Chemistry 101 : Chap. 4

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Chemistry 101 Chap. 4
Aqueous Reactions and Solution Stoichiometry

• General Properties of Aqueous Solutions
• (2) Precipitation Reactions
• (3) Acid-Base Reactions
• (4) Concentration of Solutions
• (5) Solution Stoichiometry and Chemical Analysis

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Aqueous Solutions
? Solution A homogeneous mixture of two or more
substance
? Solvent The substance present in the largest
amount ? Solute(s) The other substance in the
solution
Aqueous Solutions Solutions in which water is
the dissolving
medium (or solvent)
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Ionic Compounds in Water
When ionic compounds dissolve in water, they
dissociate completely into ions. The ions are
surrounded by water molecules
NaCl (s)
NaCl (aq) or Na (aq) Cl- (aq)
Solvation of Na and Cl-
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Molecular Compounds in Water
When molecular compounds dissolve in water, they
do not dissociate into ions in general. The
molecules remain intact and are surrounded by
water molecules
H2O
CH3OH (l)
CH3OH (aq)
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Molecular Compounds in Water
Some molecular compounds, when dissolved in
water, dissociate (ionoized) into ions.
? Strong Acid, such as hydrochloric acid,
dissociate completely
HCl (aq) ? H (aq) Cl- (aq)
? Weak Acid, such as acetic acid, dissociate only
partially
CH3COOH (aq) H (aq)
CH3COO- (aq)
NOTE All three species are present in solutions
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Electrolyte vs. Nonelectrolyte
? Electrolyte A substance whose aqueous
solutions contains ions
? Ionic compounds
strong electrolyte
? Strong acids
strong electrolyte
weak electrolyte
? Weak acids
? Nonelectrolyte A substance that does not form
ions in solution
NOTE Dont confuse the extent to which an
electrolyte dissolves with whether
it is strong or weak
Ex. CH3COOH is extremely soluble, but it is a
weak electrolyte. Ba(OH)2 is not very
soluble, but it is a strong electrolyte. ?
the dissolved Ba(OH)2 completely dissociate
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Electrolyte Solutions
Electrolyte solutions conduct electricity !!
? Electrical conductivity is the flow of charged
particles through substance, such as the flow
of electrons through a wire or the flow of
ions through a solution.
? Testing electrical conductivity of electrolyte
solutions
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Electrolyte Solutions
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Precipitation Reactions

• Recall Three types of chemical reactions
  studied in Ch. 3
• ? Combination Reaction A B ? C
• ? Decomposition Reaction A ? B C
• ? Combustion Reaction CxHy O2 ? CO2
  H2O

? Exchange Reaction (Metathesis Reaction)
AB CD ? AD CB
Reactants exchange partners to form the products
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Precipitation Reactions

• Precipitation Reactions Reactions that result
  in the formation
• of
  insoluble products

Pb(NO3)2 (aq) 2KI (aq) ? PbI2 (s)
2KNO3 (aq)
yellow ppt
AgNO3 (aq) NaCl (aq) ? AgCl (s)
NaNO3 (aq)
white ppt
Mg(NO3)2 (aq) 2NaOH (aq) ? Mg(OH)2 (s)
2NaNO3 (aq)
white ppt
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Precipitation Reactions
What is happening in molecular level (nanoscale
view)?
AgNO3 (aq) NaI (aq) ? AgI (s) NaNO3
(aq)
brown ppt

NOTE Water molecules are omitted in these
pictures
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Solubility Guidelines

• Question How can you tell if a precipitation
  will form during an
• exchange reaction?

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Solubility Guidelines
? Example Which of the following compounds is
likely soluble in water?
Fe(NO3)3 AgCl CaCO3 (NH4)2SO4 CuSO4 Cu(OH)2
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Solubility Guidelines
? Empirical Rule Salts containing following
ions are always soluble
? Group 1A ion (alkali metal) Li,
Na, K,
? Ammonium ion NH4
? Nitrate ion NO3-
? Acetate CH3COO-
? Insoluble Ionic compounds Ag,
Hg2, Pb2 Halogen anions (Cl-, Br-, I-)
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Predicting Precipitation Reactions
? Example Complete the following equation and
determine whether or not a
precipitation reaction (exchange reaction) will
occur. If a reaction will
occur, balance the equation
Pb(NO3)2 (aq) KI (aq) ?
(1) Determine the products of exchange reaction
(2) Determine if there is any insoluble product
(3) Balance the chemical equation
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Predicting Precipitation Reactions
? Example Complete the following equation and
determine whether or not a
precipitation reaction (exchange reaction) will
occur. If a reaction will
occur, balance the equation
Mg(NO3)2 (aq) KCl (aq) ?
(1) Determine the products of exchange reaction
(2) Determine if there is any insoluble product
(3) Balance the chemical equation (?)
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Net Ionic Equation
? Molecular equation
Pb(NO3)2 (aq) 2 KI (aq) ? PbI2 (s) 2 KNO3 (aq)
? (complete) Ionic equation
Pb2(aq) 2NO3-(aq) 2K(aq) 2I- (aq) ?
PbI2 (s)
2K(aq) 2NO3-(aq)
spectator ions
? Net ionic equation
Pb2(aq) 2I- (aq) ? PbI2 (s)
NOTE Mg(NO3)2 (aq) 2 KCl (aq) ? All ions are
spectators !
No net reaction !!
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Net Ionic Equation

• Summary
• ? Spectator Ions Ions present in solution
  before and after
• the reaction.
• ? Net Ionic Equation Spectators are omitted
  and only the ions
• involved in
  forming the precipitation are shown

NOTE Net ionic equation must be mass balanced
and charge balanced.
? Writing net ionic equation

• Write a balanced molecular equation for the
  reaction
• (2) Write the complete ionic equation
  precipitation or not
• (3) Identify and remove the spectator ions

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Net Ionic Equation
? Example Write the net ionic equation for the
precipitation reaction that
occurs when solutions of nickel nitrate and
sodium hydroxide are mixed.
(1) Write the balanced molecular equation
(2) Write the complete ionic equation
(3) Write the net ionic equation
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Net Ionic Equation
? Example Write the net ionic equation for the
precipitation reaction that
may occur when solutions of calcium nitrate and
sodium acetate are mixed.
(1) Write the balanced molecular equation
(2) Check the solubility of products
(3) Net ionic equation
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Acids and Bases
? Properties of Acids
? Acids ionize to produce protons HCl (aq) ?
H(aq) Cl-(aq)
? Turn blue litmus red
? Taste sour
? Aqueous solution has a pH less than 7
? React with metal to produce H2 gas
? Properties of Bases
? Bases produce OH- in water NaOH (aq) ?
Na(aq) OH-(aq)
? Turn red litmus blue
? Bitter taste
? Aqueous solution has a pH more than 7
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Acids and Bases
? Strong acids and bases Strong acids and
bases ionize completely in aqueous solution.
Strong acids and bases are strong electrolytes.
HCl (aq) ? H(aq) Cl-(aq)
100 ionized NaOH (aq) ? Na(aq)
OH-(aq) 100 ionized
strong acids
HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3
group 1A hydroxide LiOH, NaOH, KOH group 2A
hydroxide Ca(OH)2, Sr(OH)2, Ba(OH)2
strong bases
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Acids and Bases
? Weak acids and bases Weak acids and bases
ionize partially in aqueous solution. Weak
acids and bases are weak electrolytes.
HF (aq) ? H(aq) F-(aq) 10
ionized
weak acids
HF, CH3COOH, HNO2,
NH3,
weak bases
NOTE NH3 do not have OH. However, they produce
OH- by accepting a proton from
H2O. NH3 (aq) H2O (l) ?
NH4 (aq) OH- (aq)
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Acid-Base Reactions
? acid-base neutralization reactions
An exchange reaction in which a molecular
compound (H2O) is formed.
molecular equation HCl(aq)
NaOH(aq) ? H2O(l) NaCl(aq)
complete ionic equation H(aq) Cl-(aq)
Na(aq) OH-(aq) ? H2O(l) Na(aq) Cl-(aq)
net ionic equation H(aq)
OH-(aq) ? H2O(l)
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Acid-Base Reactions
NOTE (1) The products of simple acid-base
reactions are water and ionic
compounds called salts.
HNO3(aq) NaOH (aq) ? H2O(l) NaNO3(aq)
acid base
water salt
(2) The formation of water is what drives the
acid-base neutralization reaction
H(aq) OH-(aq) H2O(l)
possible, but much less favorable than the other
way
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Acid-Base Reactions
? Example Complete and balance the following
equation for an acid-base
neutralization reaction H3PO4(aq) KOH(aq) ?
What is the net ionic equation?
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Acid-Base Reactions
? Example Complete and balance the following
equation for an acid-base
neutralization reaction Mg(OH)2(s) HCl(aq) ?
What is the net ionic equation?
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Acid-Base Reactions

• acid-base reactions with gas formation
• Acids reacting with certain weak bases produce
  a salt (ionic
• compound) and gas (molecular compound). The
  most common
• reaction of this type involves carbonates
  (CO32-).

2 HCl(aq) Na2CO3(aq) ? H2CO3 (aq)
2NaCl(aq)
unstable
H2CO3(aq) ? H2O (l) CO2(g)
The overall reaction will be 2 HCl(aq)
Na2CO3(aq) ? H2O(l) 2NaCl(aq) CO2(g)
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Acid-Base Reactions
? Example What is the net ionic equation for
the reaction between
Na2CO3(aq) and HCl(aq)?
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Acid-Base Reactions
? Example Complete and balance the following
equation. HCl(aq) CaCO3(s)
? What is the net ionic
equation?
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Concentrations of Solutions
? Concentration The amount of solute dissolved
in a given amount of
solvent or solution
? Molarity The most common way of expressing
concentration in chemistry
A 1.00 molar solution ( 1.00M) contains 1.00
mol of solute in every liter of solution.
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Concentrations of Solutions
1 mol
1M
1 liter
1 mol
1M
? liter
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Concentrations of Solutions
? Example Calculate the molarity of a solution
made by dissolving 23.4 g of
Na2SO4 (MW 142) in enough water to form
125 mL of solution.
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Concentrations of Electrolyte
1L
1M solution of Na2SO4
1 mole of Na2SO4 (s)
2Na(aq) SO42-(aq)
Each formula unit of Na2SO4 that dissolve
produces two Na ions and one SO42- ion. ? The
concentration of Na (2M) will be twice the
SO42- concentration (1M).
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Concentrations

• Example How many grams of Na2SO4 (MW142) are
  required
• to make 0.350L of 0.500M
  Na(aq)?

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Dilutions
Mconc , Vconc
Mdilute , Vdilute
number of solute molecules before dilution
number of solute molecules after dilution

number of moles of solute before dilution
number of moles of solute after dilution

Mconc ? Vconc Mdilute ? Vdilute
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Dilutions
? Example What is the molarity of the resulting
solution when 15 mL of 0.65M solution is diluted
to 315mL?
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Solution Stoichiometry
Example What volume of a 0.100 M HNO3 solution
is needed to completely
neutralize 0.1 g of Ca(OH)2 ?
Stratagy (1) What is the chemical reaction
involving HNO3(aq) and
Ca(OH)2 (s)? (2) How many moles
of HNO3 is needed to neutralize
0.1 g of Ca(OH)2? (3) What
is the volume of 0.100M HNO3 solution
neutralizing that many moles of
Ca(OH)2?
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Solution Stoichiometry
aA bB ? cC dD