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Chemical Equations Chapter 8

Chemical Equations Chapter 8

Hein and Arena

Eugene PasserChemistry DepartmentBronx Community College

© John Wiley and Sons, Inc

Version 1.1

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Chapter Outline8.1 The Chemical Equation

8.2 Writing and Balancing Equations8.3 What Information Does an Equation Tell Us

8.4 Types of Chemical Equations

8.5 Heat in Chemical Reactions

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• Chemists use chemical equations to describe reactions they observe in the laboratory or in nature.

• Chemical equations provide us with the means to1.summarize the reaction

2.display the substances that are reacting

3.show the products

4.indicate the amounts of all component substances in a reaction.

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The Chemical EquationThe Chemical Equation

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• Chemical reactions always involve change.

• Atoms, molecules or ions rearrange to form new substances.

• The substances entering the reaction are called reactants.

• The substances formed in the reaction are called products.

• During reactions, chemical bonds are broken and new bonds are formed.

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• A chemical equation uses the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction.

• A chemical equation is a shorthand expression for a chemical change or reaction.

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Al + Fe2O3 Fe + Al2O3

reactants productsAl + Fe2O3 Fe + Al2O3

Chemical Equation

iron oxygen bonds break

aluminum oxygen bonds form

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Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting.

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Al + Fe2O3 Fe + Al2O3

coefficient2 2

coefficient

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Conditions required to carry out the reaction may be placed above or below the arrow.

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Al + Fe2O3 Fe + Al2O3

coefficient2 2

coefficient

heat

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The physical state of a substance is indicated by symbols such as (l) for liquid.

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2Al(s) + Fe2O3(s) 2Fe(l) + Al2O3 (s)

All atoms present in the reactant must also be present in the products.

In a chemical reaction atoms are neither created nor destroyed.

(s) (l) (s)(s)

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Symbols UsedSymbols Usedin Chemical Reactionsin Chemical Reactions

Symbols UsedSymbols Usedin Chemical Reactionsin Chemical Reactions

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placed between substances

+symbol

plusmeaning

location

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symbol

yieldsmeaning

between reactants and products

location

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(s)symbol

solidmeaning

after formulalocation

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(l)symbol

liquidmeaning

location after formula

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(g)symbol

gasmeaning

location after formula

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(aq)symbol

aqueousmeaning

after formulalocation

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symbol

heatmeaning

written above location

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hsymbol

light energymeaning

written above location

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symbol

gas formationmeaning

after formulalocation

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Writing andWriting andBalancing EquationsBalancing Equations

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• To balance an equation adjust the number of atoms of each element so that they are the same on each side of the equation.

• Never change a correct formula to balance an equation.

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Steps for Steps for Balancing EquationsBalancing Equations

Steps for Steps for Balancing EquationsBalancing Equations

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Step 1 Identify the reaction. Write a description or word equation for the reaction.

Mercury (II) oxide decomposes to form mercury and oxygen.

mercury(II) oxide → mercury + oxygen

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HgO Hg + O2

– The formulas of the reactants and products must be correct.

– The reactants are written to the left of the arrow and the products to the right of the arrow.

Step 2 Write the unbalanced (skeleton) equation.

The formulas of the reactants and products can never be changed.

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Step 3a Balance the equation.– Count and compare the number of atoms of each

element on both sides of the equation.

– Determine the elements that require balancing.

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HgO → Hg + O2

Step 3a Balance the equation.

– There is one mercury atom on the reactant side and one mercury atom on the product side.

– Mercury is balanced.

Element Reactant Side Product Side Hg 1 1

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Element Reactant Side Product Side O 1 2

Step 3a Balance the equation.

– There are two oxygen atoms on the product side and there is one oxygen atom on the reactant side.

– Oxygen needs to be balanced.

HgO Hg + O2

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Step 3b Balance the equation.– Balance each element one at a time, by

placing whole numbers (coefficients) in front of the formulas containing the unbalanced element.

– A coefficient placed before a formula multiplies every atom in the formula by that coefficient.

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Element Reactant Side Product Side O 1 2

Oxygen (O) is balanced.

Step 3b Balance the equation.

• Place a 2 in front of HgO to balance O.

There are two oxygen atoms on the reactant side and there are two oxygen atoms on the product side.

HgO Hg + O22

2

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Step 3c Balance the equation.

• Check all other elements after each individual element is balanced to see whether, in balancing one element, another element became unbalanced.

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Element Reactant Side Product Side Hg 2 1

• Count and compare the number of mercury (Hg) atoms on both sides of the equation.

Step 3c Balance the equation.

Mercury (Hg) is not balanced.

2HgO Hg + O2

• There are two mercury atoms on the reactant side and there is one mercury atom on the product side.

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2HgO Hg + O2

Step 3c Balance the equation.

• Place a 2 in front of Hg to balance mercury.

Mercury (Hg) is balanced.

There are two mercury atoms on the reactant side and there are two mercury atoms on the product side.

Element Reactant Side Product Side Hg 2 1

2

2

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2HgO 2Hg + O2

Element Reactant Side Product Side Hg 2 2 O 2 2

THE EQUATION IS BALANCED

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sulfuric acid + sodium hydroxide → sodium sulfate + water

Balance the Equation

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There is one Na on the reactant side and there aretwo Na on the product side.

Reactant Side Product Side

SO4 1 1

Na 1 2

O 1 1

H 3 2

2

H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)2

Place a 2 in front of NaOH to balance Na.

Balance the Equation

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4

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H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)

There are 4 H on the reactant side and two H on the product side.

Reactant Side Product Side

SO4 1 1

Na 2 2

O 2 1

H 4 2

2

Place a 2 in front of H2O to balance H.

2

2

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THE EQUATION IS BALANCED

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butane + oxygen → carbon dioxide + water

Balance the Equation

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)

There are four C on the reactant side and there isone C on the product side.

Reactant Side Product SideC 4 1

H 10 2

O 2 3

4

Place a 4 in front of CO2 to balance C.

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Balance the Equation

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)

There are 10 H on the reactant side and there aretwo H on the product side.

Reactant Side Product SideC 4 4

H 10 2

O 2 9

Place a 5 in front of H2O to balance H.

4 5

10

13

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C4H10 (g) + O2 (g) → CO2(g) + H2O(l)

There is no whole number coefficient that can be placed in front of O2 to balance O.

Reactant Side Product SideC 4 4

H 10 10

O 2 13

To balance O double all of the coefficients.

2010

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1052 8 10

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There are now 26 O on the product side.

Reactant Side Product SideC 8 8

H 20 20

O 2 26

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Place a 13 in front of O2 to balance O.

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THE EQUATION IS BALANCED C4H10 (g) + O2 (g) → CO2(g) + H2O(l)2 8 10

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What Information Does What Information Does an Equation Tell Usan Equation Tell Us

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The meaning of a formulais context dependent.

The formula H2O can mean:

1. 2H and 1 O atom

2. 1 molecule of water

3. 1 mol of water

4. 6.022 x 1023 molecules of water

5. 18.02 g of water

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In an equation formulas can represent units of individual chemical entities or moles.

H2 + Cl2 2HCl→

1 molecule H2 1 molecule Cl2 2 molecules HCl

1 mol H2 1 mol Cl2 2 mol HCl

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FormulasNumber of molecules

Number of atomsNumber of molesMolar masses

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Types of Chemical Types of Chemical EquationsEquations

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Combination

Decomposition

Single-Displacement

Double-Displacement

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Combination ReactionsCombination ReactionsCombination ReactionsCombination Reactions

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A + B AB

Two reactants combine to form one product.

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ExamplesExamples

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2Ca(s) + O2(g) 2CaO(s)

Metal + Oxygen → Metal Oxide

4Al(s) + 3O2(g) 2Al2O3(s)

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S(s) + O2(g) SO2(g)

Nonmetal + Oxygen → Nonmetal Oxide

N2(g) + O2(g) 2NO(g)

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2K(s) + F2(g) 2KF(s)

Metal + Nonmetal → Salt

2Al(s) + 3Cl2(g) 2AlCl3(s)

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Na2O(s) + H2O(l) 2NaOH(aq)

Metal Oxide + Water → Metal Hydroxide

CaO(s) + 2H2O(l) 2Ca(OH)2(aq)

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SO3(g) + H2O(l) H2SO4(aq)

Nonmetal Oxide + H2O(l) → Oxy-acid

N2O5(g) + H2O(l) 2HNO3(aq)

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Decomposition ReactionsDecomposition ReactionsDecomposition ReactionsDecomposition Reactions

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AB A + B

A single substance breaks down togive two or more different substances.

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ExamplesExamplesExamplesExamples

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2Ag2O(s) 4Ag(s) + O2(g)

Metal Oxide → Metal + Oxygen

Metal Oxide → Metal Oxide + Oxygen

2PbO2(s) 2PbO(s) + O2(g)

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Carbonate → CO2(g)

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

2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)

Hydrogen Carbonate → CO2(g)

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Miscellaneous Reactions

2KClO3(s) 2KCl(s) + 3O2(g)

2NaNO3(s) 2NaNO2(s) + O2(g)

2H2O2(l) 2H2O(l) + O2(g)

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Single Displacement Single Displacement ReactionsReactions

Single Displacement Single Displacement ReactionsReactions

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A + BC AC + B

One element reacts with a compound toreplace one the elements of that compound.

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Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)

2Al(s) + 3H2SO4(aq) 3H2(g) + Al2(SO4)3(aq)

salt

Metal + Acid → Hydrogen + Salt

salt

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2Na(s) + 2H2O(l) H2(g) + 2NaOH(aq)

Ca(s) + 2H2O(l) H2(g) + Ca(OH)2(aq)

Metal + Water → Hydrogen + Metal Hydroxide

metal hydroxide

metal hydroxide

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Metal + Water → Hydrogen + Metal Oxide

metal oxide

3Fe(s) + 4H2O(g) 4H2(g) + Fe3O4(s)

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The Activity SeriesThe Activity SeriesThe Activity SeriesThe Activity Series

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Metals KCaNaMgAlZnFeNiSnPbH

CuAgHg

An atom of an element in the activity series will displace an atom of an element below it from one of its compounds .

Sodium (Na) will displace an atom below it from one of its compounds.

increasing activity

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ExamplesExamples Metal Activity Series Metal Activity Series

ExamplesExamples Metal Activity Series Metal Activity Series

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Mg(s) + PbS(s) MgS(s) + Pb(s)

Metal Higher in Activity Series Displacing Metal Below It

Magnesium is above lead in the activity series.

Metals MgAlZnFeNiSnPb

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Ag(s) + CuCl2(s) no reaction

Metal Lower in Activity Cannot Displace Metal Above It

Metals PbH

CuAgHg

Silver is below copper in the activity series.

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ExampleExampleHalogen Activity SeriesHalogen Activity Series

ExampleExampleHalogen Activity SeriesHalogen Activity Series

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Cl2(g) + CaBr2(s) CaCl2(aq) + Br2(aq)

Halogen Higher in Activity Series Displaces Halogen Below It

Halogens F2

Cl2

Br2

I2

Chlorine is above bromine in the activity series.

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Double Displacement Double Displacement ReactionsReactions

Double Displacement Double Displacement ReactionsReactions

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AB + CD AD + CB

Two compounds exchange partners with each other to produce two different compounds.The reaction can be thought of as an exchange of positive and negative groups.

A displaces C and combines with DB displaces D and combines with C

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The Following Accompany Double Displacement Reactions

• formation of a precipitate

• release of gas bubbles

• release of heat

• formation of water

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ExamplesExamples

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Acid Base Neutralization

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l)

acid + base → salt + water

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Formation of an Insoluble Precipitate

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

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Metal Oxide + Acid

CuO(s) + 2HNO3(aq) Cu(NO3)2(aq) + H2O(l)

CaO(s) + 2HCl(aq) CaCl2(s) + H2O(l)

metal oxide + acid → salt + water

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Formation of a Gas

H2SO4(aq) + 2NaCN(aq) Na2SO4(aq) + 2HCN(g)

NH4Cl(aq) + NaOH(aq) NaCl(aq) + NH4OH(aq)

NH4OH(aq) NH3(g) + H2O(l)

indirect gas formation

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Heat inHeat inChemical ReactionsChemical Reactions

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Energy changes always accompany chemical reactions.

One reason why reactions occur is that the product attains a lower energy state than the reactants.

When this occurs energy is released to the surroundings.

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H2(g) + Cl2(g) → 2HCl(g) + 185 kJ (exothermic)

N2(g) + O2(g) + 185 kJ → 2NO(g) (exothermic)

Exothermic reactions liberate heat.

Endothermic reactions absorb heat.

The amounts of substances are expressed in moles.

1 mol 1 mol 2 mol

1 mol 1 mol 2 mol

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For life on Earth the sun is the major provider of energy.

The energy for plant photosynthesis is derived from the sun.

glucose

glucose6CO2 + 6H2O + 2519 kJ → C6H12O6 + 6O2

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Energy of ActivationEnergy of ActivationEnergy of ActivationEnergy of Activation

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• A certain amount of energy is always required for a reaction to occur.

• The energy required to start a reaction is called the energy of activation.

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• This reaction will not occur unless activation energy is supplied.

• The activation energy can take the form of a spark or a flame.

CH4 + 2O2 → CO2 + 2H2O + 890 kJ

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8.18.2

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