Basic Knowledge of Chemical Reaction

8/10/2019 Basic Knowledge of Chemical Reaction

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Basic knowledge of chemical reaction

*Atoms:- Are the smallest particles of which any

substance is composed that can take part in achemical reaction. An atom consists of a centralcompact nucleus which contain protons and particles

with no electrical charged called neutrons.Very smallnegatively charged particles called electrons surroundthe nucleus.Because the numbers of positively charged protonsequal the numbers of negatively charged electrons anatom is electrically neutral. Atoms of differentelements differ from one another by the number of

protons and neutrons in the nucleus of each.


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An element is a substance made up of only one kindof atom. The chemical prosperities of an atom arecontrolled by its electrons with the chemicalcombination of electrons taking place by the transferor sharing of outer electrons between combingatoms. The nuclei of atoms remain unchanged during

such reactions.*Atomic number and atomic masses of elements:-The number of protons in the nuclei of atoms

determines what is known as the atomic number ofan element. For example the number of protons in acarbon atom is 6 therefore the atomic number of anelement carbon is 6.


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The sum of the number of protons and neutronsdetermines what is called the atomic mass(weight) ofan element. This does not represent an actualweighed mass but is a relative amount. It indicateshow much heavier or lighter based on the atomicweight of an isotope of carbon which has been

assigned an atomic mass of 12 units.


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*Isotopes:- Almost all the elements have what arecalled isotopes. These are atoms of the same elementthat have the same number of protons in their nucleibut a different number of neutrons i.e they have thesame atomic number but different atomic masses. Forexample the three isotopes of the element hydrogen

(tririum,deuterium,protium) each have 1proton and 1electron but tritium has 2 neutrons, deuteriumhas1neutron and protium has no neutron.

Isotopes of elements are shown by writing the atomicmass of the element followed by the symbol of theelement for example one isotope of carbon is written

C12


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Not all isotopes remain in a stable physical state someare unstable and radioactive. Such isotopes emit rays(alpha, beta, and gamma) due to their nucleidisintegrating in order to become stable. Such raysalthough lethal to living cells can be used in acontrolled way in radiotherapy units to kill malignant

cells in the body.*Molecules and relative molecular masses:- Whereasan element is composed of only one kind of atom a

compound contains two or more different kinds ofatom. A molecule is the smallest particle of anelement or compound that can normally existseparately.


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For example 2atoms of hydrogen combine with 1atom of oxygen to form 1 molecule of water.The relative molecular mass of a molecule(Previouslycalled molecular weight) is the ratio of the average ofa molecule to1/12 of the mass of the isotope ofcarbon(12c). In mathematical terms the relative mass

of a molecule is the sum of all the atomic weights ofwhich it is composed. Of all the 104 elements known80 of these are metals.

*Valence:- The valence of a substance is the numberof hydrogen atoms which will combine with liberateor replace one atom of the substance. For examplethe valence of oxygen in water(H2O) is2.


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*Equivalent weight:- The equivalent weight is theweight that combines with liberates or replaces oneatom of hydrogen. This is equal to molecular weightdivided by the valence.

ValenceeightmolecularW

*Oxidation :- This is a reaction in which an atom loseselectrons. It is in effect the addition of oxygen or removal ofhydrogen from a substance.


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*Reduction:- This is a reaction in which oxgen isremoved or hydrogen added to the substance that isan atom gains electrons.

*Radicals:- These are units or groups of atoms whichwhen combined in a compound behave as a single

atom. For example NO3- , SO4-- , NH4+.

*Acid :- It is a substance or compound that fives off

hydrogen ion(H+) in solution and can donate a proton.

*Base:- It is a substance or compound which gives offhydroxyl ions (OH-) in solution and can accept aproton.


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Base Acid

It has a soapy feel It has a sharp taste

It turns litmus blue It turns litmus red

It neutralizes acid toproduce salt and water

It reacts with carbonates toproduce CO2

An example of acid-base reaction:

OH NaCl NaOHHCL 2*Neutralisation reaction :- The reaction between an acid andabase to produce a neutral solution of salt and water isknown as neutralisation reaction a neural solution contains

equal numbers of hydroxyl(OH-) and hydrogen(H+) ions.


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Hydrogen ion concentration and PH

Sorensen in1909 divided the units of PH (puis-sancedhydogen ) as strength of hydrogen ions orPOH(puissance dhydroxyl ) as strength of hydroxyl ions as amore convenient way to express the hydrogen ionconcentrations of solutions. PH is defined as the logarithm tothe base10 of the reciprocal of the hydrogen ionconcentration.A solution whether acid or base always contains H+ and OH-

ions. The predominance of one PH of pure

water= Neutral7Log10 7


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The PH scale is expressed from 0 to 14 units. A neutralsolution has a PH of 7 an acid has a PH less than 7and an alkali has a PH greater than 7. science strongacids like hydrochloric acid(HCL) sulphuricacid(H2SO4) or nitric acid (HNHO3) dissociatecompletely in solution they have a very low PH. Weak

acids such as lactic or acetic acid do not completelydissociate even in dilute solutions and so they have acomparatively higher pH. The same is true for strong

and weak bases where strong bases have much higherPH than weak bases.


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*Measurement of PH:-PH can be measured using:-(1)PH papers.


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(2)PH meter.

Basic Knowledge of Chemical Reaction

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