Presentation - 11 Group 17 a level chem.pdf

Chem a level group 17

1. Group 17

+ 11.1 Physical properties of the Group 17 elements

+ 11.2 The chemical properties of the elements and their hydrides
+ 11.3 Some reactions of the halide ions

+ 11.4 The reactions of chlorine

2. Introduction

@ Group 17 elements are called as halogens.

© The halogens consists of fluorine (F),
chlorine (Cl), bromine (Br), iodine (I), and
astatine (At).

Group 17 elements

3. Characteristic physical properties

© = Halogens have 7 electrons in the outermost shell.

Element Electronic configuration

Fluorine (F) 1s? 2s? 2p>

Chlorine (Cl) 152 2s? 2p6 352 3p5

Bromine (Br) 1s? 2s? 2p® 3s? 3p6 3d10 45? 4p>

lodine (I) 1s? 2s? 2p® 3s? 3p® 3d10 4s? 4p® 4d10 5s? 5p5

© = Halogens are non-metals.
e They exist as diatomic molecules and the two atoms are covalently bonded. For example: F,,
Cl,, Br, and I).

4. Characteristic physical properties

The diatomic molecules are held together by weak van der Waals’ forces.

This force increases with number of electrons in the molecules (down the group) as the
chances of dipoles arising within a molecule increase.

These dipoles would induce dipoles in the neighbouring molecules, strengthening the
intermolecular bond. Hence, melting and boiling points of halogens increases down the
group.

The melting and boiling points of all halogens are relatively low.

Chlorine is a green/yellow gas at room temperature.

Bromine is a brown liquid at room temperature.

lodine is a grey/black solid at room temperature.

5. Reactions of halogens

Halogens need one extra electron to achieve stable configuration.
They react with metallic elements in which, metallic elements lose electron and halogens gain electron.
The oxidation number of chlorine in this reaction changes from 0 to -1 in this reaction.
2 Na(s) + Cl, (g) > 2NaCl (s)
2 Na> 2Na* + 2e
Cl, +2e-Cl;

Halogens react with non-metal to form covalent bonds.
H2(8) + F2(8) > 2HF (8)

6. Reactions of halogens

The reactivity of halogens decreases down the group.

Fluorine is the element with highest reactivity in group 17. Fluorine has the least number of electrons in
this group and is the smallest in the group. The outer shell in fluorine is nearer to the nucleus and thus the
force of attraction between electrons in outermost shell and nucleus is high. Due to least shielding effect
in fluorine, the electron entering the outermost shell is accepted easily. Hence, fluorine is the strongest
electron acceptor (oxidising agent) of group 17.

The acceptance of electron decreases down the group due to increasing atomic radius, decreasing force of
attraction and increasing shielding effect.

In other words, the electronegativity of halogens decreases down the group and hence, the reactivity
decreases down the group.

7. Displacement reaction

© Amore reactive halogen replaces a less reactive halogen from its halide solution. Consider
the reaction between chlorine and sodium bromide, which forms a yellowish-brown colour
solution.

Cl, (aq) + 2NaBr (aq) > 2NaCl (aq) + Br, (aq)

@ Chlorine replaces the bromine from sodium bromide solution. The brown colour is due to
the presence of bromine molecules.

e Electronegativity of a group decreases down the group. Chlorine is more electronegative
than bromine and has a higher tendency to form negatively charged ions. The ionic
equation for this displacement reaction is,

Cl, (aq) + 2Br (aq) > 2CI (aq) + Br, (aq)

e Electronegativity of a group decreases down the group.

8. Displacement reaction

Chlorine (aq) Bromine (aq) lodine (aq)
potassium Very pale green Yellow solution, Brown solution,
chloride (aq) | solution, no reaction |no reaction no reaction
potassium Yellow solution, Yellow solution, Brown solution,
bromide (aq) | Cl has displaced Br no reaction no reaction
potassium Brown solution, Cl Brown Solution, Brown Solution,
iodide (aq) has displaced | Br has displaced | no reaction

9. Displacement
reactions

Cyclohexane
layer

Water

=
Bromine lodine Chlorine

Identification of halogen molecules

Halogen

Chlorine

Identification of halogen molecules is

difficult using the colour of the solution.

Halogens are dissolved in organic solvents
such as cyclohexane solution for

identification.

The colour of the halogen molecule when

dissolved in cyclohexane solution is given in

table below.

As cyclohexane is immiscible with water,

this layer is formed on the top of water.

Colour of free
halogen solution

Pale green

Colour in cyclohexane
solution

Pale yellow

Bromine

Yellow

Orange

lodine

Brown

Purple

10. Chlorine undergoes a redox reaction called as disproportionation, where chlorine is both
oxidised and reduced.

Reaction of chlorine with water for water treatment
Chlorine is used in water treatment to kill bacteria and make water safe for drinking.
Chlorine reacts with water to form hydrochloric acid and chloric (I) acid.

Cl, (aq) +H20 (1) > HCI (aq) +HCIO (aq)

Oxidation numbers
Reactant Product
Chlorine molecule HCl HCIO
0 “1 +1

11. Disproportion

Chloric (I) acid decomposes slowly in water to form reactive oxygen molecule that kills the
bacteria in water.
HCIO > HCI +[0]

+ Bacteria in water can cause diseases such as cholera, dysentery and typhoid. Chlorine kills
these bacteria.

+ Chlorine may react with organic compounds in water to form products that could be
hazardous to health.

+ It is important to use sodium hydroxide to remove chlorine in water before using it in
aquariums as chlorine could be harmful for aquatic organisms.

+ Before using tap-water for drinking purposes, it is important to use home filter, which
removes chlorine in water.

12. Disproportionation

b)

Reaction of chlorine with sodium hydroxide
Chlorine reacts with cold sodium hydroxide (15 °C) to form sodium chloride and sodium

chlorate (I).
Cl,(aq) + 2NaOH (aq) > NaCl (aq) +NaClO (aq) + H,0 (I)

Oxidation numbers

Reactant Product
Chlorine molecule NaCl Nacio
0 [ Sl +1

Bleach is a mixture of sodium chloride and sodium chlorate (I) and kills bacteria.
The chlorate ions oxidise the coloured dyes and other coloured molecules and helps to
remove stains.

13. Disproportionation

© = Chlorine reacts with hot sodium hydroxide to form sodium chloride and sodium chlorate (V).
3Cl,(aq) + 6NaOH (aq) > SNaCI (aq) + NaClO; (aq) + 3H,0 (I)

dation numbers

Reactant Product

Chlorine molecule NaCl NaClO,
0 a +5

14. Characteristic reactions
for halide ions

Chlorine Bromine lodine
(White) (Cream) (pale yellow)

Silver halide precipitates

15. Characteristic reactions for halide ions

Precipitate + dilute Precipitate +
Halide ion (X) Colour of Agx ue ll :
ammonia concentrated ammonia
Chloride White Soluble Soluble
Bromide Cream Insoluble Soluble

lodide Pale yellow Insoluble Insoluble

16. Reactions with hydrogen

Halogens react with hydrogen to form
hydrogen halides.

The thermal stability of hydrogen halides of
group 17 decreases down the group due to
their bond energies.

H-F> H-Cl > H-Br > H-I

Hydrogen Bond energy

halide (kJ/mol)
HF 562
HCI | 431
HBr | 366

HI | 299

17. Reactions with hydrogen

Hydrogen reacts with fluorine explosively under cool and dark
conditions,

H, (8) + F, (8)> 2HF (8)

Hydrogen reacts with chlorine explosively in the presence of
sunlight,

H (8) + Cl, (8) > 2HCI (8)
Hydrogen reacts with bromine slowly with heating,
H, (8) + Br, (8) > 2HBr (8)
Hydrogen reacts with iodine forming an equilibrium mixture,

H) (8) +1, (8) 2HI (8)

Hydroge: Bond energy
halide (kJ/mol)

HF 562

HCI 431

HBr 366

HI 299

18. Reactions with hydrogen

The thermal stabilities of hydrogen halides can also be
explained based on their decomposition.

Hydrogen iodide decomposes to form hydrogen gas and
iodine gas when a red-hot wire is inserted into its sample.
Hydrogen bromide is slightly more stable than hydrogen
iodide.

Whereas hydrogen fluoride and hydrogen chloride remain
stable up to 1500°C.

Hydrogen Bond energy
halide (kJ/mol)

HF 562

HCI 431

HBr 366

HI 299

19. Reactions of halide ions with

sulphuric acid

© Compounds containing halide ions reacts with sulphuric acid. Poisonous fumes are
produced in this reaction and hence, this reaction is performed in a fume cupboard. For
example:
NaCl (s) + H,50, (I)-> NaHSO, (s) + HCI (g)
@ Hydrogen chloride gas is poisonous. The thermal stability of hydrogen halides of group 17
decreases down the group. This is because of the trend in bond energies:
H-F> H-Cl > H-Br > H-I
© Due to this trend, bromine and iodine reacts differently with sulphuric acid. Bromine reacts
with sulphuric acid and the reaction proceeds further. Sulphuric acid is reduced to sulphur
dioxide. Fumes of HBr are white in colour. Bromine fumes are red in colour.
NaBr (s) + H,SO, (1) NaHSO, (s) + HBr(g)
2HBr(g) + H,50, (1)> Br2(g) + SO,(g) + 2H,0 (I)

20. Reactions of halide ions with

sulphuric acid

© lodine reacts with sulphuric acid and the reaction proceeds even further. Sulphuric acid is
reduced to sulphur dioxide, sulphur and hydrogen sulphide.
Nal (s) + H,50, ()> NaHSO, (s) + HI(g)
2HI(g) + H,SO, (1)> 1,(8) + SO,(8) + 24,0 (1)
and
6HI(g) + H,SO, (1) > 31,(g) + S(s) + 4H,0 (1)
and
8HI(g) + HSO, (1)> 41,(8) + H,S(s) + 4H,0 (I)
e Fumes of Hl are white in colour. lodine fumes are purple in colour. Sulphur is seen as yellow
solid. Hydrogen sulphide evolves with rotten egg smell.
e By looking at these reactions, it can be said that the reducing ability of hydrogen halides
increases down in the group.