The HSAB Concept in Mineral Ore

Indonesia is well-known as a rich country, it has many mineral, oil, natural gases,

and others. They are spread over in a lot of location in Indonesia. Some examples of

mineral product in Indonesia are iron and aluminium.

Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging

from silvery to dull grey, depending on the surface roughness. It has the symbol Al. its

atomic number is 13. It is not soluble in water under normal circumstances. Aluminium is

the most abundant metal in the Earth's crust, and the third most abundant element therein,

after oxygen and silicon. It makes up about 8% by weight of the Earth's solid surface.

Aluminium is too reactive chemically to occur in nature as a free metal. Aluminium is a

good electrical conductor. It is better than copper. Aluminium is found as its compound

because of its strong affinity to oxygen. It is found in oxides or silicates.

Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and

sapphire, respectively. Pure Al2O3, known as Corundum, is one of the hardest materials

known.

The chief source of aluminium is bauxite ore. The primary mining areas for the ore

are in Ghana, Indonesia, Jamaica, Russia and Surinam.

The last result of bauxite ore purifying process is aluminium oxide. Aluminium

oxide, Al2O3, occurs naturally as corundum, and can be made by burning aluminium in

oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only

exceeded by diamond, boron nitride, and carborundum. It is almost insoluble in water.

Aluminium sulfide, Al2S3, may be prepared by passing hydrogen sulfide over aluminium

powder. It is polymorphic.

Iron is the sixth most abundant element in the Universe, formed as the final act of

nucleosynthesis, by silicon fusing in massive stars. While it makes up about 5% of the

Earth's crust, the Earth's core is believed to consist largely of an iron-nickel alloy

constituting 35% of the mass of the Earth as a whole. Most of the iron in the crust is

found combined with oxygen as iron oxide minerals such as hematite (Fe2O3) and

magnetite (Fe3O4). Iron can be produced by reduction process according to this reaction:

Fe2O3 + 3H2 2Fe + 3H2O

The abundant of elements in our earth is usually found in their compound, such

ass oxides, sulfides, sulfates, silicates or others. For example, iron is usually found in iron

ore. It contains hematite (Fe2O3) and magnetite (Fe3O4). More iron is found as oxide

although iron could be found as pyrite (FeS2). Aluminum, silicon, titanium and

manganese are also found in an oxide. They are bauxite (Al3O3.nH2O), silica (SiO2), rutile

(TiO2) and phyrolusite (MnO2). But, copper is found in copper ore that is mostly found as

sulfide, such as chalcopyrite (CuFeS2), Chalcosite (Cu2S), and bornite (Cu5FeS4). Silver is

also found as sulfide, argentite (Ag2S).

These are some picture of the compounds:

Magnetite Bauxite

Chalcopyrite Bornite

Silica Pyrite

Chalcosite Argentite

Hematite Phyrolusite

May be there will be some questions, why is Iron ore mainly formed as oxides? Not

sulfides? And so Alumunium, why is alumininium ore formed as oxises? But, why is

copper found in its sulfides? Not formed as oxides?

The recent things above can be explained by the HSAB concept. The HSAB concept is an

acronym for 'hard and soft acids and bases'. Also known as the Pearson acid base concept, HSAB

is widely used in chemistry for explaining stability of compounds, reaction mechanisms and

pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies

to species which are small, have high charge states (the charge criterion applies mainly to acids,

to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big,

have low charge states and are strongly polarizable.

As we have seen, the Lewis theory of acid-base interactions based on electron pair

donation and acceptance applies to many types of species. As a result, the electronic

theory of acids and bases pervades the whole of chemistry. Because the formation of

metal complexes represents one type of Lewis acid-base interaction, it was in that area

that evidence of the principle that species of similar electronic character interact best

was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals

as belonging to class A if they formed more stable complexes with the first element in the

periodic group or to class B if they formed more stable complexes with the heavier

elements in that group. This means that metals are classified as A or B based on the

electronic character of the donor atom they prefer to bond to. The donor strength of the

ligands is determined by the stability of the complexes they form with metals. This

behavior is summarized in the following table.

Donor Strength

Class A metals N >> P > As > Sb > Bi

O > > S > Se > Te

F > Cl > Br > I

Class B metals N < < P > As > Sb > Bi

O < < S ≈ Se ≈ Te

F < Cl < Br < I

The HSAB theory is used in contexts where a qualitative, rather than quantitative

description would help in understanding the predominant factors which drive chemical properties

and reactions. This is especially so in transition metal chemistry, where numerous experiments

have been done to determine the relative ordering of ligands and transition metal ions in terms of

their hardness and softness.

Hard acids and hard bases tend to have:

small atomic/ionic radius

high oxidation state

low polarizability

high electronegativity

Soft acids and soft bases tend to have:

large atomic/ionic radius

low or zero oxidation state

high polarizability

low electronegativity

Borderline acids are intermediate between hard and soft acids. Thus they tend to have

lower charge and somewhat larger size than hard acids, and higher charge and somewhat smaller

size than soft acids. Borderline bases are intermediate between hard and soft bases. They tend to

be larger and less electronegative than hard bases, smaller and more electronegative than soft

bases.

Polarizability, the ability to distort the electron cloud of a molecule, and low

electronegativity depend upon these properties. Hard acids have the opposite characteristics.

Lewis Bases

Hard Soft

OH- , H2O, F- RS- , RSH, R2S

SO42- , Cl- , PO4

3-, CO32- , NO3

- I-, SCN- , CN- , S2O32-

ClO4- , RO- , ROH, R2O CO, H- , R-

NH3, RNH2 , N2H4 R3P, R3As, C2H4

Borderline Bases

C5H5N, N3-, N2 , Br- , NO2- , SO3

2-

.

Lewis Acids

Hard Soft

H+, Li+, Na+, K +, Be2+, Mg2+ Cu+, Ag+, Au+, Ru+

Ca2+, Mn2+, Al3+, Sc3+, La3+, Cr3+ Pd2+, Cd2+, Pt2+, Hg2+

Co3+, Fe3+, Si4+, Ti4+ GaCl 3, RS+, I+, Br+

Be(CH3)2, BF3, HCl, AlCl3, SO3 O, Cl, Br, I,

B(OR)3 ,CO2 , RCO+ , R2O, RO+ uncharged metals

Borderline Acids

Fe2+, Co2+, Ni2+, Zn2+, Cu2+, Sn 3+, Rh3+, (BCH3)3, Sb3+, SO2, NO+

The guiding principle regarding the interaction of electron pair donors and acceptors is that

the most favorable interactions occur when the acid and base have similar electronic character. In

accord with this observation, it is found that hard acids preferentially interact with hard bases,

and soft acids interact preferentially with soft bases. This is related to the way in which the

species interact. Hard acids interact with hard bases primarily by interactions that result from

forces between ions or polar species. Interactions of these types will be favored by high charge

and small size of both the acid and base. Soft acids and soft bases interact primarily by sharing

electron density, which is favored when the species have high polarizability. Frequently,

interactions between soft acids and soft bases involve bonding between neutral molecules. Orbital

overlap that leads to covalent bonding is most favorable when the orbitals of the donor and

acceptor atoms are of similar size and energy. The gist of this theory is that soft acids react faster

and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds

with hard bases, all other factors being equal.

Generally speaking, acids and bases interact and the most stable interactions are hard-hard

(ionogenic character) and soft-soft (covalent character).

This theory can be used to explain why Iron ore is mainly formed as oxides not as sulfides.

And so Alumunium, alumininium ore is formed as oxides. But copper is found in its sulfides not

formed as oxides.

Hematite (Fe2O3) consist of Fe3+ and O2- ions. Related to HSAB theory, Fe3+ is hard acid,

while O2- is hard base. S2- is soft bases, therefore, Fe3+ will tend to combined with oxygen ion than

sulfide ion. Fe2O3 is more stable than Fe2S3. So, iron is mainly found as oxides (hematite and

magnetite) than sulfides although there is pyrite FeS2. Aluminium ion is hard acids, so it is mostly

found in oxide. The aluminium ore, bauxite (Al2O3) combine from hard acid-hard base, so it is

stable. Besides, aluminium is very easy oxidized by atmospheric oxygen to form aluminium

oxide.

Copper is mainly found in its sulfides such as chalcopyrite (CuFeS2), chalcosite (Cu2S),

and bornite (Cu5FeS4). Cu2+ is soft acid, it will tend to combine with soft base like sulfide ion.

Thus, copper (I) sulfide is more stable than copper (I) oxide. And so silver. Silver is stable in

sulfide than oxide because silver ion is soft acid. It will prefer to combine with sulfide to form

argentite (Ag2S).

From the hard and soft acid or bases concept, we know that iron, aluminium,

silicon, titanium, and manganese are found as oxides while copper and silver are found as

sulfide.

In studying the types of minerals in which different elements are found, Berzelius

long ago noted that certain metal tend to occur as sulfides and others as carbonates or

oxides. Nowadays, geochemists classify the elements into four classes, according to their

predominant geological pattern of behavior.

1. The lithophiles, the metals and nonmetals that tend to occur as cations in oxides,

silicates, sulfates, or carbonates. Since these anions all possess oxygen as a donor

atom, the metal ions involved are the hard acids. The nonmetals that are classified

as lithophiles either are inherently hard bases or have been oxidized by

atmospheric oxygen to oxo anions, which are hard bases.

2. The chalcophiles, which occur in nature as cations in sulfides (less commonly

with other soft bases such as telluride, arsenide, etc., since sulfide is much the

most common soft base). The chalcoplhiles are mostly the borderline and some

soft acids. Logically, many of the soft bases are also listed as chalcophiles, since

they are found in such minerals.

3. The atmophiles, the chemically unreactive nonmetals that occur in the

athmosphere in elemental form (N2 and the noble gases).

4. The siderophiles, the metal that tend to occur native (in elemental form), these are

also soft acids, so the siderophiles will be treated as a subdivision of the

chalcophiles.

The other applications of HSAB concept are in predicting favorable equilibria,

toxicology (medicinal chemistry), ligand selections in metalloproteins and enzymes,

reduction potentials, characteristic of solvent, and so on.

REFERENCES

ft.unsada.ac.id

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basa-lewis-asam-basa-dan-redoks.html written by Taro Saito

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