2 mol Fe2O3

Write a mole-mole factor to determine the moles of Fe2O3.

6.0 mol O2 x 2 mol Fe2O3 = 4.0 mol Fe2O3

3 mol O2

Calculations with Mole Factors

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

4Fe(s) + 3O2(g) 2 Fe2O3(s)

A) 3.00 mol Fe B) 9.00 mol FeC) 16.0 mol Fe

Mole relations