## 18.4 Precipitation reactions (ESAFR)

Sometimes, ion in solution might react through each other to kind a new substance that is insoluble. This is referred to as a precipitate. The reaction is called a precipitation reaction.

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Precipitate

A precipitate is the hard that develops in a solution during a chemistry reaction.

## The reaction of ion in solution

### Aim

To investigate the reactions of ions in solutions.

### Apparatus

4 check tubes; copper(II) chloride solution; sodium carbonate solution; sodium sulphate solution

### Method

Prepare 2 check tubes with around $$\text5$$ $$\textmL$$ that dilute copper(II) chloride equipment in each

Prepare 1 test tube with $$\text5$$ $$\textmL$$ salt carbonate solution

Prepare 1 test tube with $$\text5$$ $$\textmL$$ salt sulphate solution

Carefully to water the sodium carbonate solution into one of the test tubes include copper(II) chloride and also observe what happens

Carefully pour the salt sulphate solution right into the second test tube containing copper(II) chloride and observe what happens

### Results

A irradiate blue precipitate creates when sodium carbonate reacts v copper(II) chloride.

No precipitate creates when sodium sulphate reacts with copper(II) chloride. The equipment is light blue.

It is important to understand what taken place in the vault demonstration. We will certainly look in ~ what happens in every reaction, step by step.

For reaction 1 you have the following ions in her solution: $$\textCu^2+$$, $$\textCl^-$$, $$\textNa^+$$ and $$\textCO_3^2-$$. A precipitate will kind if any combination of cations and also anions can come to be a solid. The adhering to table summarises which combination will type solids (precipitates) in solution.

 Salt Solubility Nitrates All are soluble Potassium, sodium and also ammonium salts All are soluble Chlorides, bromides and iodides All room soluble except silver, lead(II) and also mercury(II) salts (e.g. Silver- chloride) Sulphates All space soluble other than lead(II) sulphate, barium sulphate and also calcium sulphate Carbonates All room insoluble other than those that potassium, sodium and ammonium Compounds v fluorine Almost all room soluble other than those the magnesium, calcium, strontium (II), barium (II) and lead (II) Perchlorates and acetates All room soluble Chlorates All room soluble other than potassium chlorate Metal hydroxides and also oxides Most room insoluble

Table 18.1: general rules for the solubility that salts

Salts the carbonates, phosphates, oxalates, chromates and also sulphides are normally insoluble.

If friend look under carbonates in the table it says that all carbonates space insoluble except potassium sodium and also ammonium. This means that $$\textNa_2\textCO_3$$ will certainly dissolve in water or remain in solution, however $$\textCuCO_3$$ will type a precipitate. The precipitate the was it was observed in the reaction must because of this be $$\textCuCO_3$$. The well balanced chemical equation is:

\<2\textNa^+\text(aq) + \textCO_3^2-\text(aq) + \textCu^2+\text(aq) + 2\textCl^-\text(aq) \rightarrow \textCuCO_3\text(s) + 2\textNa^+\text(aq) + 2\textCl^-\text(aq)\>

Note that sodium chloride does no precipitate and we compose it as ions in the equation. Because that reaction 2 we have $$\textCu^2+$$, $$\textCl^-$$, $$\textNa^+$$ and also $$\textSO_4^2-$$ in solution. Most chlorides and also sulphates space soluble according to the table. The balanced chemical equation is:

\<2\textNa^+\text(aq) + \textSO_4^2-\text(aq) + \textCu^2+\text(aq) + 2\textCl^-\text(aq) \rightarrow 2\textNa^+\text(aq) + \textSO_4^2-\text(aq) + \textCu^2+\text(aq) + 2\textCl^-\text(aq)\>

Both of these reactions are ion exchange reactions.

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### Tests because that anions (ESAFS)

We often want to know which ion are current in solution. If we understand which salts precipitate, we can derive test to recognize ions in solution. Given listed below are a few such tests.

Test for a chloride

Prepare a equipment of the unknown salt using distilled water and add a tiny amount of silver nitrate solution. If a white precipitate forms, the salt is one of two people a chloride or a carbonate.

\<\textCl^-\text(aq) + \textAg^+\text(aq) + \textNO_3^-\text(aq) \rightarrow \textAgCl (s) + \textNO_3^-\text(aq)\>

(AgCl is white precipitate)

\<\textCO_3^2-\text(aq) + 2\textAg^+\text(aq) + 2\textNO_3^-\text(aq) \rightarrow \textAg_2\textCO_3\text(s) + 2\textNO_3^-\text(aq)\>

($$\textAg_2\textCO_3$$ is white precipitate)

The following step is to treat the precipitate through a small amount the concentrated nitric acid. If the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed and the precipitate disappears, the salt is a carbonate.

\<\textAgCl (s) + \textHNO_3\text(l) \rightarrow \text no reaction, precipitate is unchanged\> \<\textAg_2CO_3\text (s) + 2\textHNO_3\text(l) \rightarrow 2\textAg^+\text(aq) + 2\textNO_3^-\text(aq) + \textH_2\textO (l) + \textCO_2\text (g) precipitate disappears\> Test because that bromides and also iodides

As was the situation with the chlorides, the bromides and also iodides also form precipitates when they space reacted through silver nitrate. Silver chloride is a white precipitate, but the silver bromide and also silver iodide precipitates are both pale yellow. To identify whether the precipitate is a bromide or one iodide, we use chlorine water and also carbon tetrachloride ($$\textCCl_4$$).

Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a red brown.

\<2\textBr^-\text(aq) + \textCl_2\text(aq) \rightarrow 2\textCl^-\text(aq) + \textBr_2\text(g)\>

Chlorine water frees iodine gas indigenous an iodide and also colours the carbon tetrachloride purple.

\<2\textI^-\text(aq) + \textCl_2\text(aq) \rightarrow 2\textCl^-\text(aq) + \textI_2\text(g)\> Test for a sulphate

Add a tiny amount the barium chloride equipment to a systems of the check salt. If a white precipitate forms, the salt is one of two people a sulfate or a carbonate.

\<\textSO_4^2-\text(aq) + \textBa^2+\text(aq) + \textCl^-\text(aq) \rightarrow \textBaSO_4\text(s) + \textCl^-\text(aq)\>

($$\textBaSO_4$$ is a white precipitate)

\<\textCO_3^2-\text(aq) + \textBa^2+\text(aq) + \textCl^-\text(aq) \rightarrow \textBaCO_3\text(s) + \textCl^-\text(aq)\>

($$\textBaCO_4$$ is a white precipitate)

If the precipitate is treated with nitric acid, the is feasible to distinguish whether the salt is a sulphate or a carbonate (as in the test for a chloride).

\<\textBaSO_4\text(s) + \textHNO_3\text(l) \rightarrow \text no reaction, precipitate is unchanged\> \<\textBaCO_3\text(s) + 2\textHNO_3\text(l) \rightarrow \textBa^2+\text(aq) + 2\textNO_3^-\text(aq) + \textH_2\textO (l) + \textCO_2\text(g) precipitate disappears\> Test because that a lead carbonate

If a sample that the dried salt is treated with a little amount that acid, the production of carbon dioxide is a optimistic test because that a carbonate.

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\<2\textHCl + \textK_2\textCO_3\text(aq) \rightarrow \textCO_2\text(g) + 2\textKCl (aq) + \textH_2\textO (l)\>

If the gas is passed v limewater (an aqueous systems of calcium hydroxide) and also the equipment becomes milky, the gas is carbon dioxide.

\<\textCa^2+\text(aq) + 2\textOH^-\text(aq) + \textCO_2\text(g) \rightarrow \textCaCO_3\text(s) + \textH_2\textO (l)\>

(It is the insoluble $$\textCaCO_3$$ precipitate that makes the limewater walk milky)