Section 1 Describing Chemical Reactions

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• A chemical reaction is the process by which one or more substances change into one or more new substances.

• Reactants are the original substances in a chemical reaction.

• Products are the substances that are created in a chemical reaction.

Section 1 Describing Chemical ReactionsChapter 8

Chemical Reaction



Visual Concepts

Chemical Reaction

Chapter 8



Evidence of a Chemical Reaction




Visual Concepts

Click below to watch the Visual Concept.

Visual Concept

Signs of a Chemical Reaction

Chapter 8



Visual Concepts


Visual Concept

Precipitate

Chapter 8



Evidence of a Chemical Reaction

Section 1 Describing Chemical Reactions

• solution color changes• solution bubbles• copper is used up

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Chemical Reaction Versus Physical Change

• chemical change – new substance forms with properties that differ from original substance

• density• boiling point• melting point

• physical change - changes of state • evaporation• condensation• melting• freezing




Reactions and Energy Changes

• Energy can be released in a chemical reaction.

methane + oxygen carbon dioxide + water + energy

Energy is a product.

• Energy can be absorbed in a chemical reaction.

dinitrogen tetroxide + energy nitrogen dioxide

Energy is a reactant.




Constructing a Chemical Equation

• A chemical equation shows the chemical formulas and relative amounts of all reactants and products.

• A word equation contains the names of the reactants and products.

• Equations must be balanced.




Writing a Word Equation or Formula Equation

methane + oxygen carbon dioxide + water

?CH4 + ?O2 ?CO2 + ?H2O




Equations and Reaction Information

• Physical States

NaHCO3(s) + HC2H3O2(aq) NaC2H3O2(aq) + CO2(aq) + H2O(l) solid

liquid


catalystN2(g) + 3H2(g) 2NH3(g)

aqueous solutions

• Reaction Conditions 350°C, 25 000 kPa



Equations and Reaction Information




Visual Concepts


Visual Concept

Chemical Equation

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Reactions Conserve Mass

• Mass cannot be created or destroyed by a chemical or physical change

• Equations must be balanced.

?Na + ?H2O ?NaOH + ?H2

Section 2 Balancing Chemical EquationsChapter 8



Balancing Equations

• The number of atoms for each element must be the same on the reactants’ side and on the products’ side.

• A coefficient multiplies the number of atoms of each element in the formula that follows.

H2O: 2 hydrogen atoms, 1 oxygen atom

2H2O: 4 hydrogen atoms, 2 oxygen atoms




Visual Concepts


Visual Concept

Reading a Chemical Equation

Chapter 8



Visual Concepts


Visual Concept

Balancing a Chemical Equation by Inspection

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Balancing Equations

Sample Problem ABalance the equation for the reaction of iron(III) oxide with hydrogen to form iron and water.




Balancing EquationsSample Problem A Solution1. Identify reactants and products.

reactants products

Section 2 Balancing Chemical Equations

Reactants Products Balanced?

Unbalanced formula equation Fe2O3 + H2 Fe + H2O

Iron atoms 2 1 no

Oxygen atoms 3 1 no

Hydrogen atoms 2 2 yes

3 32

Chapter 8

3. Insert coefficients.

2. Count atomsFe2O3 + H2 Fe + H2O




• Balanced equations show mass conservation

?Na + ?H2O ?NaOH + ?H2

2Na + 2H2O 2NaOH + H2

• Never change subscripts to balance equationsUnbalanced: H2 + O2 H2O

Incorrect: H2 + O2 H2O2

H2O H2O2

Correct: 2H2 + O2 2H2O




Polyatomic Ions can be balanced as a group




Balancing Equations

Sample Problem CAluminum reacts with arsenic acid, HAsO3, to form H2 and aluminum arsenate. Write a balanced equation for this reaction.




Balancing EquationsSample Problem C Solution1. Identify reactants and products.

reactants productsAl + HAsO3 H2 + Al(AsO3)3

2. Count Atoms

Section 2 Balancing Chemical Equations

Reactants Products Balanced?

Unbalanced formula equation Al + HAsO3 H2 + Al(AsO3)3

Iron atoms 1 1 yes

Oxygen atoms 1 2 no

Hydrogen atoms 1 3 no

3 22 6

Chapter 8

3. Insert coefficients



Section 3 Classifying Chemical Reactions

Combustion Reactions

• A combustion reaction is a reaction of a carbon-based compound with oxygen.

Combustion of propane:

C3H8 + 5O2 3CO2 + 4H2O

Combustion of ethanol:

CH3CH2OH + 3O2 2CO2 + 3H2O

Chapter 8



Visual Concepts


Visual Concept

Chapter 8

Combustion Reaction




Synthesis Reactions

• In a synthesis reaction a single compound forms from two or more reactants.

• Two elements form a binary compound

C + O2 CO2

2C + O2 2CO

• Two compounds form a ternary compoundCaO(s) + H2O(l) Ca(OH)2(s)

CO2(g) + H2O(l) H2CO3(aq)

Chapter 8



Visual Concepts


Visual Concept

Chapter 8

Synthesis Reactions




Decomposition Reactions

• In a decomposition reaction a single compound breaks down, often with the input of energy, into two or more elements or simpler compounds.

• Decomposition of waterelectricity

heat

Chapter 8

CaCO3(s) CaO(s) + CO2(g)

• A metal carbonate decomposes to form a metal oxide and carbon dioxide.

2H2O(l) O2(g) + 2H2(g)



Visual Concepts


Visual Concept

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Decomposition Reaction



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Electrolysis

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Decomposition Reactions, continued Sample Problem D

Predicting Products Predict the product(s) and write a balanced equation for the reaction of potassium with chlorine.

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Decomposition Reactions, continued Sample Problem D Solution

• Reaction is most likely a synthesis reaction, so the product will be binary

• Potassium will lose one electron to become a 1+ ion.

• Chlorine will gain one electron to become a 1– ion.

K + Cl2 KCl

• Balance the equation.

2K + Cl2 2KCl

Chapter 8




Displacement Reactions

• In a displacement reaction a single element reacts with a compound and displaces another element from the compound.

2Al(s) + 3CuCl2(aq) 2AlCl3(aq) + 3Cu(s)

Aluminum displaces copper.

Chapter 8



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Single Displacement Reaction

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Displacement Reactions, continued

• The activity series ranks the reactivity of elements

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Visual Concept

Activity Series

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Displacement Reactions, continuedSample Problem E

Determining Products by Using the Activity Series Magnesium is added to a solution of lead(II) nitrate. Will a reaction happen? If so, write the equation and balance it.

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Displacement Reactions, continuedSample Problem E Solution

1. Identify the reactants. Magnesium will attempt to displace lead from lead(II) nitrate.

2. Check the activity series.Magnesium is more active than lead and displaces it.

3. Write the balanced equation.

Mg + Pb(NO3)2 Pb + Mg(NO3)2

Chapter 8




Double-Displacement Reactions

• In a double-displacement reaction two compounds in aqueous solution appear to exchange ions and form two new compounds.

• One of the products must be a solid precipitate, a gas, or a molecular compound, such as water.

HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq)

Chapter 8



Visual ConceptsChapter 8

Double-Displacement Reaction



Visual Concepts


Visual Concept

Precipitation Reaction

Chapter 8



Identifying Reactions and Predicting Products

Chapter 8

Section 1 Describing Chemical Reactions

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