ACIDS, BASES, AND SALTS; NEUTRALIZATION
~Acids, bases, and salts.~ The three classes of compounds known
respectively as acids, bases, and salts include the great majority of
the compounds with which we shall have to deal. It is important,
therefore, for us to consider each of these classes in a systematic way.
The individual members belonging to each class will be discussed in
detail in the appropriate places, but a few representatives of each
class will be described in this chapter with special reference to the
common properties in accordance with which they are classified.
~The familiar acids.~ _Hydrochloric acid_ is a gas composed of hydrogen and
chlorine, and has the formula HCl. The substance is very soluble in
water, and it is this solution which is usually called hydrochloric
acid. _Nitric acid_ is a liquid composed of hydrogen, nitrogen, and
oxygen, having the formula HNO_{3}. As sold commercially it is mixed
with about 32% of water. _Sulphuric acid_, whose composition is
represented by the formula H_{2}SO_{4}, is an oily liquid nearly twice
as heavy as water, and is commonly called _oil of vitriol_.
~Characteristics of acids.~ (1) All acids contain hydrogen. (2) When
dissolved in water the molecules of the acid dissociate into two kinds
of ions. One of these is always hydrogen and is the cation (+), while
the other consists of the remainder of the molecule and is the anion
(-). (3) The solution tastes sour. (4) It has the power to change the
color of certain substances called _indicators_. Thus blue litmus is
changed to red, and yellow methyl orange is changed to red. Since all
acids produce hydrogen cations, while the anions of each are different,
the properties which all acids have in common when in solution, such as
taste and action on indicators, must be attributed to the hydrogen ions.
DEFINITION: _An acid is a substance which produces hydrogen ions when
dissolved in water or other dissociating liquids._
~Undissociated acids.~ When acids are perfectly free from water, or are
dissolved in liquids like benzene which do not have the power of
dissociating them into ions, they should have no real acid properties.
This is found to be the case. Under these circumstances they do not
affect the color of indicators or have any of the properties
characteristic of acids.
The familiar bases. The bases most used in the laboratory are sodium
hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide
(Ca(OH)_{2}). These are white solids, soluble in water, the latter
sparingly so. Some bases are very difficultly soluble in water. The very
soluble ones with most pronounced basic properties are sometimes called
the _alkalis_.
~Characteristics of bases.~ (1) All bases contain hydrogen and oxygen. (2)
When dissolved in water the molecules of the base dissociate into two
kinds of ions. One of these is always composed of oxygen and hydrogen
and is the anion. It has the formula OH and is called the _hydroxyl
ion_. The remainder of the molecule, which usually consists of a single
atom, is the cation. (3) The solution of a base has a soapy feel and a
brackish taste. (4) It reverses the color change produced in indicators
by acids, turning red litmus blue, and red methyl orange yellow. Since
all bases produce hydroxyl anions, while the cations of each are
different, the properties which all bases have in common when in
solution must be due to the hydroxyl ions.
DEFINITION: _A base is a substance which produces hydroxyl ions when
dissolved in water or other dissociating liquids._
~Undissociated bases.~ Bases, in the absence of water or when dissolved in
liquids which do not dissociate them, should have none of the properties
characteristic of this class of substances. This has been found to be
the case. For example, they have no effect upon indicators under these
circumstances.
~Neutralization.~ When an acid and a base are brought together in solution
in proper proportion, the characteristic properties of each disappear.
The solution tastes neither sour nor brackish; it has no effect upon
indicators. There can therefore be neither hydrogen nor hydroxyl ions
present in the solution. A study of reactions of this kind has shown
that the hydrogen ions of the acid combine with the hydroxyl ions of the
base to form molecules of water, water being a substance which is not
appreciably dissociated into ions. This action of an acid on a base is
called _neutralization_. The following equations express the
neutralization of the three acids by three bases, water being formed in
each case.
Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
K^{+}, OH^{-} + H^{+}, NO_{3}^{-} = K^{+}, NO_{3}^{-} + H_{2}O.
Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
= Ca^{++}, SO_{4}^{--} + 2H_{2}O.
DEFINITION: _Neutralization consists in the union of the hydrogen ion of
an acid with the hydroxyl ion of a base to form water._
~Salts.~ It will be noticed that in neutralization the anion of the acid
and the cation of the base are not changed. If, however, the water is
expelled by evaporation, these two ions slowly unite, and when the water
becomes saturated with the substance so produced, it separates in the
form of a solid called a _salt_.
DEFINITION: _A salt is a substance formed by the union of the anion of
an acid with the cation of a base._
~Characteristics of salts.~ (1) From the definition of a salt it will be
seen that there is no element or group of elements which characterize
salts. (2) Salts as a class have no peculiar taste. (3) In the absence
of all other substances they are without action on indicators. (4) When
dissolved in water they form two kinds of ions.
~Heat of neutralization.~ If neutralization is due to the union
of hydrogen ions with hydroxyl ions, and nothing more, it
follows that when a given weight of water is formed in
neutralization, the heat set free should always be the same, no
matter from what acid and base the two kinds of ions have been
supplied. Careful experiments have shown that this is the case,
provided no other reactions take place at the same time. When
18g. of water are formed in neutralization, 13,700 cal. of heat
are set free. This is represented in the equations
Na^{+}, OH^{-} + H^{+}, Cl^{-}
= Na^{+}, Cl^{-} + H_{2}O + 13,700 cal.
K^{+}, OH^{-} + H^{+}, NO_{3}^{-}
= K^{+}, NO_{3}^{-} + H_{2}O + 13,700 cal.
Ca^{++}, (OH)_{2}^{--} + H_{2}^{++}, SO_{4}^{--}
= Ca^{++}, SO_{4}^{--} + 2H_{2}O + 2 × 13,700 cal.
~Neutralization a quantitative act.~ Since neutralization is a definite
chemical act, each acid will require a perfectly definite weight of each
base for its neutralization. For example, a given weight of sulphuric
acid will always require a definite weight of sodium hydroxide, in
accordance with the equation
H_{2}, SO_{4} + 2Na, OH = Na_{2}, SO_{4} + 2H_{2}O.
~Determination of the ratio in neutralization.~ The quantities of
acid and base required in neutralization may be determined in
the following way. Dilute solutions of the two substances are
prepared, the sulphuric acid being placed in one of the
burettes (Fig. 34) and the sodium hydroxide in the other. The
levels of the two liquids are then brought to the zero marks of
the burettes by means of the stopcocks. A measured volume of
the acid is drawn off into a beaker, a few drops of litmus
solution added, and the sodium hydroxide is run in drop by drop
until the red litmus just turns blue. The volume of the sodium
hydroxide consumed is then noted. If the concentrations of the
two solutions are known, it is easy to calculate what weight of
sodium hydroxide is required to neutralize a given weight of
sulphuric acid. By evaporating the neutralized solution to
dryness, the weight of the sodium sulphate formed can be
determined directly. Experiment shows that the weights are
always in accordance with the equation in the preceding
paragraph.
[Illustration: Fig. 34]
~Extent of dissociation.~ The question will naturally arise, When an acid,
base, or salt dissolves in water, do all the molecules dissociate into
ions, or only a part of them? The experiments by which this question can
be answered cannot be described here. It has been found, however, that
only a fraction of the molecules dissociate. The percentage which will
dissociate in a given case depends upon several conditions, the chief of
which are: (1) The concentration of the solution. In concentrated
solutions only a very small percentage of dissociation occurs. As the
solution is diluted the percentage increases, and in very dilute
solutions it may be very large, though it is never complete in any
ordinary solution. (2) The nature of the dissolved compound. At equal
concentrations substances differ much among themselves in the percentage
of dissociation. The great majority of salts are about equally
dissociated. Acids and bases, on the contrary, show great differences.
Some are freely dissociated, while others are dissociated to but a
slight extent.
~Strength of acids and bases.~ Since acid and basic properties are due to
hydrogen and hydroxyl ions respectively, the acid or base which will
produce the greatest percentage of these ions at a given concentration
must be regarded as the strongest representative of its class. The acids
and bases described in the foregoing paragraphs are all quite strong. In
10% solutions they are dissociated to about 50%, and this is also
approximately the extent to which most salts are dissociated at this
same concentration.
~Partial neutralization.~ 1. _Basic salts._ The chemical action
between an acid and a base is not always as complete as has
been represented in the foregoing paragraphs. For example, if
the base magnesium hydroxide (Mg(OH)_{2}) and hydrochloric acid
(HCl) are brought together in the ratio of an equal number of
molecules of each, there will be only half enough hydrogen ions
for the hydroxyl ions present.
Mg, (OH)_{2} + H, Cl = Mg, OH, Cl + H_{2}O.
Magnesium, hydroxyl, and chlorine ions are left at the close of
the reaction, and under the proper conditions unite to form
molecules of the compound Mg(OH)Cl. This compound, when
dissolved, can form hydroxyl ions and therefore possesses basic
properties; it can also form the ions of a salt (Mg and Cl),
and has properties characteristic of salts. Substances of this
kind are called _basic salts._
DEFINITION: _A basic salt is a substance which can give the
ions both of a base and of a salt when dissolved in water._
2. _Acid salts._ In a similar way, when sulphuric acid and
sodium hydroxide are brought together in the ratio of equal
numbers of the molecules of each, it is possible to have a
reaction expressed by the equation
Na, OH + H_{2}, SO_{4} = Na, H, SO_{4} + H_{2}O.
The ions remaining after all the hydroxyl ions have been used
up are those of an acid (H) and those of a salt (Na and
SO_{4}). These unite to form the substance NaHSO_{4}, and as
the solution becomes saturated with this substance through
evaporation, it separates in the form of crystals. In solution
this substance can give hydrogen ions, and therefore possesses
acid properties; it can also give the ions characteristic of a
salt. It is therefore called an _acid salt_.
DEFINITION: _An acid salt is one which can give the ions of an
acid and of a salt when in solution._
3. _Normal salts._ Salts which are the products of complete
neutralization, such as Na_{2}SO_{4}, and which in solution can
give neither hydrogen nor hydroxyl ions, but only the ions of a
salt, are called _normal salts_ to distinguish them from acid
and basic salts.
~Methods of expressing reactions between compounds in solution.~ Chemical
equations representing reactions between substances in solution may
represent the details of the reaction, or they may simply indicate the
final products formed. In the latter case the formation of ions is not
indicated. Thus, if we wish to call attention to the details of the
reaction between sodium hydroxide and hydrochloric acid in solution, the
equation is written as follows:
Na^{+}, OH^{-} + H^{+}, Cl^{-} = Na^{+}, Cl^{-} + H_{2}O.
On the other hand, if we wish simply to represent the final products
formed, the following is used.
NaOH + HCl = NaCl + H_{2}O.
Both of these methods will therefore be used:
~Radicals.~ It has been emphasized that the hydroxyl group (OH) always
forms the anion of a base, while the group NO_{3} forms the anion of
nitric acid and sodium nitrate; the group SO_{4}, the anion of sulphuric
acid and calcium sulphate. A group of elements which in this way
constitutes a part of a molecule, acting as a unit in a chemical change,
or forming ions in solution, is called a _radical_. Some of these
radicals have been given special names, the names signifying the
elements present in the radical. Thus we have the hydroxyl radical (OH)
and the nitrate radical (NO_{3}).
DEFINITION: _A radical is a group of elements forming part of a
molecule, and acting as a unit in chemical reactions._
~Names of acids, bases, and salts.~ Since acids, bases, and salts are so
intimately related to each other, it is very advantageous to give names
to the three classes in accordance with some fixed system. The system
universally adopted is as follows:
~Naming of bases.~ All bases are called _hydroxides_. They are
distinguished from each other by prefixing the name of the element which
is in combination with the hydroxyl group. Examples: sodium hydroxide
(NaOH); calcium hydroxide (Ca(OH)_{2}); copper hydroxide (Cu(OH)_{2}).
~Naming of acids.~ The method of naming acids depends upon whether the
acid consists of two elements or three.
1. _Binary acids._ Acids containing only one element in addition to
hydrogen are called _binary acids_. They are given names consisting of
the prefix _hydro-_, the name of the second element present, and the
termination _-ic_. Examples: hydrochloric acid (HCl); hydrosulphuric
acid (H_{2}S).
2. _Ternary acids._ In addition to the two elements present in binary
acids, the great majority of acids also contain oxygen. They therefore
consist of three elements and are called _ternary acids_. It usually
happens that the same three elements can unite in different proportions
to make several different acids. The most familiar one of these is given
a name ending in the suffix _-ic_, while the one with less oxygen is
given a similar name, but ending in the suffix _-ous_. Examples: nitric
acid (HNO_{3}); nitrous acid (HNO_{2}). In cases where more than two
acids are known, use is made of prefixes in addition to the two suffixes
_-ic_ and _-ous_. Thus the prefix _per-_ signifies an acid still richer
in oxygen; the prefix _hypo-_ signifies one with less oxygen.
~Naming of salts.~ A salt derived from a binary acid is given a name
consisting of the names of the two elements composing it, with the
termination _-ide_. Example: sodium chloride (NaCl). All other binary
compounds are named in the same way.
A salt of a ternary acid is named in accordance with the acid from which
it is derived. A ternary acid with the termination _-ic_ gives a salt
with the name ending in _-ate_, while an acid with termination _-ous_
gives a salt with the name ending in _-ite_. The following table will
make the application of these principles clear:
ACIDS SYMBOL SALTS SYMBOL
Hydrochloric HCl Sodium chloride NaCl
Hypochlorous HClO Sodium hypochlorite NaClO
Chlorous HClO_{2} Sodium chlorite NaClO_{2}
Chloric HClO_{3} Sodium chlorate NaClO_{3}
Perchloric HClO_{4} Sodium perchlorate NaClO_{4}
EXERCISES
1. 25 cc. of a solution containing 40 g. of sodium hydroxide per liter
was found to neutralize 25 cc. of a solution of hydrochloric acid. What
was the strength of the acid solution?
2. After neutralizing a solution of sodium hydroxide with nitric acid,
there remained after evaporation 100 g. of sodium nitrate. How much of
each substance had been used?
3. A solution contains 18 g. of hydrochloric acid per 100 cc. It
required 25 cc. of this solution to neutralize 30 cc. of a solution of
sodium hydroxide. What was the strength of the sodium hydroxide solution
in parts per hundred?
4. When perfectly dry sulphuric acid is treated with perfectly dry
sodium hydroxide, no chemical change takes place. Explain.
5. When cold, concentrated sulphuric acid is added to zinc, no change
takes place. Recall the action of dilute sulphuric acid on the same
metal. How do you account for the difference?
6. A solution of hydrochloric acid in benzene does not conduct the
electric current. When this solution is treated with zinc, will hydrogen
be evolved? Explain.
7. (a) Write equation for preparation of hydrogen from zinc and dilute
sulphuric acid. (b) Rewrite the same equation from the standpoint of
the theory of electrolytic dissociation, (c) Subtract the common
SO_{4} ion from both members of the equation, (d) From the resulting
equation, explain in what the preparation of hydrogen consists when
examined from the standpoint of this theory.
8. In the same manner as in the preceding exercise, explain in what the
action of sodium on water to give hydrogen consists.