CARBOXYLIC ACIDS AS ACIDS
This page looks at the simple reactions of carboxylic acids as
acids, including their reactions with metals, metal hydroxides,
carbonates and hydrogencarbonates, ammonia and amines.
Note: This page covers all the reactions likely to be asked by
any of the UK syllabuses for 16 - 18 year olds, but no single
syllabus is likely to want all of them. Check your syllabus and
past papers to find out what you need to know. If you are a
studying a UK-based syllabus and haven't got these, follow this
link to find out how to get hold of them.
The acidity of the carboxylic acids
Why are carboxylic acids acidic?
Using the definition of an acid as a "substance which donates
protons (hydrogen ions) to other things", the carboxylic acids are
acidic because of the hydrogen in the -COOH group.
In solution in water, a hydrogen ion is transferred from the COOH group to a water molecule. For example, with ethanoic
acid, you get an ethanoate ion formed together with a
hydroxonium ion, H3O+.
This reaction is reversible and, in the case of ethanoic acid, no
more than about 1% of the acid has reacted to form ions at any
one time. (This is a rough-and-ready figure and varies with the
concentration of the solution.)
These are therefore weak acids.
This equation is often simplified to:
However, if you are going to use this second equation, you must
include state symbols. They imply that the hydrogen ion is
actually attached to a water molecule.
The pH of carboxylic acid solutions
The pH depends on both the concentration of the acid and how
easily it loses hydrogen ions from the -COOH group.
Ethanoic acid is typical of the acids where the -COOH group is
attached to a simple alkyl group. Typical lab solutions have pH's
in the 2 - 3 range, depending on their concentrations.
Methanoic acid is rather stronger than the other simple acids,
and solutions have pH's about 0.5 pH units less than ethanoic
acid of the same concentration.
Note: You will find factors affecting the acidity of organic acids
discussed in detail if you follow this link.
If you should know how to calculate the pH of weak acids like
ethanoic acid, but aren't happy about it, you might be interested
in my chemistry calculations book.
If you choose to follow either of these links, use the BACK
button on your browser to return to this page.
Reactions of the carboxylic acids
With metals
Carboxylic acids react with the more reactive metals to produce
a salt and hydrogen. The reactions are just the same as with
acids like hydrochloric acid, except they tend to be rather
slower.
For example, dilute ethanoic acid reacts with magnesium. The
magnesium reacts to produce a colourless solution of
magnesium ethanoate, and hydrogen is given off. If you use
magnesium ribbon, the reaction is less vigorous than the same
reaction with hydrochloric acid, but with magnesium powder,
both are so fast that you probably wouldn't notice much
difference.
Warning! Students have a high tendency to get the formulae
of salts like magnesium ethanoate wrong. Remember that these
simple carboxylic acids form ions with a single negative charge,
and that the magnesium ion has two positive charges. In
essence, this is no different from writing the formula for
magnesium chloride - except that with these salts the metal is
written at the end of the formula rather than at the beginning.
With metal hydroxides
These are simple neutralisation reactions and are just the same
as any other reaction in which hydrogen ions from an acid react
with hydroxide ions. They are most quickly and easily
represented by the equation:
Note: This assumes that the hydroxide is in solution. (It
probably will be, because almost the only reaction ever done of
this type is with sodium hydroxide solution!) If you were using a
solid hydroxide, however, you would have to swap the
hydroxide state symbol from "(aq)" to "(s)".
If you mix dilute ethanoic acid with sodium hydroxide solution,
for example, you simply get a colourless solution containing
sodium ethanoate. The only sign that a change has happened is
that the temperature of the mixture will have increased.
This change could well be represented by the ionic equation
above, but if you want it, the full equation for this particular
reaction is:
With carbonates and hydrogencarbonates
In both of these cases, a salt is formed together with carbon
dioxide and water. Both are most easily represented by ionic
equations.
For carbonates:
. . . and for hydrogencarbonates:
Note: This time, this assumes that the carbonate or
hydrogencarbonate will be a solid - which it often is. If you used
a solution of, say, sodium carbonate or hydrogencarbonate, you
would have to remember to change the state symbol.
If you pour some dilute ethanoic acid onto some white sodium
carbonate or sodium hydrogencarbonate crystals, there is an
immediate fizzing as carbon dioxide is produced. You end up
with a colourless solution of sodium ethanoate.
With sodium carbonate, the full equation is:
. . . and for sodium hydrogencarbonate:
There is very little obvious difference in the vigour of these
reactions compared with the same reactions with dilute
hydrochloric acid.
However, you would notice the difference if you used a slower
reaction - for example with calcium carbonate in the form of a
marble chip. With ethanoic acid, you would eventually produce a
colourless solution of calcium ethanoate.
In this case, the marble chip would react noticeably more slowly
with ethanoic acid than with hydrochloric acid.
With ammonia
Ethanoic acid reacts with ammonia in exactly the same way as
any other acid does. It transfers a hydrogen ion to the lone pair
on the nitrogen of the ammonia and forms an ammonium ion.
If you mix together a solution of ethanoic acid and a solution of
ammonia, you will get a colourless solution of ammonium
ethanoate.
With amines
Amines are compounds in which one or more of the hydrogen
atoms in an ammonia molecule have been replaced by a
hydrocarbon group such as an alkyl group. For simplicity, we'll
just look at compounds where only one of the hydrogen atoms
has been replaced. These are called primary amines.
Simple primary amines include:
The small amines are very similar indeed to ammonia in many
ways. For example, they smell very much like ammonia and are
just as soluble in water. Because all you have done to an
ammonia molecule is swap a hydrogen for an alkyl group, the
lone pair is still there on the nitrogen atom.
That means that they will react with acids (including carboxylic
acids) in just the same way as ammonia does.
For example, ethanoic acid reacts with methylamine to produce
a colourless solution of the salt methylammonium ethanoate.
Note: I have deliberately not tried to write this as a one line
equation. It is much easier to see what is happening, to write it,
and to remember it, if you draw the structures.
Don't be put off by the name of the product - methylammonium
ethanoate. "Methylammonium" just means an ammonium ion in
which one of the hydrogens is replaced by a methyl group.
However complicated the amine, because all of them have got a
lone pair on the nitrogen atom, you would get the same sort of
reaction.
Note: If you want to find out more about amines you could
explore the amines menu by following this link.