Dictionary O

Octane Number

The octane number of a fuel is an experimentally determined value for the fuel, which describes its combustion properties (i.e. knocking characteristics) relative to octane.

The octet rule states that the chemical properties of the elements repeat on a regular basis with increasing atomic mass, and that the chemical properties of each eight element is similar. Since the inert gases, with the exception of helium have eight electrons in their outer shells, this stable electronic configuration is called the octet rule. In chemical reactions atoms of elements tend to react in such a way as to achieve the electronic configuration of the inert gas nearest them in the periodic table. There are a number of exceptions to the octet rule.

The noble gases are known to be very inert chemically. In chemical reactions, the atoms of elements tend to react in such a way as to achieve the electronic configuration of the inert gas nearest them in the periodic table. Since all the inert gases, except helium, have eight electrons in the outer level, this concept is often referred to as the Octet Rule, to which there are exceptions. This rule is a guide to understanding bonding. In order to become stable, atoms with incomplete outer shells will attempt to join chemically with another atom, and to share the electrons of that atom in its own outer shell, so as to fill it. This is called chemical bonding.

Oil Pollution

Oil pollution is the degradation of the quality of the water, when crude oil, lubricants, fuel oil, etc. contaminate the aquatic environment.

Order of Filling the Atomic Orbitals

The sequence in which the atomic orbitals in an atom are filled is as follows :
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

Organic Chemistry

Organic chemistry is the study of the chemistry of carbon compounds.

Organic compounds form the chemical basis for life and are more abundant than inorganic compounds. Carbon compounds are made up of carbon atoms which form covalent bonds with each of its neighboring carbon atoms in either a chain or a ring and also form bonds with other atoms mainly hydrogen, oxygen, nitrogen or sulphur.

Compounds containing only carbon and hydrogen are known as Hydrocarbons.

Osmosis

Osmosis is the passage of a solvent through a semipermeable membrane which acts as a barrier to the passage of the solute dissolved in the solution. The passage of water through the walls of living cells, which consist of very thin membranes that act as a barrier to substances dissolved in the water, is an essential process in living cells and is an example of osmosis. In general, the molecules of the solute are too large to pass through such semi-permeable membranes.

Osmotic Pressure

The osmotic pressure of a solution is the pressure which must be applied to the solution to stop osmosis. Osmotic pressure is similar to gas pressure and is caused by the dissolved substances (i.e. the solutes) behaving as if they were gases occupying the space volume of the solvent. Under osmosis, a solvent always diffuses from the less concentrated solution to the more concentrated solution.

Oswald Process

The Oswald Process is the three stage process by which nitric acid is manufactured. Firstly, ammonia is oxidised, at high temperature (900 deg.C.) over a platinum-rhodium catalyst, to form nitrogen monoxide.


               	4 NH3 (g)   +   5O2 (g)   ==>   4 NO (g)   +   6H2O

The nitrogen monoxide cools and reacts with oxygen to produce nitrogen dioxide.


                         2 NO (g)   +   O2   ==>   2 NO2 (g)

Finally, the nitrogen dioxide reacts with water and oxygen to produce nitric acid.


       NO2 (g)   +   2 H2O (l)   + O2   ==>   4 HNO3 (l)

Oxidation

Oxidation is the process in which there is a loss of electrons from an atom or ion. This definition applies only to reactions in which electron transfer occurs. Originally, oxidation was simply regarded as a chemical reaction with oxygen.

Oxidation Number

The oxidation number describes the oxidation state of an atom or ion, and is the positive or negative charge which each atom in a molecule would possess if the bonds were purely ionic. The oxidation number is the same as the valency of an element. The oxidation number of an element in its neutral state, such as fluorine gas or solid copper, is zero. The sum of the oxidation states of all the atoms in a compound will be equal to zero. When balancing redox equations an increase in oxidation number means oxidation; decrease in oxidation number means reduction.

Oxides

Oxides are the compounds obtained when other elements combine chemically with oxygen. Examples of oxides include sodium oxide, Magnesium Oxide, etc.

An Oxide can be classified as

a basic oxide,
an acidic oxide,
an amphoteric oxide, or
a Neutral Oxide,

depending on how it reacts with water, acids and bases.

There is a change from strongly basic oxides to strongly acidic oxides for the elements on going across the periodic table as can be seen from the following table.


  Group in  	Element		     Oxide	  	Classification
    Table
	1	Sodium, Na	Sodium Oxide, NaO      Strongly Basic
	2	Magnesium, Mg 	Magnesia, MgO	       Basic
	3	Aluminum, Al	Alumina, Al2O3	       Amphoteric
	4	Silicon, Si	Silica, SiO2	       Weakly Acidic
	5	Phosphorus, P	Phosphorus Pentoxide,  Acidic
                                          P4O10
	6	Sulphur, S	Sulphur Dioxide, SO2    Strongly Acidic
	7	Chlorine, Cl	Chlorine Superoxide,     Very Strong Acidic
                                           Cl2O7

Because of the increase of the electronegativity of the elements from left to right across the periodic table, with the consequent decrease in electronegativity difference between oxygen and the elements, the oxides go from being strongly basic for elements on the extreme left of the table, to being strongly acidic for elements on the extreme right of the table.

The oxides of elements high in the electrochemical series are strongly basic with ionic bonding, (e.g. alkali and alkaline-earth metals), while the less electropositive elements have bonds that are partly covalent and therefore the oxide is amphoteric (e.g. aluminium oxide).

The metals below hydrogen in the electrochemical series show variable valency (e.g. cuprous oxide, Cu2O, and cupric oxide, CuO). The oxides of these metals are not easily prepared by direct action of oxygen on the metal and heat must be used. These metallic oxides are basic.

The other elements below hydrogen in the series are the non-metals and these non-metals form oxides which are entirely covalent and are usually acidic oxides. It should be noted that the more ionic oxides are formed by elements on the left side of the periodic table.

The principal oxides of the Alkali Metals Group I Elements are

	Lithium Oxide, 	
	Sodium Oxide, and 	
	Potassium Oxide.

The principal oxides of Copper are

	Cupric Oxide, and 	
	Cuprous Oxide.

The principal oxides of Alkaline Earths Group II Elements are

	Magnesium Oxide, and	
	Calcium Oxide or Quicklime,

The principal oxides of Group III Elements are

	Aluminium Oxide

The principal oxides Group IV Elements are

	Carbon Dioxide, 	
	Carbon Monoxide, and 	
	Silicon Dioxide or Silica

The principal oxides of Group V Elements are

	Nitrous Oxide, 	
	Nitric Oxide, 	
	Nitrogen Dioxide, 	
	Nitrogen Pentoxide, 	
	Dinitrogen Tetroxide,	
	Phosphorus Trioxide, and	
	Phosphorus Pentoxide.

The principal oxides of Sulphur, S, are

	Sulphur Dioxide, and 	
	Sulphur Trioxide

The principal oxides of Group VII Elements are

	Ferrous Oxide, 	
	Ferric Oxide, 	
	Ferro-Ferric Oxide.

The oxides of nitrogen are formed during high temperature combustion, are present in exhaust gases from these processes and are responsible for the degradation of the quality of air. The principal oxides of nitrogen, NOx, are

	Nitrous Oxide, 		
	Nitric Oxide,	
	Nitrogen Dioxide, 	
	Nitrogen Pentoxide, and 	
	Dinitrogen Tetroxide.

The lower oxides of nitrogen are neutral oxides.

The higher oxides of nitrogen are acidic oxides.

All oxides of nitrogen are polar covalent compounds.

Oxidising Agent

An oxidising agent is a substance which oxidises another substance.

Ozone

Ozone, O3, is the triatomic isomer of oxygen, O2, and it is a powerful oxidising agent. The presence of ozone in the stratosphere is responsible for the absorption of ultra-violet wavelengths from the sunlight.

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