2.7 Inorganic chemistry of group 7 (limited to chlorine, bromine and iodine)

Cro2012

Group 7 – the halogensThe elements in group 7 of the periodic table, on the right, are called the halogens.

fluorine

chlorine

bromine

iodine

astatine

I

Br

Cl

F

At

a. recall the characteristic physical properties of the elements limited to the appearance of solutions of the elements in water and hydrocarbon solvents

In water:• Chlorine solution is pale green• Bromine solution is yellow to

orange• Iodine solution is brown.

In hydrocarbon solvents:• Chlorine solution is pale green• Bromine solution is yellow to orange• Iodine solution is pink-purple.

b. describe and carry out the following chemical reactions of halogens:i. oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution

Oxidation Reaction of HalogensOxidation Reaction of Halogens

With Metals:React strongly with electropositive metals, removing the outer electrons to become reduced themselves.

b. describe and carry out the following chemical reactions of halogens:i. oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution

Oxidation Reaction of HalogensOxidation Reaction of Halogens

With Non-Metals:The halogen usually achieves a noble gas configuration by forming a covalent bond:

Reactivity decreases

b. describe and carry out the following chemical reactions of halogens:i. oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution

Oxidation Reaction of HalogensOxidation Reaction of Halogens

With Iron(II) chloride solution:

b. describe and carry out the following chemical reactions of halogens:ii. disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide

with iodine to produce potassium iodate(V)

DisproportionationDisproportionation

Give the oxidation numbers for the chlorine containing species.

b. describe and carry out the following chemical reactions of halogens:ii. disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide

with iodine to produce potassium iodate(V)

DisproportionationDisproportionation

c. carry out an iodine/thiosulfate titration, including calculation of the results and evaluation of the procedures involved,

eg determination of the purity of potassium iodate(V) by liberation of iodine and titration with standard sodium thiosulfate solution

Iodine/thiosulfate titration

Remember that when carrying out this titration the starch is added near the end point (when the solution is pale yellow). 

Addition of starch to a solution that contains iodine or triiodide ion forms a reversible blue complex. The disappearance of this blue coloured complex is a much more sensitive method of determining the end point.  However, if the starch is added to a solution which contains a great deal of iodine, the complex which forms may not be reversible. Therefore, the starch is not added until shortly before the end point is reached.

Reaction with conc. Sulphuric AcidReaction with conc. Sulphuric Acid

• With sodium bromide/iodide, the hydrogen halides are oxidised by the acid:

White steamy fumes

Orange/brown fumes or colour

Purple fumes and/or black solid

d. describe and carry out the following reactions:i. potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution

Displacement (Redox) ReactionsDisplacement (Redox) Reactions

• Potassium halides react with other halogens:

• What gets reduced and what gets oxidised?

d. describe and carry out the following reactions:i. potassium halides with concentrated sulfuric acid, halogens and silver

nitrate solution

A more reactive halogen will displace a less reactive halogen from it’s halide in solution.So:

• Cl2 displaces Br2 from Br- and I2 from I-

• Br2 displaces I2 from I-

Testing for Halides: Silver Testing for Halides: Silver HalidesHalides

Addition of dilute nitric acidThen add aqueous silver

Nitrate solution.

d. describe and carry out the following reactions:i. potassium halides with concentrated sulfuric acid, halogens and silver nitrate solutionii. silver halides with sunlight and their solubilities in aqueous ammonia solution

Testing for Halides: Silver Testing for Halides: Silver HalidesHalides

Partial decomposition turns AgCl intoPartial decomposition turns AgCl intograyish precipitate in sunlight. grayish precipitate in sunlight.

Equations

d. describe and carry out the following reactions:ii. silver halides with sunlight and their solubilities in aqueous ammonia

solution

The silver halides are unstable in the presence of sunlight. They decompose forming silver (seen as dark specs) and the halogen, for example:  2AgI(s) 2Ag(s) + I2(s) 

Reaction of Hydrogen HalidesReaction of Hydrogen Halides

• Hydrogen halides react with ammonia gas to form ammonium halides:

• Hydrogen chloride dissolves in water to form hydrochloric acid.

d. describe and carry out the following reactions:

iii. hydrogen halides with ammonia and with water (to produce acids)

e. make predictions about fluorine and astatine and their compounds based on the trends in the physical and chemical properties of halogens.

poisonous and smelly.

brittle and crumbly when solid

What are the general properties of What are the general properties of the halogens?the halogens?

All the halogens are:

They become darker in colour down the group:

Diatomic non-metals and so do not conduct electricity

is pale yellow

is yellow-green

is grey

is red-brown

Very reactive and strong oxidising agents.

All halogens have seven electrons in their outer shell. This means that:

What is the electron structure of the What is the electron structure of the halogens?halogens?

fluorine2,7

chlorine2,8,7

bromine2,8,8,7

They can easily obtain a full outer shell by gaining one electron.

They have similar chemical properties.

They all gain an electron in reactions to form negative ions with a -1 charge.

How does electron structure affect reactivity?The reactivity of alkali metals decreases going down the group. What is the reason for this? The atoms of each element get

larger going down the group. This means that the outer shell gets

further away from the nucleus and is shielded by more electron shells.

The further the outer shell is from the positive attraction of the nucleus, the harder it is to attract another electron to complete the outer shell.

This is why the reactivity of the halogens decreases going down group 7.

decr

ease

in re

activ

ity

F

Cl

Br

What is the physical state of the What is the physical state of the halogens?halogens?

The melting and boiling points of the halogens increase down the group, as the molecules become bigger.

What is the state of each halogen at room temperature?

Halogen Relative size

Melting point (°C)

Boiling point (°C)

State

-220 -118

-101

-7

114

-34

59

184

gas

gas

liquid

solid

Halogen SolubilityHalogen Solubility• Solubility in water decreases down group.

• Chlorine reacts in water to form chlorine water ( a mixture of HCl and chloric (I) acid:

• Chloric (I) acid is what gives a solution of chlorine its bleaching properties.

• Bromine reacts in a similar way, but to a lesser extent.