MANGANESE AND CHROMIUM
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| | | | |
| SYMBOL | ATOMIC | DENSITY | MELTING | FORMULAS OF ACIDS
| | WEIGHT | | POINT |
__________|________|________|_________|_________|___________________
| | | | |
Manganese | Mn | 55.0 | 8.01 | 1900° | H_{2}MnO_{4} and
| | | | | HMnO_{4}
Chromium | Cr | 52.1 | 7.3 | 3000° | H_{2}CrO_{4} and
| | | | | H_{2}Cr_{2}O_{7}
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~General.~ Manganese and chromium, while belonging to different families,
have so many features in common in their chemical conduct that they may
be studied together with advantage. They differ from most of the
elements so far studied in that they can act either as acid-forming or
base-forming elements. As base-forming elements each of the metals forms
two series of salts. In the one series, designated by the suffix "ous,"
the metal is divalent; in the other series, designated by the suffix
"ic," the metal is trivalent. Only the manganous and the chromic salts,
however, are of importance. The acids in which these elements play the
part of a non-metal are unstable, but their salts are usually stable,
and some of them are important compounds.
MANGANESE
~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO_{2},
called pyrolusite. In smaller amounts it occurs as the oxides
Mn_{2}O_{3} and Mn_{3}O_{4}, and as the carbonate MnCO_{3}. Some iron
ores also contain manganese.
~Preparation and properties.~ The element is difficult to prepare in pure
condition and has no commercial applications. It can be prepared,
however, by reducing the oxide with aluminium powder or by the use of
the electric furnace, with carbon as the reducing agent. The metal
somewhat resembles iron in appearance, but is harder, less fusible, and
more readily acted upon by air and moisture. Acids readily dissolve it,
forming manganous salts.
~Oxides of manganese.~ The following oxides of manganese are known: MnO,
Mn_{2}O_{3}, Mn_{3}O_{4}, MnO_{2}, and Mn_{2}O_{7}. Only one of these,
the dioxide, needs special mention.
~Manganese dioxide~ (_pyrolusite_) (MnO_{2}). This substance is the most
abundant manganese compound found in nature, and is the ore from which
all other compounds of manganese are made. It is a hard, brittle, black
substance which is valuable as an oxidizing agent. It will be recalled
that it is used in the preparation of chlorine and oxygen, in
decolorizing glass which contains iron, and in the manufacture of
ferromanganese.
~Compounds containing manganese as a base-forming element.~ As has been
stated previously, manganese forms two series of salts. The most
important of these salts, all of which belong to the manganous series,
are the following:
Manganous chloride MnCl_{2}·4H_{2}O.
Manganous sulphide MnS.
Manganous sulphate MnSO_{4}·4H_{2}O.
Manganous carbonate MnCO_{3}.
Manganous hydroxide Mn(OH)_{2}.
The chloride and sulphate may be prepared by heating the dioxide with
hydrochloric and sulphuric acids respectively:
MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl,
MnO_{2} + H_{2}SO_{4} = MnSO_{4} + H_{2}O + O.
The sulphide, carbonate, and hydroxide, being insoluble, may be prepared
from a solution of the chloride or sulphate by precipitation with the
appropriate reagents. Most of the manganous salts are rose colored. They
not only have formulas similar to the ferrous salts, but resemble them
in many of their chemical properties.
~Compounds containing manganese as an acid-forming element.~ Manganese
forms two unstable acids, namely, manganic acid and permanganic acid.
While these acids are of little interest, some of their salts,
especially the permanganates, are important compounds.
~Manganic acid and manganates.~ When manganese dioxide is fused with an
alkali and an oxidizing agent a green compound is formed. The equation,
when caustic potash is used, is as follows:
MnO_{2} + 2KOH + O = K_{2}MnO_{4} + H_{2}O.
The green compound (K_{2}MnO_{4}) is called potassium manganate, and is
a salt of the unstable manganic acid (H_{2}MnO_{4}). The manganates are
all very unstable.
~Permanganic acid and the permanganates.~ When carbon dioxide is passed
through a solution of a manganate a part of the manganese is changed
into manganese dioxide, while the remainder forms a salt of the unstable
acid HMnO_{4}, called permanganic acid. The equation is
3K_{2}MnO_{4} + 2CO_{2} = MnO_{2} + 2KMnO_{4} + 2K_{2}CO_{3}.
Potassium permanganate (KMnO_{4}) crystallizes in purple-black needles
and is very soluble in water, forming an intensely purple solution. All
other permanganates, as well as permanganic acid itself, give solutions
of the same color.
~Oxidizing properties of the permanganates.~ The permanganates are
remarkable for their strong oxidizing properties. When used as an
oxidizing agent the permanganate is itself reduced, the exact character
of the products formed from it depending upon whether the oxidation
takes place (1) in an alkaline or neutral solution, or (2) in an acid
solution.
1. _Oxidation in alkaline or neutral solution._ When the solution is
either alkaline or neutral the potassium and the manganese of the
permanganate are both converted into hydroxides, as shown in the
equation
2KMnO_{4} + 5H_{2}O = 2Mn(OH)_{4} + 2KOH + 3O.
2. _Oxidation in acid solution._ When free acid such as sulphuric is
present, the potassium and the manganese are both changed into salts of
the acid:
2KMnO_{4} + 3H_{2}SO_{4} = K_{2}SO_{4} + 2MnSO_{4} + 3H_{2}O + 5O.
Under ordinary conditions, however, neither one of these reactions takes
place except in the presence of a third substance which is capable of
oxidation. The oxygen is not given off in the free state, as the
equations show, but is used up in effecting oxidation.
Potassium permanganate is particularly valuable as an oxidizing agent
not only because it acts readily either in acid or in alkaline solution,
but also because the reaction takes place so easily that often it is not
even necessary to heat the solution to secure action. The substance
finds many uses in the laboratory, especially in analytical work. It is
also used as an antiseptic as well as a disinfectant.
CHROMIUM
~Occurrence.~ The ore from which all chromium compounds are made is
chromite, or chrome iron ore (FeCr_{2}O_{4}). This is found most
abundantly in New Caledonia and Turkey. The element also occurs in small
quantities in many other minerals, especially in crocoisite (PbCrO_{4}),
in which mineral it was first discovered.
~Preparation.~ Chromium, like manganese, is very hard to reduce from its
ores, owing to its great affinity for oxygen. It can, however, be made
by the same methods which have proved successful with manganese.
Considerable quantities of an alloy of chromium with iron, called
ferrochromium, are now produced for the steel industry.
~Properties.~ Chromium is a very hard metal of about the same density as
iron. It is one of the most infusible of the metals, requiring a
temperature little short of 3000° for fusion. At ordinary temperatures
air has little action on it; at higher temperatures, however, it burns
brilliantly. Nitric acid has no action on it, but hydrochloric and
dilute sulphuric acids dissolve it, liberating hydrogen.
~Compounds containing chromium as a base-forming element.~ While chromium
forms two series of salts, chromous salts are difficult to prepare and
are of little importance. The most important of the chromic series are
the following:
Chromic hydroxide Cr(OH)_{3}.
Chromic chloride CrCl_{3}·6H_{2}O.
Chromic sulphate Cr_{2}(SO_{4})_{3}.
Chrome alums
~Chromic hydroxide~ (Cr(OH)_{3}). This substance, being insoluble, can be
obtained by precipitating a solution of the chloride or sulphate with a
soluble hydroxide. It is a greenish substance which, like aluminium
hydroxide, dissolves in alkalis, forming soluble salts.
~Dehydration of chromium hydroxide.~ When heated gently chromic
hydroxide loses a part of its oxygen and hydrogen, forming the
substance CrO·OH, which, like the corresponding aluminium
compound, has more pronounced acid properties than the
hydroxide. It forms a series of salts very similar to the
spinels; chromite is the ferrous salt of this acid, having the
formula Fe(CrO_{2})_{2}. When heated to a higher temperature
chromic hydroxide is completely dehydrated, forming the
trioxide Cr_{2}O_{3}. This resembles the corresponding oxides
of aluminium and iron in many respects. It is a bright green
powder, and when ignited strongly becomes almost insoluble in
acids, as is also the case with aluminium oxide.
~Chromic sulphate~ (Cr_{2}(SO_{4})_{3}). This compound is a violet-colored
solid which dissolves in water, forming a solution of the same color.
This solution, however, turns green on heating, owing to the formation
of basic salts. Chromic sulphate, like ferric and aluminium sulphates,
unites with the sulphates of the alkali metals to form alums, of which
the best known are potassium chrome alum (KCr(SO_{4})_{2}·12H_{2}O) and
ammonium chrome alum (NH_{4}Cr(SO_{4})_{2}·12H_{2}O).
These form beautiful dark purple crystals and have some practical uses
in the tanning industry and in photography. A number of the salts of
chromium are also used in the dyeing industry, for they hydrolyze like
aluminium salts and the hydroxide forms a good mordant.
~Hydrolysis of chromium salts.~ When ammonium sulphide is added
to a solution of a chromium salt, such as the sulphate,
chromium hydroxide precipitates instead of the sulphide. This
is due to the fact that chromic sulphide, like aluminium
sulphide, hydrolyzes in the presence of water, forming chromic
hydroxide and hydrosulphuric acid. Similarly, a soluble
carbonate precipitates a basic carbonate of chromium.
~Compounds containing chromium as an acid-forming element.~ Like
manganese, chromium forms two unstable acids, namely, chromic acid and
dichromic acid. Their salts, the chromates and dichromates, are
important compounds.
~Chromates.~ When a chromium compound is fused with an alkali and an
oxidizing agent a chromate is produced. When potassium hydroxide is used
as the alkali the equation is
2Cr(OH)_{3} + 4KOH + 3O = 2K_{2}CrO_{4} + 5H_{2}O.
This reaction recalls the formation of a manganate under similar
conditions.
~Properties of chromates.~ The chromates are salts of the unstable chromic
acid (H_{2}CrO_{4}), and as a rule are yellow in color. Lead chromate
(PbCrO_{4}) is the well-known pigment chrome yellow. Most of the
chromates are insoluble and can therefore be prepared by precipitation.
Thus, when a solution of potassium chromate is added to solutions of
lead nitrate and barium nitrate respectively, the reactions expressed by
the following equations occur:
Pb(NO_{3})_{2} + K_{2}CrO_{4} = PbCrO_{4} + 2KNO_{3},
Ba(NO_{3})_{2} + K_{2}CrO_{4} = BaCrO_{4} + 2KNO_{3}.
The chromates of lead and barium separate as yellow precipitates. The
presence of either of these two metals can be detected by taking
advantage of these reactions.
~Dichromates.~ When potassium chromate is treated with an acid the
potassium salt of the unstable dichromic acid (H_{2}Cr_{2}O_{7}) is
formed:
2K_{2}CrO_{4} + H_{2}SO_{4} = K_{2}Cr_{2}O_{7} + K_{2}SO_{4} + H_{2}O.
The relation between the chromates and dichromates is the same as that
between the phosphates and the pyrophosphates. Potassium dichromate
might therefore be called potassium pyrochromate.
~Potassium dichromate~ (K_{2}Cr_{2}O_{7}). This is the best known
dichromate, and is the most familiar chromium compound. It forms large
crystals of a brilliant red color, and is rather sparingly soluble in
water. When treated with potassium hydroxide it is converted into the
chromate
K_{2}Cr_{2}O_{7} + 2KOH = 2K_{2}CrO_{4} + H_{2}O.
When added to a solution of lead or barium salt the corresponding
chromates (not dichromates) are precipitated. With barium nitrate the
equation is
2Ba(NO_{3})_{2} + K_{2}Cr_{2}O_{7} + H_{2}O
= 2BaCrO_{4} + 2KNO_{3} + 2HNO_{3}.
Potassium dichromate finds use in many industries as an oxidizing agent,
especially in the preparation of organic substances, such as the dye
alizarin, and in the construction of several varieties of electric
batteries.
~Sodium chromates.~ The reason why the potassium salt rather than
the sodium compound is used is that sodium chromate and
dichromate are so soluble that it is hard to prepare them pure.
This difficulty is being overcome now, and the sodium compounds
are replacing the corresponding potassium salts. This is of
advantage, since a sodium salt is cheaper than a potassium
salt, so far as raw materials go.
~Oxidizing action of chromates and dichromates.~ When a dilute solution of
a chromate or dichromate is acidified with an acid, such as sulphuric
acid, no reaction apparently takes place. However, if there is present a
third substance capable of oxidation, the chromium compound gives up a
portion of its oxygen to this substance. Since the chromate changes into
a dichromate in the presence of an acid, it will be sufficient to study
the action of the dichromates alone. The reaction takes place in two
steps. Thus, when a solution of ferrous sulphate is added to a solution
of potassium dichromate acidified with sulphuric acid, the reaction is
expressed by the following equations:
(1) K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4}
= K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 4H_{2}O + 3O,
(2) 6FeSO_{4} + 3H_{2}SO_{4} + 3O = 3Fe_{2}(SO_{4})_{3} + 3H_{2}O.
The dichromate decomposes in very much the same way as a permanganate
does, the potassium and chromium being both changed into salts in which
they play the part of metals, while part of the oxygen of the dichromate
is liberated.
By combining equations (1) and (2), the following is obtained:
K_{2}Cr_{2}O_{7} + 7H_{2}SO_{4} + 6FeSO_{4}
= K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 3Fe_{2}(SO_{4})_{3} + 7H_{2}0.
This reaction is often employed in the estimation of iron in iron ores.
~Potassium chrome alum.~ It will be noticed that the oxidizing
action of potassium dichromate leaves potassium sulphate and
chromium sulphate as the products of the reaction. On
evaporating the solution these substances crystallize out as
potassium chrome alum, which substance is produced as a
by-product in the industries using potassium dichromate for
oxidizing purposes.
~Chromic anhydride~ (CrO_{3}). When concentrated sulphuric acid is added
to a strong solution of potassium dichromate, and the liquid allowed to
stand, deep red needle-shaped crystals appear which have the formula
CrO_{3}.This oxide of chromium is called chromic anhydride, since it
combines readily with water to form chromic acid:
CrO_{3} + H_{2}O = H_{2}CrO_{4}.
It is therefore analogous to sulphur trioxide which forms sulphuric acid
in a similar way:
SO_{3} + H_{2}O = H_{2}SO_{4}.
Chromic anhydride is a very strong oxidizing agent, giving up oxygen and
forming chromic oxide:
2CrO_{3} = Cr_{2}O_{3} + 3O.
~Rare elements of the family.~ Molybdenum, tungsten, and uranium
are three rather rare elements belonging in the same family
with chromium, and form many compounds which are similar in
formulas to the corresponding compounds of chromium. They can
play the part of metals and also form acids resembling chromic
acid in formula. Thus we have molybdic acid (H_{2}MoO_{4}), the
ammonium salt of which is (NH_{4})_{2}MoO_{4}. This salt has
the property of combining with phosphoric acid to form a very
complex substance which is insoluble in nitric acid. On this
account molybdic acid is often used in the estimation of the
phosphoric acid present in a substance. Like chromium, the
metals are difficult to prepare in pure condition. Alloys with
iron can be prepared by reducing the mixed oxides with carbon
in an electric furnace; these alloys are used to some extent in
preparing special kinds of steel.
EXERCISES
1. How does pyrolusite effect the decolorizing of glass containing iron?
2. Write the equations for the preparation of manganous chloride,
carbonate, and hydroxide.
3. Write the equations representing the reactions which take place when
ferrous sulphate is oxidized to ferric sulphate by potassium
permanganate in the presence of sulphuric acid.
4. In the presence of sulphuric acid, oxalic acid is oxidized by
potassium permanganate according to the equation
C_{2}H_{2}O_{4} + O = 2CO_{2} + H_{2}O.
Write the complete equation.
5. 10 g. of iron were dissolved in sulphuric acid and oxidized to ferric
sulphate by potassium permanganate. What weight of the permanganate was
required?
6. What weight of ferrochromium containing 40% chromium must be added to
a ton of steel to produce an alloy containing 1% of chromium?
7. Write the equation representing the action of ammonium sulphide upon
chromium sulphate.
8. Potassium chromate oxidizes hydrochloric acid, forming chlorine.
Write the complete equation.
9. Give the action of sulphuric acid on potassium dichromate (a) in
the presence of a large amount of water; (b) in the presence of a
small amount of water.