AqueousReactions

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

1. Note ions present in reaction

2. Consider possible combinations of cations and anions

3. 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

1. Determine ions present

2. Write chemical formulas of products by combining cation from one reactant with anion of the other

3. Use charges of ions to determine subscripts

4. 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

1. Write a balanced molecular equation.

2. Dissociate all strong electrolytes.

3. 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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Ca 2+ + 2Cl- + 2Na+ + CO3

2- CaCO3 + 2Na+ + 2Cl-

All are strong electrolytes, but CaCO3 is insoluble in water

Step 2: Dissociate strong electrolytes

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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