Introduction to Reactions in Aqueous Solutions

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Introduction to Reactions in Aqueous Solutions

• Chapter 4

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Objectives

• 4-1 The Nature of Aqueous Solutions
• 4-2 Precipitation Reactions
• 4-3 Acid-Base Reactions
• 4-4 Oxidation-Reduction Some General Principles
• 4-4 Balancing Oxidation-Reduction Equations
• 4-5 Oxidizing and Reducing Agents
• 4-7,8 Stoichiometry of Reactions in Aqueous
  Solutions Titrations

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4-1 The Nature of Aqueous Solutions

• Strong electrolyte dissociates completely.
• Good electrical conduction.

• Weak electrolyte partially dissociates.
• Fair conductor of electricity.
• Non-electrolyte does not dissociate.
• Poor conductor of electricity.

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• All soluble ionic compounds and few molecular
  compounds are strong electrolytes.
• Most molecular compounds are either
  nonelectrolytes or weak electrolytes.

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Example 1
Calculating Ion concentrations in a Solution of a
Strong Electolyte. What are the aluminum and
sulfate ion concentrations in 0.0165 M Al2(SO4)3?.
Balanced Chemical Equation
Al2(SO4)3 (s) ? 2 Al3(aq) 3 SO42-(aq)
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Example 1
Aluminum Concentration
2 mol Al3
0.0165 mol Al2(SO4)3
0.0330 M Al3
Al

1 L
1 mol Al2(SO4)3
0.0495 M SO42-
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4-2 Precipitation Reactions

• Soluble ions can combine to form an insoluble
  compound.
• Precipitation occurs.

Ag(aq) Cl-(aq) ? AgCl(s)
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Net Ionic Equation
Overall Precipitation Reaction
AgNO3(aq) NaI (aq) ? AgI(s) NaNO3(aq)
Complete ionic equation
Ag(aq) NO3-(aq) Na(aq) I-(aq) ? AgI(s)
Na(aq) NO3-(aq)
Net ionic equation
Ag(aq) I-(aq) ? AgI(s)
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Solubility Rules

• Compounds that are soluble

• Alkali metal ion and ammonium ion salts
• Nitrates, perchlorates and acetates

Li, Na, K, Rb, Cs NH4 NO3-
ClO4- CH3CO2-
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Solubility Rules

• Chlorides, bromides and iodides Cl-, Br-,
  I-
• Except those of Pb2, Ag, and Hg22.
• Sulfates
  SO42-
• Except those of Sr2, Ba2, Pb2 and Hg22.
• CaSO4 is slightly soluble.

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Solubility Rules

• Compounds that are insoluble

• Hydroxides and sulfides HO-, S2-
• Except alkali metal and ammonium salts
• Sulfides of alkaline earths are soluble
• Hydroxides of Sr2 and Ca2 are slightly soluble.
• Carbonates and phosphates CO32-, PO43-
• Except alkali metal and ammonium salts

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Problem

• Sodium phosphate aluminum chloride ? ?
• Aluminum sulfate barium chloride ? ?
• Ammonium carbonate lead(II) nitrate ? ?

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4-3 Acid-Base Reactions

• Latin acidus (sour)
• Sour taste
• Arabic al-qali (ashes of certain plants)
• Bitter taste

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Properties of Acid

• Sour taste
• Cause color change in plant dyes
• React with certain metals, such as zinc,
  magnesium, and iron to produce hydrogen gas
• React with carbonates and bicarbonates to produce
  carbon dioxide gas
• Aqueous acid solutions conduct electricity

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Properties of Bases

• Bitter taste
• Feel slippery
• Cause color change in plant dyes litmus changes
  from red to blue
• Aqueous base solutions conduct electricity

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Svante Arrhenius 1884 Acid-Base theory.
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Acids

• Acids provide H in aqueous solution.
• Strong acids
• Weak acids

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Bases

• Bases provide OH- in aqueous solution.
• Strong bases
• Weak bases

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Recognizing Acids and Bases.

• Acids have ionizable hydrogen ions.
• CH3CO2H or HC2H3O2
• Bases have OH- combined with a metal ion.
• KOH
• or are identified by chemical equations
• Na2CO3(s) H2O(l)? HCO3-(aq) 2 Na(aq)
  OH-(aq)

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More Acid-Base Reactions

• Milk of magnesia Mg(OH)2

Mg(OH)2(s) 2 H(aq) ? Mg2(aq) 2 H2O(l)
Mg(OH)2(s) 2 HCH3CO2(aq) ? Mg2(aq) 2
CH3CO2-(aq) 2 H2O(l)
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More Acid-Base Reactions

• Limestone and marble.

CaCO3(s) 2 H(aq) ? Ca2(aq) H2CO3(aq)
But H2CO3(aq) ? H2O(l) CO2(g)
CaCO3(s) 2 H(aq) ? Ca2(aq) H2O(l) CO2(g)
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Johannes Brønsted1932
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Brønsted acids bases

• Brønsted Acids are proton donors
• Brønsted Bases are proton acceptors
• HCl (aq) ? H (aq) Cl- (aq)
• Brønsted acid
• HCl (aq) H2O (l) ? H3O (aq) Cl-
  (aq)
• Brønsted base
• 
  hydronium ion

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Oxidation-Reduction Reactions

• Oxidation-Reduction reactions are classified as
  electron transfer process.
• 2Ca(s) O2(g) ? 2CaO(s)
• 2Ca ? 2Ca2 4e- Oxidation
• O2 4e- ? 2O2- Reduction

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Types of Reactions

• Acid/base
• Combustion of hydrocarbon
• Redox
• Complex ions
• Precipitation
• Hydrolysis
• Decomposition/synthesis

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Acid/Base

• Formation of water

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Combustion of hydrocarbon

• Products are carbon dioxide and water

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Redox

• Look for words like acidified
• Look for metal ions with multiple oxidation
  states like Fe3/2, Sn4/2, Pb4/2

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Complex Ions

• Look for transition metal ions or Al3, NH3, OH-,
  Cl-, SCN-, CN-
• Look for excess concentrated
• Ex.
• Excess concentrated aqueous ammonia is added to
  solid silver chloride.
• NH3 AgCl ? Ag(NH3)2 Cl-

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Precipitation

• Look for two aqueous solutions as reactants
• You need to know the solubility rules

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Hydrolysis

• Hydrolysis adds water to split
• Solid sodium oxide is added to distilled water.
• Na2O H2O ? Na OH-

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Decomposition/Synthesis

• Synthesis example
• Carbon dioxide gas is passed over hot, solid
  sodium oxide
• Decomposition example
• Solid sodium bicarbonate is strongly heated