Solubility and Ionic Equations

Aqueous Solutions

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Adding compounds to water

• Many molecular and ionic compounds dissolve in water.

• Ionic compounds dissolve and dissociate into their component ions.

• Ionize: another word for the dissociation of ionic compounds

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• A strong electrolyte dissociates completely.– A strong electrolyte is present in solution almost

exclusively as ions.– Strong electrolyte solutions are good conductors.

• A nonelectrolyte does not dissociate.– A nonelectrolyte is present in solution almost exclusively

as molecules.– Nonelectrolyte solutions do not conduct electricity.

• A weak electrolyte dissociates partially.– Weak electrolyte solutions are poor conductors.– Different weak electrolytes dissociate to different extents.

Types of Electrolytes

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• Electrolytes dissociate to produce ions.

Electrolytic Properties of

Aqueous Solutions

The more the electrolyte dissociates, the more ions it produces.4

Identifying “Strong” and “Weak” Electrolytes

1. Most salts are strong electrolytes. (ionic compounds)

2. Most acids are weak electrolytes (Exceptions: the strong acids)

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Identifying “Strong” and “Weak” Electrolytes (Cont.)

3. The common strong bases are the hydroxides of the alkali metals and the heavy alkaline earths. Ammonia (NH3) is a weak base.

4. Most other substances are nonelectrolytes. (Molecular compounds)

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Four –Step Path to writing Ionic Equations (Precipitates)

1. Decide what ions are present in the solutions

2. Identify the two possible precipitates formed by combining a cation from one solution with the anion from the other.

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Four –Step Path to writing Ionic Equations (Precipitates, Cont.)

3. Use the solubility rules to decide whether either or both of the possible solids will precipitate.

4. Write a balanced net ionic equation for any precipitation reaction that occurs. - no reaction occurs if all of the possible products

are soluble.

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Opposite Charges Attract!

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Driving Forces

• “Driving forces” pull ions out of solution because insoluble products of reactions are formed.

• Driving forces:– Formation of a solid– Formation of a gas (H2S, CO2, H2 etc.)

– Formation of water– Formation of a weak acid or base

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Formation of a Solid

• Double replacement reactions resulting in the formation of a precipitate that is insoluble.

General Form: AX + BY AY + BX

Ex. CaCl2(aq) + Na2SO4(aq) CaSO4(s) + 2 NaCl(aq)

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Acids and Bases

• Arrhenius Acid – a substance that produces H+ ions• Arrhenius Base – a substance that produces OH- ions

when dissolved in water

• BrØnsted-Lowrey Acid – donates protons• BrØnsted-Lowrey Base – accepts protons

* Acids – sour taste * Base – bitter taste, slippery feel

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Chemistry Students “Play Ball”

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Acid-Base Reactions• Strong Acids and Bases ionize completely

– HNO3(aq) H+(aq) + NO3-(aq)

– HCl(aq) H+(aq) + Cl-(aq)

– NaOH(aq) Na+(aq) + OH-(aq)

Ionization of strong acids and bases

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Conductivity Testing

• Strong acids and bases are strong electrolytes.

• When a conductivity tester is inserted in an aqueous solution of a strong acid or base, the light bulb readily lights up.

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Acid-Base Neutralization Reactions

– HCl (aq) + NaOH(aq) H2O(l) + NaCl (aq)

– HNO3 (aq) + NaOH(aq) H2O(l) + NaNO3 (aq)

• Neutralization occurs when a solution of an acid and a base are mixed.

• Notice we form a salt (NaCl, NaNO3 ) and water.• Salt = ionic compound whose cation comes from a

base and anion from an acid.• Neutralization between acid and metal hydroxide

produces water and a salt.16

HC2H3O2(aq) H+(aq) + C2H3O2-(aq)

Equations for weak acids and bases

have “double arrows” indicating that

there are very few ions in solution

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

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• In the reaction above, the HCl, NaOH, and NaCl all are strong electrolytes and dissociate completely.

• The actual reaction occurs between ions.

Acid–Base Reactions:Net Ionic Equations

HCl + NaOH H2O + NaCl

H+ + Cl– + Na+ + OH– H2O + Na+ + Cl–

H+ + OH– H2OA net ionic equation shows the species actually involved in the reaction.

Na+ and Cl– are spectator ions.

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• Indicators are commonly used to tell when a neutralization is complete, or if a solution is acidic or basic. Phenol red is …

Indicators

… yellow in acidic solution …

… orange in neutral solution …

… and red in basic solution.

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• Neutralization of a strong acid with strong base gives a salt and water:

• This net ionic equation applies only to strong acids and bases

OH)(OH)(H :ionicNet

)(Cl)(KOH)(OH)(K)(Cl)(H :Ionic

OH)KCl()KOH()HCl( :Molecular

2-

-2

--

2

aqaq

aqaqaqaqaqaq

aqaqaq

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Weak acids and bases

• Weak acids and bases only partially ionize when in aqueous solution.

• Conductivity tester lights up “weakly”

• The brightness of light is experimental verification of the classification as a strong or weak electrolyte.

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Weak acids and bases are weak electrolytes because less than 100% of the molecules ionize.

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• Weak acids and bases are in dynamic equilibrium in solution

• Consider the case of acetic acid:Two opposing reactions occur in solution: the ionization of the acid, called the forward reaction, and the recombination of ions into molecules, called the reverse reaction.

Chemical or dynamic equilibrium results when the rate of the forward and reverse reaction are equal.

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Weak acid + Strong base• The neutralization of a weak acid with a strong base

involves a strong and weak electrolyte• Consider the neutralization of acetic acid with NaOH:

OH)(OHC)(OH)(OHHC :ionicNet

OH)(OHC)(Na )(OH)(Na)(OHHC :Ionic

OH)(OHNaC)NaOH()(OHHC :Molecular

2-232

-232

2-232

-232

2232232

aqaqaq

aqaqaqaqaq

aqaqaq

24

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Note that in ionic equations the formulas of weak electrolytes are written in

“molecular” form

OH)(OHC)(OH)(OHHC :ionicNet

OH)(OHC)(Na

)(OH)(Na)(OHHC :Ionic

OH)(OHNaC)NaOH()(OHHC :Molecular

2-232

-232

2-232

-232

2232232

aqaqaq

aqaq

aqaqaq

aqaqaq

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Acid-base reactions may produce a gas

Sulfide and carbonate ions can react with H+ in a similar way to OH-. (OH- makes water)

• 2HCl (aq) + Na2S(aq) H2S(g) + 2NaCl(aq)

2H+(aq) + S2-(aq) H2S(g)

• HCl(aq) + NaHCO3(aq) NaCl(aq) + H2O(l) + CO2(g)

The reactions are driven to completion because a gas escapes and is unavailable for the reverse reaction.

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(CO2 and SO2 are produced by the decomposition of H2CO3 and H2SO3, respectively)

OHNHOHNH Salts Ammonium NH

OHSOHSOH SulfitesHydrogen

OHSOSO2H Sulfites SO

OHCOHCOH CarbonatesHydrogen

OHCOCO2H Carbonates CO

HCNCNH Cyanides HCN

SHS2H Sulfides SH

Equation IonicNet Compounds Gas

23-

43

22-2

3

22-2

32

22-3

22-2

32

-

2-2

2

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Polyprotic Acids

)(PO)(HOH)(HPO

)(HPO)(HOH)(POH

)(POH)(HOH)(POH :Triprotic

)(CO)(HOH)(HCO

)(HCO)(HOH)(COH :Diprotic

)(Cl)(HOH)HCl( :Monoprotic

-342

-24

-242

-42

-42243

-232

-3

-3232

-2

aqaqaq

aqaqaq

aqaqaq

aqaqaq

aqaqaq

aqaqaq

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