Tutorial 1

Can you think of an application (to make use of) for a compound having a high specific heat and a high boiling point?      Answer

Alkali metals react actively (or vigorously) with water or any protonated compounds (in their liquid form). Protonated compounds are compounds that have active protons (H⁺).

Note: From now on the word heat will be replaced by the symbol ΔH(enthalpy).

Hydrogen and heat is a dangerous combination as hydrogen burns by itself. (oxygen only supports burning). So every chemist knows that it is important to keep the alkali metal away from protonated compounds. In the lab these metals are stored in plastic bottles (it will attack glass) and submerged in paraffin oil, to keep them from the water moisture in the air. Do not forget to check on the bottle once in a while. Plastics do not last forever in the atmosphere. Sometimes it turns brittle and leaks, becoming a time bomb in the lab.

DO NOT DROP ALKALI METALS INTO THE SINK.

SIZE & REACTIVITY

Although all alkali metals react with water and protonated compounds, they do so with different degree of aggressiveness. Here I would like to introduce the importance of size in chemical reaction. Lithium is the smallest of all the alkali metals, and so the valence electron is attracted more strongly to the nucleus. The first ionisation potential (kJ/mol) for the alkali metals are in the order of:

It is not surprising that lithium reacts gently with water, sodium vigorously, potassium inflames, and rubidium and cesium explosively.

This trend in reactivity is also observed for all other reactions, and for the elements in the other Groups of the Periodic Table.

In Applied (or Industrial) Chemistry the interest is how we can make use of the chemistry to make things happen. In this case it appears that the reaction has not much value. This is where creativity comes into play. If we can slow down the reaction, we can use the hydrogen released for reduction processes. What we can do is to find a compound, having a hydrogen, that will not react so readily.

Let us take sodium and ammonia (NH₃). Sodium can dissolve in ammonia to about 10 ‾⁴ g/litre. By using photochemical irradiation the sodium can be made to react with the ammonia to release hydrogen.

If it can happen with ammonia it should be able to do so with the cousins of ammonia, like the amines.

This reaction will take place when the solution is heated to about 60�C.

This series of reactions are very useful for reducing organic compounds in the laboratory.

REACTION with OXYGEN

All metals (that is elements in Group IA to Group IIIA) when left in air will react with oxygen (in the air) to give the oxides. Iron will give iron oxide, known as rust. When we polish our silver wares, copper antiques and tin (Pewter ware) souvenirs because they appear dull or tarnished, we are actually removing the layer of oxide formed on the surface.

So we can assume that all metals will form oxides with oxygen when left to stand in air, the only difference will be the reactivity. Lithium would react with oxygen in the air;

The oxide then reacts with water to give the hydroxide

So store lithium, immersed in oil, to keep it away from air and water.

REACTION with WATER

Lithium reacts with water to give the hydroxide and hydrogen gas

Lithium hydroxide is a strong base. It can react with various acids to give the corresponding ionic compounds. Example it will react with hydrochloric acid to give the chloride.

Using this chemistry we can prepare lithium halides (that is lithium chloride, bromide and iodide), lithium nitrate, lithium sulphate, and lithium carbonate. The corresponding acids used will be hydrochloric, hydrobromic, hydroiodic, nitric, sulphuric and carbonic acids.

REACTION of LITHIUM HYDROXIDE with CARBON DIOXIDE

Lithium hydroxide can absorb carbon dioxide to form lithium carbonate.

SOLUBILITY

All ionic lithium compounds are soluble in water.

Needless to say if you mixed a solution of an ionic lithium compound with a solution of another ionic compound there will be a "reaction". Actually it will be more of a dance; "changing partners".

I will illustrate this with solutions of lithium chloride and barium nitrate. (Remember all nitrates are soluble in water.) The system will then be:

It would be a nightmare to try to isolate the lithium nitrate from the barium chloride, although it could be done.

• Unless otherwise stated all solutions of ionic compound would be assumed to be in water.
• The symbol "aq" is used here to show that the ions were surrounded by water molecules.

Let us try the mixture of lithium sulphate and barium nitrate.

The equilibrium will be in favour of the formation of barium sulphate since it is not soluble in water and settles out as a solid. (Pay attention to the second set of arrows.) At the end of the reaction the barium sulphate is filtered off leaving the lithium nitrate and a little barium sulphate in the filtrate (filtrate is the solution which passes through the filtration system.)

This is the best procedure to prepare an ionic compound. If the ionic compound to be prepared is insoluble in water this method should be your first consideration. This is not difficult as all alkali compounds are soluble in water and all nitrates are soluble in water, since the other components should be an alkali compound it will also remain soluble in water.

The problem is, most of the common ionic compounds are soluble in water, making this method rather limited.

Tutorial 2

• What are halides?
• Why was there a little barium sulphate in the filtrate for the reaction discussed above?
• What would be the products if you left a piece of lithium standing in the air?      Answer

LITHIUM HYDRIDE

Lithium hydride, LiH, is the most stable hydride of the Group IA elements. It is unaffected by oxygen, chlorine or HCl. But it is very sensitive to water and of course reacts vigorous with protonated acids. So in a laboratory hydrides must be kept in a very dry atmosphere, and away from protonated acids. It is very useful as a source of hydrogen and for practical reason it is complexed with Group IIIB elements for this application.