Halogens and Noble Gases: Quick Revision

A brief note on Halogens and Noble Gases. Read it before a test. Try each question at the end before you open its answer.

Group 17

• The halogens are $ns^2np^5$. F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid.
• Chlorine has the most negative electron gain enthalpy. Fluorine is the strongest oxidising agent ($E^\circ = +2.87$ V), because of its weak F-F bond and the hydration of the fluoride ion.
• Fluorine shows only $-1$. HF is a weak acid and forms hydrogen bonds. HF attacks glass.
• Displacement: a halogen with a higher $E^\circ$ oxidises the halide of a lower one.

Chlorine and hydrogen chloride

• Preparation: $\text{MnO}_{2}\ +\ 4\text{HCl}\ \rightarrow \ \text{MnCl}_{2}\ +\ \text{Cl}_{2}\ +\ 2\text{H}_{2}\text{O}$. Deacon: $4\text{HCl}\ +\ \text{O}_{2}$ over CuCl2 at about 723 K. Electrolysis of brine.
• Cold dilute NaOH: NaCl and NaOCl. Hot concentrated NaOH: $6\text{NaOH}\ +\ 3\text{Cl}_{2}\ \rightarrow \ 5\text{NaCl}\ +\ \text{NaClO}_{3}\ +\ 3\text{H}_{2}\text{O}$. Slaked lime: bleaching powder.
• Excess ammonia: $8\text{NH}_{3}\ +\ 3\text{Cl}_{2}\ \rightarrow \ 6\text{NH}_{4}\text{Cl}\ +\ \text{N}_{2}$. Bleaching by chlorine is by oxidation and is permanent.
• HCl: $\text{NaCl}\ +\ \text{H}_{2}\text{SO}_{4}$ at about 420 K, then $\text{NaHSO}_{4}\ +\ \text{NaCl}$ at about 823 K. Aqua regia is 3 parts HCl to 1 part HNO3.
• Hydrogen halides: acid strength HF < HCl < HBr < HI.

Interhalogens and oxoacids

• XY linear, XY3 T-shaped (2 lone pairs, $sp^3d$), XY5 square pyramidal (1 lone pair, $sp^3d^2$), XY7 pentagonal bipyramidal ($sp^3d^3$).
• Oxoacid strength rises from $\text{HOCl}$ to $\text{HClO}_{2}$, $\text{HClO}_{3}$ and $\text{HClO}_{4}$. $\text{ClO}_{4}^{-}$ is tetrahedral and $\text{ClO}_{3}^{-}$ is pyramidal.

Noble gases

• $ns^2np^6$. Highest ionisation enthalpy in each period. Boiling point rises down the group (He 4.2 K).
• Bartlett (1962): Xe and PtF6. XeF2 (linear, 3 lone pairs), XeF4 (square planar, 2), XeF6 (distorted octahedral, 1).
• Hydrolysis: $\text{XeF}_{6}\ +\ 3\text{H}_{2}\text{O}\ \rightarrow \ \text{XeO}_{3}\ +\ 6\text{HF}$. XeO3 is pyramidal. XeOF4 is square pyramidal.

A worked example

How many moles of sodium chlorate form from $0.90\ \text{mol}$ of chlorine in hot concentrated alkali?

1. Equation. $6\text{NaOH}\ +\ 3\text{Cl}_{2}\ \rightarrow \ 5\text{NaCl}\ +\ \text{NaClO}_{3}\ +\ 3\text{H}_{2}\text{O}$.
2. Ratio. Three Cl2 give one NaClO3.
3. Answer. $0.90 \times \dfrac{1}{3} = 0.30\ \text{mol}$.

Practice with new numbers

A program to try

The program lists the oxidation state of chlorine in its oxoacids and the rough pKa that comes from the rule of Pauling.

Mistakes to avoid

• Hot and cold alkali mixed up. Cold dilute gives hypochlorite, hot concentrated gives chlorate.
• Fluorine with the most negative electron gain enthalpy. It is chlorine.
• Leaving out the lone pairs of the central atom. They decide the shape.
• Using the covalent radius for the noble gases. The value is the van der Waals radius.

Check yourself

1. Which noble gas has the lowest boiling point, and why?

2. What is the shape of ClF3, and how many lone pairs does chlorine have?

3. Why is HClO4 a stronger acid than HOCl?

4. What are the products of the complete hydrolysis of XeF6?

Sources

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements.
• OpenStax, Chemistry 2e, Chapter 18.