chapter 7 chemical reactions table of · pdf file•a product is a substance that forms in...

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Chemical ReactionsChapter 7

Table of Contents

Section 1 The Nature of Chemical Reactions

Section 2 Reaction Types

Section 3 Balancing Chemical Equations

Section 4 Rates of Change



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 7



Signs of a Chemical Reaction






Chemical Reactions

Energy and Reactions

• Energy must be added to break bonds.

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

• Forming bonds releases energy because bonds of a molecule store chemical 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, and new molecules are formed.

Chapter 7



Reaction Model






Chemical Reactions

Energy and Reactions, continued

• Energy 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, and an increase in temperature occurs.

• An endothermic reaction is a chemical reaction that absorbs heat and energy.

Chapter 7




Chemical Reactions

Energy and Reactions, continued

• The graphs below represent the changes in chemical

energy for an exothermic reaction and an

endothermic reaction.

Chapter 7



Classifying Reactions

• A 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 7



Synthesis Reaction




Classifying Reactions, continued

• A 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.




Decomposition Reaction




Electrolysis





• A combustion reaction is the oxidation reaction of

an organic compound, in which heat is released and

water is formed.

• 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.




Combustion Reaction





• A 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




Single Displacement





• A 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




Double Displacement Reaction




Electrons and Chemical Reactions

• An 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.




Redox Reactions




Section 3 Balancing Chemical

Equations

Describing Reactions

• One 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 7

• 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.




Equations

Describing Reactions

• When 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 7



Law of Conservation of Mass


EquationsChapter 7




Equations

Describing Reactions, continued

• When the numbers of atoms for each element are the

same on each side, the equation is balanced, as

shown below.

Chapter 7



Reading a Chemical Equation


EquationsChapter 7



Chemical Equation


EquationsChapter 7



Balancing a Chemical Equation by Inspection


EquationsChapter 7




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.

Magnesium and oxygen gas are the reactants that

form the product, magnesium oxide.

2. Write a word equation for the reaction.

magnesium + oxygen → magnesium oxide.

Chapter 7



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.


Equations

Math Skills, continued

Chapter 7

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



4. Balance the equation one element at a time, continued

To balance the number of oxygen atoms, you need to double the amount of magnesium oxide:

Mg + O2 → 2MgO


Equations


Chapter 7

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



4. Balance the equation one element at a time,

continued


Equations


Chapter 7

Now the equation is balanced. It has an equal

number of each type of atom on both sides.

2Mg + O2 → 2MgO



Factors Affecting Reaction Rates

• For 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.

• Adding a catalyst.

Section 4 Rates of ChangeChapter 7



Factors Affecting Reaction Rate




Factors Affecting Reaction Rates, continued

• A 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.




Catalyst




Factors Affecting Reaction Rates, continued

• Enzymes 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.




Inhibitors




Enzyme




Equilibrium Systems

• Some changes are reversible.


• 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 since the pressure decreases.



Equilibrium




Equilibrium Systems, continued

• Le 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.




Factors Affecting Equilibrium




Le Châtelier’s Principle


chapter 7 chemical reactions table of · pdf file•a product is a substance that forms in...

Documents