Chapter 7Chapter 7

Chemical ReactionsChemical Reactions

HomeworkHomework►Assigned Problems (odd numbers only)Assigned Problems (odd numbers only)►““Questions and Problems” 7.1 to 7.31 Questions and Problems” 7.1 to 7.31

(begins on page 200)(begins on page 200)►““Additional Questions and Problems” Additional Questions and Problems”

7.41 to 7.49 (page 221)7.41 to 7.49 (page 221)►““Challenge Questions” 7.51-7.57 (page Challenge Questions” 7.51-7.57 (page

222)222)

Chemical ReactionsChemical Reactions►Physical changes: Physical changes:

Involves no changes in chemical identity of a Involves no changes in chemical identity of a substancesubstance

No changes in physical properties (color, No changes in physical properties (color, physical state, freezing point, boiling point)physical state, freezing point, boiling point)

►Chemical changes: Chemical changes: A chemical reaction in which one or more A chemical reaction in which one or more

substances changes to a different substancesubstances changes to a different substance Properties that matter exhibits as it Properties that matter exhibits as it

undergoes changes in chemical compositionundergoes changes in chemical composition

Chemical ReactionsChemical Reactions►Chemical properties determine Chemical properties determine

whether or not a substance can be whether or not a substance can be changed to another substancechanged to another substance

►Reactions involve chemical changes in Reactions involve chemical changes in matter resulting in new substancesmatter resulting in new substances

►Reactions involve rearrangement and Reactions involve rearrangement and exchange of atoms to produce new exchange of atoms to produce new moleculesmolecules Elements are not changed during a Elements are not changed during a

reactionreactionReactants Products

Changes DuringChanges During Chemical Reactions Chemical Reactions

►A chemical change occurs when A chemical change occurs when new substances are madenew substances are made Conversion of material(s) into one or Conversion of material(s) into one or

more new substancesmore new substances These substances will have different These substances will have different

properties from the original materialproperties from the original material

►New properties are visible (visual New properties are visible (visual clues)clues) Color change, precipitate formation, gas Color change, precipitate formation, gas

bubbles, flames, heat releasebubbles, flames, heat release

Changes During Chemical Changes During Chemical ReactionsReactions

Fe Fe FeFe22OO3 3

HCOHCO33-- COCO2 2

Li LiOH, HLi LiOH, H22

Na NaOH, HNa NaOH, H22

Changes During Chemical Changes During Chemical ReactionsReactions

►In a chemical reaction:In a chemical reaction: At least one new substance is At least one new substance is

producedproduced Atoms are never created or Atoms are never created or

destroyeddestroyed Every atom present as a Every atom present as a

reactant has to be present as a reactant has to be present as a productproduct

The atoms in reactants The atoms in reactants rearrangerearrange to form new products to form new products

Chemical EquationsChemical Equations

►A chemical equation is a written A chemical equation is a written statement that uses symbols and statement that uses symbols and formulas (no words) to describe the formulas (no words) to describe the changes during a chemical reactionchanges during a chemical reaction

► It shows substances at the beginning It shows substances at the beginning of a reaction (of a reaction (reactantsreactants))

► It shows substances formed in the It shows substances formed in the reaction (reaction (productsproducts))

Writing a Chemical EquationWriting a Chemical Equation

►Chemical reactions can be written as:Chemical reactions can be written as: Word equationsWord equations Formula equationsFormula equations

productsreactants

Balancing Chemical Balancing Chemical EquationsEquations

►A balanced chemical reaction has the A balanced chemical reaction has the same number of atoms of each element same number of atoms of each element on on both sides of the arrowboth sides of the arrow

►Atoms are neither created nor Atoms are neither created nor destroyeddestroyed

►Every atom must be accounted forEvery atom must be accounted for►Equations are balanced by placing a Equations are balanced by placing a

coefficient in front one or more of the coefficient in front one or more of the substances in the equationsubstances in the equation

Symbols Used in EquationsSymbols Used in Equations

►Symbols used after chemical Symbols used after chemical formula to indicate physical stateformula to indicate physical state ((gg) = gas) = gas ((ll) = liquid ) = liquid ((ss) = solid) = solid ((aqaq) = aqueous, dissolved in water) = aqueous, dissolved in water

Writing Chemical EquationsWriting Chemical Equations► When magnesium metal burns in air When magnesium metal burns in air

it produces a white, powdery it produces a white, powdery compound magnesium oxidecompound magnesium oxide

Burning in air means reacting with OBurning in air means reacting with O22

► Write the word equationWrite the word equation The reactants are to the left of the arrowThe reactants are to the left of the arrow The products are to the right of the arrowThe products are to the right of the arrow Two or more reactants or products are Two or more reactants or products are

separated by a plus sign separated by a plus sign

magnesium + oxygen magnesium oxidemagnesium + oxygen magnesium oxide

Writing Chemical EquationsWriting Chemical Equations

► Indicate the physical state of each Indicate the physical state of each substancesubstance Use the correct chemical symbol to Use the correct chemical symbol to

indicate liquids and solidsindicate liquids and solids Metals are solids, except for Hg which is Metals are solids, except for Hg which is

liquidliquid Use molecular form for gases (HUse molecular form for gases (H22, O, O22, N, N22, ,

all halogens)all halogens) Identify Identify polyatomicpolyatomic ions ionsmagnesium(s) + oxygen(g) magnesium oxide(s)magnesium(s) + oxygen(g) magnesium oxide(s)

Writing Chemical EquationsWriting Chemical Equations

►Convert the word equation into a Convert the word equation into a formula equation formula equation Use the correct chemical symbol to Use the correct chemical symbol to

indicate liquids and solidsindicate liquids and solids There must be the same number of each There must be the same number of each

kind of atom on the reactant and product kind of atom on the reactant and product side of the equationside of the equation

Determine if the equation is balancedDetermine if the equation is balanced If not equal, must If not equal, must BALANCEBALANCE

___Mg (s) +___O___Mg (s) +___O22 (g) ___MgO(s) (g) ___MgO(s)

Balancing Chemical Balancing Chemical EquationsEquations

►Balance equations by the use of a Balance equations by the use of a coefficient placed to the left of a coefficient placed to the left of a substancesubstance

►NEVERNEVER change the subscripts of a change the subscripts of a compound to balance an elementcompound to balance an element It changes the identity of the It changes the identity of the

compoundcompound►Can change coefficients but never Can change coefficients but never

subscript numberssubscript numbers

1 O

Balancing a Chemical EquationBalancing a Chemical EquationExample 1Example 1

Mg(s) O2 (g) MgO(s)

1 Mg 1 Mg2 O

2

22

2

2CoefficientCoefficient

Balancing a Chemical Balancing a Chemical Equation: Example 2Equation: Example 2

►When solid ammonium nitrite When solid ammonium nitrite is heated it produces nitrogen is heated it produces nitrogen gas and water vaporgas and water vapor Write the formula equationWrite the formula equation

NH4NO2 (s) N2 (g) H2O(g)

NH4NO2 (s) N2 (g) H2O(g)

Balancing a Chemical Equation Balancing a Chemical Equation Example 2 Example 2

2 x N2 x O4 x H

2 x N1 x O2 x H

2

2

4

Balancing a Chemical Balancing a Chemical Equation: Example 3Equation: Example 3

►Nitrogen monoxide gas Nitrogen monoxide gas decomposes to produce dinitrogen decomposes to produce dinitrogen monoxide gas and nitrogen monoxide gas and nitrogen dioxide gasdioxide gas Write the formula equationWrite the formula equation

NO(g) N2O(g) NO2 (g)

Balancing a Chemical Equation Balancing a Chemical Equation Example 3Example 3

1 x N1 x O

3 x N3 x O

3

3 (g)NO (g)ON (g)NO 22

3

Balancing a Chemical Equation Balancing a Chemical Equation Example 4Example 4

►Liquid nitric acid decomposes to Liquid nitric acid decomposes to reddish-brown nitrogen dioxide reddish-brown nitrogen dioxide gas, liquid water and oxygen gas.gas, liquid water and oxygen gas. Write the formula equationWrite the formula equation

HNO3(l) NO2 (g) H2O(l) + O2 (g)

Balancing a Chemical Balancing a Chemical Equation Example 4Equation Example 4

1 x N3 x O1 x H

1 x N5 x O2 x H2

2HNO3(l) NO2 (g) H2O(l) + O2 (g)

62

2

27

4124

4

4

4

24

12

Types of ReactionsTypes of Reactions

►Reactions are separated into groups of Reactions are separated into groups of similar reactions similar reactions

►Based on the form of the equation for Based on the form of the equation for the reactionthe reaction Synthesis (combination)Synthesis (combination) DecompositionDecomposition Single replacementSingle replacement Double replacementDouble replacement CombustionCombustion

Types of ReactionsTypes of Reactions►Synthesis ReactionsSynthesis Reactions

Reactions in which two or more Reactions in which two or more substances combine to form a third substances combine to form a third substancesubstance

(one product forms)(one product forms)►General form of equation:General form of equation:

A + BA + B ABAB

Synthesis ReactionsSynthesis Reactions► The combinations can includeThe combinations can include

Two elementsTwo elements An element and a compoundAn element and a compound Two compoundsTwo compounds

► ExamplesExamples

(s) NaCl 2 (g)Cl(s)Na2 2

Cl(s)NH (g)HCl(g)NH 43

(g) NO 2 (g)O(g)NO2 22

Types of ReactionsTypes of Reactions►Decomposition ReactionsDecomposition Reactions

Reactions in which Reactions in which one reactantone reactant breaks breaks down into simpler (smaller) substancesdown into simpler (smaller) substances

Generally initiated by addition of energy Generally initiated by addition of energy (electric current or heating substances to (electric current or heating substances to high temperature)high temperature)

Opposite of a Synthesis ReactionOpposite of a Synthesis Reaction

►General Form of EquationGeneral Form of Equation

ABAB A + B A + B

Decomposition ReactionsDecomposition Reactions► Can be broken down to: Can be broken down to:

Smaller compoundsSmaller compounds Elements Elements BothBoth

► ExamplesExamples

(g)O (g)NO4 (g)ON2 2252

)(Br (s) Ag2(s) AgBr2 2 light

(g)CO(s)CaO(s)CaCO 23

Types of ReactionsTypes of Reactions

►Single replacement reactionsSingle replacement reactions One element replaces another elementOne element replaces another element Forms a new compound which frees the Forms a new compound which frees the

replaced element replaced element Most reactions occur in an aqueous Most reactions occur in an aqueous

solutionsolution

►General Form of EquationGeneral Form of EquationA + BCA + BC AC + B AC + B

Single Replacement Single Replacement ReactionsReactions

► Three typesThree types Metal replaces a metalMetal replaces a metal Metal replaces hydrogenMetal replaces hydrogen Nonmetal replaces nonmetalNonmetal replaces nonmetal

► ExamplesExamples

(s)Ag2 (aq))Cu(NO (aq) AgNO2(s)Cu233

metalmetal metalmetalreplacesreplaces

metalmetal hydrogenhydrogenreplacesreplaces

(g)H (s)ZnSO (aq)SOH(s)Zn 2442

(aq)I (aq)NaCl 2 (aq)Cl(aq)NaI2 22 nonmetalnonmetal replacesreplaces nonmetalnonmetal

Type of ReactionsType of Reactions

►Two compounds exchange ions or Two compounds exchange ions or atoms to form new compoundsatoms to form new compounds

►Also called Also called exchange exchange reactionsreactions Shows the exchange of “associates” when Shows the exchange of “associates” when

comparing the reactants and productscomparing the reactants and products

►General Form of EquationGeneral Form of Equation

AB + CDAB + CD AD + BC AD + BC

Double Replacement Double Replacement ReactionsReactions

►Most of these reactions occur in Most of these reactions occur in aqueous solutionaqueous solution

►Most involve acids, bases, and ionic Most involve acids, bases, and ionic compoundscompounds

►Products formedProducts formed Precipitate (a solid that is insoluble)Precipitate (a solid that is insoluble) A gasA gas WaterWater

Double Replacement Double Replacement ReactionsReactions

► ExamplesExamples

(s) AgCl(aq)NaNO (aq)NaCl(aq)AgNO 33

)(OH (aq)NaCl (aq)NaOH(aq)HCl 2

precipitateprecipitate

waterwater

(g)SH (aq)NaCl2 (aq)HCl2(aq)SNa 22 gasgas

Summary of Reaction TypesSummary of Reaction Types

Combustion ReactionsCombustion Reactions

►Occurs when a hydrocarbon combines Occurs when a hydrocarbon combines with oxygen which produces carbon with oxygen which produces carbon dioxide, water and heat (flame)dioxide, water and heat (flame)

►The reaction of oxygen with any The reaction of oxygen with any substancesubstance

► If a combustion reaction is possible If a combustion reaction is possible then the substance will burnthen the substance will burn

Combustion ReactionsCombustion Reactions

► ExamplesExamples The combustion of propane gasThe combustion of propane gas

►Produces carbon dioxide and waterProduces carbon dioxide and water►Produces heat (flame)Produces heat (flame)

The combustion of sulfur The combustion of sulfur ►Also a combination reactionAlso a combination reaction►Also produces heat (flame)Also produces heat (flame)

(g)OH 4(g)3CO (g)O5(g)HC 22283

(g)SO (g)O(s)S 22

hydrocarbonhydrocarbon

Energy in Chemical ReactionsEnergy in Chemical Reactions

► In a chemical reactionIn a chemical reaction A change in energy occurs as bonds are A change in energy occurs as bonds are

broken (broken (reactantsreactants) and new ones form ) and new ones form ((productsproducts))

Nearly all chemical reactions absorb or Nearly all chemical reactions absorb or produce heatproduce heat

Measured by the Measured by the heat of reactionheat of reaction or or enthalpyenthalpy

►Enthalpy change Enthalpy change is the amount of heat is the amount of heat produced or consumed in a process (produced or consumed in a process (∆H H ))

► In a chemical reactionIn a chemical reaction A change in energy occurs as bonds are A change in energy occurs as bonds are

broken (broken (reactantsreactants) and new ones form ) and new ones form ((productsproducts))

Nearly all chemical reactions absorb or Nearly all chemical reactions absorb or produce heatproduce heat

Measured by the Measured by the heat of reactionheat of reaction or or enthalpyenthalpy

►Enthalpy change Enthalpy change is the amount of heat is the amount of heat produced or consumed in a process (produced or consumed in a process (∆H H ))

Heat of ReactionHeat of Reaction

►Endothermic reactions absorb heat as Endothermic reactions absorb heat as they occurthey occur If (If (∆H H ) is positive, then heat is added to ) is positive, then heat is added to

the reactionthe reaction

►Exothermic reactions produce heat as Exothermic reactions produce heat as they occurthey occur If (If (∆H H )) is negative, then heat is evolved is negative, then heat is evolved

by the reactionby the reaction

Heat of ReactionHeat of Reaction► Photosynthesis reactionPhotosynthesis reaction

Carbon dioxide reacts with water to produce Carbon dioxide reacts with water to produce glucose and oxygenglucose and oxygen

► Cell metabolismCell metabolism Glucose reacts with oxygen to produce carbon Glucose reacts with oxygen to produce carbon

dioxide and water dioxide and water

(g)O6(s)OHC )(OH6(g)CO6 2612622

∆H = +2801 H = +2801 kJkJ

)(OH6(g)CO6 (g)O6(s)OHC 2226126

∆H = -2801 H = -2801 kJkJ

Calculation of Heat in Calculation of Heat in ReactionsReactions

► The combustion of sulfur dioxide The combustion of sulfur dioxide ► It reacts with oxygen to produce sulfur It reacts with oxygen to produce sulfur

trioxidetrioxide

► Calculate the heat produced when 75.2 g of Calculate the heat produced when 75.2 g of sulfur trioxide is produced sulfur trioxide is produced

(g)SO2 (g)O(g)SO2 322 ∆H = -99.1 H = -99.1 kJkJ

Given 75.2 g SOGiven 75.2 g SO33

Heat in kJ produced Heat in kJ produced

when SOwhen SO3 3 is formedis formed

Calculation of Heat in Calculation of Heat in ReactionsReactions

Relation between g of SORelation between g of SO33 and heat and heat released released

Grams Grams

of SOof SO33

MolesMoles

of SOof SO33

kkjj

Write the necessary conversion factorsWrite the necessary conversion factors

3

3

SOmol2

kJ99.1

kJ99.1

SOmol2and

3

3

SOmol1

g80.07

g 80.07

SOmol1and

Set up the problemSet up the problem

33

33 SOmol2

kJ99.1

SOg80.07

SOmol1SOg75.2 46.5 kJ46.5 kJ

Heat Heat of rxnof rxn

MolarMolarmassmass

►EndEnd