7.8: Precipitation Reactions

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⚙️ Learning Objectives

To identify a precipitation reaction and predict solubility.

A precipitation reaction is a reaction that yields an insoluble product – a precipitate – when two solutions are mixed. When a colorless solution of silver nitrate is mixed with a yellow-orange solution of potassium dichromate, a reddish precipitate of silver dichromate is produced (see Video \(\PageIndex{1}\)).

2 AgNO₃ (aq) + K₂Cr₂O₇ (aq) → Ag₂Cr₂O₇ (s) + 2 KNO₃ (aq)

This chemical equation has the general form of an exchange reaction:

\[\ce{ AC + BD \rightarrow }\underset{insoluble}{\ce{AD}} + \ce{BC}\]

Thus precipitation reactions are a subclass of exchange reactions that occur between ionic compounds when one of the products is insoluble. Because both components of each compound change partners, such reactions are sometimes called double-replacement reactions. Precipitation reactions are used to isolate metals that have been extracted from their ores, and to recover precious metals for recycling.

Video \(\PageIndex{1}\): Mixing potassium dichromate and silver nitrate together to initiate a precipitation reaction.

Just as important as predicting the product of a reaction is knowing when a chemical reaction will not occur. Simply mixing solutions of two different chemical substances does not guarantee that a reaction will take place. For example, if 500 mL of an aqueous sodium chloride solution is mixed with 500 mL of an aqueous potassium bromide solution, the final solution has a volume of 1000 mL and contains a mixture of Na⁺ (aq), Cl⁻ (aq), K⁺ (aq), and Br⁻ (aq) ions. As you will see in (Figure \(\PageIndex{1}\)), none of these species reacts with any of the others. When these solutions are mixed, the only effect is to dilute each solution with the other.

Three beakers; beaker on left has KBr solution, beaker on right has NaCl solution, and beaker in middle shows the two solutions mixed together. — Figure \(\PageIndex{1}\): The effect of mixing aqueous KBr and NaCl solutions. Because no net reaction occurs, the only effect is to dilute each solution with the other. (Water molecules are omitted from molecular views of the solutions for clarity.)

Predicting Precipitation Reactions

A precipitation reaction occurs when a solid precipitate forms after mixing two strong electrolyte solutions. As stated previously, if none of the species in the solution reacts then no net reaction occurs.

⚡️ A Case for No Reaction

When two aqueous ionic compounds are mixed together, there will be no reaction when both of the potential products would also be aqueous ionic compounds.

Let's see what happens when aqueous solutions of barium chloride and lithium sulfate are mixed:

BaCl₂ (aq) + Li₂SO₄ (aq) →

Since barium chloride and lithium sulfate are both strong electrolytes, each dissociates completely in water resulting in a solution that contains all of the constituent anions and cations. Barium chloride is comprised of barium ions, Ba²⁺, and chloride ions, Cl^–, while lithium sulfate is comprised of lithium ions, Li⁺, and sulfate ions, SO₄²⁻. The anions exchange partners.

Barium ions pair up with sulfate ions forming barium sulfate, BaSO₄ (Ba²⁺ balances the charge of SO₄²⁻). Lithium ions pair up with chloride ions forming LiCl (Li⁺ balances the charge of Cl^–).

BaCl₂ (aq) + Li₂SO₄ (aq) → BaSO₄ + LiCl

Referring to the solubility rules found in the previous section, BaSO₄ is insoluble in water resulting in a phase label of (s), while LiCl is soluble in water resulting in a phase label of (aq).

BaCl₂ (aq) + Li₂SO₄ (aq) → BaSO₄ (s) + LiCl (aq)

At this point, the equation may be balanced by placing a coefficient of "2" in front of LiCl (aq).

BaCl₂ (aq) + Li₂SO₄ (aq) → BaSO₄ (s) + 2 LiCl (aq)

Although soluble barium salts are toxic, BaSO₄ is so insoluble that it can be used to diagnose stomach and intestinal problems without being absorbed into tissues. An outline of the digestive organs appears on x-rays of patients who have been given a “barium milkshake” or a “barium enema”, a suspension of very fine BaSO₄ particles in water.

✅ Example \(\PageIndex{1}\)

Complete and balance the following equation. If no reaction takes place, write "No Reaction" after the arrow.

MgBr₂ (aq) + (NH₄)₂CO₃ (aq) →

Solution

MgBr₂ is called magnesium bromide and is comprised of magnesium ions, Mg²⁺, and bromide ions, Br⁻, while (NH₄)₂CO₃ is called ammonium carbonate and is comprised of ammonium ions, NH₄⁺, and carbonate ions, CO₃²^–. The anions exchange partners.

Magnesium ions pair up with carbonate ions forming magnesium carbonate, MgCO₃ (Mg²⁺ balances the charge of CO₃²^–). Ammonium ions pair up with bromide ions forming NH₄Br (NH₄⁺ balances the charge of Br^–).

MgBr₂ (aq) + (NH₄)₂CO₃ (aq) → MgCO₃ + NH₄Br

Referring to the solubility rules, MgCO₃ is insoluble in water resulting in a phase label of (s), while NH₄Br is soluble in water resulting in a phase label of (aq).

MgBr₂ (aq) + (NH₄)₂CO₃ (aq) → MgCO₃ (s) + NH₄Br (aq)

At this point, the equation may be balanced by placing a coefficient of "2" in front of NH₄Br (aq).

MgBr₂ (aq) + (NH₄)₂CO₃ (aq) → MgCO₃ (s) + 2 NH₄Br (aq)

✅ Example \(\PageIndex{2}\)

Write a balanced chemical equation that shows what happens when aqueous solutions of sodium carbonate and potassium iodide are mixed. If there is no reaction, write "No Reaction" after the arrow.

Solution

Sodium carbonate is comprised of sodium ions, Na⁺, and carbonate ions, CO₃²^–, while potassium iodide is comprised of potassium ions, K⁺, and iodide ions, I⁻. The anions exchange partners.

Sodium ions pair up with iodide ions forming sodium iodide, NaI (Na⁺ balances the charge of I⁻). Potassium ions pair up with carbonate ions forming K₂CO₃ (two K⁺ balances the charge of one CO₃²^–).

Na₂CO₃ (aq) + KI (aq) → NaI + K₂CO₃

Referring to the solubility rules, both NaI and K₂CO₃ are soluble in water resulting in a phase label of (aq).

Na₂CO₃ (aq) + KI (aq) → NaI (aq) + K₂CO₃ (aq)

Since this equation shows two aqueous ionic compounds being mixed together to potentially yield two aqueous ionic compounds, no reaction will take place.

Na₂CO₃ (aq) + KI (aq) → No Reaction

✅ Example \(\PageIndex{3}\)

Write a balanced chemical equation that shows what happens when aqueous solutions of iron(III) nitrate and sodium hydroxide are mixed. If there is no reaction, write "No Reaction" after the arrow.

Solution

Iron(III) nitrate is comprised of iron(III) ions, Fe³⁺, and nitrate ions, NO₃^–, while sodium hydroxide is comprised of sodium ions, Na⁺, and hydroxide ions, OH⁻. The anions exchange partners.

Iron(III) ions pair up with hydroxide ions forming iron(III) hydroxide, Fe(OH)₃ (one Fe³⁺ balances the charge of three OH⁻). Sodium ions pair up with nitrate ions forming NaNO₃ (Na⁺ balances the charge of NO₃^–).

Fe(NO₃)₃ (aq) + NaOH (aq) → Fe(OH)₃ + NaNO₃

Referring to the solubility rules, Fe(OH)₃ is insoluble in water resulting in a phase label of (s), while NaNO₃ is soluble in water resulting in a phase label of (aq).

Fe(NO₃)₃ (aq) + NaOH (aq) → Fe(OH)₃ (s) + NaNO₃ (aq)

At this point, the equation may be balanced by placing a coefficient of "3" in front of NaOH (aq) and NaNO₃ (aq).

Fe(NO₃)₃ (aq) + 3 NaOH (aq) → Fe(OH)₃ (s) + 3 NaNO₃ (aq)

✏️ Exercise \(\PageIndex{1}\)

Complete and balance each equation. If no reaction takes place, write "No Reaction" after the arrow.

Li₃PO₄ (aq) + KOH (aq) →
MgSO₄ (aq) + SrI₂ (aq) →

Answer A: Li₃PO₄ (aq) + KOH (aq) → No Reaction

Answer B: MgSO₄ (aq) + SrI₂ (aq) → SrSO₄ (s) + MgI₂ (aq)

✏️ Exercise \(\PageIndex{2}\)

Write a balanced chemical equation that shows what happens when aqueous solutions of each pair are mixed. If no reaction occurs, write "No Reaction" after the arrow.

Potassium phosphate and calcium chloride
Zinc nitrate and sodium carbonate

Answer A: 2 K₃PO₄ (aq) + 3 CaCl₂ (aq) → Ca₃(PO₄)₂ (s) + 6 KCl (aq)

Answer B: Zn(NO₃)₂ (aq) + Na₂CO₃ (aq) → ZnCO₃ (s) + 2 NaNO₃ (aq)

Summary

In a precipitation reaction, a subclass of exchange reactions, an insoluble material (a precipitate) forms when two electrolyte solutions are mixed.
To predict the product of a precipitation reaction, all species initially present in the solutions are identified, as are any combinations likely to produce an insoluble salt.

This page is shared under a CK-12 license and was authored, remixed, and/or curated by Joshua Halpern (Howard University), Melissa Alviar-Agnew, Henry Agnew, and Lance S. Lund (Anoka-Ramsey Community College). Original source: https://www.ck12.org/c/chemistry/.

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