Chemistry

Find free online Chemistry Topics covering a broad range of concepts from research institutes around the world.

s-Block Elements

The elements belonging to the group 1 and 2 in the modern periodic table are called s-block elements. The elements belonging to these two groups are commonly known as alkali and alkaline earth metals respectively. In this unit, we study their properties, uses, important compounds and biological importance.

S-block comprises 14 elements namely hydrogen (H), lithium (Li), helium (He), sodium (Na), beryllium (Be), potassium (K), magnesium (Mg), rubidium (Rb), calcium (Ca), cesium (Cs), strontium (Sr), francium (Fr), barium (Ba), and radium (Ra).

s-block elements are the elements found in Group 1 and Group 2 on the periodic table. Group 1 are the alkali metals which have one valence electron. They have low ionization energies which makes them very reactive. Group 2 is the alkali earth metals which have two valence electrons, filling their s sublevel.

The s-block and p-block elements are so called because their valence electrons are in an s orbital or p orbital respectively. They are also called Typical Elements to distinguish them from the transition and inner transition series.

Elements in which all inner electron shells are completely filled, and the last electron enters the s-orbital of the outermost shell, are called s-block elements. Thus, for s-block elements, the differentiating electron enters the ns-orbital.

The s-block in the periodic table of elements occupies the alkali metals and alkaline earth metals, also known as groups 1 and 2. Helium is also part of the s block. The principal quantum number “n” fills the s orbital. There is a maximum of two electrons that can occupy the s orbital.

Physical properties of S-Block Elements

1. Electronic Configuration.
2. Large Atomic Radii.
3. Large Ionic Radii.
4. Low Ionization Enthalpy.
5. Hydration Enthalpy.
6. Unipositive ions.
7. Metallic Character.
8. Melting and Boiling Points.

The s block has two columns corresponding to one of the s orbitals holding a maximum of two electrons. The p block has six columns corresponding to the three p orbitals with two electrons each. The lettered group number of a main-group element is equal to the number of valence electrons for that element.

The s-block is on the left side of the conventional periodic table and is composed of elements from the first two columns plus one element in the rightmost column, the nonmetals hydrogen and helium and the alkali metals (in group 1) and alkaline earth metals (group 2). Their general valence configuration is ns^1-2.

S Block Elements are a family of elements with similar characteristcs. S Block Elements include Alkali Metals. They include Lithium, Sodium, Potassium, Rubidium, Cesium, and Francium. They display many of the physical properties similar to metals.

As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table. Group 1 of the Periodic Table consists of the elements: lithium, sodium, potassium, rubidium, caesium and francium. They are collectively known as the alkali metals.

There can be two electrons in one orbital maximum. The s sublevel has just one orbital, so can contain 2 electrons max. The p sublevel has 3 orbitals, so can contain 6 electrons max. The d sublevel has 5 orbitals, so can contain 10 electrons max.

Recall that the four different sublevels each consist of a different number of orbitals. The s sublevel has one orbital, the p sublevel has three orbitals, the d sublevel has five orbitals, and the f sublevel has seven orbitals. In the first period, only the 1s sublevel is being filled.

Find free online Chemistry Topics covering a broad range of concepts from research institutes around the world.

Hydrogen Bonding

Hydrogen bonding is one of the most important natural phenomena occurring in chemical and biological sciences. These interactions play a major role in the structure of proteins and DNA. When a hydrogen atom (H) is covalently bonded to a highly electronegative atom such as flourine (F) or oxygen (O) or nitrogen (N), the bond is polarized.

Due to this effect, the polarized hydrogen atom is able to form a weak electrostatic interaction with another electronegative atom present in the vicinity. This interaction is called as a hydrogen bond (20-50 kJ mol^-1) and is denoted by dotted lines (…).

It is weaker than covalent bond (>100 kJ mol^-1) but stronger than the van der Waals interaction (< 20 kJ mol^-1). Hydrogen bond has profound effct on various physical properties including vapour pressure (H₂O and H₂S), boiling point, miscibility of liquids (H₂O and C₂H₅OH), surface tension, densities, viscosity, heat of vapourization and fusion, etc. Hydrogen bonds can occur within a molecule (intramolecular hydrogen bonding) and between two molecules of the same type or different type (intermolecular hydrogen bonding).

Intramolecular Hydrogen Bond

Intramolecular hydrogen bonds are those which occur within a single molecule.

Intermolecular Hydrogen Bond

Intermolecular hydrogen bonds occur between two separate molecules. They can occur between any numbers of like or unlike molecules as long as hydrogen donors and acceptors are present in positions which enable the hydrogen bonding interactions. For example, intermolecular hydrogen bonds can occur between ammonia molecule themselves or between water molecules themselves or between ammonia and water.

Water molecules form strong hydrogen bonds with one another. For example, each water molecule is linked to four others through hydrogen bonds. The shorter distances (100 pm) correspond to covalent bonds (solid lines), and the longer distances (180 pm) correspond to hydrogen bonds (dotted lines).

In ice, each atom is surrounded tetrahedrally by four water molecules through hydrogen bonds. That is, the presence of two hydrogen atoms and two lone pairs of electron on oxygen atoms in each water molecule allows formation of a three-dimensional structure. This arrangement creates an open structure, which accounts for the lower density of ice compared with water at 0°C. While in liquid water, unlike ice where hydrogen bonding occurs over a long-range, the strong hydrogen bonding prevails only in a short range and therefore the denser packing.

Hydrogen bond occurs not only in simple molecules but also in complex biomolecules such as proteins, and they are crucial for biological processes. For example, hydrogen bonds play an important role in the structure of deoxyribonucleic acid (DNA), since they hold together the two helical nucleic acid chains (strands).

Find free online Chemistry Topics covering a broad range of concepts from research institutes around the world.

Hydrides

Hydrogen forms binary hydrides with many electropositive elements including metals and non-metals. It also forms ternary hydrides with two metals. E.g., LiBH₄ and LiAlH₄. The hydrides are classified as ionic, covalent and metallic hydrides according to the nature of bonding. Hydrides formed with elements having lower electronegativity than hydrogen are often ionic, whereas with elements having higher electronegativity than hydrogen form covalent hydrides.

Ionic (Saline) Hydrides:

These are hydrides composed of an electropositive metal, generally, an alkali or alkaline-earth metal, except beryllium and magnesium, formed by transfer of electrons from metal to hydrogen atoms. They can be prepared by the reaction of elements at about 400°C. These are salt-like, high-melting, white crystalline solids having hydride ions (H^–) and metal cations (Mⁿ⁺).

2 Li + H₂ → 2 LiH
2 Ca + 2H₂ → 2 CaH₂

Covalent (Molecular) Hydrides:

They are compounds in which hydrogen is attached to another element by sharing of electrons. The most common examples of covalent hydrides of non-metals are methane, ammonia, water and hydrogen chloride. Covalent hydrides are further divided into three categories, viz., electron precise (CH₄, C₂H₆, SiH₄, GeH₄), electrondeficient (B₂H₆) and electron-rich hydrides (NH₃, H₂O). Since most of the covalent hydrides consist of discrete, small molecules that have relatively weak intermolecular forces, they are generally gases or volatile liquids.

Metallic (Interstitial) Hydrides:

Metallic hydrides are usually obtained by hydrogenation of metals and alloys in which hydrogen occupies the interstitial sites (voids). Hence, they are called interstitial hydrides; the hydrides show properties similar to parent metals and hence they are also known as metallic hydrides.

Most of the hydrides are non-stoichiometric with variable composition (TiH_1.5-1.8 and PdH_0.6-0.8), some are relatively light, inexpensive and thermally unstable which make them useful for hydrogen storage applications. Electropositive metals and some other metals form hydrides with the stoichiometry MH or sometimes MH₂ (M = Ti, Zr, Hf, V, Zn).

Find free online Chemistry Topics covering a broad range of concepts from research institutes around the world.

Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) is one of the most important peroxides. It can be prepared by treating metal peroxide with dilute acid.

BaO₂ + H₂SO₄ → BaSO₄ + H₂O₂
Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

On an industrial scale, hydrogen peroxide is now prepared exclusively by autoxidation of 2-alkyl anthraquinol.

Physical Properties:

Pure hydrogen peroxide is almost a colorless liquid (pale blue), less volatile and more viscous than water.

A 30 % solution of hydrogen peroxide is marketed as ‘100-volume’ hydrogen peroxide indicating that at S.T.P., 100 ml of oxygen is liberated by 1 ml of this solution on heating.

Chemical Properties:

Hydrogen peroxide is highly unstable and the aqueous solution spontaneously disproportionates to give oxygen and water. The reaction is, however, slow but is explosive when catalyzed by metal. If it is stored in glass container, it dissolves the alkali metals from the glass, which catalyzes the disproportionation reaction. For this reason, H₂O₂ solutions are stored in plastic bottles.

H₂O₂ → H₂O + ½O₂

Hydrogen peroxide can act both as an oxidizing agent and a reducing agent. Oxidation is usually performed in acidic medium while the reduction reactions are performed in basic medium.

In Acidic Conditions:

H₂O₂ + 2 H⁺ + 2e^– → 2 H₂O (E° = + 1.77 V)

For Example

2FeSO₄ + H₂SO₄ + H₂O₂ → Fe₂(SO₄)₃ + 2H₂O

In Basic Conditions:

HO₂^– + OH^– → 2H₂O

For Example,

2KMnO₄(aq) + 3 H₂O₂(aq)
↓
2MnO₂ + 2KOH + 2H₂O + 3O₂(g)

Uses of Hydrogen Peroxide:

The oxidizing ability of hydrogen peroxide and the harmless nature of its products, i.e., water and oxygen, lead to its many applications. It is used in water treatment to oxidize pollutants, as a mild antiseptic, and as bleach in textile, paper and hair-care industry.

Hydrogen peroxide is used to restore the white colour of the old paintings which was lost due to the reaction of hydrogen sulphide in air with the white pigment Pb₃(OH)₂(CO₃)₂ to form black colored lead sulphide. Hydrogen peroxide oxidises black coloured lead sulphide to white coloured lead sulphate, there by restoring the colour.

PbS + 4H₂O₂ → PbSO₄ + 4 H₂O

Structure of Hydrogen Peroxide:

Both in gas-phase and solid-phase, the molecule adopts a skew conformation due to repulsive interaction of the OH bonds with lone-pairs of electrons on each oxygen atom. Indeed, it is the smallest molecule known to show hindered rotation about a single bond.

H₂O₂ has a non-planar structure. The molecular dimensions in the gas phase and solid phase differ as shown in figure 4.5. Structurally, H₂O₂ is represented by the dihydroxyl formula in which the two OH groups do not lie in the same plane.

One way of explaining the shape of hydrogen peroxide is that the hydrogen atoms would lie on the pages of a partly opened book, and the oxygen atoms along the spine. In the solid phase of molecule, the dihedral angle reduces to 90.2° due to hydrogen bonding and the O-O-H angle expands from 94.8° to 101.9°.

Find free online Chemistry Topics covering a broad range of concepts from research institutes around the world.

Heavy Water of Hydrolysis

Heavy water (D₂O) is the oxide of heavy hydrogen. One part of heavy water is present in 5000 parts of ordinary water. It is mainly obtained as the product of electrolysis of water, as D₂O does not undergo electrolysis as easily as H₂O.

D₂O is a colorless, odorless and tasteless liquid. However, there is a marked difference between physical properties of water and heavy water as shown in Table 4.2.

Table: Properties of water, heavy water and super heavy water.

2 NaOH + D₂O → 2NaOD + HOD
HCl + D₂O → DCl + HOD
NH₄Cl + 4D₂O → ND₄Cl + 4HOD

These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound. For example, when D₂O is treated with of hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that, it is a monobasic acid.

H₃PO₂ + D₂O → H₂DPO₂ + HDO

It is also used to prepare some deuterium compounds:

Al₄C₃ + 12D₂O → 4Al(OD)₃ + 3CD₄
CaC₂ + 2 D₂O → Ca(OD)₂ + C₂D₂
Mg₃N₂ + 6D₂O → 3Mg(OD)₂ + 2ND₃
Ca₃P₂ + 6D₂O → 3Ca(OD)₂ + 2PD₃

Uses of Heavy Water:

1. Heavy water is widely used as moderator in nuclear reactors as it can lower the energies of fast neutrons
2. It is commonly used as a tracer to study organic reaction mechanisms and mechanism of metabolic reactions
3. It is also used as a coolant in nuclear reactors as it absorbs the heat generated

The heavy water produced is used as a moderator of neutrons in nuclear power plants. In the laboratory heavy water is employed as an isotopic tracer in studies of chemical and biochemical processes.

Heavy water is a form of water with a unique atomic structure and properties coveted for the production of nuclear power and weapons. Like ordinary water-H₂O-each molecule of heavy water contains two hydrogen atoms and one oxygen atom. The difference, though, lies in the hydrogen atoms.

The hydrogen is then liquefied and distilled to separate the two components, then the deuterium is reacted with oxygen to form heavy water. Producing heavy water requires advanced infrastructure, and heavy water is actively produced in Argentina, Canada, India, and Norway.

The heavy water is not manufactured, but rather it is extracted from the quantity that is found naturally in lake water. The water is separated through a series of towers, using hydrogen sulphide as an agent.

It was accepted by Norsk Hydro, and production began in 1935. The technology is straightforward. Heavy water (D₂O) is separated from normal water by electrolysis, because the difference in mass between the two hydrogen isotopes translates into a slight difference in the speed at which the reaction proceeds.

Heavy water is indeed heavier than normal water (which contains a tiny amount of heavy water molecules naturally), and heavy-water ice will sink in normal water.

Since the chemical properties of the heavier hydrogen-nucleus-with-a-neutron are slightly different, heavy water starts to gum up all manner of body parts. Eventually, if you drank enough purified heavy water-more than 20 gallons, at least a quarter heavy-you’d die.