Practical investigations
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I am doing my coursework on vinegars, and am confused by the distillation process. If heating up the vinegars separate the volatile acids from the non-volatile acids, then why does the non-volatile acid nitric acid have a boiling point of 83°C when the volatile acid ethanoic have a boiling temp of 118°C. If this is the case, won’t the nitric acid come off before the ethanoic?
Igloo writes ...The boiling point of concentrated (pure) nitric acid is about 80°C whereas that of concentrated (pure) ethanoic acid is about 30°C higher. So as you would expect, concentrated nitric acid is likely to be more volatile than concentrated ethanoic acid.
However, these two acids behave very differently when diluted with water. Nitric acid is a strong acid and is virtually completely ionised:
HNO3 -> H+(aq) + NO3-(aq)
Dilute nitric acid consists entirely of ions, apart from the water molecules present, of course, and there are no HNO3 molecules present at all. When this solution is gently boiled water is evolved – not nitric acid. We can say therefore that in solution nitric acid is non volatile.
However, when ethanoic acid dissolves in water, the ionising reaction which takes place is reversible, and an equilibrium is set up:
CH3COOH(aq) <=> CH3COO-(aq) + H+(aq)
And we say that the acid is weak since it not totally ionised in solution. Only about one molecule in a hundred is ionised.
When dilute ethanoic acid is boiled water molecules and covalent ethanoic acid molecules vaporise away and the above equilibrium shifts to the left in order to replace the ethanoic acid molecules which have left the system.
Because of all this we say that ethanoic acid (in solution) is a volatile acid whereas nitric acid (in solution) is non volatile.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 24 February 2006
