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Section 6.1 Chemical Equations

Section 6.2 Types of Reactions

Section 6.3 Nature of Reactions

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Section 6.1

Chemical Equations

• Relate chemical changes and macroscopic properties.

• Demonstrate how chemical equations describe chemical reactions.

• Illustrate how to balance chemical reactions by changing coefficients.

Section 6.1

Chemical Equations

energy: the capacity to do work

Section 6.1

Chemical Equations

reactant

product

coefficient

Balanced chemical equations represent chemical reactions.

Section 6.1

Chemical Reactions

• When substances undergo chemical changes, observable differences usually occur.

– color changes

– precipitation of a solid

– energy changes

– odor changes

– gas release

Section 6.1

Chemical Equations

• Chemists use statements called equations to represent chemical reactions.

• A substance that undergoes a reaction is called a reactant.

• When reactants undergo a chemical change, each new substance formed is called a product.

Section 6.1

Chemical Equations (cont.)

• To express chemical reactions, scientists use word equations, chemical equations, and symbols to indicate the physical state of the substances.

• In word equations, acetic acid + sodium hydrogen carbonate → sodium acetate + water + carbon dioxide reads as “acetic acid and sodium hydrogen carbonate react to produce sodium acetate, water, and carbon dioxide”.

Section 6.1

Chemical Equations (cont.)

• Chemical equations use symbols and formulas to represent the reactants and products.

HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2

Section 6.1

Chemical Equations (cont.)

• An endothermic reaction is a reaction in which energy is absorbed.

• An exothermic reaction is a reaction in which energy is released.

Section 6.1

Balanced Chemical Equations

• The most fundamental law in chemistry is the law of conservation of mass: Matter is neither created nor destroyed.

Section 6.1

Balanced Chemical Equations (cont.)

• The law of conservation of atoms: The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the product.

Section 6.1

Balanced Chemical Equations (cont.)

• A coefficient is a number, which is always positive, that is placed in front of the parts of a chemical reaction to indicate how many units are involved.

Section 6.1

Section Assessment

Which is an indicator that a chemical change is taking place?

A. color change

B. gas release

C. precipitation of a solid

D. all of the above

Section 6.1

Section Assessment

What is the product of a chemical reaction between iron and oxygen?

A. rust

B. liquid mercury

C. precipitation

D. steel

End of Section 6.1

Section 6.2

Types of Reactions

• Distinguish among the five major types of chemical reactions.

• Classify a reaction as belonging to one of five major types.

Section 6.2

Types of Reactions

reactant: a substance that undergoes a reaction

Section 6.2

synthesis

decomposition

single displacement

There are five types of chemical reactions: synthesis, decomposition, single-displacement, double-displacement, and combustion reactions.

double displacement

combustion

Types of Reactions

Section 6.2

Why Reactions Are Classified

• Classifying reactions can help in understanding the reaction and making predictions about it.

Section 6.2

Major Classes of Reactions

• Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction.

• A decomposition reaction is one in which a compound breaks down into two or more simpler substances.

Section 6.2

Major Classes of Reactions (cont.)

• A single-displacement reaction is one in which one element takes the place of another in a compound.

• A double-displacement reaction is one in which the positive ions of two ionic compounds are interchanged.

• A combustion reaction is one in which a substance rapidly combines with oxygen to form one more oxides.

Section 6.2

Major Classes of Reactions (cont.)

Section 6.2

Section Assessment

A reaction in which one element takes the place of another is what type of reaction?

A. combustion reaction

B. synthesis reaction

C. single displacement reaction

D. double displacement reaction

Section 6.2

Section Assessment

Which reaction forms oxides?

A. single displacement reaction

B. decomposition reaction

C. synthesis reaction

D. combustion reaction

End of Section 6.2

Section 6.3

Nature of Reactions

• Demonstrate factors that influence the direction of a reaction.

• Classify factors that influence the rate of a reaction.

Section 6.3

Nature of Reactions

synthesis: reaction in which two or more substances combine to form a single product.

Section 6.3

Nature of Reactions

equilibrium

dynamic equilibrium

Le Châtelier’s principle

soluble

insoluble

activation energy

concentration

limiting reactant

catalyst

enzyme

inhibitor

External factors modify the direction and rate of chemical reactions.

Section 6.3

Reversible Reactions

• When no net change occurs in the amount of reactants and products, a system is said to be in equilibrium.

• A dynamic equilibrium is a system in which opposite actions are taking place at the same rate.

Section 6.3

Reversible Reactions (cont.)

• The reversible reaction will favor the direction that produces the most stable products, which are those that are least likely to change.

• Le Châtelier’s principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

Section 6.3

Reversible Reactions (cont.)

• A product that does not dissolve in water can be removed if all other products and the reactants dissolve in water.

– A compound is soluble in a liquid if it dissolves in it.

– A compound is insoluble if it does not dissolve in a liquid.

Section 6.3

Reversible Reactions (cont.)

• Adding or removing energy, usually in the form of heat, can also influence the direction of a reaction.

Section 6.3

Activation Energy

• For a reaction to occur between two substances, particles of those substances must collide with each other with enough force to cause a change to take place.

• The amount of energy the particles must have when they collide is called the activation energy of the reaction.

Section 6.3

Speed Rate

• To determine how fast a reaction is taking place, you can measure how quickly one of the reactants disappears or how quickly one of the products appears.

• Most reactions go faster at higher temperatures.

Section 6.3

Speed Rate (cont.)

• Raising the concentration, or amount of the substance present in a certain volume, will speed up a reaction because there are more particles per volume.

Section 6.3

Speed Rate (cont.)

• When the limiting reactant, or the reactant in limited supply, is used up, the reaction stops and no new product is formed.

Section 6.3

Speed Rate (cont.)

• A catalyst is a substance that speeds up the rate of a reaction without being permanently changed or used up itself.

– Biological catalysts are called enzymes.

– A substance that slows a reaction is called an inhibitor.

Section 6.3

Section Assessment

What is a substance that slows a reaction called?

A. inhibitor

B. enzyme

C. limiting reactant

D. insoluble

Section 6.3

Section Assessment

Which of the following does not affect the speed rate of a reaction?

A. raising the temperature

B. raising the concentration

C. removing a catalyst

D. none of the above

Section 6.3

Section Assessment

Lowering concentration ___ the rate of reaction.

A. decreases

B. increases

C. equalizes

D. does not affect

End of Section 6-3

Study Guide 1

Key Concepts

• Chemical equations—used to represent reactions—are written using symbols and formulas for elements and compounds.

• Chemical equations can tell you how elements and compounds change during a reaction and whether a reaction is endothermic or exothermic.

• Equations are balanced by changing coefficients.

• A balanced chemical equation reflects the law of conservation of mass.

Study Guide 2

Key Concepts

• Although thousands of individual chemical reactions are known, most can be classified into five major classes that are based on patterns of behavior of reactants and products.

• The five general classes of reactions are synthesis, decomposition, single-displacement, double-displacement, and combustion.

• Sometimes classes of reactions overlap. For example, some combustion reactions are also synthesis reactions.

Study Guide 3

Key Concepts

• Reversible reactions are those in which the products can react to reform the reactants.

• Equilibrium occurs when forward and reverse reactions take place at the same rate.

• At equilibrium, there is no net change in the amounts of products and reactants.

• According to Le Châtelier’s principle, if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

Chapter Assessment 1

In which kind of reaction is energy absorbed?

A. endothermic

B. exothermic

C. esothermic

D. ergothermic

Chapter Assessment 2

What do scientists use to represent the number of parts involved in a chemical reaction?

A. product

B. coefficient

C. reactant

D. symbols

Chapter Assessment 3

What do scientists use to express chemical reactions?

A. word equations

B. chemical equations

C. symbols to indicate the physical state of the substances

D. all of the above

Chapter Assessment 4

A water-based solution is indicated by which symbol?

A. (s)

B. (l)

C. (g)

D. (aq)

Chapter Assessment 5

Which of the following is not a chemical reaction?

A. a piece of wood burning

B. a car rusting

C. an ice cube melting

D. red litmus paper turning blue

STP 1

What is the coefficient of bromine in the equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?

A. 1

B. 2

C. 3

D. 6

STP 2

The law of conservation of atoms requires what in a chemical reaction equation?

A. both side of the equation to contain the same substances

B. the reactants to have the same amount of molecules

C. both sides to have the same amount of atoms of each element

D. the products to have fewer molecules than the reactants

STP 3

How can the rate of a reaction be determined?

A. by how quickly a reactant disappears

B. by how quickly the product disappears

C. by the amount of activation energy it releases

D. by how quickly it reverses direction

STP 4

Reactions that release heat energy are called____.

A. esothermic

B. exothermic

C. endothermic

D. ergothermic

STP 5

Atoms in a chemical reaction do not change but are rearranged.

A. true

B. false