Chemical Reactions Chapter 5 Dr. Victor Vilchiz. Solution Components In order to have a solution we...

Chemical Chemical ReactionsReactions

Chapter 5Chapter 5

Dr. Victor Vilchiz

Solution ComponentsSolution Components

• In order to have a solution we must In order to have a solution we must have at least have at least “TWO”“TWO” components. components.– The SolventThe Solvent which is the compound which is the compound

present in biggest abundance, water in present in biggest abundance, water in most cases presented in this chapter.most cases presented in this chapter.

– The SoluteThe Solute which is the “impurity” or which is the “impurity” or compound present in the smallest compound present in the smallest amount.amount.

– The resulting mixture is The resulting mixture is the solutionthe solution..

Dissolving Ionic Dissolving Ionic CompoundsCompounds

• Some compounds dissociate when Some compounds dissociate when placed in water.placed in water.– This does not mean we will get the This does not mean we will get the

constituent elements once in water.constituent elements once in water.– For example: when table salt (NaCl) is For example: when table salt (NaCl) is

added to a container with water as soon added to a container with water as soon as the grains of salt touch the water as the grains of salt touch the water NaCl ceases to exist. In solution, we NaCl ceases to exist. In solution, we have the component ions Nahave the component ions Na++ and Cl and Cl--

Ions in Aqueous SolutionIons in Aqueous Solution

• Many ionic compounds Many ionic compounds dissociate into into independent ions when dissolved in independent ions when dissolved in waterwater

These compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytes.

Their aqueous solutions are capable of conducting an electric current. Figure 4.2 illustrates this.

Ionic Theory of Solutions

)aq(Cl)aq(Na)s(NaClOH 2


• Electrolytes are substances that Electrolytes are substances that dissolve in water to give an dissolve in water to give an electrically conducting solution.electrically conducting solution.

Thus, in general, ionic solids that dissolve in water are electrolytes.

Some molecular compounds, such as acids, also dissociate in aqueous solution and are considered electrolytes.



• Molecular compounds that dissolve Molecular compounds that dissolve usually do not dissociate into ions.usually do not dissociate into ions.

These compounds are referred to as nonelectrolytes. They dissolve in water to give a nonconducting solution.

)aq(OHC )glucose( )s(OHC 6126 OH

61262


Conductivity Test


• There are few There are few molecular compoundsmolecular compounds (acids & alcohols) that upon solvation (acids & alcohols) that upon solvation dissociate into ions, this is due to the dissociate into ions, this is due to the weak interaction between the atoms.weak interaction between the atoms.

The resulting solution is electrically conducting, and so we say that the molecular substance is an electrolyte.

)aq(Cl)aq(H)aq(HCl


Does water conduct?Does water conduct?

• As we have seen current/energy does As we have seen current/energy does not flow in a circuit unless there are not flow in a circuit unless there are “free” ions in solution.“free” ions in solution.– This means that a sample or “pure” water This means that a sample or “pure” water

will not conduct electricity or current.will not conduct electricity or current.– Tap water conducts electricity and Tap water conducts electricity and

current because there are dissolved ions current because there are dissolved ions present put there on purpose and some present put there on purpose and some from pipes dissolving. from pipes dissolving.

Strong ElectrolytesStrong Electrolytes

• Electrolytes can be separated into two Electrolytes can be separated into two different categoriesdifferent categories

– Strong electrolytes.Strong electrolytes.

A A strong electrolytestrong electrolyte is an electrolyte that is an electrolyte that exists in solution almost entirely as ions.exists in solution almost entirely as ions.

The solvation process is represented by a The solvation process is represented by a one-way arrow in the chemical reaction one-way arrow in the chemical reaction implying a path of no return.implying a path of no return.

)aq(Cl)aq(Na)s(NaCl OH 2


Weak ElectrolytesWeak Electrolytes

– Weak electrolytes.Weak electrolytes.

A A weak electrolyteweak electrolyte is an electrolyte is an electrolyte that dissolves in water to give a that dissolves in water to give a relatively small percentage of ions.relatively small percentage of ions.

The solvation process is presented by a double sided arrow implying an equilibrium between reactants and products.

Most soluble molecular compounds are either nonelectrolytes or weak electrolytes.

(aq)SO(aq)Mg (aq)MgSO 24

24


Strength Test

Why dissociation?Why dissociation?

• Dissociation takes place because the Dissociation takes place because the attractive force between the ions can attractive force between the ions can be overcome by other forces.be overcome by other forces.– The solvent is able to surround ions and The solvent is able to surround ions and

provide stronger forces of attraction. provide stronger forces of attraction. Why?Why?• If water has no charge how can it create this If water has no charge how can it create this

attractive forces that compete with coulombic attractive forces that compete with coulombic interaction?interaction?

– This can be explained if we look at the electron This can be explained if we look at the electron distribution in water.distribution in water.

PolarityPolarity• Electron distributionElectron distribution

– If we draw a water molecule representing If we draw a water molecule representing its true shape we will see that the its true shape we will see that the electrons are not evenly distributed.electrons are not evenly distributed.

The Red represents a high

density of electrons (-); the blue represents a

low density of electrons (+).

While there are no real charges the

difference in electron density

acts as a separation of charges which

leads to a pseudo ionic behavior.

The separation of charge we see in water is labeled as the polarity of the molecule; the higher the difference in

electron density the higher the polarity of the molecule.

PolarityPolarity

• The polarity of a molecule depends mainly on two The polarity of a molecule depends mainly on two factorsfactors– Shape of the moleculeShape of the molecule– CompositionComposition

• It is represented by an arrow with its head It is represented by an arrow with its head pointing towards the negative charge side and a pointing towards the negative charge side and a crossed tail on the positive side of the moleculecrossed tail on the positive side of the molecule

Polarity and SolvationPolarity and Solvation

• Molecules that have a separation of Molecules that have a separation of charges are called polar moleculescharges are called polar molecules

• When ionic compounds are added to When ionic compounds are added to water, the ions break apart and the water, the ions break apart and the water molecules arrange themselves water molecules arrange themselves so the negative end (O) points so the negative end (O) points towards the cations and the positive towards the cations and the positive end (H’s) point towards the anions.end (H’s) point towards the anions.

Representation

Insoluble CompoundsInsoluble Compounds

• No compound is really insoluble.No compound is really insoluble.– However, if the amount that dissolves is However, if the amount that dissolves is

compared to the starting amount it is compared to the starting amount it is found to be insignificant that they are found to be insignificant that they are said to be insoluble.said to be insoluble.•ExampleExample

– NaCl in HNaCl in H22O @20°C =365g/LO @20°C =365g/L

– AgCl in HAgCl in H22O @20°C =0.009g/LO @20°C =0.009g/L

Chemical EquationsChemical Equations

• A A molecular equationmolecular equation is one in is one in which the reactants and products are which the reactants and products are written as if they were molecules, written as if they were molecules, even though they may actually even though they may actually exist in solution as ionsexist in solution as ions..

Note that Ca(OH)2, Na2CO3, and NaOH are all soluble compounds but CaCO3 is not.

Molecular and Ionic Equations

)aq(CONa)aq()OH(Ca 322 )aq(NaOH2)s(CaCO3


• An An ionic equationionic equation, however, , however, represents strong electrolytes as represents strong electrolytes as separate independent ions. This is a separate independent ions. This is a more accurate representation of the more accurate representation of the way electrolytes behave in solution.way electrolytes behave in solution.


)aq(CO)aq(Na2)aq(OH2)aq(Ca 23

2

)aq(OH2)aq(Na2)s(CaCO3

The downward arrow represents a precipitate which will fall to the bottom of

the container

A complete ionic equation is a chemical equation in which strong electrolytes (such as soluble ionic compounds) are written as separate ions in solution.


• Complete and net ionic equationsComplete and net ionic equationsMolecular and Ionic Equations

)aq(CO)aq(K2)aq(NO2)aq(Ca 233

2

)aq(COK)aq()NO(Ca 3223(strong) (strong) (strong)(insoluble)

)aq(KNO2)s(CaCO 33

)aq(NO2)aq(K2)s(CaCO 33


• Complete and net ionic equations.Complete and net ionic equations.Molecular and Ionic Equations

A net ionic equation is a chemical equation from which the spectator ions have been removed.

A spectator ion is an ion in an ionic equation that does not take part in the reaction (present on both sides of the arrow in the same state.

)aq(CO)aq(K2)aq(NO2)aq(Ca 233

2

)aq(NO2)aq(K2)s(CaCO 33


• Complete and net ionic equationsComplete and net ionic equationsLet’s try an example. First, we start with Let’s try an example. First, we start with a molecular equation.a molecular equation.


Nitric acid, HNO3, and magnesium nitrate, Mg(NO3)2, are both strong electrolytes.

)s()OH(Mg)aq(HNO2 23 )aq()NO(Mg)l(OH2 232


• Complete and net ionic equationsComplete and net ionic equationsSeparating the strong electrolytes into Separating the strong electrolytes into separate ions, we obtain the complete separate ions, we obtain the complete ionic equation.ionic equation.


)s()OH(Mg)aq(NO2)aq(H2 23

)aq(NO2)aq(Mg)l(OH2 32

2

Note that the nitrate ions did not participate in the reaction. These are spectator ions.


• Complete and net ionic equationsComplete and net ionic equationsEliminating the spectator ions results in Eliminating the spectator ions results in the net ionic equation.the net ionic equation.


)s()OH(Mg)aq(NO2)aq(H2 23

)aq(NO2)aq(Mg)l(OH2 32

2

)aq(Mg)l(OH2)s()OH(Mg)aq(H2 222

This equation represents the “essential” reaction.

Types of Chemical Types of Chemical ReactionsReactions

Precipitation ReactionsPrecipitation Reactions

Acid-Base ReactionsAcid-Base Reactions

Oxidation-Reduction ReactionsOxidation-Reduction Reactions

Most of the reactions we will study fall into one of the following categories

Types of Chemical Types of Chemical ReactionsReactionsPrecipitation

Reactions• In a In a precipitation reaction we start with 2 we start with 2 soluble compounds dissolved in water and soluble compounds dissolved in water and when mixed they produce at least one when mixed they produce at least one insoluble compound, which insoluble compound, which precipitates precipitates (falls to the bottom). (falls to the bottom).

For example, the reaction of sodium chloride with silver nitrate forms AgCl(s), an insoluble precipitate.

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

Precipitation ReactionsPrecipitation Reactions

• Does this mean that if we mixed two Does this mean that if we mixed two soluble ionic compounds we will always soluble ionic compounds we will always form a precipitate?form a precipitate?

NONO

• What is the driving force for What is the driving force for precipitation reactions?precipitation reactions?– While the formation of the solid can be While the formation of the solid can be

viewed as the driving force, it is the viewed as the driving force, it is the removal of ions from solution that is the removal of ions from solution that is the true driving force.true driving force.


• Solubility rulesSolubility rules

Precipitation Reactions

Substances vary widely in their solubility, or ability to dissolve, in water.

For example, NaCl is very soluble in water whereas calcium carbonate, CaCO3, is insoluble in water. (see Figure 4.5)


• Predicting Precipitation Reactions.Predicting Precipitation Reactions.


To predict whether a precipitate will form, we need to look at potential insoluble products.

Table 4.1 lists eight solubility rules for ionic compounds. These rules apply to the most common ionic compounds.




Suppose you mix together solutions of nickel(II) chloride, NiCl2, and sodium phosphate, Na3PO4.

How can you tell if a reaction will occur, and if it does, what products to expect?

432 PONaNiCl




Precipitation reactions have the form of an “exchange reaction.”

An exchange (or metathesis) reaction is a reaction between compounds that, when written as a molecular equation, appears to involve an exchange of cations and anions.

432 PONaNiCl NaCl)PO(Ni 243

3 2 6




Now that we have predicted potential products, we must balance the equation.

We must verify that NiCl2 and Na3PO4 are soluble and then check the solubilities of the products.

432 PONa NiCl NaCl )PO(Ni 243

(aq) (aq)




Looking at the potential products we find that nickel(II) phosphate is not soluble although sodium chloride is.

NaCl6 )PO(Ni 243

Table 4.1 indicates that our reactants, nickel(II) chloride and sodium phosphate are both soluble.

PONa2 NiCl3 432

(s) (aq)




We predict that a reaction occurs because nickel(II) phosphate is insoluble and precipitates from the reaction mixture.

To see the reaction that occurs on the ionic level, we must rewrite the molecular equation as an ionic equation.




First write strong electrolytes (the soluble ionic compounds) in the form of ions to obtain the complete ionic equation

)aq(PO2)aq(Na6)aq(Cl6)aq(Ni3 34

2

)aq(Cl6)aq(Na6)s()PO(Ni 243




After canceling the spectator ions, you obtain the net ionic equation..

)aq(PO2)aq(Na6)aq(Cl6)aq(Ni3 34

2

)aq(Cl6)aq(Na6)s()PO(Ni 243

)s()PO(Ni)aq(PO2)aq(Ni3 2433

42

This equation represents the “essential” reaction.


• Acid-Base ReactionsAcid-Base ReactionsAcids and bases are some of the most important electrolytes. (see Table 4.2)

They can cause color changes in certain dyes called acid-base indicators.

Household acids and bases. (see Figure 4.7)

Red cabbage juice as an acid-base indicator. (see Figure 4.8)


• The Ancient ConceptThe Ancient ConceptIn ancient times acids and bases had a different meaning.

An acid was defined as a sour substance.

A Base was defined as a substance that was both bitter and slippery

Acid-Base Reactions


• The Arrhenius ConceptThe Arrhenius ConceptThe Arrhenius concept defines acids as substances that contain H and produce hydrogen ions, H+, when dissolved in water.

An example is nitric acid, HNO3, a molecular substance that dissolves in water to give H+ and NO3

-.

Acid-Base Reactions

)aq(NO)aq(H)aq(HNO 3 OH

32

Types of Chemical Types of Chemical ReactionsReactionsAcid-Base Reactions

• The Arrhenius ConceptThe Arrhenius ConceptThe Arrhenius concept defines bases as substances that contain OH and produces hydroxide ions, OH-, when dissolved in water.

An example is sodium hydroxide, NaOH, an ionic substance that dissolves in water to give sodium ions and hydroxide ions.

He really meant contain OH-

)aq(OH)aq(Na)s(NaOH OH 2

• The Arrhenius ConceptThe Arrhenius ConceptHowever, there are substances that we now classify as bases or acids but they do not follow the Arrhenius definition.

For example ammonia, NH3, is a base but it does not contain OH-,

Therefore we need a second definition that can take compounds like ammonia into account.


)aq(OH)aq(NH)l(OH)aq(NH 4

23


The Brønsted-Lowry concept of acids and bases avoids the problems of composition inherent in the Arrhenius definitions by basing the definitions on the transfer of protons (H+) instead.

In this view, acid-base reactions are proton-transfer reactions and there must be two reactions taking place at once.

• The Brønsted-Lowry ConceptThe Brønsted-Lowry Concept


• The Brønsted-Lowry ConceptThe Brønsted-Lowry ConceptThe Brønsted-Lowry concept defines an acid as the species (molecule or ion) that donates a proton (H+) to another species in a proton-transfer reaction.

A base is defined as the species (molecule or ion) that accepts the proton (H+) in a proton-transfer reaction.


– The H2O molecule is the acid because it donates a proton. The NH3 molecule is a base, because it accepts a proton.

– Likewise NH4+ is an acid because it can donate one of the

protons, and OH- is a base since it can accept a proton.

)( )( )( )( 43 aqOHaqNHlHOHaqNH

H+

In the reaction of ammonia with water,


• The Brønsted-Lowry ConceptThe Brønsted-Lowry Concept– The H+(aq) ion due to its small size has a very

high positive charge density.– The polarity of the water molecules allowed for

the water molecules to be closely associated with the proton making it appear as if the hydrogen ion was bonded to water molecules as it moves.

• This “mode of transportation” for the H+ ion is called the hydronium ion.


)aq(OH)l(OH)aq(H 32


)aq(OH)aq(NO)l(OH)aq(HNO 3323

The dissolution of nitric acid, HNO3, in water is therefore a proton-transfer reaction


where HNO3 is an acid (proton donor) and H2O is a base (proton acceptor).

H+

Water and Acid/Base Water and Acid/Base RxnsRxns

• As we have seen there are cases in As we have seen there are cases in which water:which water:– Donates a Proton acting as an acid.Donates a Proton acting as an acid.– Accepts a Proton acting as a base.Accepts a Proton acting as a base.

• Molecules can act both as acid or Molecules can act both as acid or base depending on the environment base depending on the environment they are in are called they are in are called amphiproticamphiprotic..

Acid-Base Reactions


Arrhenius Concept

acid: proton (H+) donor to the water

base: hydroxide ion (OH-) donor to the water

• In summary, the Arrhenius concept is In summary, the Arrhenius concept is very basic and the Brønsted-Lowry very basic and the Brønsted-Lowry concept was developed to cover concept was developed to cover cases left out; however, in water cases left out; however, in water they are almost the same.they are almost the same.


The Brønsted-Lowry concept

acid: proton (H+) donor to anything

base: proton (H+) acceptor from anything

• In summary, any Arrhenius acid/base In summary, any Arrhenius acid/base is also a Brønsted-Lowry acid/base is also a Brønsted-Lowry acid/base but not the other way around.but not the other way around.

Acid/Base ConceptsAcid/Base Concepts

Lewis Acid/Base Concept

Bronsted-Lowry Acid/Base Concept

Arrhenius Acid/Base Concept

Acid-Base Reactions


• Arrhenius Acids/BasesArrhenius Acids/BasesAcids can be separated into two subcategories depending on the strength of the acid.

The Strength of the acid is determined by how easily it releases the proton. The easier it is to give the proton away the stronger the acid.

A strong acid is an acid that ionizes completely in water; it is a strong electrolyte.

Strong AcidsStrong Acids

• Furthermore, if the proton comes off a Furthermore, if the proton comes off a molecule easier than it comes off from molecule easier than it comes off from the solvent molecule then that acid is the solvent molecule then that acid is treated as having the same strength as treated as having the same strength as the solvent, this is the the solvent, this is the leveling effectleveling effect..

HNOHNO33 HClHCl

HClOHClO

44HBrHBr

HH22SOSO

44HIHI

These are 6 compounds that

give up their proton readily, hence they are strong acids in

water.

Acid-Base Reactions

Weak AcidsWeak Acids

• Weak AcidsWeak Acids

Acid-Base Reactions

A weak acid is a molecule that holds on to its proton tightly allowing for a very small percentage of ionization, it is a weak electrolyte.

HCNHCN HFHF

HCHC22HH33OO

22

HH22SS

HH33POPO44 NHNH44++

These are 6 common weak

acids

Strong BasesStrong Bases

• Strong BasesStrong Bases

Acid-Base Reactions

A strong base just like a strong acid is a compound that dissociates completely. Moreover, it readily accepts the proton given up by an acid.

These are 6 compounds that

are happy to accept any proton

given up by an acid.

LiOHLiOHCa(OH)Ca(OH)

22

NaOHNaOH Sr(OH)Sr(OH)22

KOHKOHBa(OH)Ba(OH)

22

Weak BasesWeak BasesAcid-Base Reactions

• Weak BasesWeak BasesA weak base is a base that is only partially ionized in water; it is a weak electrolyte.

It does not want to accept a proton and if it does the new compound/ion is likely to give it up the first chance it has.

)aq(OH )aq(NH )l(OH )aq(NH 423


• Strong and Weak Acids and BasesStrong and Weak Acids and BasesYou will find it important to be able to identify an acid or base as strong or weak.

When you write an ionic equation, strong acids and bases are represented as separate ions.

Weak acids and bases are represented as undissociated “molecules” in ionic equations since they hardly dissociate.


• Neutralization ReactionsNeutralization ReactionsOne of the chemical properties of acids and bases is that they neutralize one another.

A neutralization reaction is a reaction of an acid and a base that results in an ionic compound and water.

The ionic compound that is the product of a neutralization reaction is called a salt.

)l(OH)aq(KCN)aq(KOH)aq(HCN 2acid base salt


• Neutralization ReactionsNeutralization ReactionsThe net ionic equation for each acid-base neutralization reaction involves a transfer of a proton.

Consider the reaction of the strong acid , HCl(aq) and a strong base, LiOH(aq).

)l(OH)aq(KCl)aq(KOH)aq(HCl 2


• Neutralization ReactionsNeutralization ReactionsWriting the strong electrolytes in the form of ions (refer to Table 4.1 and 4.3) gives the following complete ionic equation.

)aq(OH)aq(K)aq(Cl)aq(H

)l(OH)aq(Cl)aq(K 2


• Neutralization ReactionsNeutralization ReactionsCanceling the spectator ions results in the net ionic equation. Note the proton transfer.

)aq(OH)aq(K)aq(Cl)aq(H

)l(OH)aq(Cl)aq(K 2

H+

)l(OH)aq(OH)aq(H 2


• Neutralization ReactionsNeutralization ReactionsIn a reaction involving HCN(aq), a weak acid, and KOH(aq), a strong base, the product is KCN, a strong electrolyte

Referring to Tables 4.1, 4.2 and 4.3, we obtain this net ionic equation:

)l(OH)aq(CN)aq(OH)aq(HCN 2

H+Note the proton transfer.


• Acid-Base Reactions with Gas FormationAcid-Base Reactions with Gas Formation

Carbonates react with acids to form CO2, carbon dioxide gas.

2232 COOHNaCl2HCl2CONa

Sulfites react with acids to form SO2, sulfur dioxide gas.

2232 SOOHNaCl2HCl2SONa

Gas Production in Gas Production in Neutralization ReactionsNeutralization Reactions

• The previous two reactions are overall The previous two reactions are overall reactions of the actual molecular events.reactions of the actual molecular events.

NaClNaCl

ClOHHClOH

COOHHCO

HCONaNaHCO

NaHCONaClHClCONa

2

23

33

332

2232 COOHNaCl2HCl2CONa


• Acid-Base Reactions with Gas FormationAcid-Base Reactions with Gas Formation

• The Driving Force of Neutralization The Driving Force of Neutralization reactions like that in precipitation reactions like that in precipitation reactions is the removal of ions from reactions is the removal of ions from solution in this case to form water.solution in this case to form water.

Sulfides react with acids to form H2S, hydrogen sulfide gas.

SHNaCl2HCl2SNa 22

Working with SolutionsWorking with Solutions

• The majority of chemical reactions The majority of chemical reactions discussed so far occur in discussed so far occur in aqueous aqueous solutionsolution..

When you run reactions in liquid solutions, it is convenient to dispense the amounts of reactants by measuring out volumes of reactant solutions and not mass.

Solution StoichiometrySolution Stoichiometry• MolarityMolarity is the measurement of the is the measurement of the

concentration of a chemical in solution.concentration of a chemical in solution.– The unit of molarity is the Molar (M).The unit of molarity is the Molar (M).

solution of L

solute of molesolarityM

Example: Calculate the molarity of a solution made by dissolving 12.94g of Ca(OH)2 in enough water to make 1.23L of solution.

22

22

22

Ca(OH) 0.142Msolution of 1.23L

Ca(OH) 0.175molMolarity

Ca(OH) 0.175molCa(OH) 74.1g

Ca(OH) 1molxCa(OH) 12.94g

(see Figure 4.19)

Types of Chemical Types of Chemical ReactionsReactions• Types of Oxidation-Reduction ReactionsTypes of Oxidation-Reduction Reactions

Oxidation-Reduction Reactions

Most of the oxidation-reduction reactions fall into one of the following simple categories:

Combination Reactions

Decomposition Reactions

Displacement Reactions

Combustion Reactions

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions


A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance..

)(2)()(2 2 sNaClgClsNa Sodium and chlorine combine in a fiery reaction. (see Figure)

Types of Chemical Types of Chemical ReactionsReactions• Combination ReactionsCombination Reactions


Other combination reactions involve compounds as reactants.

)s(CaSO)g(SO)s(CaO 32

Types of Chemical Types of Chemical ReactionsReactions• Decomposition ReactionsDecomposition Reactions


A decomposition reaction is a reaction in which a single compound reacts to give two or more substances.

)g(N)g(OH4)s(OCr)s(OCr)NH( 22327224

Types of Chemical Types of Chemical ReactionsReactions• Displacement ReactionsDisplacement Reactions


)g(H)aq(ZnCl)aq(HCl2)s(Zn 22

A displacement reaction (also called a single- replacement reaction) is a reaction in which an element reacts with a compound, displacing an element from it.

Types of Chemical Types of Chemical ReactionsReactions• Combustion ReactionsCombustion Reactions


A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame.

)g(OH 10)g(CO 8)g(O 13)g(HC 2 222104

MolarityMolarity

ExampleExample: how many grams of ammonium : how many grams of ammonium nitrate are in a 172.7mL sample of nitrate are in a 172.7mL sample of 1.21M NH1.21M NH44NONO33 solution? solution?

34

343434

34

3434

34

3434

3434

NONH 16.7g

NONH 80.0gx NONH 0.209molesNONH mass

NONH 80.0g

NONH massNONH 0.209moles

NONH 0.209moles

solution of 0.1727Lx NONH 1.21MNONH molessolution of 0.1727L

MNONH molesNONH 1.21M

Diluting SolutionsDiluting Solutions

• When diluting a solution the When diluting a solution the number of moles is constant.number of moles is constant.

(Molarity)(Volume) =moles(Molarity)(Volume) =moles

MM11xVxV11 = n = M = n = M22xVxV22

•So, as water is added, increasing the final volume, V2, the final molarity, M2, decreases.

MM22=M=M11xVxV11/V/V22

Acid Base TitrationsAcid Base Titrations

• A A titrationtitration is a laboratory technique is a laboratory technique used to determine the concentration used to determine the concentration of a solution sample from the volume of a solution sample from the volume of a known concentration solution of a known concentration solution required to complete a given reaction.required to complete a given reaction.

• Titrations are usually used to Titrations are usually used to determine the concentration of acids determine the concentration of acids or bases.or bases.

Acid/Base IndicatorsAcid/Base Indicators

• Most acids and bases as well as the Most acids and bases as well as the resulting salt solution are colorless.resulting salt solution are colorless.– In order to determine when the reaction is In order to determine when the reaction is

complete, we must use chemical complete, we must use chemical indicators.indicators.•Chemical IndicatorsChemical Indicators in the case of acid/base in the case of acid/base

reactions are weak acids that have the reactions are weak acids that have the property of changing colors when going from property of changing colors when going from basic to acidic solutions or vice-versa.basic to acidic solutions or vice-versa.

•The most used acid/base indicator is The most used acid/base indicator is phenolphthalein. phenolphthalein.

IndicatorsIndicators

• The job of the indicator is to signal to you The job of the indicator is to signal to you the point when you are done with the the point when you are done with the experiment. The point when the color experiment. The point when the color changes is defined as the changes is defined as the end pointend point..

• The The equivalence pointequivalence point is not the same as is not the same as the end point, ideally it should be but those the end point, ideally it should be but those occasions are rare. The occasions are rare. The equivalence equivalence pointpoint is when the amount of is when the amount of titranttitrant added is exactly the amount needed to added is exactly the amount needed to “neutralize” the “neutralize” the analyteanalyte in the flask. in the flask.

Types of Chemical Types of Chemical ReactionsReactions• Oxidation-Reduction (RedOx) ReactionsOxidation-Reduction (RedOx) Reactions

RedOx reactions are by far the most important type of reactions.

RedOx reactions involve the transfer of electrons from one species to another.

Oxidation is defined as the loss of electrons.

Reduction is defined as the gain of electrons.

Oxidation and reduction always occur simultaneously, since the electrons lost in the Oxidation must go somewhere.


• Oxidation-Reduction ReactionsOxidation-Reduction ReactionsThe reaction of an iron nail with a solution of copper(II) sulfate, CuSO4, is an oxidation- reduction reaction. (see Figure 4.11)

The molecular equation for this reaction is:

)s(Cu)aq(FeSO)aq(CuSO)s(Fe 44


• Oxidation-Reduction ReactionsOxidation-Reduction ReactionsThe net ionic equation shows the reaction of iron metal with Cu2+

(aq) to produce iron(II) ion and copper metal.

)s(Cu)aq(Fe)aq(Cu)s(Fe 22

Loss of 2 e-1 oxidation

Gain of 2 e-1 reduction

RedOx ReactionsRedOx Reactions

• The species that is reduced itself causes The species that is reduced itself causes another species to be oxidized and is another species to be oxidized and is therefore known as the therefore known as the oxidizing agent.oxidizing agent.

• Similarly, the species that are oxidized Similarly, the species that are oxidized causes another to be reduced and is causes another to be reduced and is therefore known as the therefore known as the reducing agent.reducing agent.– Note: An element is reduced/oxidized the Note: An element is reduced/oxidized the

compound containing that element is the compound containing that element is the oxidizing/reducing agent.oxidizing/reducing agent.


• Oxidation NumbersOxidation NumbersThe concept of oxidation numbers is a simple way of keeping track of electrons in a reaction.

The oxidation number (or oxidation state) of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion.

Alternatively, it is hypothetical charge assigned to the atom in the substance by simple rules.


Oxidation Number RulesOxidation Number Rules

Oxidation NumbersOxidation Numbers

• It is possible to predict the upper and It is possible to predict the upper and lower limits of main group lower limits of main group elements…elements…– The upper limit is equal to the group The upper limit is equal to the group

numbernumber– The lower limit is the group number-8The lower limit is the group number-8

•Keep in mind couple things like… Oxygen Keep in mind couple things like… Oxygen will never have an ON=+6 and Fluorine will will never have an ON=+6 and Fluorine will never have an ON=+7never have an ON=+7

Types of Chemical Types of Chemical ReactionsReactions• Describing Oxidation-Reduction ReactionsDescribing Oxidation-Reduction Reactions


Look again at the reaction of iron with copper(II) sulfate.

)s(Cu)aq(Fe)aq(Cu)s(Fe 22

The Iron losses 2 electrons so it is oxidized and at the same time it is the reducing agent.

The Copper gains 2 electrons and so it is reduced and at the same time it is the oxidizing agent.

Writing RedOx reactionsWriting RedOx reactions

• There are two ways to deal with RedOx There are two ways to deal with RedOx reactions (balancing purposes):reactions (balancing purposes):– Treat them as any other reactionTreat them as any other reaction– We can write this reaction in terms of two We can write this reaction in terms of two

half-reactionshalf-reactions..

• Driving force for these type of reactions Driving force for these type of reactions is the exchange of electrons. is the exchange of electrons.

Types of Chemical Types of Chemical ReactionsReactions• Describing Oxidation-Reduction ReactionsDescribing Oxidation-Reduction Reactions


A half-reaction is one of the two parts of an oxidation-reduction reaction. One involves the loss of electrons (oxidation) and the other involves the gain of electrons (reduction).

e2)aq(Fe)s(Fe 2

)s(Cue2)aq(Cu2

oxidation half-reaction

reduction half-reaction

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple Oxidation-Reduction ReactionsBalancing Simple Oxidation-Reduction Reactions


At first glance, the equation representing the reaction of zinc metal with silver(I) ions might appear to be balanced.

)s(Ag)aq(Zn)aq(Ag)s(Zn 2

However, a balanced equation must have a charge balance as well as a mass balance.

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple RedOx ReactionsBalancing Simple RedOx Reactions


As mentioned before we can split the reaction into two half-cells before balancing. You will learn this method in Chem 102 when you cover chapter 20.

We will balance RedOx reactions using the Oxidation Number method.

Types of Chemical Types of Chemical ReactionsReactions• Balancing Simple RedOx ReactionsBalancing Simple RedOx Reactions

– There are some steps to follow to balance There are some steps to follow to balance these reactions.these reactions.•Assign ON to ALL elements in the reactionAssign ON to ALL elements in the reaction

• Identify the species that are oxidized/reducedIdentify the species that are oxidized/reduced

•Compute the number of eCompute the number of e--s lost in OX and gained s lost in OX and gained in RED draw lines between the two pairs including in RED draw lines between the two pairs including the number of ethe number of e--s lost/gaineds lost/gained

•Multiply one or both reactions so that both Multiply one or both reactions so that both numbers match, use these factors as balancing numbers match, use these factors as balancing coefficientscoefficients

•Balance any other species that were not involved Balance any other species that were not involved in the electron exchange. in the electron exchange.


Quantitative AnalysisQuantitative Analysis

• Analytical chemistry deals with the Analytical chemistry deals with the determination of composition of determination of composition of materials-that is, the analysis of materials-that is, the analysis of materials. materials. Quantitative analysis involves the

determination of the amount of a substance or species present in a sample of material.


• Gravimetric analysisGravimetric analysis is a type of is a type of quantitative analysis in which the quantitative analysis in which the amount of a species in a material is amount of a species in a material is determined by converting the determined by converting the species into a product that can be species into a product that can be isolated and isolated and weighed.weighed.Precipitation reactions are often used in

gravimetric analysis.

The precipitate from these reactions is then filtered, dried, and weighed.

Gravimetric Analysis


• Consider the problem of determining Consider the problem of determining the amount of lead in a sample of the amount of lead in a sample of drinking water.drinking water.


Adding sodium sulfate (Na2SO4) to the sample will precipitate lead(II) sulfate.

)s(PbSO)aq(Na2)aq(Pb)aq(SONa 42

42

The PbSO4 can then be filtered, dried, and weighed. This figure shows a similar procedure.


• Suppose a 1.00 L sample of polluted water Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pbwas analyzed for lead(II) ion, Pb2+2+, by adding , by adding an excess of sodium sulfate to it. The mass an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was of lead(II) sulfate that precipitated was 229.8 mg. 229.8 mg. What is the mass of lead in a What is the mass of lead in a liter of the water?liter of the water? Express the answer as Express the answer as mg of lead per liter of solution.mg of lead per liter of solution.


)s(PbSO)aq(Na2)aq(Pb)aq(SONa 42

42

Quantitative AnalysisQuantitative AnalysisGravimetric Analysis

%32.68100g/mol 303.3g/mol 207.2

Pb%

• First we must obtain the mass percentage of lead First we must obtain the mass percentage of lead in lead(II) sulfate, by dividing the molar mass of in lead(II) sulfate, by dividing the molar mass of lead by the molar mass of PbSOlead by the molar mass of PbSO44, then multiplying , then multiplying by 100.by 100.

Then, calculate the amount of lead in the PbSO4 precipitated.

Pb mg 157.0 0.6832 4PbSO mg 229.8 sample in PbAmount

Quantitative AnalysisQuantitative AnalysisVolumetric Analysis

• An important method for determining the An important method for determining the amount of a particular substance is based amount of a particular substance is based on measuring the on measuring the volumevolume of the reactant of the reactant solution.solution.Titration is a procedure for determining the

amount of substance A by adding a carefully measured volume of a solution with known concentration of B until the reaction of A and B is just complete.

Volumetric analysis is a method of analysis based on titration.

Chemical ReactionsChemical Reactions

• Reactions often involve ions in aqueous solution. Reactions often involve ions in aqueous solution. Many of these compounds are electrolytes.Many of these compounds are electrolytes.

We can represent these reactions as molecular equations, complete ionic equations (with strong electrolytes represented as ions), or net ionic equations (where spectator ions have been canceled).

Most reactions are either precipitation reactions, acid-base reactions, or oxidation-reduction reactions.

Acid-base reactions are proton-transfer reactions.

Oxidation-reduction reactions involve a transfer of electrons from one species to another.

Summary


• Oxidation-reduction reactions are the most important type of reactions.Oxidation-reduction reactions are the most important type of reactions.• Oxidation-reduction reactions usually fall into the following categories: Oxidation-reduction reactions usually fall into the following categories:

combination reactions, decomposition reactions, displacement reactions, combination reactions, decomposition reactions, displacement reactions, and combustion reactions.and combustion reactions.

Molarity is defined as the number of moles of solute per liter of solution. Knowing the molarity allows you to calculate the amount of solute in a given volume of solution.

Quantitative analysis involves the determination of the amount of a species in a material.

Summary


In gravimetric analysis, you determine the amount of a In gravimetric analysis, you determine the amount of a species by converting it to a product you can weigh.species by converting it to a product you can weigh.

Summary

In volumetric analysis, you determine the amount of a species by titration.

Operational SkillsOperational SkillsUsing solubility rules.Using solubility rules.Writing net ionic equations.Writing net ionic equations.Deciding whether precipitation occurs.Deciding whether precipitation occurs.Classifying acids and bases as weak or strong.Classifying acids and bases as weak or strong.Writing an equation for a neutralization.Writing an equation for a neutralization.Writing an equation for a reaction with gas formation.Writing an equation for a reaction with gas formation.Assigning oxidation numbers.Assigning oxidation numbers.Balancing simple oxidation-reduction reactions.Balancing simple oxidation-reduction reactions.Calculating molarity from mass and volume.Calculating molarity from mass and volume.Using molarity as a conversion factor.Using molarity as a conversion factor.Diluting a solution.Diluting a solution.Determining the amount of a substance by Determining the amount of a substance by gravimetric analysis.gravimetric analysis.Calculating the volume of reactant solution needed.Calculating the volume of reactant solution needed.Calculating the quantity of a substance by titration.Calculating the quantity of a substance by titration.

DissociationDissociation

Return to Lecture

Electrolytic SolutionsElectrolytic Solutions

Negatively charged ions move towards

the positive electrode and

positively charged ions moves towards

the negative electrode

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ConductivityConductivity

Since there are no “free” ions on non-electrolytic solutions there is no flow of energy and the bulb does not lit. The “free” ions on

electrolytic solutions completer the circuit and allow energy to flow lighting the bulb.

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Weak vs Strong Weak vs Strong ElectrolytesElectrolytes

While both types of electrolytes conduct electricity the While both types of electrolytes conduct electricity the amount of “free” ions manifests itself by the brightness of amount of “free” ions manifests itself by the brightness of

the light emitted by the bulb.the light emitted by the bulb.Return to Lecture

Water Solvation of IonsWater Solvation of Ions

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PrecipitationPrecipitation

Mixing KI (aq) Mixing KI (aq) and Pb(NOand Pb(NO33))2 2 (aq) (aq)

leading to leading to precipitation of precipitation of

PbIPbI22

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Return to Lecture

Figure 4.7: Household acids and bases. Figure 4.7: Household acids and bases. Photo courtesy of American Color.Photo courtesy of American Color.

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Figure 4.8: Figure 4.8: Preparation of red Preparation of red cabbage juice as an cabbage juice as an acid-base acid-base indicator.indicator.Photo courtesy Photo courtesy

of James Scherer.of James Scherer.

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Neutralization ReactionNeutralization Reaction

Neutralization reaction between Acetic Acid (Vinegar) and Baking Soda (Sodium

Bicarbonate).

Making a SolutionMaking a Solution

a) Measure the mass of the compound to dissolve, b)make sure all solid makes it into the volumetric flask using the solvent, then dilute to the bottom of the neck, and c) add the last amount of solvent carefully making sure not to go

past the volumetric mark.

a) b) c)

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TitrationTitration

Titration of an acid with a base. a) the flask contains acid and a few drops of phenolphthalein, which is colorless in acidic conditions, b) the endpoint has

been reached (notice the faint pink color of the solution), finally in c), the solution is well beyond the end point since more

base was added.

a) b) c)

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Electron ExchangeElectron Exchange

Electrons from the iron nail are transferred to the copper in solution, the solid copper plates the nail.

Notice the color change in the solution, this is due to the lower amount of copper ions in solution.

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RedOx SummaryRedOx Summary

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Figure 4.13: A representation of an oxidation reduction reaction. Figure 4.13: A representation of an oxidation reduction reaction.

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Figure 4.14: Oxidation reduction reaction Figure 4.14: Oxidation reduction reaction of mercury (III) oxide into its elements. of mercury (III) oxide into its elements. Photo courtesy of James Scherer.Photo courtesy of James Scherer.

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Figure 4.15: Figure 4.15: Oxidation Oxidation reduction reaction reduction reaction of zinc metal and of zinc metal and hydrochloric acid. hydrochloric acid. Photo courtesy of American Photo courtesy of American

Color.Color.

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Figure 4.16: Oxidation Figure 4.16: Oxidation reduction reaction of iron reduction reaction of iron wool and oxygen. wool and oxygen. Photo courtesy Photo courtesy

of James Schererof James Scherer. .

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Calcium/Chlorine Calcium/Chlorine ReactionReaction

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Gravimetric AnalysisGravimetric Analysis

A solution of potassium chromate is poured down a stirring rod into a solution containing an unknown amount

of barium ion to precipitate Barium Chromate which is then filtered dried and weighed.

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Chemical Reactions Chapter 5 Dr. Victor Vilchiz. Solution Components In order to have a solution we...

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