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We see that each of the reactions shown are very similar. This is typical of a homologous series. Effervescence occurs as hydrogen gas is evolved in each of the reactions. It is rare to see C-H bonds being broken in the carbon skeleton. The reaction is usually focused on the functional group, meaning that the O-H bond in the alcohol has been broken. An ionic solid is also formed which remains dissolved in the alcohol. Reaction becomes slower with the larger alcohols as these are more viscous (less runny) and it takes longer for new alcohol molecules to reach the surface of the sodium metal. |
The ionic solid is called an alkoxide. With ethanol, sodium ethoxide is produced:
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C2H5OH(l) + Na(s) ⇒C2H5ONa + ½H2(g) |
This reaction is is very easy to work out - just remove the -OH hydrogen and replace it with a sodium atom, -ONa. You may be asked to indicate the ionic charges - just remember that sodium always forms a 1+ ion, and you get -O-Na+
You should compare the reaction of sodium with water. A similar, but much more reactive, reaction takes place. I have written water as HOH rather than H2O so that you can more easily see the comparison with an alcohol:
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HOH(l) + Na(s) ⇒NaOH(aq) + ½H2(g) |
The sodium melts and floats on the surface of the water (it is one of only three metals which are less dense than water - lithium and potassium being the other two). Full range indicator placed in the water is initially yellow but turns purple when the alkali, sodium hydroxide, is produced. Most books suggest that potassium will catch fire on the water, but sodium does not. As you will see in the video sodium can also catch fire!
Video - the reaction of sodium with water
