Precipitation Reactions

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Section 4.2

• Precipitation Reactions

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Precipitation Reactions

• Mixing ions that form insoluble compounds
• The insoluble solid formed is a precipitate

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Solubility

• The amount of substance that can dissolve in a
  given quantity of solvent at a given temperature

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Solubility Guidelines
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Steps for Predicting Precipitation

• Note ions present in reaction
• Consider possible combinations of cations and
  anions
• Use Table 4.1 to determine if any of those
  combinations are insoluble

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Example

• Will a precipitate form when solutions of
  Mg(NO3)2 and NaOH are mixed?

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Step 1 Ions present

• Mg 2
• NO3
• Na
• OH -

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Step 2 Possible combinations

• Mg 2 with OH
• Na with NO3

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Step 3 Table 4.1

• Hydroxides generally insoluble, and Mg is not an
  exception
• NaNO3 is soluble
• Mg(NO3)2 (aq) 2NaOH (aq) ? Mg(OH)2 (s) 2NaNO3
  (aq)

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Metathesis (Exchange) Reactions

• Metathesis- Greek, to transpose
• Pattern of precipitation formation
• AX BY ? AY BX

• AgNO3 (aq) KCl (aq) ?? AgCl (s) KNO3 (aq)

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Metathesis (Exchange) Reactions

• AgNO3 (aq) KCl (aq) ?? AgCl (s) KNO3 (aq)

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Balancing Metathesis Reactions

• Determine ions present
• Write chemical formulas of products by combining
  cation from one reactant with anion of the other
• Use charges of ions to determine subscripts
• Balance the equation

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Sample Problem

• Predict the identity of the precipitate that
  forms when solutions of BaCl2 and K2SO4 are mixed.

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Step 1 Determine ions

• BaCl2 and K2SO4 are mixed
• Ba 2
• Cl -
• K
• SO4 2-

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Step 2 Combine

• Reactants BaCl2 and K2SO4
• BaCl2 K2SO4 ? BaSO4 K2Cl2

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Step 3 Subscripts

• Ba 2 Cl - K SO4 2-
• BaCl2 K2SO4 ? BaSO4 KCl

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Step 4 Balance

• BaCl2 K2SO4 ? BaSO4 2KCl
• Did a precipitate form?

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Solution

• BaCl2 (aq) K2SO4 (aq) ? BaSO4 (s) 2KCl (aq)

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Molecular Equation

• Reactants and products in their molecular form
• NOT ionic character

• AgNO3 (aq) KCl (aq) ?? AgCl (s) KNO3 (aq)

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Complete Ionic Equation

• All soluble strong electrolytes shown as ions
• Spectator ions- appear as reactants and products
  in identical form

• Ag (aq) NO3- (aq) K (aq) Cl- (aq) ??
• AgCl (s) K (aq) NO3- (aq)

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Net Ionic Equation

• Omit spectator ions
• Ag (aq) Cl- (aq) ?? AgCl (s)

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Writing Net Ionic Equations

• Write a balanced molecular equation.
• Dissociate all strong electrolytes.
• Identify and cancel spectator ions

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Sample Problem

• Write the net ionic equation for mixing calcium
  chloride and sodium carbonate.

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Step 1 Molecular equation

• Calcium chloride and sodium carbonate
• CaCl2 Na2CO3 ? CaCO3 2Na Cl

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Step 2 Dissociate strong electrolytes

Ca 2 2Cl- 2Na CO3 2- ? CaCO3 2Na
2Cl- All are strong electrolytes, but CaCO3 is
insoluble in water
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Step 3 Omit spectators

• Ca 2 (aq) CO3 2- (aq)? CaCO3 (s)

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Homework

• 4.19-4.24 on page 158