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ChemicalChemicalReactionsReactions

Chapter 4, part 2:Reactions in

Aqueous Solution

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The The dissolvingdissolving process process When a solid is put into a liquid, solute-solute attractions

compete with solute-solvent & solvent-solvent attractions

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SolubilitySolubility Let’s assume our solvent is water . . .

If solute-water attractions > solute-solute & water-water attractions, solute particles are pulled out one by one into the water:

The solute is SOLUBLE in water

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SolubilitySolubility

But if solute-water attractions < solute-solute & water-water attractions, solute particles remain together:

The solute is INSOLUBLE in water

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Solution conductivitySolution conductivity Solution conductivity depends on type of solute

particles

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Solution conductivitySolution conductivity Ionic solutes (salts) made of cations

& anions Ions DISSOCIATE (separate) during

dissolving Molecular solutes made of

molecules each solute particle that moves into

solution is identical

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Solution conductivitySolution conductivity Solutions of ionic solutes

contain independent mobile ions Solution conducts electricity Solute is an ELECTROLYTE Solute also conducts when melted,

but not when solid (ions can’t move)

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Solution conductivitySolution conductivity Solutions of most molecular

solutes contain independent neutral molecules Solution does not conduct

electricity Solute is a NONELECTROLYTE Solute also does not conduct when

melted or solid

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Solution conductivitySolution conductivity Some molecular compounds (acids & bases) react

with water to produce ions, as if they dissociated A few acids & bases do this very well, producing lots

of ions = strong electrolytes, strong acids & bases Most acids & bases do this weakly, producing a few

ions = weak electrolytes, weak acids & bases

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Strong acids & basesStrong acids & bases

HCl HNO3 LiOH Mg(OH)2

HBr H2SO4 NaOH Ca(OH)2

HI HClO4 KOH Sr(OH)2

etc. Ba(OH)2

STRONG ACIDS STRONG BASES

All other acids & bases are WEAK

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Predicting ElectrolytesPredicting Electrolytes All soluble salts and strong acids & bases are

strong electrolytes Weak acids & bases are weak electrolytes All other molecular compounds are nonelectrolytes

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Precipitation reactionsPrecipitation reactions

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Precipitation reactionsPrecipitation reactions

spectator ions

precipitate

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Predicting Precipitation ReactionsPredicting Precipitation Reactions To predict whether a precipitate will form, you

need to know which compounds are soluble (no ppt) and which are insoluble (ppt forms)

Memorize the guidelines in Table 4.1 on page 154 in your text! Add this guideline: CrO4

2– acts like SO42–

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Solubility guidelines: solubleSolubility guidelines: soluble Compounds of these ions are generally soluble and

do NOT form precipitates: Alkali metals (group 1A, except Li1+) Ammonium (NH4

1+) Nitrates (NO3

1–) and acetates (C2H3O21–)

Chlorides, bromides, iodides, except Pb2+, Ag1+, Hg22+

Sulfates (SO42–) & chromates (CrO4

2–), except Sr2+, Ba2+, Pb2+, and Hg2

2+

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Solubility guidelines: insolubleSolubility guidelines: insoluble Compounds of these ions are generally insoluble

and DO form precipitates: Hydroxides (OH1–) and Sufides (S2–)

Alkali metals (group 1A) and ammonium (NH41+) are soluble

Sulfides of group 2A metals are generally soluble Hydroxides of Ca2+, Sr2+, and Ba2+ are soluble

Carbonates (CO32–) and phosphates (PO4

3–)

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Net ionic equationsNet ionic equations AgNO3 (aq) + NaCl (aq)

AgNO3 (aq) + NaCl (aq) AgCl + NaNO3

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

The (aq) substances are dissociated:Ag1+ (aq) + NO3

1– (aq) + Na1+ (aq) + Cl1– (aq) AgCl (s) + Na1+ (aq) + NO3

1– (aq)

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Net ionic equationsNet ionic equationsNa1+ and NO3

1– are spectator ions

Ag1+ (aq) + NO31– (aq) + Na1+ (aq) + Cl1– (aq)

AgCl (s) + Na1+ (aq) + NO31– (aq)

The net ionic equation omits spectators:Ag1+ (aq) + Cl1– (aq) AgCl (s)

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Is that really a spectator?Is that really a spectator? An ion is a spectator if and only if it is in exactly

the same form in the products and reactants:Na2CO3 (aq) + BaCl2 (aq) BaCO3 (s) + 2 NaCl (aq)

CO32– (aq) + Ba2+ (aq) BaCO3 (s)

Na2CO3 (s) + 2 HCl (aq) 2 NaCl (aq) + H2O + CO2 (g)

Na2CO3 (s) + 2 H1+ (aq) 2 Na1+ (aq) + H2O + CO2 (g)

Only Cl1– is a spectatorNa1+ is not a spectator because it was (s), then (aq)

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ExamplesExamples Indicate whether a ppt forms and if so, complete

the reaction as a balanced net ionic equation: AlCl3 (aq) + KOH (aq) K2SO4 (aq) + FeBr3 (aq) CaI2 (aq) + Pb(NO3)2 (aq) Na3PO4 (aq) + AlCl3 (aq) Al2(SO4)3 (aq) + BaCl2 (aq) (NH4)2CO3 (aq) + Pb(NO3)2 (aq)

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AcidsAcids Acids produce H3O1+ in aqueous solution:

Strong acids are molecular compounds that react completely with water to produce H3O1+:

HCl (g) + H2O H3O1+ (aq) + Cl1– (aq) For convenience, we often show acids as simply

dissociating to produce H1+:HCl (g) H1+ (aq) + Cl1– (aq)

There are only 6 strong acids (memorize them, pg 161)

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AcidsAcids Acids produce H3O1+ in aqueous solution:

Weak acids are molecular compounds that react incompletely with water: HC2H3O2 (aq) + H2O H3O1+ (aq) + C2H3O2

1– (aq)

HC2H3O2 (aq) H1+ (aq) + C2H3O21– (aq)

All acids that are not strong are weak

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AcidsAcids H1+ is a proton

The reaction with water is proton transfer: HCl (g) + H2O H3O1+ (aq) + Cl1– (aq)

HC2H3O2 (aq) + H2O H3O1+ (aq) + C2H3O21– (aq)

Acids with one H1+ to transfer are monoprotic acids HCl HNO3 HC2H3O2 Acids with more than one H1+ to transfer are polyprotic

acids H2SO4 H3PO4 H2C3H2O4

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AcidsAcids Polyprotic acids produce H3O1+ in steps:

H2SO4 (aq) + H2O H3O1+ (aq) + HSO41– (aq)

HSO41– (aq) + H2O H3O1+ (aq) + SO4

2– (aq) For H2SO4 the first step is strong and the second weak

H2C2O4 (aq) + H2O H3O1+ (aq) + HC2O41– (aq)

HC2O41– (aq) + H2O H3O1+ (aq) + C2O4

2– (aq) For all other polyprotic acids, the first step is weak and the

second step is weaker

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BasesBases Bases produce OH1– in aqueous solution:

Strong bases are ionic hydroxide compounds that are completely dissociated in water:

NaOH (s) Na1+ (aq) + OH1– (aq) The strong bases are the hydroxides of group 1A and

2A metals (memorize them) Weak bases are molecular compounds that react

incompletely with water to produce OH1–: NH3 (aq) + H2O (l) NH4

1+ (aq) + OH1– (aq)

Amines (–NH2 ) are weak bases

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NeutralizationNeutralization Acids and bases neutralize each other

The H1+ from the acid is transferred to the base: HCl (aq) + NaOH (aq) HOH (l) + NaCl

(aq) H1+ (aq) + OH1– (aq) HOH (l) The base is not always an OH1– compound:

HCl (aq) + NH3 (aq) NH41+ (aq) + Cl1– (aq)

H1+ (aq) + NH3 (aq) NH41+ (aq)

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Neutralization and net ionic equationsNeutralization and net ionic equations It is important to recognize strong acids and bases

when writing net ionic equations HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq)

H1+ (aq) + OH1– (aq) H2O (l)

HC2H3O2 (aq) + NaOH (aq) H2O (l) + NaC2H3O2 (aq)

HC2H3O2 (aq) + OH1– (aq) H2O (l) + C2H3O21– (aq)

The weak acid is not significantly ionized,so acetate is not a spectator (even if it is aq)

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ExamplesExamples Write the molecular and net ionic equations for

HNO3 + NaOH HF + KOH HC2H3O2 + NH3 H2SO4 + Ba(OH)2 H2C2O4 + NaOH HCHO2 + Ca(OH)2

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Gas forming reactionsGas forming reactions Some neutralizations produce a gas:

CO32– + 2 H1+ H2CO3

H2CO3 is unstable and decomposes immediately H2CO3 CO2 (g) + H2O

The overall reaction is CO3

2– + 2 H1+ CO2 (g) + H2O

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Gas forming reactionsGas forming reactions Memorize these gas-formers (pg 166):

SO32– + 2 H1+ SO2 (g) + H2O

HSO31– + H1+ SO2 (g) + H2O

CO3

2– + 2 H1+ CO2 (g) + H2O HCO3

1– + H1+ CO2 (g) + H2O S2– + 2 H1+ H2S (g)

NH41+ + OH1– NH3 (g) + H2O

burning sulfur smell

rotten egg smell

ammonia

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RedoxRedox Redox (oxidation-reduction) reactions are those in

which electrons are transferred The loss of electrons is oxidation The gain of electrons is reduction

LEO says GER

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Oxidation statesOxidation states The oxidation state (O.S.) or oxidation number is a

convenient but artificial way to describe the electron environment around an atom

It is related to the number of electrons gained, lost, or apparently used in forming compounds

Oxidation states are assigned using the rules on page 169 of your text (memorize these in order)

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Assigning oxidation statesAssigning oxidation states1. The O.S. of each atom in an element is zero2. The O.S. of a monoatomic ion is equal to its charge3. The total of the O.S. of all atoms in any species (formula unit,

molecule or ion) equals the charge on that species4. In compounds, metals always have a positive O.S.

• Group 1A metals are always O.S. +1 and Group 2A metals are always O.S. +2

5. For nonmetals in compounds,• the O.S. of fluorine is –1.• the O.S. of hydrogen is +1.• the O.S. of oxygen is –2.

6. In binary compounds, the O.S. of a Group 7A element is –1, Group 6A element –2, and Group 5A element –3.

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ExamplesExamples What is the oxidation state of each element in

S8 Cr2O72– Cl2O KO2

What is the oxidation state of each element in S2O3

2– Hg2Cl2 KMnO4 H2CO

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RedoxRedox In a redox reaction, atoms change (O.S.)

The element oxidized loses electrons and its O.S. becomes more positive

The element reduced gains electrons and its O.S. becomes more negative

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ExamplesExamples Which of these reactions is a redox reaction?

Identify the species oxidized and reduced.HCl (aq) + NaOH (aq) H2O + NaCl (aq)

2 Pb(NO3)2 (s) 2 PbO (s) + 4 NO2 (g) + O2 (g)

NH4Cl (s) + NaOH (aq) NH3 (g) + H2O + NaCl (aq)

Identify the species oxidized and reduced:5 VO2+ (aq) + MnO4

1– (aq) + H2O 5 VO2

1+ (aq) + Mn2+ (aq) + 2 H1+ (aq)

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Agents of Oxidation and ReductionAgents of Oxidation and Reduction An agent makes something happen

An oxidizing agent makes oxidation happen by being reduced

A reducing agent makes reduction happen by being oxidized

The “agent” is the entire species in which the oxidized or reduced atom appears

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ExamplesExamples Identify the elements oxidized & reduced and the

oxidizing & reducing agents in 2 NO2 (g) + 7 H2 (g) 2 NH3 (g) + 4 H2O (g) 5 H2O2 (aq) + 2 MnO4

1– (aq) + 6 H1+ (aq) 8 H2O + 2 Mn2+ (aq) + 5 O2 (g)

S2O32– (aq) + 4 Cl2 (aq) + 5 H2O 2 HSO4

1– (aq) + 8 H1+ (aq) + 8 Cl1– (aq)

6 Fe2+ (aq) + 14 H1+ (aq) + Cr2O72– (aq) 6 Fe3+

(aq) + 2 Cr3+ (aq) + 7 H2O S2O3

2– (aq) + 2 H1+ (aq) S (s) + SO2 (g) + H2O

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ReactionsReactions You now know how to write & balance 4 types of

reactions combustion precipitation acid-base neutralization gas-forming

and how to recognize redox reactions

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Reaction quizzesReaction quizzes Reaction quizzes (RQ) will replace NQ

I give you the names of the reactants You write the formulas of the reactant and the formulas

of the products Cross out spectator ions Write the balanced net ionic equation You need not include state symbols such as (aq) or (g)

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ExampleExample Solutions of silver nitrate and potassium chloride

are mixed AgNO3 (aq) + KCl (aq) AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) cross out K1+ and NO3

1– as spectators Ag1+ + Cl1– AgCl check: it is already balanced

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ExampleExample Hydrochloric acid solution is added to solid

sodium hydrogen carbonate HCl (aq) + NaHCO3 (s) HCl (aq) + NaHCO3 (s) H2O + CO2 (g) + NaCl (aq) cross out Cl1– as a spectator do NOT cross out Na1+ because NaHCO3 is solid H1+ + NaHCO3 H2O + CO2 + Na1+ check: it is already balanced

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ExampleExample Ethoxyethane burns in air C2H5OC2H5 + O2 C2H5OC2H5 + O2 CO2 + H2O there are no spectators in combustion balance C2H5OC2H5 + 6 O2 4 CO2 + 5 H2O

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ExampleExample Solutions of nitrous acid and sodium hydroxide are

mixed HNO2 (aq) + NaOH (aq) HNO2 (aq) + NaOH (aq) H2O + NaNO2 (aq) cross out Na1+ as a spectator do NOT cross out NO2

1– because HNO2 is a weak acid and is not significantly dissociated in solution!

HNO2 + OH1– H2O + NO21–

check: it is balanced