Halide Ions – Properties, Reactions & Qualitative Tests

14 question-and-answer flashcards covering definitions, formation, redox behaviour, reactions with concentrated sulfuric acid, silver nitrate qualitative tests, and periodic trends of halide ions.

What is a halide ion?

A negatively charged ion formed when a Group 7 element gains one electron (e.g., F⁻, Cl⁻, Br⁻, I⁻).

Why do all halide ions carry a −1 charge?

Because each halogen gains exactly one electron to complete its outer shell.

Write the general half-equation for the formation of halide ions from halogen molecules.

X₂ + 2 e⁻ → 2 X⁻

How does reducing ability change down the halide group?

It increases: F⁻ < Cl⁻ < Br⁻ < I⁻.

Give two reasons why I⁻ is a stronger reducing agent than F⁻.

(1) Larger ionic radius so outer electrons are farther from the nucleus. (2) Greater electron shielding, so electrons are lost more easily.

What happens when NaCl reacts with concentrated H₂SO₄? Include observation.

Acid–base reaction only: NaCl + H₂SO₄ → HCl + NaHSO₄; misty white fumes of HCl, no redox products.

State the main observations when NaBr is treated with concentrated H₂SO₄.

Misty white fumes (HBr), brown Br₂ vapour, choking SO₂ gas—evidence of both acid–base and redox reactions.

List the observations when NaI reacts with concentrated H₂SO₄.

Purple I₂ vapour, yellow solid S, rotten-egg smell of H₂S—indicating full reduction of H₂SO₄.

Describe the two steps in the silver nitrate test for halide ions.

1) Add dilute HNO₃ to remove interfering ions. 2) Add AgNO₃ to form characteristic AgX precipitates.

What colour precipitate forms with AgNO₃ for Cl⁻, Br⁻, and I⁻ respectively?

Cl⁻: white (AgCl); Br⁻: cream (AgBr); I⁻: yellow (AgI).

How do you confirm halide precipitates using ammonia?

AgCl dissolves in dilute NH₃; AgBr dissolves only in concentrated NH₃; AgI is insoluble in NH₃.

Why is nitric acid added before AgNO₃ in the halide test?

To remove CO₃²⁻ or OH⁻ ions that could form unwanted white precipitates with Ag⁺ and cause false positives.

Why does the solubility of silver halides in ammonia decrease from AgCl to AgI?

Lattice energy increases with larger halide ions, making the lattice harder to break apart with NH₃.

State four overall trends for halide ions down Group 7.

Ionic radius ↑, reducing ability ↑, nucleophilic reactivity ↑, AgX lattice strength ↑ while solubility of AgX in NH₃ ↓.