Solution Composition

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Solutions

• Solution Composition
• A solution is a homogeneous mixture
• Solutions are prepared by dissolving components
  in a solvent
• The solvent is usually the component of the
  solution in highest quantity
• When solids are dissolved in a liquid, the liquid
  is the solvent
• If a liquid is dissolved in another liquid, the
  one in highest amount is the solvent
• When water is one of the components, it is the
  solvent regardless of how much is present
• The solutes are the other components in a
  solution they are what is dissolved
• Water is a most important solvent
• Dissolves a wide variety of substances
• Important in biological process

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Solutions

• Properties of Aqueous Solutions
• Ionic compounds dissolve by dissociating into
  ions
• NaCl(s) ?H2O Na(aq) Cl-(aq)
• Water molecules have a strong affinity for ions
• The end is attracted to anions the - end is
  attracted to cations
• This attraction overcomes the attraction of the
  cations and anions for each other

Water molecules have a - end water molecules
have a end
d-
electric dipole
d
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Solutions

• Ionic Compounds in Solution
• Ions in solution conduct electricity
• Electrolytes are ionic substances whose aqueous
  solutions conduct electricity.
• Non-electrolytes are substances whose aqueous
  solutions do not conduct electricity.
• Most Molecular compounds when dissolved to not
  produce ions
• Some important exceptions occur when molecules
  interact with water to produce ions
• HCl(aq) H(aq) Cl-(aq) there are
  no HCl molecules in water
• NH3(aq) H2O NH4(aq) OH-(aq)
• Strong Electrolytes are substances that dissolve
  by forming electrolytes.
• Almost all ionic compounds and some molecular
  compounds are strong electrolytes
• Some ionic compounds are not very soluble, yet,
  because the dissolve by forming electrolytes,
  they are strong electrolytes even though the
  concentration of ions in solution is small.
• HCl is a strong electrolyte in water because it
  completely ionizes when it dissolves

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Solutions

• Weak Electrolytes are substances that dissolve
  but do not form many ions
• Weak electrolyte solutions do not conduct
  electricity very well
• Example acetic acid, HC2H3O2
• Acetic acid is very soluble in water
• A 1 M solution of acetic acid is only about 1
  ionized
• HC2H3O2(aq) H(aq) C2H3O2-(aq)
• The double arrow represents and equilibrium the
  reaction is occurring in both directions
• A single arrow implies the reaction is complete
  in the direction of the arrow
• Acids, Bases and Salts
• Acids are substances that produce H in aqueous
  solution
• Examples HCl, HC2H3O2, H2SO4, H3PO4
• HCl, HC2H3O2 are monoprotic acids they produce
  only 1 H
• H2SO4 is a diprotic acid it produces 2 H, but
• H2SO4(aq) H(aq) HSO4-(aq)
• HSO4-(aq) H(aq) SO42-(aq)

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Solutions

• Bases are substances that react with H
• OH- is a base H(aq) OH-(aq) H2O(l )
• Metal hydroxides are common bases NaOH, KOH,
  Ca(OH)2. Ba(OH)2
• Some non-hydroxides produce OH-(aq) NH3
• NH3(aq) H2O(l ) NH4(aq) OH-(aq)
• Strong and Weak Acids and Bases
• Strong acids and bases are acids or bases that
  are strong electrolytes
• Examples Acids HCl, HBr, HI, HNO3, HClO4,
  H2SO4
• Bases Group 2A hydroxides, Ca(OH)2, Sr(OH)2,
  Ba(OH)2
• Weak acids and bases are acids or bases that are
  weak electrolytes
• See next slide
• Strong acids are more reactive when the
  reactivity depends on the concentration of H
  sometimes its the anion
• HCl(aq) Cu(s) N.
  R.
• 4H(aq) 2NO3-(aq) Cu(s) Cu2
  2NO2(g) 2H2O(l )

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Some Acids and Bases
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Solutions

• Neutralization reactions involve the reaction
  between an acid and a base to produce water and
  a salt
• HCl(aq) NaOH(aq) H2O(l ) NaCl(aq)
• A salt is an ionic compound whose cation comes
  from a base and whose anion comes from an acid
  NaCl
• H2SO4(aq) Ba(OH)2(aq) 2H2O(l )
  BaSO4(s)
• Note BaSO4 is not soluble in water
• NH3(aq) HCl(aq) NH4(aq) Cl-(aq)
• NH3(aq) H2O(l )
  NH4(aq) OH-(aq)
• NH4(aq) OH-(aq) HCl(aq) NH4(aq)
  Cl-(aq) H2O(l )
• add eqns NH3(aq) HCl(aq) NH4(aq)
  Cl-(aq)

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Ionic Reactions in Solution

• Ionic Equations
• Example AgNO3(aq) NaCl(aq) AgCl(s)
  NaNO3(aq)
• This is called a molecular equation even though
  all the reactants and products are ionic
  compounds
• The ionic character of the reactants and product
  are not shown in the equation
• Another way to write the equation
• Ag(aq) NO3-(aq) Na(aq) Cl-(aq)
  AgCl(s) Na(aq) NO3- (aq)
• This is called a complete ionic equation because
  it shows the ionic nature of all the reactants
  and products
• Notice AgCl forms as a solid and is not
  dissociated. It looks like a molecular
  compound but in fact is ionic but is not very
  soluble in water.
• Both Na(aq) and NO3-(aq) show up on both sides
  of the equation in the same form they can be
  canceled
• Ag(aq) Cl-(aq) AgCl(s)
• This is called a net ionic equation
• It shows the net chemical change that takes place

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Ionic Reactions in Solution

• Net Ionic Equations
• Another Example
• HNO3(aq) NaOH(aq)
  NaNO3(aq) H2O molecular equation
• H(aq) NO3-(aq) Na(aq) OH-(aq)
  H2O(l ) Na(aq) NO3-(aq)
• H(aq)
  OH-(aq) H2O(l )
  net ionic equation
• HCl(aq) NaOH(aq)
  NaCl(aq) H2O (l ) molecular
  equation
• H(aq)
  OH-(aq) H2O(l )
  net ionic equation
• The net ionic equations for the two
  neutralization reactions are the same
• The net ionic equations for any neutralization
  reaction between a strong acid and a strong base
  will be the same.
• Spectator ions are the ions not involved in the
  net ionic equations but show up in complete
  ionic equations.

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Ionic Reactions in Solution
What species are written in ionic form? Species
that are soluble and
write
in ionic form Species that are strong
electrolytes What species are not written in
ionic form? Soluble weak electrolytes Soluble
non-electrolytes write
in molecular form Insoluble substances -
solids, liquids, gases Metathesis reactions
also called double replacement reactions Cations
in two compounds exchange anion partners
AX BY AY BX Pb(NO3)2(aq)
2HCl(aq) PbCl2(s) 2HNO3(aq)
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Metathesis Reactions

• Three processes lead to a metathesis reaction
• Formation of a precipitate, an insoluble product
• Formation of a weak electrolyte or
  non-electrolyte
• Formation of a gas that escapes from solution
• Precipitation Reactions
• Ag(aq) Cl-(aq) AgCl(s)
• AgCl is not very soluble in water
• Solubility is the amount of substance that
  dissolves in 1 L water
• Solubility of AgCl is about 10-5 mol/L
• Insoluble substances have solubilities that are
  low

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Ca2
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Solubility Guidelines
Predict metathesis precipitation reactions
Na2SO4 CaCl2 2NaCl CaSO4 Cl-s
are soluble SO42- s are soluble except Ca2
Na2SO4(aq) CaCl2(aq) 2NaCl(aq)
CaSO4(s) Ca2 (aq) SO42- (aq)
CaSO4(s) Metathesis Reactions Formation
of a weak electrolyte or non-electrolyte Water
is a very weak electrolyte
HNO3(aq) NaOH(aq) NaNO3(aq)
H2O H(aq)
OH-(aq) H2O(l ) net
Mg(OH)2(s)
2HCl(aq) MgCl2(aq) 2H2O(l )
Mg(OH)2(s) 2H (aq)
Mg2 (aq) 2H2O(l) net
?
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Metathesis Reactions
Formation of a weak electrolyte or
non-electrolyte Other weak electrolytes
HC2H3O2, HNO2, and other weak acids Formation of
a gas Anion Reaction S2- S2-(aq)
2H(aq) H2S(g) CO32- CO32-(aq)
2H(aq) H2CO3(aq) CO2 (g)
H2O(l ) SO32- SO32-(aq) 2H(aq)
H2SO3(aq) SO2 (g) H2O(l ) NO2-
NO2-(aq) H(aq) HNO2(aq)
2HNO2(aq) H2O(l)
NO2(g) NO (g) Cation Reaction NH4 NH4
(aq) OH- (aq) NH3(g) H2O(l )
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Metathesis Reactions
Formation of a gas Examples Reaction
between PbS(s) and HNO3(aq)
PbS(s) HNO3 (aq) Pb(NO3)2 (aq)
H2S(g) PbS(s)
2H (aq) Pb (aq) H2S(g)
Reaction between Fe2(CO3)3(s) and HCl(aq)
Fe2(CO3)3(s) 6HCl(aq)
2FeCl3 3CO2(g) 3H2O(l )
Fe2(CO3)3(s) 6H (aq) 2Fe3 (aq)
3CO2 (g) 3H2O(l ) Introduction to Oxidation
Reduction Reactions Example 2Na(s) Cl2(g)
2NaCl(s) The Na atoms have lost
electrons to become Na The Cl atoms have gained
electrons to become Cl-
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Introduction to Oxidation Reduction Reactions

• Oxidation is the loss of electrons by a substance
• Na Na 1e-
• Reduction is the gain of electrons by a substance
• Cl 1e- Cl-
• Oxidation must be accompanied by reduction
• If a species loses electrons another species must
  pick them up
• The number of electrons gained must equal the
  number lost in a balanced chemical equation
• Oxidation of metals by acids
• Many metals react with acids to produce a metal
  ion (or a metal salt) and H2(g)
• Fe(s) 2HCl(aq) FeCl2(aq) H2(g)
• Fe(s) 2H (aq)
  Fe2 (aq) H2(g)