Inorganic - Group 7 (17), the Halogens

Question 1

What are the physical appearances of halogens?

Answer 1

The gaseous halogens vary in appearance. At room temperature:

• fluorine is a pale yellow gas (poisonous)
• chlorine is a greenish gas (poisonous and dense)
• bromine is a red-brown liquid (very easily forms orange vapour, often used as a solution in water)
• iodine is a black solid (very easily forms purple vapour, often used as a sort of solution in water)

They get darker and denser going down the group. They all have a characteristics ‘swimming-bath’ smell.

Question 2

Why are a number of the properties of fluorine untypical?

Answer 2

Many of these untypical properties stem from the fact that the F-F bond in unexpectedly weak, compared with the trend for the rest of the halogens. The small size of the fluorine atom leads to repulsion between non-bonding electrons because they are close together.

Question 3

How does bond energy change down the group?

Answer 3

The bond energy generally decreases, with fluorine as the exception. Fluorine’s bond energy is lower than chlorine and bromine, but higher than iodine.

F-F: 158
Cl-Cl: 243
Br-Br: 193
I-I: 151

Question 4

How does the atomic radius change down the group?

Answer 4

• increases
• on moving down the group, there are more energy levels of electrons
• the outer energy levels of electrons are further from the nucleus
• this increases the size of the atoms down the group

Question 5

How does electronegativity change down the group?

Answer 5

• decreases
• for the smaller atoms, the bonding electrons in a covalent bond are closer to the positive nucleus (smaller atomic radius)
• there is less shielding (fewer inner shells of electrons)
• these factors are more important than the decreasing nuclear charge
• so the positive nucleus attracts the bonding pair of electrons more strongly
• and so the element has a greater electronegativity value

Question 6

What is electronegativity?

Answer 6

Electronegativity is a measure of the ability of an atom to attract electrons, or electron density, towards itself within a covalent bond.

Question 7

What does electronegativity depend on?

Answer 7

Electronegativity depends on the attraction between the nucleus and bonding electrons in the outer shell. This, in turn, depends on a balance between:

• the number of protons in the nucleus (nuclear charge)
• the distance between the nucleus and the bonding electrons (atomic radius)
• the shielding effect of inner shells of electrons

Question 8

How do the melting and boiling points change down the group?

Answer 8

• increases
• halogens are simple covalent molecules
• the size of the diatomic molecules increases down the group
• larger molecules have more electrons
• more electrons lead to greater individual dipole-dipole forces
• this means greater van der Waal’s forces between the molecules
• more energy is needed to overcome the forces to change them from a liquid to a gas
• the higher the boiling point, the less volatile the element

Question 9

How does reactivity change down the group?

Answer 9

• decreases
• halogens form negative ions, gaining an electron to fill their outer shell and become stable
• outer electrons are further from the nucleus as the group is descended as an extra energy level is added each time
• increase in shielding
• the atoms gain electrons less easily as there is a weaker attraction between the nucleus and the incoming electron
• smaller halogen atoms are more reactive

Question 10

How does the first ionisation energy change down the group?

Answer 10

• decreases
• greater atomic radius
• increased amounts of shielding
• easier to lose an electron

Question 11

How does the oxidising power change down the group?

Answer 11

• decreases
• halogens act as good oxidising agents as they accept electrons from the species being oxidised and are reduced
• their ability to attract electrons decreases due to shielding and a greater atomic radius down the group
• so oxidising power decreases
• the relative oxidising strengths mean a halogen will displace any halide beneath it in the periodic table

Question 12

What is a displacement reaction?

Answer 12

Halogens will react with metal halides in solution in such a way that the halide in the compound will be displaced by a more reactive halogen but not by a less reactive one. This is called a displacement reaction.

Question 13

How do halide ions act as reducing agents?

Answer 13

In these reactions, the halide ions lose electrons and become halogen molecules.

Question 14

What is the trend in reducing ability of halide ions?

Answer 14

• increases down a group
• the larger the ion, the more easily it loses an electron
• because the electron is lost from the outer shell which is further away from the nucleus as the ion gets larger so attraction to outer electron is less
• more shielding
• the greater the reducing power, the longer the reaction as the halide is powerful enough to reduce more species
• trend can be seen in the reactions of solid sodium halides with concentrated sulphuric acid

Question 15

What is the reaction of sodium halides with concentrated sulphuric acid?

Answer 15

Solid sodium halides react with concentrated sulphuric acid. The products are different and reflect the reducing powers of the halide ions.

Question 16

What is the reaction of solid sodium chloride with concentrated sulphuric acid?

Answer 16

Drops of concentrated sulphuric acid are added to the solid sodium chloride. Steamy fumes of hydrogen chloride are seen. The solid product is sodium hydrogensulphate.

The reaction is:
NaCl + H2SO4 -> NaHSO4 + HCl

This is not a redox reaction because no oxidation state has changed. The chloride ion is too weak a reducing agent to reduce the sulphur in sulphuric acid. It is an acid-base reaction.

This reaction can be used to prepare hydrogen chloride gas which, because of this reaction, was once called salt gas.

A similar reaction occurs with sodium fluoride to produce hydrogen fluoride, an extremely dangerous gas that will etch glass. The fluoride ion is an even weaker reducing agent than the chloride ion.

Question 17

What is the reaction of solid sodium bromide with concentrated sulphuric acid?

Answer 17

In this case you will see steamy fumes of hydrogen bromide and brown fumes of bromine. Colourless sulphur dioxide is also formed. Two reactions occur.

First sodium hydrogensulphate and hydrogen bromide are produced (in a similar acid-base reaction to sodium chloride).

NaBr + H2SO4 -> NaHSO4 + HBr

However, bromide ions are strong enough reducing agents to reduce the sulphuric acid to sulphur dioxide. The oxidation state of the sulphur is reduced from +6 to +4 and that of the bromine increases from -1 - 0.

This is a redox reaction. The reactions are exothermic and some of the bromine vaporises.

Question 18

What is the reaction of solid sodium iodide with concentrated sulphuric acid?

Answer 18

In this case you see steamy fumes of hydrogen iodide, the black solid of iodine, and the bad egg smell of hydrogen sulphide gas is present. Yellow solid sulphur may also be seen. Colourless sulphur dioxide is also evolved.

Several reactions occur. Hydrogen iodide is produced in an acid-base reaction as before.

Na + H2SO4 -> NaHSO4 + HI

Iodide ions are better reducing agents than bromide ions so they reduce the sulphur in sulphuric acid even further so that sulphur dioxide, sulphur, and hydrogen sulphide gas are produced.

Question 19

What are the features of chlorine?

Answer 19

• poisonous gas
• soluble in water
• used in the treatment of water

Question 20

How does chlorine react with water?

Answer 20

Chlorine reacts with water in a reversible reaction to form chloric acid (oxidising agent that kills bacteria) and hydrochloric acid.

Cl2 + H2O -> HClO + HCl

Question 21

How does chlorine react with water in sunlight?

Answer 21

2Cl2 + 2H2O -> 4HCl + O2

Chloride is rapidly lost from the pool water in sunlight so that shallow pools need frequent addition of chlorine.

Question 22

What is an alternative to the direct chlorination of swimming pools?

Answer 22

Add solid sodium (or calcium) chlorate. This dissolves in water to form chloric acid in a reversible reaction.

NaClO + H2O -> HClO + NaOH

Question 23

How does chlorine react with alkali?

Answer 23

Chloride reacts with cold, dilute sodium hydroxide to form sodium chlorate. This is an oxidising agent and the active ingredient in household bleach. This is also a disproportionation reaction.

Cl2 + 2NaOH -> NaClO + NaCl + H2O

The other halogens behave similarly.