Why can't HI be prepared by using NaI and Conc. H2SO4 ? Give the balanced chemical reaction for the preparation of HCl . How would you test the presence of Cl or Br in its aqueous solution ? [2+1+2]

Why HBr cannot be prepared using NaBr and conc. H₂SO₄

Hydrogen bromide ($\mathrm{HBr}$) cannot be prepared by reacting sodium bromide ($\mathrm{NaBr}$) with concentrated sulfuric acid ($\mathrm{Conc}\cdot \mathrm{H}{}_2\mathrm{SO}{}_4$) because $\mathrm{H}{}_2\mathrm{SO}{}_4$ is a strong oxidizing agent. When $\mathrm{NaBr}$ reacts with $\mathrm{Conc}\cdot \mathrm{H}{}_2\mathrm{SO}{}_4$, the bromide ion ($\mathrm{Br}{}^{-}$) is oxidized to bromine ($\mathrm{Br}{}_2$) instead of forming $\mathrm{HBr}$. The reaction proceeds as follows:

NaBr+2H₂SO₄conc.Na₂SO₄+Br₂+2SO₂+2H₂O

$$2\mathrm{NaBr}+2\mathrm{H}{}_2\mathrm{SO}{}_4\to \mathrm{Br}{}_2+\mathrm{SO}{}_2+\mathrm{Na}{}_2\mathrm{SO}{}_4+2\mathrm{H}{}_2\mathrm{O}$$

Here, $\mathrm{H}{}_2\mathrm{SO}{}_4$ oxidizes $\mathrm{Br}{}^{-}$ to $\mathrm{Br}{}_2$ (reddish-brown fumes), and itself gets reduced to $\mathrm{SO}{}_2$ (choking gas). Thus, $\mathrm{HBr}$ is not obtained.

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Balanced chemical reaction for the preparation of HCl

Hydrogen chloride ($\mathrm{HCl}$) can be prepared by reacting sodium chloride ($\mathrm{NaCl}$) with concentrated sulfuric acid ($\mathrm{Conc}\cdot \mathrm{H}{}_2\mathrm{SO}{}_4$). The balanced chemical equation is:

NaCl+H₂SO₄conc.NaHSO₄+HCl

$$\mathrm{NaCl}+\mathrm{H}{}_2\mathrm{SO}{}_4\to \mathrm{NaHSO}{}_4+\mathrm{HCl}$$

On further heating, sodium bisulfate ($\mathrm{NaHSO}{}_4$) reacts with more $\mathrm{NaCl}$ to produce sodium sulfate ($\mathrm{Na}{}_2\mathrm{SO}{}_4$) and $\mathrm{HCl}$:

$$\mathrm{NaHSO}{}_4+\mathrm{NaCl}\to \mathrm{Na}{}_2\mathrm{SO}{}_4+\mathrm{HCl}$$

The overall reaction is:

$$2\mathrm{NaCl}+\mathrm{H}{}_2\mathrm{SO}{}_4\to \mathrm{Na}{}_2\mathrm{SO}{}_4+2\mathrm{HCl}$$

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Testing for $\mathrm{Cl}{}^{-}$ and $\mathrm{Br}{}^{-}$ ions in aqueous solution

To identify the presence of $\mathrm{Cl}{}^{-}$ and $\mathrm{Br}{}^{-}$ ions in an aqueous solution, the following tests can be performed:

CHCl₃CHCl₃

Test for $\mathrm{Cl}{}^{-}$ ions:

1. Take a small amount of the solution in a test tube.
2. Add a few drops of dilute nitric acid ($\mathrm{HNO}{}_3$) followed by silver nitrate ($\mathrm{AgNO}{}_3$) solution.
3. A white precipitate of silver chloride ($\mathrm{AgCl}$) confirms the presence of $\mathrm{Cl}{}^{-}$ ions.

Cl⁻+AgNO₃+HNO₃AgCl+HNO₃

$$\mathrm{AgNO}{}_3+\mathrm{NaCl}\to \mathrm{AgCl}(\mathrm{white}\mathrm{ppt})+\mathrm{NaNO}{}_3$$

Test for $\mathrm{Br}{}^{-}$ ions:

1. Take a small amount of the solution in a test tube.
2. Add chlorine water ($\mathrm{Cl}{}_2$ water) and shake well.
3. If $\mathrm{Br}{}^{-}$ ions are present, bromine ($\mathrm{Br}{}_2$) is liberated, which dissolves in water to give a brown/orange color.
4. To confirm, add an organic solvent like hexane or chloroform. The $\mathrm{Br}{}_2$ layer dissolves in the organic layer, giving a reddish-brown color in the organic phase.

Br⁻+Cl₂Br₂+Cl⁻

$$2\mathrm{NaBr}+\mathrm{Cl}{}_2\to 2\mathrm{NaCl}+\mathrm{Br}{}_2$$

Alternatively, add $\mathrm{AgNO}{}_3$ solution to the solution containing $\mathrm{Br}{}^{-}$ ions. A pale yellow precipitate of $\mathrm{AgBr}$ confirms the presence of $\mathrm{Br}{}^{-}$ ions.

$$\mathrm{AgNO}{}_3+\mathrm{NaBr}\to \mathrm{AgBr}(\mathrm{pale}\mathrm{yellow}\mathrm{ppt})+\mathrm{NaNO}{}_3$$