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SolutionsPart I: Reactions of Ions &

Molecules in Aqueous Solns

(from Chapter 5 Sec 1 thru 5of Jespersen 6TH ed)

Dr. C. Yau

Spring 2014

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What is a Solution?A "solution" is a homogeneous mixture of one or more solute(s) and a solvent.

solute + solvent solution

The physical state of the solute and the solvent can be quite varied:

salt solution NaCl (s) + H2O (l)

air N2 (g) + O2 (g)

bronze Cu (s) + Sn (s)

booze CH3CH2OH(l) + H2O (l)

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What is a Solution?

The fact that it is a "homogeneous mixture" tells us that the components …

• are present in a uniform mixture throughout the sample.

• cannot be written with a formula. (Components are not in a fixed ratio as in a compound.)

• cannot have a molar mass.

• are not joined by "chemical bonds."

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Concentration of a Solution

The "concentration" of a solution refers to how much solute is present.

A "concentrated solution" contains a large amount of solute.

A "dilute solution" contains a small amount of solute.

A "saturated solution" contains the maximum amount of dissolved solute. Any amount beyond the saturation point will remain undissolved.

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Concentration of a Solution"Concentrated" and "dilute" are relative

terms: 1% 3% 5%A 3% solution is more concentrated than a

1% solution but more dilute than a 5% solution.

A "saturated solution," however, is dependent on the solubility of the solute, under a particular condition, such as a specified temperature.

e.g. Saturated soln of NaCl at 25oC is about 60%.

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Solubility• A solute is said to be "soluble" if it dissolves

appreciably in a given solvent.

• It is said to be "insoluble" if it does not.

• Solubility generally is affected by temperature.

• Solids generally (but not always) dissolve better at a higher temperature.

• Gases generally dissolve better at a lower temperature.

• Sugar dissolves faster in hot water, but • you can “boil” off the gas from soda to make it flat.

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What does a compound dowhen it dissolves?

It depends…

An ionic compound dissociates into ions which are then surrounded by solvent molecules.

A molecular compound stays intact as molecules as each molecule becomes surrounded by solvent molecules…

EXCEPT for certain ACID molecules.

How do you recognize acids from a given formula?

Its formula generally has H in front: HCl, H2SO4

REMEMBER THIS!!!!!

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Ionic Compound in Solution

Molecular Compound in Solution

Na+Cl Na+Cl Na+Cl

Cl Na+Cl Na+ClNa+ Na+Cl Na+Cl Na+Cl

Na+

Cl

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

C12H22O11

NaCl (aq)C12H22O11(aq)

Note: Sugar molecules are still intact.

Note: Na+ and Cl- ions have separated in the water.

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ElectrolytesAn "electrolyte" is a substance that

conducts electricity when dissolved in water.

What conducts electricity?

…mobile, charged particles.

Common charged particles involved in conducting electricity are…

cations, anions, electrons

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Common Electrolytes

1. Water-soluble ionic compounds.They separate into ions that become mobile in solution.

2. Strong acidsMolecules change into ions when dissolved in water and become mobile.

REMEMBER THIS!!! Electrolytes consist of...• Water soluble ionic compounds, or• STRONG acids

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What Else Conducts Electricity?

• Ionic compounds are generally solids. As solids, the ions are NOT mobile, and therefore they do NOT conduct.

• Ionic compounds in solution conduct because the ions become mobile.

• Ionic compounds in their molten state (as a liquid) will conduct, because their ions become mobile.

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What Else Conducts Electricity?

• Metals conduct electricity because their electrons are mobile (move from atom to atom)

• This is totally different from molecular and ionic compounds where electrons are confined within the molecule or ion.

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Types of Electrolytes

• Strong electrolytes are ones that conduct electricity well.(They fully dissociate into ions.)

• Weak electrolytes are ones that conduct electricity weakly.(They dissociate only partially into ions.)

• Nonelectrolytes are ones that do not conduct electricity.(They do not dissociate into ions.)

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Ionic Compounds in SolutionWhen table salt dissolves in water, we write...

NaCl (s) NaCl (aq)or NaCl (s) Na+ (aq) + Cl- (aq)One f.u. of NaCl dissociates into two ions.One mole of NaCl dissociates into two moles of

ions.

Ex 5.1 (NH4)2SO4 (s) (NH4)2SO4 (aq)

or (NH4)2SO4 (s) 2 NH4+ (aq) + SO4

2-(aq)

In contrast, one mole of (NH4)2SO4 dissociates into three moles of ions.

Pract.Exercises 5.1 & 5.2 on p.160

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Ionic Compounds in SolutionHow many moles of ions will 1 mole of

ammonium phosphate form when dissolved in water?

A. one mole

B. two moles

C. three moles

D. four moles

E. none of the above

Write the equation showing the dissolution of ammonium phosphate. Remember to always include physical states.

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Ionic Compounds in SolutionHow many moles of ions will 4 moles of zinc

nitrate form when dissolved in water?

A. 2 moles

B. 3 moles

C. 4 moles

D. 12 moles

E. none of the above

Write the equation showing the dissolution of zinc nitrate. Remember to always include physical states.

How many moles of ions will form when one mole of barium sulfate is placed in water?

Trick question! Answer is none!...hardly any to be exact. WHY?

BaSO4 is not soluble in water!

How do we know that?....

You need to memorize the solubility rules!

(Back in your previous chemistry you were probably given these rules at exams...not any more!)

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Know these solubility rules WELL.MEMORIZE THEM!Ionic compounds are solids to begin with.

Insoluble Compounds5. All hydroxides and oxides are insoluble except those of NH4

+, Group IA and Ca2+, Sr2+ and Ba2+.

6. Assume all other ionic compounds to be insoluble unless specified otherwise.

Soluble Compounds1.All cmpds of the alkali metals (Group IA) are soluble. Most common is Na+.2.All salts containing NH4

+, NO3-, ClO4

-, ClO3- and C2H3O2

- are soluble.3.All cmpds of Cl-, Br-, I- are soluble except combined with Ag+, Pb2+ and Hg2

2+ (mercury(I) ion).4.All sulfates (salts containg SO4

2-) are soluble exept those of Ca2+, Sr2+, Ba2+, Pb2+ and Hg2

2+.

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Using the Solubility Rules

Which of the following are soluble in water?

A. FeSO4

B. K2CO3

C. ammonium phosphate

D. HgCl2

E. PbBr2

Which of the above are electrolytes?

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Precipitation ReactionsA "precipitate" is a solid formed when two

aqueous solutions are mixed.

It is NOT just any solid formed in a reaction.

e.g.AgNO3 + NaCl AgCl + NaNO3

(aq) (aq) (s) (aq)

AgCl is a precipitate (commonly abbreviated as ppt).

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

CaO is not considered a "precipitate." Why?

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3 Types of Equations for each reaction.When aqueous solutions of lead(II) nitrate and

potassium iodide are mixed, a precipitate forms.

First predict the products, then check solubility.

Molecular Equation

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

Total Ionic Equation or Ionic Equation

Pb2+ + 2NO3- + 2K+ + 2I- PbI2 + 2K+ + 2NO3

-

(aq) (aq) (aq) (aq) (s) (aq) (aq)

Net Ionic Equation

Pb2+ (aq) + 2I- (aq) PbI2 (s)

Spectator Ions: 2K+ and 2NO3-

Check List: 1) Coefficients are at lowest ratio.

2) Net charge on left = net charge on right.

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Fig. 5.7 p.161

Pb2+ NO3- K+ I-

PbI2 (s)

NO3-(aq)

K+ (aq)

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Example 5.2 p.162

Write the molecular, ionic and net ionic equations for the rxn of aqueous solns of lead(II) acetate and sodium iodide, which yields a precipitate.

Remember to check 1) Coefficients are at lowest ratio.

2) What is the other check?

Do Practice Exercises 5.3 & 5.4 on p.163.

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Acids and BasesAcids • taste sour

• release H+ ions (H3O+) when placed in water

• turn blue litmus paper red.• react with bases to form a salt.

Bases• taste bitter• feel soapy (slippery)• turn red litmus paper blue.• increase OH- ion concentration when placed

in water• react with acids to form a salt.

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Acids in Solution

Simplified depiction of acid ionization:

HCl (g) H+ (aq) + Cl- (aq)

What exactly is H+?

What is in a H atom?

How does it become H+ ion?

H+ is simply a proton.

H+ is often referred to as a "proton."

An acid is often called a "proton-donor."

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H+ in SolutionH+ ions do not exist as such in solution!The nuclear charge is totally exposed and is

highly attractive to the water molecules surrounding it.

Instead of H+ we will consider it as a "hydrated proton" and write it as

H3O+ (aq)This is the hydronium ion.

Learn this name and formula!

O H

H

H O H

H

H+

+

proton water hydronium ion

....

:

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Ionization of Acids in Water

HCl H2O Cl- H3O+

Collision of HCl molecule with water molecule

Note that H+ ions are never formed. They immediately became part of the hydronium ion.

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Ionization of Acids in WaterInstead of…

HCl (g) H+ (aq) + Cl- (aq)

we write…

HCl (g) + H2O (l) H3O+ (aq) + Cl- (aq)

Write the equation for the ionization of nitric acid.

Write the equation for the ionization of hydrobromic acid.

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3 Definitions for Acids and Bases

Arrhenius Definition

An acid is a substance that reacts with water to produce a hydronium ion, H3O+.

A base is a substance that produces OH- ions in water.

Brønsted-Lowry Definition

An acid is a proton donor.

A base is a proton acceptor.

Lewis Definition...(must wait breathlessly for next semester)

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Monoprotic and Polyprotic Acids

A monoprotic acid can produce only one H+ per molecule.

A polyprotic acid can produce more than one H+ per molecule.

e.g.

H2SO4 + H2O H3O+ + HSO4-

HSO4- + H2O H3O+ + SO4

2-

Overall Rxn:

H2SO4 + 2H2O 2H3O+ + SO42-

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Ionization of Phosphoric AcidExample 5.3 p.166

Phosphoric acid is a triprotic acid. Write the three ionization equations and name each ion formed. Watch out for the charges of these ions!

Do Pract. Exer. 5, 6 & 7 on p.167.

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Nonmetal Oxides in Water

Nonmetal Oxides are considered "acidic anhydrides."

"Anhydride" means without water.

When water is added to an acidic anhydride it becomes an acid.

SO3 + H2O H2SO4

NO2 + H2O HNO3

CO2 + H2O H2CO3

Acid rain!!

Metal Oxides in WaterIn contrast, metal oxides are considered

bases.When placed in water, metal oxides become

metal hydroxides:

Na2O + H2O 2NaOH

CaO + H2O Ca(OH)2

REMEMBER!!Nonmetal oxides in water become ACIDS.Metal oxides in water become BASES.

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Strong & Weak AcidsStrong acids are ones that dissociate

100% when placed in water. They are strong electrolytes.

Weak acids are ones that dissociate only partially. They are weak electrolytes.

Do not confuse "strong acid" with "concentrated acid."

One has to do with... the concentration of acid in solution.The other reflects the nature of the acid itself.

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Strong & Weak AcidsHCl is a strong acid:

HCl (aq) + H2O (l)

H3O+ (aq)

+ Cl-(aq)

MEMORIZE THESE

7 COMMON STRONG ACIDS:

HNO3

H2SO4

HClO4 HCl HBr HI

HClO3

(HClO3 is borderline and we will considered it as being a strong acid.)

All the other acids are weak acids.

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Strong & Weak AcidsAcetic acid is a weak acid. Write its

ionization equation.

HC2H3O2 + H2O H3O+ + C2H3O2-

The two arrows show this is a reversible reaction, favoring the left side of the equation.

Acetic acid is a weak electrolyte. Why?

Write the ionization equation for another weak acid.

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Bases1. Metal oxides and compounds that

contain OH- are bases because when placed in water they produce OH-.

e.g. NaOH, Ba(OH)2, Na2O (Why?)

Na2O + H2O 2NaOH

2. Molecular compounds that react with water to produce OH- are bases (even though they do not contain OH- themselves).NH3 (aq) + H2O (l) NH4OH (aq)Learn this!

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Bases

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

NH3 is ammonia.

NH4+ is the ammonium ion.

Note that ammonia has no OH- and yet it is a base. Why is it a base?

Note the two arrows. This is a reversible reaction favoring the left side.NH3 is a weak electrolyte.

NH3 as a baseNH3 (aq) + H2O (l) NH4OH (aq)

NH4OH (aq) is actually NH4+

(aq) + OH- (aq)

What exactly happened in this reaction?

H N H

H

H

O

HO H

H N H

H

H

_

+

H+

A proton (H+) is transferred from H2O to NH3.H2O is acting as a proton donor.NH3 is acting as a proton acceptor.

proton donor

proton acceptor

Brønsted Definition!39

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N-Bases (nitrogen-bases)

N-bases are the molecular compounds that behave as bases. They are similar to ammonia.

CH3NH2 + H2O CH3NH3+ + OH-

proton acceptor proton donor

Note methylamine has no OH- but it is a base because it produces OH- when placed in water.

Write the equation for the ionization of ethylamine:

CH3CH2NH2

Ex 5.4 p.169

Dimethylamine, (CH3)2NH, is a base in water. It is used commercially as an attractant for boll weevils so they can be destroyed. This insect has caused more than 414 billion loss to the yield of cotton in the U.S. since it arrived from Mexico in 1892. Write an eqn for the ionization of (CH3)2NH in water.

Practice Exer 8, 9 & 10 on p. 16941

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HCl (aq) HC2H3O2(aq) NH3 (aq)

Fig 5.10 p.171

brightly lit dimly lit dimly lit

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Neutralization Reaction

The reaction of an acid with a base is called a neutralization reaction.

(…but not all neutralizations end up with neutral solutions!)

An acid-base reaction is often taught as producing water and a salt.

This is not always true!

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Neutralization Reaction

It always produces a salt, but not always water.

What is a salt? A salt is a compound formed by the reaction of an acid with a base.

It is any ionic compound excluding ones that contain OH- or O2- (hydroxide or oxide ions).

HCl + NaOH HOH + NaCl

HCl + NH3 NH4Cl

Neutralization Based on Brønsted-Lowry Definition

An acid is a proton donor

A base is a proton acceptor.

HCl + NaOH HOH + NaCl

proton proton

donor acceptor

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HCl + NH3 NH4Cl

proton proton salt (no water)

donor acceptor

Neutralization Forming Only a Salt (& No Water)

Double Replacement Rxn

AB + CD AD + CB

“Switching partners”

Why AD and not AC?

Why CB and not BC?

Generally, AB + CD AD + CB

involves A+B- C+D-

Rxn also known as “metathesis,”

from Greek meaning “different order.”47

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Predicting ReactionsFor precipitation reactions, we can predict whether the reaction will “go” or “not go” by examining the solubility of the products.

If one or more of the products is a solid (the precipitate) the reaction will go.

Prediction of precipitation reactions hinges on the knowledge of the solubility rules.

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Writing Precipitation ReactionsExample 5.6 p. 176

Predict whether a rxn will occur when aqueous solns of Pb(NO3)2 and Fe2(SO4)3 are mixed. Write molecular, ionic and net ionic equations for it.

Do Pract Exer. 19 & 20 p. 177

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Writing Neutralization Reactions

You can assume all acid-base reactions will go. We will not deal with the exceptions in this class. Exceptions will be covered in Gen Chem II.

Writing ionic and net ionic equations is a little different from those of precipitation reactions.

You MUST know which acids are strong and which are weak.

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Writing Neutralization ReactionsWrite the 3 equations for the reaction of aqueous nitric acid with solid magnesium

hydroxide.

Write the 3 equations for the reaction of aqueous acetic acid with calcium hydroxide.

Write the 3 equations for the reaction of aqueous solutions of acetic acid with ammonia.

Do Pract. Exer. 21, 22, 23 p.179

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Reactions that Produce Gases

Reactions that occur have a driving force…

• Precipitation reactions form precipitates.

• Acid-Base reactions form molecular products such as H2O.

• Reactions that produce gases.

HCl + NaHCO3 NaCl + H2CO3

HCl + NaHCO3 NaCl + H2O + CO2(g)

KOH + NH4Cl KCl + NH4OH

KOH + NH4Cl KCl + NH3(g)+ H2O

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Reactions that Produce a Gas

Know that these are gases: H2S, HCN, CO2, SO2, NH3 …. Why should you?

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Predicting and Writing Eqns

Write the 3 equations for the reaction between aqueous solutions of sodium sulfide with nitric acid. Will the reaction go? Why?

Write the 3 equations for the reaction between aqueous solutions of potassium nitrate and ammonium chloride. Will the reaction go? Why?

Do Pract. Exer. 24, 25 p.183

ReminderSpecial reactions you should remember:Metallic oxides react with water to form hydroxides.

• Na2O (s) + H OH (l) 2 NaOH (aq)

• MgO (s) + H2O (l) Mg(OH)2 (s)

NH4OH is soluble but most of it converts to

NH3 (aq) + H2O (l)

H2CO3 (aq) converts immediately to CO2 (g) + H2O (l)

H2SO3 (aq) converts immediately to SO2 (g) + H2O (l)

This means that if your product is one of the above (NH4OH, H2CO3 or H2SO3) it should be replaced with

NH3 (aq) + H2O (l), H2O (l) + CO2 (g) , H2O (l) + SO2 (g)

respectively.55

Summary: Predicting Rxns

How do we know whether a reaction will go?

First identify the type of rxn:

•Precipitation type: at least one of product must be insoluble (formation of ppt)

•Acid-Base rxn: all will go except for weak acid + weak base (none this semester)

•Gas forming rxn: all will go

•Oxidation-reduction: covered next chapter

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What reaction might we use to synthesize lead(II) sulfate?

Use soluble reactants. (Why?)

Match cation with NO3- or C2H3O2

-

Match anion with Na+ or K+

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Synthesizing a Compound

Synthesizing a CompoundExample 5.9 p.184

What reaction might we use to synthesize nickel sulfate? How is this problem different from the previous one on lead(II) sulfate?

Note that nickel sulfate is soluble. How might this pose a problem?

Why would this work?

Sulfuric acid + nickel hydroxide

Sulfuric acid + nickel carbonate

Practice Exer 26 & 27 p.184

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