Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.27 Reactions of halides-Study Notes - New Syllabus

8.27 (i) Reaction of Solid Group 1 Halides with Concentrated Sulfuric Acid

These reactions show the trend in reducing ability of hydrogen halides (\( \mathrm{HX} \)) down Group 7.

Key Trend

• Reducing ability increases down the group:

\( \mathrm{HF < HCl < HBr < HI} \)

• Due to weaker H–X bond → easier to lose electrons.

(A) Chloride (e.g. NaCl)

Reaction (Acid–Base Only)

\( \mathrm{NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl} \)

Observation

• Steamy white fumes of \( \mathrm{HCl} \)

Key Point

• No redox reaction (Cl⁻ is a weak reducing agent)

(B) Bromide (e.g. NaBr)

Step 1 (Acid–Base)

\( \mathrm{NaBr + H_2SO_4 \rightarrow NaHSO_4 + HBr} \)

Step 2 (Redox)

\( \mathrm{2HBr + H_2SO_4 \rightarrow Br_2 + SO_2 + 2H_2O} \)

Observations

• Brown fumes of \( \mathrm{Br_2} \)
• Colourless \( \mathrm{SO_2} \) gas

Key Point

• Bromide is a stronger reducing agent → reduces \( \mathrm{H_2SO_4} \)

(C) Iodide (e.g. NaI)

Step 1 (Acid–Base)

\( \mathrm{NaI + H_2SO_4 \rightarrow NaHSO_4 + HI} \)

Further Redox Reactions

• \( \mathrm{2HI + H_2SO_4 \rightarrow I_2 + SO_2 + 2H_2O} \)
• \( \mathrm{6HI + H_2SO_4 \rightarrow 3I_2 + S + 4H_2O} \)
• \( \mathrm{8HI + H_2SO_4 \rightarrow 4I_2 + H_2S + 4H_2O} \)

Observations

• Purple iodine vapour (\( \mathrm{I_2} \))
• Yellow sulfur solid (\( \mathrm{S} \))
• Rotten egg smell (\( \mathrm{H_2S} \))

Key Point

• Iodide is the strongest reducing agent
• Reduces sulfur from +6 to +4, 0, and −2

Summary Table

Summary

• Reducing ability increases down Group 7.
• Chloride → no redox, bromide → partial, iodide → extensive redox.
• Shows increasing ease of electron donation.

Therefore, reactions with sulfuric acid clearly demonstrate the trend in reducing power of halides.

8.27 (ii) Test for Halide Ions using Silver Nitrate

Halide ions (\( \mathrm{Cl^- , Br^- , I^-} \)) are identified using aqueous silver nitrate after acidifying with nitric acid.

(A) Why Nitric Acid is Used

• Removes interfering ions such as \( \mathrm{CO_3^{2-}} \)
• Prevents formation of other precipitates (e.g. \( \mathrm{Ag_2CO_3} \))
• Ensures test is specific for halides

(B) Precipitation Reactions

General Equation

\( \mathrm{Ag^+ + X^- \rightarrow AgX(s)} \)

Observations

(C) Solubility in Ammonia

• Used to confirm identity of halide

Explanation

• Ammonia forms a complex ion:

\( \mathrm{Ag^+ + 2NH_3 \rightarrow [Ag(NH_3)_2]^+} \)

• Strength of Ag–X bond increases from Cl → I
• Therefore solubility decreases down the group

Summary

• Silver nitrate forms characteristic precipitates.
• Colours distinguish halides.
• Ammonia confirms identity via solubility.

Therefore, this test is a key qualitative method for identifying halide ions.

8.27 (iii) Reactions of Hydrogen Halides with Ammonia and Water

Hydrogen halides (\( \mathrm{HX} \), where X = Cl, Br, I) show important reactions with ammonia gas and water.

(A) Reaction with Ammonia Gas

General Equation

\( \mathrm{NH_3 + HX \rightarrow NH_4X} \)

Examples

• \( \mathrm{NH_3 + HCl \rightarrow NH_4Cl} \)
• \( \mathrm{NH_3 + HBr \rightarrow NH_4Br} \)

Observation

• White fumes of ammonium halide (e.g. \( \mathrm{NH_4Cl} \))

Explanation

• Ammonia acts as a base (accepts \( \mathrm{H^+} \))
• Hydrogen halide acts as an acid
• Forms ammonium salt via proton transfer

(B) Reaction with Water (Formation of Acids)

General Equation

\( \mathrm{HX + H_2O \rightarrow H_3O^+ + X^-} \)

Examples

• \( \mathrm{HCl + H_2O \rightarrow H_3O^+ + Cl^-} \)
• \( \mathrm{HBr + H_2O \rightarrow H_3O^+ + Br^-} \)

Observation

• Solution becomes acidic (low pH)

Explanation

• Hydrogen halide donates a proton to water
• Forms hydronium ions (\( \mathrm{H_3O^+} \))
• Therefore behaves as a strong acid (HCl, HBr, HI)

Key Comparison

Summary

• Hydrogen halides react with ammonia to form ammonium salts.
• They dissolve in water to form acidic solutions.
• Both reactions involve proton transfer.

Therefore, hydrogen halides behave as acids in both gaseous and aqueous systems.