Precipitation Reactions
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Precipitation occurs when ions in solution “swap partners”
to form a new compound of low solubility in water.
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This low-solubility compound forms as solid particles that
eventually settle.
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It is called a precipitate.
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To predict whether a precipitate will form, you need to
know which substances are soluble in water and which
are insoluble.
Solutions of sodium chloride and silver nitrate react to
form a precipitate of silver chloride.
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Precipitation Reactions
Barium Sulphate Contrasting in Medicine
Predic'ng Precipita'on Reac'ons •  Predict the products and write a balanced equa'on for the reac'on occurring between sodium carbonate and copper(II) sulfate. •  Step 1 Write the reactants and check their solubility •  Soluble = (aq) Insoluble = (s) •  Na2CO3(aq) + CuSO4(aq) •  Step 2 Swap ion partners. Write products. Balance equa'on. Check solubility •  Na2CO3(aq) + CuSO4(aq)  Na2SO4(aq) + CuCO3(s) Ionic Equations
•  Ionic equa'ons are equa'ons that show only the species that are formed or changed in a reac'on. •  Any ions that remain unchanged in a reac'on are not included in an ionic equa'on. •  Ions that are present in a reac'on but do not react are called spectator ions. •  Spectator ions are not shown in the ionic equation.
Ionic Equations - Rules
1.  Write the balanced chemical equation.
2.  Decide which substances are soluble and which will form
precipitates.
3.  Expand the chemical equation by dissociating all the
soluble compounds into their free ions.
4.  Cancel all free ions that are unchanged on both sides of
the equation (the spectators).
5.  Write the resulting ionic equation.
Ionic Equations - Example
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Write a balanced ionic equation for the reaction between
barium chloride and sodium sulphate:
1.  BaCl2 + Na2SO4 → BaSO4 + 2NaCl
2.  BaSO4 is insoluble and forms the precipitate.
NaCl is soluble and forms the spectator ion.
3. Ba2+ (aq) + 2Cl- (aq) + 2Na+ (aq) + SO42- (aq) →
BaSO4 (s) + 2Na+ (aq) + 2Cl- (aq)
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Hence we get:
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)