1. Chapter 19: Chemical Reactions
Section 1: Chemical Change
Section 2: Classifying Chemical Reactions
Section 3: Chemical Reactions and Energy
Section 4: Reaction Rates and Equilibrium

2. Section 1: Chemical Change
Chemical Reaction – a change in which one or more substances are converted into new substances
A chemical reaction consists of two parts:
Reactants – the substances that react
Products – the new substances produced
Chemical reactions obey the conservation laws
The mass of the reactants always equal the mass of the products
Examples of chemical reactions:
Digestion.
Internal combustion engines.
Cooking – sometimes

3. Section 1: Chemical Change
Chemical equations
The exact nature of chemical reactions are difficult to describe using words
Example: Aluminum metal can react with iron oxide to give us two new substances, iron and aluminum oxide
How much aluminum is needed to react with how much iron oxide?
Which iron oxide is involved in the reaction?
What form –solid, liquid, gas—are the reactants and products?
Chemists have developed a shorthand way to describe chemical reactions
Chemical equation – a way to describe a chemical reaction using chemical formulas and other symbols
General form:
Example:
The equation tells us the 2 units of aluminum will react with 1 unit of iron (III) oxide to form 2 units of iron and 1 unit of aluminum (III) oxide
The equation also tells us that the reactants and products are in the solid state during the reaction

4. Section 2: Classifying Chemical Reactions
Types of Chemical Reactions
Most reactions may be grouped into three basic types: synthesis, decomposition, and replacement reactions
Synthesis reaction – a reaction where two or more substances combine to form one new substance
General form: 𝐀+𝐁→𝐀𝐁
Example: 𝟐 𝐇 𝟐 + 𝐎 𝟐 →𝟐 𝐇 𝟐 𝐎
Decomposition reaction – involves breaking a compound into two or more substances
General form: 𝐀𝐁→𝐀+𝐁
Example: 𝟐𝐇𝐠𝐎→𝟐𝐇𝐠+ 𝐎 𝟐
Replacement reaction – one atom or group of atoms in a compound is replaced with another atom or group of atoms
A single replacement reaction is when on atom or group of atoms is replaced
General form: 𝐀+𝐁𝐗→𝐀𝐗+𝐁
Example: 𝐂𝐮+𝟐𝐀𝐠𝐍 𝐎 𝟑 →𝐂𝐮 𝐍 𝐎 𝟑 𝟐+𝟐𝐀𝐠
A double replacement reaction occurs when two different types of atoms or groups of atoms exchange place
General form: 𝐀𝐗+𝐁𝐘→𝐀𝐘+𝐁𝐗
Example: 𝟐𝐊𝐂𝐥+𝐏𝐛 𝐍 𝐎 𝟑 𝟐→𝐏𝐛 𝐂𝐥 𝟐 +𝟐𝐊𝐍 𝐎 𝟑
A precipitate is a solid that is formed in a double replacement reaction

5. Section 2: Classifying Chemical Reactions
Balancing Chemical Equations
Chemical equations obey the Law of Conservation of Mass
A balanced chemical equation has the same number of atoms of each element of both sides of the equation
To balance a chemical equation, coefficients must be used until the number of atoms of each element is balanced.
Coefficients – represent the number of units of each substance taking part in the reaction
Important: You cannot balance a chemical equation by changing subscripts. If you change the subscript you change the composition of the substance

6. Section 2: Classifying Chemical Reactions
Ex.: Balance the following equation: Step 1: Check the equation for atom balance: The Mg atoms are balanced but the O atoms are not, therefore the equation is not balanced Step 2: Choose coefficients that balance the equation Try putting a coefficient of 2 in front of the MgO Step 3: Recheck the number of atoms on each side of the equation and adjust if necessary Now, there are two Mg’s on the right side but only one of the left, so a coefficient of 2 is needed for the Mg also The equation is now balanced
Atom Mg + O2 
MgO 1 O 2
Atom Mg + O2 
MgO 1 2 O
Atom Mg + O2 
MgO 2 O

7. Section 3: Chemical Reactions and Energy
All chemical reactions release or absorb energy
Exergonic reactions – less energy is required to break the original bonds than is released when new bonds form
Exothermic reaction – a reaction where the energy given off in the reaction is primarily in the form of heat
Endergonic reactions – a chemical reaction that requires more energy to break bonds than is released when new bonds are formed
Endothermic reaction – a reaction where the energy needed is in the form of heat
Catalyst – a substance that speeds up a chemical reaction without being permanently changed itself
Inhibitors – substances used to prevent certain reactions from occurring

8. Section 4: Reaction Rates and Equilibrium
Reaction rate – the rate at which reactants change into products
Factors that affect reaction rate include:
Temperature at which reaction occurs
Concentration of the reactants
Volume and pressure (in reactions that involve gases)
Surface area over which the reaction occurs
The presence of catalysts or inhibitors
Equilibrium – a state in which forward and reverse reactions or process proceed at equal rates
LeChatelier’s principle – if a stress is applied to a system at equilibrium, the equilibrium shifts in the direction that opposes the stress
Common stresses include: changing the concentration of the reactants, changing the temperature, and changing volume and pressure

9. The Mole and Molar Mass
Because atoms are so small and it requires a tremendous number of atoms to make any one thing, it is very difficult to count individual atoms.
A counting unit is used to count out a very large number of a very small thing
Example 1: When you order popcorn at the movies you do not buy a specific number of popcorn kernels; you buy a size of container for the popcorn. The size of the container—small, medium, and large—are the counting units for popcorn.
Example 2: You do not go to the grocery store and buy one egg; you buy a dozen (12) eggs. The dozen is the counting unit for buying eggs
Mole – the counting unit chemists use for counting atoms and other small particles.
One mole is a collection of a very large number of particle
One mole is equal to x 1023 particles
The mole is usually written as x 1023/mole, and is known as Avogadro’s constant

10. The Mole and Molar Mass
Molar mass – the mass in grams of one mole of a substance
The molar mass of an element in grams is the same as its atomic mass listed in the periodic table
Example: The average atomic mass of carbon is 12.01, so one mole of carbon has a mass of grams
Calculations involving molar mass
Finding number of atoms if you know mass Example: 85.5-g sample of Hg in a barometer
Finding mass if know the number of atoms Example: Determine the mass of a piece of titanium if it contains 2.24x1028 atoms