Halogens
- Fluorine
- Chlorine
- Bromine
- Iodine
- Astatine

Brief introduction on Group VII elements

Halogens can be found in Group VII of the periodic table. Thus, they have 7 valence electrons and form '-1' ions known as anions. Halogens are fundamentally diatomic covalent non-metal molecules and have low melting and boiling points. They are reactive non-metals as they only need one more electron to attain the noble gas configuration. Halogens react with most metals to form salts known as halides. This is known as the displacement reaction whereby one element takes the place of another alement in a compound. A more reactive halogen will displace the less reactive halogen from its halide solution. A less reactive halogen cannot displace a more reactive halogen from its halide solution. The reactivity of halogen decreases down the group.



CLICK THE IMAGE for clearer view.

Redox reaction with solutions of other halide ions

Redox reactions with other halide ion solutions can also be known as the displacement reaction. As its name suggest, it is the reaction in which one element takes the place of another element in a compound, A more reactive halogen will displace a less reactive halogen from its halide solution. A less reactive halogen CANNOT displace a more reactive halogen from its halide solution. The reactivity of halogens decrease down the group.
F>Cl>Br>I>At





As demonstrated by the above picture as well as the comparison of reactivity of halogens: F>Cl>Br>I>At, Chlorine is the most reactive amongst the three solutions used which is why is manages to displace the other two elements, Bromine and Iodine. We can infer the element that is being displaced from the colour change of the solution.

Down the group:
-Oxidising power of halogens decrease

-Melting and boiling points increase this is because… the boiling points of alkyl chlorides, bromides and iodides follow the order RI > RBr > RCl where R is an alkyl group. With the increase in the size of halogen, the magnitude of Van der Waals force increases and consequently, the boiling points increase. This works like wise for the increase in melting points of halogens.

-Halogens become darker in colour

Test for and identification of halide ions

Carrying out the test:
This test has to be done in solution. If you start from a solid, it must first be dissolved in pure water.
The solution is acidified by adding dilute nitric acid. (Remember: silver nitrate + dilute nitric acid.) The nitric acid reacts with, and removes, other ions that might also give a confusing precipitate with silver nitrate.

Silver nitrate solution is then added to give:




The chloride, bromide and iodide precipitates are shown in the photograph:



The chloride precipitate is obviously white, but the other two aren't really very different from each other. You couldn't be sure which you had unless you compared them side-by-side.
All of the precipitates change colour if they are exposed to light - taking on grey or purplish tints.
The absence of a precipitate with fluoride ions doesn't prove anything unless you already know that you must have a halogen present and are simply trying to find out which one. All the absence of a precipitate shows is that you haven't got chloride, bromide or iodide ions present.

The chemistry of the test
The precipitates are the insoluble silver halides - silver chloride, silver bromide or silver iodide.


Silver fluoride is soluble, and so you don't get a precipitate.

Ammonia solution is added to the precipitates to confirm the precipitate.



Explaining what happens

Look at the way the solubility products vary from silver chloride to silver iodide. (You can't quote a solubility product value for silver fluoride because it is too soluble. Solubility products only work with compounds which are very, very sparingly soluble.)


You can see that the compounds are all pretty insoluble, but become even less soluble as you go from the chloride to the bromide to the iodide.

What is the ammonia doing?
The ammonia combines with silver ions to produce a complex ion called the diamminesilver(I) ion, [Ag(NH3)2]+. This is a reversible reaction, but the complex is very stable, and the position of equilibrium lies well to the right.




A solution in contact with one of the silver halide precipitates will contain a very small concentration of dissolved silver ions. The effect of adding the ammonia is to lower this concentration still further.

What happens if you multiply this new silver ion concentration by the halide ion concentration? If the answer is less than the solubility product, the precipitate will dissolve.

That happens with the silver chloride, and with the silver bromide if concentrated ammonia is used. The more concentrated ammonia tips the equilibrium even further to the right, lowering the silver ion concentration even more.

The silver iodide is so insoluble that the ammonia won't lower the silver ion concentration enough for the precipitate to dissolve.


Here's a video to demonstrate the chemical properties of halogens!