Section 1 The Nature of Chemical Reactions

ObjectivesRecognize some signs that a chemical

reaction may be taking place.

Explain chemical changes in terms of the structure and motion of atoms and molecules.

Describe the differences between endothermic and exothermic reactions.

Identify situations involving chemical energy.

Chapter 6

Section 1 The Nature of Chemical Reactions

Chemical Reactions Change Substances

Chemical reactions occur when substances undergo chemical changes to form new substances.

Production of gas and change of color are signs of chemical reactions.

Chemical reactions rearrange atoms.• A reactant is a substance or molecule that

participates in a chemical reaction.• A product is a substance that forms in a

chemical reaction.

Chapter 6

Section 1 The Nature of Chemical ReactionsChapter 6

Section 1 The Nature of Chemical ReactionsChapter 6

Section 1 The Nature of Chemical Reactions

Energy and ReactionsEnergy must be added to break bonds.

• Many forms of energy can be used to break bonds: heat electricity sound light

Forming bonds releases energy.• Example: When gasoline burns, energy in the

form of heat and light is released as the products of the isooctane-oxygen reaction and other gasoline reactions form.

Chapter 6

Reaction Model

Section 1 The Nature of Chemical ReactionsChapter 6

Section 1 The Nature of Chemical Reactions

Energy and Reactions, continuedEnergy is conserved in chemical reactions.

• Chemical energy is the energy released when a chemical compound reacts to produce new compounds.

• The total energy that exists before the reaction is equal to the total energy of the products and their surroundings.

An exothermic reaction is a chemical reaction in which heat is released to the surroundings.

An endothermic reaction is a chemical reaction that absorbs heat.

Chapter 6

Section 1 The Nature of Chemical Reactions

Energy and Reactions, continuedThe graphs below represent the changes

in chemical energy for an exothermic reaction and an endothermic reaction.

Chapter 6

Section 2 Reaction Types

ObjectivesDistinguish among five general types of

chemical reactions.

Predict the products of some reactions based on the reaction type.

Describe reactions that transfer or share electrons between molecules, atoms, or ions.

Chapter 6

Classifying ReactionsA synthesis reaction is a reaction in which two

or more substances combine to form a new compound.

• Synthesis reactions have the following general form:

A + B → AB

• Example: In the following synthesis reaction, the metal sodium reacts with chlorine gas to form sodium chloride, or table salt.

• 2Na + Cl2 → 2NaCl

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Classifying Reactions, continuedA decomposition reaction is a reaction in

a single compound breaks down to form two or more simpler substances.• Decomposition reactions have the following

general form: AB → A + B • Example: The following shows the

decomposition of water.• 2H2O → 2H2 + O2

• Electrolysis is the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water.

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Classifying ReactionsA combustion reaction is the oxidation

reaction of an organic compound, in which heat is released.

• Combustion reactions use oxygen as a reactant.

• Water is a common product of combustion reactions.

In combustion the products depend on the amount of oxygen available for the reaction.

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Classifying ReactionsA single-displacement reaction is a

reaction in which one element or radical takes the place of another element or radical in the compound.

• Single-displacement reactions have the following general form: AX + B → BX + A

• Example: The single-displacement reaction between copper(II) chloride and aluminum is shown as follows.

3CuCl2 + 2Al → 2AlCl3 + 3Cu

Section 2 Reaction TypesChapter 6

Single Displacement

Section 2 Reaction TypesChapter 6

Classifying ReactionsA double-displacement reaction is a

reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds.

• Double-displacement reactions have the following general form: AX + BY → AY + BX

• Example: The double-displacement reaction that forms lead chromate is as follows.

Pb(NO3)2 + K2CrO4 → PbCrO4 + 2KNO3

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Electrons and Chemical ReactionsAn oxidation-reduction reaction is any

chemical change in which one species gains electrons and another species loses electrons.• Oxidation-reduction reactions are often called redox

reactions for short.• Substances that accept electrons in a redox reaction are

said to be reduced.• Substances that give up electrons in a redox reaction are

said to be oxidized.

A radical is an organic group that has one or more electrons available for bonding.• Polymerization reactions can occur when radicals

are formed.

Section 2 Reaction TypesChapter 6

Section 2 Reaction TypesChapter 6

Section 3 Balancing Chemical Equations

ObjectivesDemonstrate how to balance chemical equations.

Interpret chemical equations to determine the relative number of moles of reactants needed and moles of products formed.

Explain how the law of definite proportions allows for predictions about reaction amounts.

Identify mole ratios in a balanced chemical equation.

Calculate the relative masses of reactants and products from a chemical equation.

Chapter 6

Section 3 Balancing Chemical Equations

Describing ReactionsOne way to record the products and reactants of a

reaction is to write a word equation.• Example: methane + oxygen → carbon dioxide + water

A chemical equation is a representation of a chemical reaction that uses symbols to show the relationship between the reactants and the products.

Chapter 6

• In a chemical equation, such as the one above, the reactants, which are on the left-hand side of the arrow, form the products, which are on the right-hand side.

Section 3 Balancing Chemical Equations

Describing ReactionsWhen the number of atoms of reactants

matches the number of atoms of products, then the chemical equation is said to be balanced.• Balancing equations follows the law of

conservation of mass.

You cannot balance chemical equations by changing chemical formulas themselves, because that would change the substances involved.

To balance chemical equations, numbers called coefficients must be placed in front of the chemical formulas.

Chapter 6

Section 3 Balancing Chemical EquationsChapter 6

Section 3 Balancing Chemical Equations

Describing ReactionsWhen the numbers of atoms for each

element are the same on each side, the equation is balanced, as shown below.

Chapter 6

Section 3 Balancing Chemical EquationsChapter 6

Section 3 Balancing Chemical EquationsChapter 6

Section 3 Balancing Chemical EquationsChapter 6

Section 3 Balancing Chemical Equations

Math Skills Balancing Chemical Equations Write the

equation that describes the burning of magnesium in air to form magnesium oxide.

1. Identify the reactants and products.

2. Write a word equation for the reaction.magnesium + oxygen → magnesium oxide.

Chapter 6

Math Skills, continued 3. Write the equation using formulas for

the elements and compounds in the word equation.Remember that some gaseous elements, like oxygen, are molecules, not atoms. Oxygen in air is O2, not O.

Mg + O2 → MgO

4. Balance the equation one element at a time.The same number of each kind of atom must appear on both sides. So far, there is one atom of magnesium on each side of the equation.

Section 3 Balancing Chemical EquationsChapter 6

But there are two oxygen atoms on the left and only one on the right.

Math Skills, continued 4. Balance the equation one element at

a time, continuedTo balance the number of oxygen atoms, you need to double the amount of magnesium oxide:

Mg + O2 → 2MgO

Section 3 Balancing Chemical EquationsChapter 6

This equation gives you two magnesium atoms on the right and only one on the left. So you need to double the amount of magnesium on the left, as follows.

2Mg + O2 → 2MgO

Math Skills, continued 4. Balance the equation one element at

a time, continued

Section 3 Balancing Chemical EquationsChapter 6

Now the equation is balanced. It has an equal number of each type of atom on both sides.

2Mg + O2 → 2MgO

Section 3 Balancing Chemical Equations

Determining Mole RatiosThe law of definite proportions states that

a compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed.

reaction.

Chapter 6

Section 3 Balancing Chemical EquationsChapter 6

Section 4 Rates of Change

ObjectivesDescribe the factors affecting reaction rates.

Explain the effect a catalyst has on a chemical reaction.

Explain chemical equilibrium in terms of equal forward and reverse reaction rates.

Apply Le Châtelier’s principle to predict the effect of changes in concentration, temperature, and pressure in an equilibrium process.

Chapter 6

Factors Affecting Reaction RatesFor any reaction to occur, the particles of

the reactants must collide with one another. Therefore, whatever will help particles collide with one another will speed up the reaction rate.

• Most reactions go faster at higher temperatures.

• Greater surface area speeds up reactions.• Concentrated solutions react faster.• Reactions are faster at higher pressure.• Massive, bulky molecules react slower.

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Factors Affecting Reaction RatesA catalyst is a substance that changes the

rate of a chemical reaction without being consumed or changed significantly.

• Catalysts are not reactants or products, because they are not used up in the reaction.

• Catalysts are often used in industry to make reactions go faster.

• Catalysts that slow reactions are called inhibitors.

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Factors Affecting Reaction RatesEnzymes are proteins that serve as

biological catalysts. • An enzyme is very specific, controlling one

reaction or set of similar reactions.• Most enzymes are fragile, and stop working

above certain temperatures.

The substrate is the reactant in reactions catalyzed by enzymes.• Example: hydrogen peroxide is the

substrate for catalase:catalase

2 2 2 22H O 2H O + O

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Equilibrium SystemsSome changes are reversible.

• Example: the physical change represented below can go in either direction.

increase pressure

2 2decrease pressureCO (gas above liquid) CO (gas dissolved in liquid)

Section 4 Rates of ChangeChapter 6

• Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction.

• Systems in equilibrium respond to minimize change.• Example: when the top is removed from a carbonated drink,

the system is no longer at equilibrium, and CO2 leaves as bubbles.

Equilibrium

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Equilibrium SystemsLe Châtelier’s principle predicts changes

in equilibrium.

• Le Châtelier’s principle is a general rule that states that if a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached.

Le Châtelier’s principle can be used to control reactions.

• Example: in a reaction that releases energy, if you raise the temperature, the equilibrium will shift to the left and make less products.

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6

Section 4 Rates of ChangeChapter 6