AqueousReactions

Chapter 4Aqueous Reactions and Solution Stoichiometry

Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.;

and Bruce E. Bursten

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4.1

Properties of Aqueous Solutions

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Solutions (abrev. sol’n)

• Homogeneous mixtures of two or more pure substances – can be liquids or gases.

• Solvent - substance present in the greatest quantity

• Solute(s) - All other substances present in lesser quantities

• Solutions in which water is the solvent are called aqueous solutions

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Water – a great solvent!• H2O molecules can attract, separate, and

surround the cations and anions that make up many ionic compounds– Solvation - the process of a solvent surrounding

and dissolving an ionic compound the cations & anions become dissociated (or separated) as they are surrounded by the H2O molecules

• The dissociated ions are free to move and become evenly dispersed throughout the solution.– the moving ions can carry a current throughout

the solution resulting in electrical conductivity (movement of charged particles through a medium).

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NaCl +

H2O

1 2

3 4

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Ionic Compounds in Water• While water is an excellent solvent, not ALL

ionic substances will readily dissolve and dissociate in the presence of H2O

• Those that DO dissociate are known as electrolytes:–Electrolyte - a substance which contains

ions and conducts electricity in it’s aqueous form

–There are 3 classifications of electrolytes: strong, weak, and nonelectrolytes

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Strong Electrolytes• dissociates completely into

ions when dissolved in H2O

• sol’n exists as 100% ions (no bonded ionic compounds left undissolved)

• good conductor of electricity • Includes:

– all soluble * ionic compounds

– all strong acids & bases*– a few covalent compounds

* explained on a later slide

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Weak Electrolytes• dissociate only partially into ions when

dissolved in H2O

• sol’n exists mostly as molecules surrounded by water (very, very few ions present)

• poorly conducts electricity• Includes:

– weak acids & bases*

(ex: acetic acid, HC2H3O2 [only 1% as ions])

– slightly soluble ionic compounds*– some covalent compounds

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Nonelectrolytes• do NOT dissociate into ANY

ions• dissolved substance consists

of intact molecules surrounded by H2O

• does not conduct electricity• Includes:

– most covalent compounds (w/the exception of acids & bases which are either strong or weak)

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Electrolytes: a visual

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Solubility – ability to be dissolved• Soluble (aq)

– a substance which easily dissolves in water• Slightly Soluble (s)

– a substance that only dissolves a tiny bit in water

• Insoluble (s)– substance that does NOT dissolve in water – substance remains separate from the H2O

molecules (no interaction)– substances are insoluble when the ions attract

so strongly that they CANNOT be pulled apart by H2O molecules

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Solubility: a visual

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Solubility Rules

• To know if an ionic substance is soluble or not, you must consult the solubility rules– lists which ions are typically soluble and

insoluble–also lists exceptions to the rules

Note: Acids are not explicitly listed in the rules because all acids are aqueous solutions (thus soluble)

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Solubility Rules for Common Ionic Compounds in Water

Soluble Ionic Compounds Exceptions: these are insoluble w/ respective anionsNitrates, NO3

- noneAcetates, C2H3O2

- none

Chlorides, Cl- Ag+, Hg22+ (2 Hg atoms), Pb2+

Bromides, Br- Ag+, Hg22+, Pb2+

Iodides, I- Ag+, Hg22+, Pb2+

Sulfates, SO42- Ag+, Hg2

2+, Pb2+, Sr2+, Ba2+

Alkali metal cations noneAmmonium, NH4

+ none

Insoluble Ionic Compounds Exceptions: these are soluble w/ respective anions

Sulfides, S2- alkali metal cations, NH4+, Ca2+, Sr2+, Ba2+

Carbonates, CO32- alkali metal cations, NH4

+

Phosphates, PO43- alkali metal cations, NH4

+

Hydroxides, OH- alkali metal cations, NH4+, Ca2+, Sr2+, Ba2+

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Solubility Rules - Practice

• Are the following soluble or insoluble?

– Be(C2H3O2)2

– MgS

– BaSO4

– K3PO4

soluble

soluble

insoluble

insoluble

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Acids

• Substances that produce H+ ions when dissolved in water (Arrhenius).

• Ex) HNO3 H+ + NO3

–

• Substances that produce OH− ions when dissolved in water (Arrhenius).

• Ex) NaOH Na+ + OH–

Bases

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Strong vs. Weak

• Both strong & weak acids and bases exist–Strong acids & bases dissociate

completely in solution• Exist as 100% ions • Strong electrolytes & conductors

–Weak acids & bases only partially dissociate in solution• Few ions exits• Weak electrolytes & conductors

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Strong vs. Weak Electrolytes

Strong Acid: Weak Acid:

HCl HNO2

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Seven Strong AcidsMUST BE MEMORIZED!

• Hydrochloric (HCl)• Hydrobromic (HBr)• Hydroiodic (HI)• Nitric (HNO3)

• Sulfuric (H2SO4)

• Chloric (HClO3)

• Perchloric (HClO4)

**When dissolved in water, they dissociate 100% into H+ ions and their respective anion.**

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Strong BasesMUST BE MEMORIZED!

Soluble hydroxide compounds• Alkali metal hydroxides

• LiOH, NaOH, KOH, etc…

• Calcium hydroxide, Ca(OH)2

• Strontium hydroxide, Sr(OH)2

• Barium hydroxide, Ba(OH)2

• When dissolved in water, they dissociate 100% into their respective metal cation and OH–

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Predicting Ions in Solution• If a substance is soluble, you can predict

what ions present are in the solution

– Ex) Ammonium sulfate

(NH4)2SO4 soluble

Dissociates into: 2 NH4+ ions

1 SO42- ion

– Ex) Sulfuric Acid

H2SO4 soluble

Dissociates into: 2 H+ ions

1 SO42- ion

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How to Classify ElectrolytesAsk… Is the substance ionic, covalent, or an

acid/base?• If ionic, is it soluble?

– If soluble = strong electrolyte – If insoluble = nonelectrolyte

• If covalent– If covalent and NOT an acid or base =

generally nonelectrolyte (ex: sugar, organic compounds)

• If acid or a base, is it a strong acid/base?– If strong acid or base = strong electrolyte– if it is any other acid or base = weak electrolyte

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Classify the following as: strong, weak, or nonelectrolyte

1) CaCl2

2) HNO3

3) Ag3PO4

4) HCHO2

5) KOH

6) C2H5OH

– ionic – soluble = Strong

– acid = Strong

– ionic – insoluble = Nonelectrolyte

– acid = Weak

– ionic – soluble = Strong (also strong base)

– covalent– NOT a base = Nonelectrolyte

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Classify the following as: strong, weak, or nonelectrolyte

1) Mg(OH)2

2) HCl

3) H3PO4

4) (NH4)2CO3

5) H2SO4

– ionic – insoluble = nonelectrolyte

– strong acid = strong

– weak acid = weak

– ionic – soluble = strong

– strong acid = strong

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4.2

Precipitation

Rxns

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Precipitation Rxns

• Double replacement reactions that specifically produce a solid product–This solid product is called a precipitate

(abbreviated ppt.) and is known as insoluble (because it cannot readily dissolve in water)

• To predict if an insoluble ppt will form, use the solubility rules

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Predicting Precipitations Rxns

1. Predict the products of the double replacement reaction (cations switch partners)

2. Use solubility rules to determine the states of matter of the products: insoluble = (s); soluble = (aq)If a solid is formed, a ppt rxn has

occurred

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1. Fe(NO3)3 (aq) + K2S (aq)

2 Fe(NO3)3 (aq) + 3 K2S (aq)

Fe2S3 (s) + 6 KNO3 (aq)

2. Li2CO3 (aq) + KC2H3O2 (aq)

Li2CO3 (aq) + KC2H3O2 (aq)

2 LiC2H3O2 (aq) + K2CO3 (aq)

NR (no reaction)

= a bunch of ions in a puddle

Predicting Precipitation Rxns Practice

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DEMO!!!

Will precipitates form?

• Solutions Used:0.1 M KI0.1 M Pb(NO3)2

0.1 M CaCl2

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Balanced Chemical Equations• Balanced chemical equations show chemical

formulas without considering the ionic character of any substances

• However… recall that soluble ionic compounds are strong electrolytesstrong acids, strong bases, and soluble ionic

salts

• This means they dissociate and are truly present as ions in a sol’n

(which leads us to…)

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…Ionic Equations• Complete Ionic (CIE): explicitly indicates all

the IONS that are in sol’n throughout the rxnSpectator Ions = ions present on BOTH

sides of the chemical equation • Ions are present but play NO DIRECT

ROLE in chemical rxn• When ALL reactants and products are

AQUEOUS (no reaction), every ion is a spectator ion (so CIE is not usually written)

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…Ionic Equations

• Net Ionic (NIE): includes only ions and compounds

directly involved in the chemical rxn This means NO spectators are

included

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CIE & NIE Practice

Mg(NO3)2 (aq) + Li2SO4 (aq)

Balanced Equation:

Mg(NO3)2 (aq) + Li2SO4 (aq) MgSO4 (aq) + 2 LiNO3 (aq) = NR

Complete Ionic Equation:

Mg2+ (aq) + 2 NO3- (aq) + 2 Li+ (aq) + SO4

2- (aq)

Mg2+ (aq) + SO42-

(aq) + 2 Li+ (aq) + 2 NO3- (aq)

Net Ionic Equation:

NONE! – all spectators

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6 NH4+ (aq) + 2 PO4

3- (aq) + 3 Ba2+ (aq) + 3 S2- (aq)

6 NH4+ (aq) + 3 S2-

(aq) + Ba3(PO4)2 (s)

(NH4)3PO4 (aq) + BaS (aq)

Balanced Equation:

2 (NH4)3PO4 (aq) + 3 BaS (aq)

3 (NH4)2S (aq) + Ba3(PO4)2 (s)

Complete Ionic Equation:

Net Ionic Equation:

2 PO43- (aq) + 3 Ba2+ (aq) Ba3(PO4)2 (s) … NO spectators!

CIE & NIE Practice

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4.4

Oxidation-Reduction Reactions

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Single Replacement Rxns• General Rxn Formula:

A + BC AC + B• The reactants:

– One reactant is a single element by itself– The other reactant is an ionic compound

• The products:– If the single element is a metal, it will replace

the cation in the ionic compound– If the single element is a halogen gas, it will

replace the anion in the ionic compound

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Oxidation-Reduction Rxns (aka: Redox)• All single replacement

rxns can be classified as Redox rxns (but not all redox rxns are single replacement)

• Redox rxns occur when electrons are transferred between reactants

• Oxidation occurs when an atom, ion, or molecule loses electrons. (LEO)

• Reduction occurs when an atom, ion, or molecule gains electrons. (GER)

* Both processes happen simultaneously in a rxn

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Activity Series of Metals- Ease of Oxidation

• When a single replacement rxn occurs, the metal that is alone will be oxidized.

• However, some metals are easier to oxidize than others (they lose electrons more easily than others, thus forming cations)– Active Metals:

• metals at the top of the activity series are more reactive and more easily oxidized

– Noble Metals:• metals at the bottom of the activity series

which have very low reactivity

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In a single replacement rxn:

----------------------------------------------------------------------

(1) a solid metal element can be OXIDIZED by any elements listed below it in the activity series

(2) Look to see if the metal solid is ABOVE the metal cation (in the 2nd reactant) If yes, the rxn

does happen predict products

If not, NR (b/c the solid metal cannot be oxidized)

Activity Series of Metals

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Using the Activity Series Examples

1) magnesium sulfate+ zinc metal

2) iron metal + copper (II) nitrate

NR

Fe (s) + Cu(NO3)2 (aq) Cu (s) + Fe(NO3)2

CIE: Fe (s) + Cu2+ (aq) + 2 NO3– (aq)

Cu (s) + Fe2+ (aq) + 2 NO3– (aq)

We can also write CIE & NIE!!! (only aqueous dissociate)

NIE: Fe (s) + Cu2+ (aq) Cu (s) + Fe2+ (aq)

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Metals Reacting w/Acids• Only metals listed ABOVE hydrogen in the

series are able to react with acids • If the reaction does occur, H2 gas is

produced. • Example #1:

___Ni (s) + ___HCl (aq) Ni is above H2, so the rxn occurs!

• Example #2: ___Cu (s) + ___HCl (aq) Cu is below H2, so no rxn occurs!

NiCl2(aq) + H2 (g)

NR

2

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Activity Series of Halogens:

• The idea is the same here…– Fluorine is the most reactive– Iodine is the least reactive

• Only a MORE reactive halogen can replace a LESS reactive one

• In these single replacement rxns, the single element (halogen gas) must be listed ABOVE the halogen anion in the 2nd reactant for a rxn to occur

• ___Cl2 + ___NaBr (aq) ___NaCl (aq) + ___Br2 (g)2

 

ACTIVITY SERIES OF HALOGENS:

 

     Fluorine

     Chlorine

     Bromine 

     Iodine

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