Chapter 5 Molecular View of Reactions in Aqueous Solutions Part I Chemistry: The Molecular Nature of...

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Chapter 5 Molecular View of

Reactions in Aqueous Solutions

Part IChemistry: The Molecular Nature of Matter, 6E

Jespersen/Brady/Hyslop

Jespersen/Brady/Hyslop Chemistry: The Molecular Nature of Matter, 6E

Reactions in Solution

For reaction to occur Reactants needs to come into physical

contact

Happens best in gas or liquid phase Movement occurs

Solution Homogeneous mixture

Two or more components mix freely

Molecules or ions completely intermingled

Contains at least two substances

Definitions:Solvent Medium that dissolves solutes

Component present in largest amount Can be gas, liquid, or solid Aqueous solution—water is solvent

Solute Substance dissolved in solvent

Solution is named by solute Can be gas—CO2 in soda Liquid—ethylene glycol in antifreeze Solid—sugar in syrup

Iodine Molecules in Ethanol

Crystal of solute placed in solvent

Solute molecules dispersed throughout solvent

Solutions May be characterized using

Concentration Solute-to-solvent ratio

Percent concentration

Relative ConcentrationDilute solution

Small solute to solvent ratio

e.g. Eye drops

Concentrated solution Large solute to solvent

e.g. Pickle brine

Dilute solution contains less solute per unit volume than more concentrated solution

ConcentrationSolubility

Temperature dependent

Saturated solution Solution in which no more solute can be

dissolved at a given temperature

Unsaturated solution Solution containing less solute than

maximum amount Able to dissolve more solute

Solubilities of Some Common Substances

Substance Formula

Solubility (g/100 g water)

Sodium chloride

NaCl 35.7 at 0 °C39.1 at 100 °C

Sodium hydroxide

NaOH 42 at 0 °C347 at 100 °C

Calcium carbonate

CaCO3 0.0015 at 25 °C

ConcentrationsSupersaturated Solutions

Contains more solute than required for saturation at a given temperature

Formed by careful cooling of saturated solutions

Unstable Crystallize out when add seed crystal –

results in formation of solid or precipitate (ppt.)

PreciptatesPrecipitate

Solid product formed when reaction carried out in solutions and one product has low solubility

Insoluble product

Separates out of solution

Precipitation reaction Reaction that produces precipitate

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

1 mol Pb(NO3)2 2 mol KI

0.100 mol Pb(NO3)2 0.200 mol KI

Electrolytes in Aqueous Solution Ionic compounds conduct electricity

Molecular compounds don’t conduct electricityWhy?

Bright light

No light

Molecular

Ions present

CuSO4 and water

Sugar and water

Ionic Compounds (Salts) in Water Water molecules arrange themselves

around ions and remove them from lattice.Dissociation

Salts break apart into ions when entering solution

Separated ions Hydrated Conduct electricity

Note: Polyatomic ions remain intact e.g. KIO3 K+ + IO3

NaCl(s) Na+(aq) + Cl–

Molecular Compounds In Water When molecules dissolve in water

Solute particles are surrounded by water Molecules do not dissociate

Electrical ConductivityElectrolyte

Solutes that yield electrically conducting solutions

Separate into ions when enter into solution

Strong electrolyte Electrolyte that dissociates 100% in water

Yields aqueous solution that conducts electricity

Good electrical conduction

Ionic compounds, e.g. NaCl, KNO3

Strong acids and bases, e.g. HClO4, HCl14

Electrical Conductivity

Weak electrolyte When dissolved in water only a small percentage

ionize

Common examples are weak acids and bases

Solutions weakly conduct electricity

e.g. Acetic acid (HC2H3O2), ammonia (NH3)

Non-electrolyte Aqueous solution that doesn’t conduct electricity

Molecules remain intact in solution

e.g. Sugar, alcohol

Strong vs. Weak Electrolyte

HCl(aq) CH3COOH(aq) NH3(aq)

Your Turn

How many ions form on the dissociation of Na3PO4?

Your Turn

How many ions form on the dissociation of

Al2(SO4)3?

Equations for Dissociation Reactions

Ionic compound dissolves to form hydrated ions Hydrated = surrounded by water molecules

In chemical equations, hydrated ions are indicated by Symbol (aq) after each ions

Ions are written separately

KBr(s) K+(aq) + Br–(aq)

Mg(HCO3)2(s) Mg2+(aq) + 2HCO3–(aq)

Learning CheckWrite the equations that illustrate the dissociation of the following salts:

Na3PO4(aq) →

Al2(SO4)3(aq) →

CaCl2(aq) →

Ca(MnO4)2(aq) →

2Al3+(aq) + 3SO42–(aq)

3Na+(aq) + PO43–(aq)

Ca2+(aq) + 2Cl–(aq)

Ca2+(aq) + 2MnO4–

Equations of Ionic Reactions Consider the reaction of Pb(NO3)2 with KI

PbI2(s)Pb2+ NO3

– K+ I–

Equations of Ionic Reactions When two soluble ionic solutions are

mixed, sometimes an insoluble solid forms. Three types of equations used to describe

1. Molecular equation Substances listed as complete formulas

2. Ionic equation All soluble substances broken into ions

3. Net ionic equation Only lists substances that actually take

part in reaction

Equations of Ionic Reactions

1. Molecular Equation Complete formulas for all reactants and products

Formulas written with ions together

Does not indicate presence of ions (no charges)

Gives identities of all compounds

Good for planning experiments

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

Equations of Ionic Reactions2. Ionic Equation

Emphasizes the reaction between ions

All strong electrolytes dissociate into ions

Used to visualize what is actually occurring in solution

Insoluble solids written together as they don’t dissociate to any appreciable extent

Pb2+(aq) + 2NO3–(aq) + 2K+(aq) + 2I–(aq)

PbI2(s) + 2K+(aq) + 2NO3–(aq)

Equations of Ionic ReactionsSpectator Ions

Ions that don’t take part in reaction

They hang around and watch

K+ and NO3– in our example

3. Net Ionic Equation Eliminate all spectator ions

Emphasizes the actual reaction

Focus on chemical change that occurs

e.g. Pb2+(aq) + 2I–(aq) PbI2(s)

Net Ionic Equations Many ways to make PbI2

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

2. Pb(C2H3O2)2(aq) + 2NH4I(aq)

PbI2(s) + 2NH4C2H3O2(aq)

Different starting reagents

Same net ionic equation

Pb2+(aq) + 2I–(aq) PbI2(s) 26

Converting Molecular Equations to Ionic Equations

Strong electrolytes exist as dissociated ions in solution

Strategy

1. Identify strong electrolytes

2. Use subscript coefficients to determine total number of each type of ion

3. Separate ions in all strong electrolytes

4. Show states as recorded in molecular equations

Learning Check: Convert Molecular to Ionic Equations:

Write the correct ionic equation for each:

Pb(NO3)2(aq) + 2NH4IO3(aq) →

Pb(IO3)2(s) + 2NH4NO3(aq)

2NaCl (aq) + Hg2(NO3)2 (aq) → 2NaNO3 (aq) + Hg2Cl2 (s)

2Na+(aq) + 2Cl–(aq) + Hg22+(aq) + 2NO3

–(aq) →

2Na+(aq) + 2NO3–(aq) + Hg2Cl2(s)

Pb2+(aq) + 2NO3–(aq) + 2NH4

+(aq) + 2IO3–(aq) →

Pb(IO3)2(s) + 2NH4+(aq) + 2NO3

–(aq)

Your TurnConsider the following reaction :

Na2SO4(aq) + BaCl2(aq) → 2NaCl(aq) + BaSO4(s)

Which is the correct ionic equation?

A. 2Na+(aq) + SO42–(aq) + Ba2+(aq) + Cl22–(aq) →

2Na+(aq) + 2Cl–(aq) + BaSO4(s)

B. 2Na+(aq) + SO42–(aq) + Ba2+(aq) + 2Cl–(aq) →

2Na+(aq) + 2Cl–(aq) + BaSO4(s)

C. 2Na+(aq) + SO42–(aq) + Ba2+(aq) + Cl22–(aq) →

2Na+(aq)+ 2Cl–(aq) + Ba2+(s) + SO42–(s)

D. Ba2+(aq) + SO42–(aq) → BaSO4(s)

E. Ba2+(aq) + SO42–(aq) → Ba2+(s) + SO4

2–(s) 29

Converting Ionic Equations to Net Ionic Equations

Strategy1. Identify spectator ions

2. Cancel from both sides

3. Rewrite equation using only substances that actually react.

4. Show states as recorded in molecular and ionic equations

Learning Check: Convert Ionic Equation to Net Ionic Equation

Write the correct net ionic equation for each.

Pb2+(aq) + 2NO3–(aq) + 2K+(aq) + 2IO3

–(aq) →

Pb(IO3)2(s)+ 2K+(aq) + 2NO3–

2Na+(aq) + 2Cl–(aq) + Hg22+(aq) + 2NO3

–(aq) →

2Na+(aq)+ 2NO3–(aq) + Hg2Cl2(s)

2Cl–(aq) + Hg22+(aq) → Hg2Cl2(s)

Pb2+(aq) + 2IO3–(aq) → Pb(IO3)2(s)

Your TurnConsider the following molecular equation:

(NH4)2SO4(aq) + Ba(CH3CO2)2(aq) →

2NH4CH3CO2(aq) + BaSO4(s)

Which is the correct net ionic equation?

A. Ba2+(aq) + SO42–(aq) → BaSO4(s)

B. 2NH4+(aq) + 2CH3CO2

–(aq) → 2NH4CH3CO2(s)

C. Ba2+(aq) + SO42–(aq) → BaSO4(aq)

D. 2NH4+(aq) + Ba2+(aq) + SO4

2–(aq) + 2CH3CO2–(aq) →

2NH4+(aq) + 2CH3CO2

–(aq) + BaSO4(s)

E. 2NH4+(aq) + 2CH3CO2

–(aq) → 2NH4CH3CO2(aq)32

Criteria for Balancing Ionic and Net Ionic Equations

Material Balance There must be the same number of

atoms of each kind on both sides of the arrow

Electrical Balance The net electrical charge on the left must

equal the net electrical charge on the right

Charge does not have to be zero

Learning Check: Balancing Equations for Mass & Charge

Balance molecular equation for mass 2Na3PO4(aq) + 3Pb(NO3)2(aq)

6NaNO3(aq) + Pb3(PO4)2(s)

Can keep polyatomic ions together when counting

Balance ionic equation for charge 6Na+(aq) + 2PO4

3–(aq) + 3Pb2+(aq) + 6NO3–(aq)

6Na+(aq) + 6NO3–(aq) + Pb3(PO4)2(s)

Charge must add up to zero on both sides.

Net ionic equation balanced for mass and charge

3Pb2+(aq) + 2PO43–(aq) Pb3(PO4)2(s)

Acids and Bases as Electrolytes Many common laboratory chemicals and

household products

Indicators Dye molecules that change color

in presence of acids or bases

Acids Turn blue litmus red Lemon juice, vinegar, H2SO4

Bases Turn red litmus blue Drano (lye, NaOH), ammonia (NH3)

Neutralization Reaction Important reaction of acids and bases Acid reacts with base to form water and salt

(ionic compound).

Acid + base salt + H2O

e.g. HCl(aq) + NaOH(aq) NaCl(aq) + H2O

HBr(aq) + LiOH(aq) LiBr(aq) + H2O

1:1 mole ratio of acid:base gives neutral solution

Ionization reactions Ions form where none have been before

Reactions of acids or bases with water

Arrhenius Strong acid-base neutralization is

H+(aq) + OH–(aq) H2O

In solution, H+ is hydrated and we often present this as H3O+ and call it the hydronium ion H+ does not ever exist in aqueous solution

We often use just H+ for simplicity

)()(OH

)( Cl HHCl 2 aqaqg

Arrhenius Acid Substance that reacts with water to produce

the hydronium ion, H3O+

Acid + H2O Anion + H3O+

HA + H2O A– + H3O+

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2−(aq)

HCl(g) + H2OCl–(aq) + H3O+(aq)

Acids Categorized by Number of H+sMonoprotic Acids

Furnish only one H+

HNO3(aq) + H2O H3O+(aq) + NO3–(aq)

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2–(aq)

Diprotic acids — furnish two H+

H2SO3(aq) + H2O H3O+(aq) + HSO3–(aq)

HSO3–(aq) + H2O H3O+(aq) + SO3

2–(aq)

Polyprotic acids Furnish more than one H+

Acids Categorized by Number of H+sPolyprotic acids

Triprotic acids — furnish three H+

H3PO4 H2PO4– HPO4

2– PO43–

Stepwise equations

H3PO4(aq) + H2O H3O+(aq) + H2PO4–(aq)

H2PO4–(aq) + H2O H3O+(aq) + HPO4

2–(aq)

HPO42–(aq) + H2O H3O+(aq) + PO4

3–(aq)

H3PO4(aq) + 3H2O 3H3O+(aq) + PO43–(aq)

– H+ – H+ – H+

Acidic Anhydrides Nonmetal Oxides

Act as Acids

React with water to form molecular acids that contain hydrogen

SO3(g) + H2O H2SO4(aq)

sulfuric acid

N2O5(g) + H2O 2HNO3(aq)

nitric acid

CO2(g) + H2O H2CO3(aq)

carbonic acid41

Arrhenius Bases Ionic compounds that contain hydroxide ion,

OH–, or oxide ion, O2–.or

Molecular compounds that react with water to give OH–.

1. Ionic compounds containing OH– or O2–

a. Metal hydroxides Dissociate into metal and hydroxide ions

NaOH(s) Na+(aq) + OH–(aq)

Mg(OH)2(s) Mg2+(aq) + 2OH–(aq) 42

Ionic Oxidesb. Basic Anhydrides

Soluble metal oxides Undergo ionization (hydrolysis) reaction to

form hydroxide ions Oxide reacts with water to form metal

hydroxide

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

Then metal hydroxide dissociates in water

Ca(OH)2(aq) Ca2+(aq) + 2OH–(aq) 43

2OH–O2– H2O

Strong Acids

Dissociate completely when dissolved in watere.g. HBr(g) + H2O H3O+(aq) + Br–(aq)

Good electrical conduction Any acid not on this list, assume weak

HClO4(aq) perchloric acid

HClO3(aq) chloric acid

HCl(aq) hydrochloric acidHBr(aq) hydrobromic acidHI(aq) hydroiodic acid

HNO3(aq) nitric acid

H2SO4(aq) sulfuric acid

Arrhenius Bases2. Molecular Bases

Undergo ionization (hydrolysis) reaction to form hydroxide ions

Base + H2O BaseH+(aq) + OH–(aq)

B + H2O BH+(aq) + OH–(aq)

NH3(aq) + H2O NH4+(aq) + OH–(aq)

NH3 H2O NH4+ OH–

Strong Bases Bases that dissociate completely in water

Soluble metal hydroxides KOH(aq) K+(aq) + OH–(aq)

Good electrical conductors Behave as aqueous ionic compounds Common strong bases are:

Group 1A metal hydroxides LiOH, NaOH, KOH, RbOH, CsOH

Group 2A metal hydroxides

Ca(OH)2, Sr(OH)2, Ba(OH)2

Weak Acids Any acid other than seven strong acids Only ionize partially (<100%)

Organic acids HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2

–(aq)

e.g. HCO2H(aq) + H2O H3O+(aq) + HCO2

–(aq)47

Only this H comes off as H+

Acetic AcidMolecule,HC2H3O2

Acetate ion, C2H3O2

Why is Acetic Acid Weak?

H3O+(aq) + C2H3O2–(aq) HC2H3O2(aq) +

H2O + C2H3O2–(aq) HC2H3O2(aq) + H3O+

Dynamic Equilibrium Two opposing reactions occurring at same

rate Also called chemical equilibrium

Equilibrium Concentrations of substances present in

solution do not change with time

Dynamic Both opposing reactions occur continuously

Represented by double arrowHC2H3O2(aq) + H2O H3O+(aq) + C2H3O2

–(aq)

Forward reaction – forms ions

Reverse reaction – forms molecules 49

Weak Bases Molecular bases Do not dissociate Accept H+ from water inefficiently Accept H+ from acids preferentially

NH3(aq) + HCl(aq) NH4Cl(aq)

NH3(aq) + H2O NH4+(aq) + OH–(aq)

Or for general base

B(aq) + H2O BH+(aq) + OH–(aq)

Equilibrium for Weak BaseForward reaction

Reverse reaction

Net is dynamic equilibriumNH3(aq) + H2O NH4

+(aq) + OH–(aq)

Position of Equilibrium Extent of completion Depends on electrolyte

Strong electrolyte Completely ionizes Forward reaction

dominates Mostly products

Strong acids & bases Little reverse reaction Write eqn. as

Weak electrolyte Small % ionizes Reverse rxn

dominates Mostly reactants

Weak acids and bases

Lots of reverse reaction

Write eqn. as

Learning Check Write the ionization equation for each of

the following with water:

1.Weak base methylamine, CH3NH2.

2.Weak acid nitrous acid, HNO2.

3.Strong acid chloric acid, HClO3.

4.Strong base strontium hydroxide, Sr(OH)2.

CH3NH2(aq) + H2O CH3NH3+(aq) + OH–(aq)

HClO3(aq) + H2O H3O+(aq) + ClO3–(aq)

Sr(OH)2(aq) Sr2+(aq) + 2OH–(aq)

HNO2(aq) + H2O H3O+(aq) + NO2–

Your Turn

Which of the following is a weak acid?

B.HNO3

C.HClO4

D.HC2H3O2

E.H2SO4

Your Turn

Which of the following is not a strong base?

A.NaOH

B.CH3NH2

C.Cs2O

D.Ba(OH)2

Your Turn

Which of the following is not a product of the reaction:

NH3(aq) + HCN(aq) ?

A. CN–(aq)

B. NH4+(aq)

C. NH3CN(s)

Acid—Base Nomenclature System for naming acids and bases

Acids Binary acid system e.g. HCl(aq), H2S(aq)

Oxoacid system e.g. H2SO4, HClO2

Acid salt system e.g. NaHSO4, NaHCO3

Bases Metal hydroxide/oxide system e.g. NaOH,

Molecular base system e.g. NH3, (CH3)3N

Naming AcidsA. Binary Acids — hydrogen + nonmetal

Take molecular name Drop –gen from H name Merge hydro– with nonmetal name Replace –ide with –ic acid

Name of Molecular Compound

Name of Aqueous Binary Acid

HCl(g)

hydrogen chloride

HCl(aq)

hydrochloric acid

H2S(g)

hydrogen sulfide

H2S(aq)

hydrosulfuric acid

Naming AcidsB. Oxo Acids

Acids with hydrogen, oxygen and another nonmetal element

Most of the polyatomic ions in Table 3.5

To name: Based on parent oxoanion name

Take parent ion name

Anion ends in –ate change to –ic (more O's)

Anion ends in –ite change to –ous (less O's)

End name with acid to indicate H+

Oxoacids (Aqueous)Named according to the anion suffix

Anion ends in -ite, acid name is -ous acid

Anion ends in -ate, acid name is -ic acid

Name of Parent Oxoanion

Name of Oxoacid

NO3 HNO3

SO42 H2SO4

ClO2 HClO2

PO32 H2PO3

sulfate

chlorite

phosphite

sulfuric acid

chlorous acid

phosphorous acid

nitrate nitric acid

Learning Check: Name Each Aqueous Acid

nitrous acid hydrocyanic acid perchloric acid hydrofluoric acid carbonic acid

Your Turn

Which of the following is the correct name for HClO4 (aq)?

A. chloric acid

B. hydrochloric acid

C. perchloric acid

D. hypochlorous acid

E. chlorous acid

Your Turn

Which of the following is the correct name for H2SO3(aq)?

A.sulfuric acid

B.sulfurous acid

C.hydrosulfuric acid

D.hydrosulfurous acid

E.hydrogen sulfite acid

Acid Salts Polyprotic acids can be neutralized

stepwise Can halt neutralization at each step

Name must specify number of hydrogens remaining in the salt

Acid salt Formula contains a cation, a hydrogen, and an

The acid salt can react with a base

H2SO4(aq) + KOH(aq) KHSO4(aq) + H2O(l )

acid salt64

Naming Acid Salts—Polyprotic Must specify number of hydrogens still

attached to the anion Can be neutralized by additional base

e.g. Na2HPO4

NaH2PO4

Some acid salts have common names

NaHCO3

sodium hydrogen carbonate or sodium bicarbonate

sodium hydrogen phosphate

sodium dihydrogen phosphate

potassium hydrogen sulfate

C. Naming BasesOxides & Hydroxides

Ionic compounds Named like ionic compounds

Ca(OH)2 calcium hydroxide

Li2O lithium oxide

Molecular Bases Named like molecules

NH3 ammonia

CH3NH2 methylamine

(CH3)2NH dimethylamine

(CH3)3N trimethylamine66

Chapter 5 Molecular View of Reactions in Aqueous Solutions Part I Chemistry: The Molecular Nature of...

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