Chemical properties of Group 7

In a nutshell

Halogens are reactive non-metals which are found in Group $7$ . Their boiling points increase as you go down the group whilst their reactivity decreases as you go down the group. A more reactive halide will displace a less reactive halide. Halides can be tested for with silver nitrate.

The halogens

Halogen	Formula	State at $(20 \degree C)$	Colour	Electronic structure
Fluorine	$F_2$	Gas	Pale yellow	$1s^22s^22p^5$
Chlorine	$Cl_2$	Gas	Green	$1s^22s^22p^63s^23p^5$
Bromine	$Br_2$	Liquid	Brown-red	$1s^22s^22p^63s^23p^63d^{10}4s^24p^5$
Iodine	$I_2$	Solid	Grey-black	$1s^22s^22p^63s^23p^63d^{10}4s^24p^64d^{10}5s^25p^5$

Chemistry; Inorganic chemistry and the periodic table; KS5 Year 12; Chemical properties of Group 7

Boiling points of halogens

The Group $7$ elements are simple covalent molecules which are held together by weak van der Waals forces. The strength of these forces increases as the atomic radius increases, this means that more energy would be required to overcome these forces. As you go down the group this would lead to an increase in boiling point.

Halogens reactivity

Halogens are generally highly reactive non-metals and gain an electron to form a $1-$ ion, to achieve a full outer shell. As the atomic radius increases down the group, it is more difficult to attract an electron as the shielding effect increases thus weakening the attraction between the nucleus and outer electrons. Halogen ions are reduced as they gain an electron, they oxidise another substance, making them oxidising agents.

Displacement reactions

Halogens displace less reactive halides. Halogens' relative oxidation strengths can be seen in their displacement reactions with halide ions. The relative oxidising strengths of halogens mean a halogen will displace any halide beneath it in the periodic table.

Example

$Cl_2(aq)\ +\ 2KBr(aq)\ \rightarrow\ 2KCl(aq)\ +\ Br_2(aq)$

$Cl_2(aq)\ +\ 2Br^-(aq)\ \rightarrow\ 2Cl^-(aq)\ +\ Br_2(aq)$

Displacement reactions to identify halides

Displacement reactions can be used to help identify which halogen is present in a solution. When a displacement reaction occurs, there are colour changes. These colour changes can be easily seen when shaking and mixing with hexane. A clear distinct layer will form. The halogen which is displaced will dissolve into the hexane layer whilst the more reactive halogen will remain in the aqueous layer.

Chemistry; Inorganic chemistry and the periodic table; KS5 Year 12; Chemical properties of Group 7

Halogen	Colour
Halogen	Aqueous	Organic
Chlorine	Colourless	Colourless
Bromine	Yellow	Orange
Iodine	Orange/brown	Purple

Halogen	Displacement reaction	Ionic equation
$Cl$	Chlorine $(Cl_2)$ will displace $Br^-$ and $I^-$	$Cl_2(aq)\ +\ 2Br^-(aq)\ \rightarrow\ 2Cl^-(aq)\ +\ Br_2(aq)$ $Cl_2(aq)\ +\ 2I^-(aq)\ \rightarrow\ 2Cl^-(aq)\ +\ I_2(aq)$
$Br$	Bromine $(Br_2)$ will displace $I^-$	$Br_2(aq)\ +\ 2I^-(aq)\ \rightarrow\ 2Br^-(aq)\ +\ I_2(aq)$
$I$	No reaction with $F^-, Cl^-, Br^-$

Testing for halides

To test for halides using silver nitrate, add dilute nitric acid to remove ions which may interfere with the test. Next, add silver nitrate solution $(AgNO_3)$ . A precipitate of the silver halide is formed.

$Ag^+(aq)\ +\ X^-(aq)\ \rightarrow\ AgX(s)$ : where $X$ is $Cl$ , $Br$ or $I$ .

The colour of the precipitate identifies the halide.

Halide	Precipitate colour
Chloride $(Cl^-)$	White precipitate
Bromide $(Br^-)$	Cream precipitate
Iodide $(I^-)$	Yellow precipitate

Ammonia $(NH_3)$ can be further used to indicate the halide ions present with different concentrations of ammonia giving different results. The greater the atomic radius of a halide, the more difficult it is to dissolve.

Halide	Solubility
Chloride ions $(Cl^-)$	Dissolves in dilute $NH_3\ (aq)$
Bromide ions $(Br^-)$	Dissolves in conc. $NH_3\ (aq)$
Iodide ions $(I^-)$	Insoluble in conc. $NH_3\ (aq)$