Question Paper

AHSEC (Assam higher secondary education council), HS final exam question paper 2012

HS final exam question paper 2012

AHSEC (Assam higher secondary education council), HS final exam question paper 2012  And also the some important question answer of the papers.

AHSEC (Assam higher secondary education council), HS final exam question paper 2012class 12 Assam, HS 2nd year final exam question paper 2012 are available on this site. The question papers are available in the pdf format. The questions papers of the subjects are physics, chemistry, mathematics, biology, and English. And also some important question answers to the papers.

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Mathematics

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Physics

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Chemistry

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Biology

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English

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Assamese

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Some important topics of chemistry

 

Imperfection or defect :

The irregularities that occur in the arrangement of particles are called Imperfection or defect.
There are two types of defect —

1) Point defect
2) Line defect

The irregularity on deviations of ideal arrangement around a point or an atom in the crystalline substance is called point defect.
On the other hand, the irregularities of deviations from Ideal arrangement in the entire row of lattice point is called the line defect.

Types of point defect —

There are three types of points defect –

1) Stoichiometric defect :
Due to this defect the stoichiometric of the compound does not change this defect is also called thermodynamic defect.
Stoichiometric defect can be divided into two types in molecular solid —

a) Vacancy defect :
When some of the lattic site are vecan the crystal is said to have vacancy defect. Due to this defect density of solid decreases.

b) Interstitial defect :
When some constituent particle occupie an interstitial site, the crystal is said to have interstitial defect. Due to this defect
density increases.
In case of ionic solid stoichiometric defect may be divided into two types —

a) Frenkel defect :
This defect arises in ionic solid when there is a large difference in the size of cation and anion. Smaller iron is dislocated from it normal site to an interstitial site. This is also called this dislocation defect. Due to this defect density of the Crystal does not altar.
Eg : ZnS, AgCl, AgBr and AgI

b) Schottky defect :
This defect arises in ionic solid having similar size of cation and anion. In this defect equal number of cations and anions are missing from the crystal as a result density of the solid decreases.
Eg : NaCl, KCl, CsCl, AgBr etc.

2) Impurity defect :
Vacancy producers in alkali metal halides due to presence of external cation is called impurity defect.
Eg : When NaCl is crystallize in the presence of small amount of SrCl2, some of the site of Na+ ions are occupied by Sr2+ ion. Each Sr2+ ion replaces to Na+ ion out of which one of the site of Na+ ion is occupied by Sr2+ ion and the other side remain vacant. The number of cation vacant is equal to the number of Sr2+ ion.

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3) Non-stoichiometric defect :
Due to this defect stoichiometric of the compound changes. There are two types of non-stoichiometric defect —
i) Metal excess defect ii)Metal deficiency defect

i) Metal excess defect:

Metal excess defect arises due to the —
a) Presence of anion vacancy : eg : When NaCl crystal is heated with in the presence of Na+ vapour, Na atoms are deposited at the surface of crystal. Cl- Ion diffused from the crystal and combined with Na to from NaCl. The electron lose by Na atom occupies the site of Cl- ion. The anomic vacancy of Cl- ion which are occupied by electron is called F-centre. Due to this F centre colour of NaCl becomes yellow. Similarly F-centre defects occurs in other alkali metal halides.

b) Due to presence of extra cation : eg : When ZnO is heated is looses O2 molecule. The vacancies are occupied by Zn2+ ion and the electron. Eg: ZnO Zn2 + O2 + 2e-

ii) Metal deficiency defect :

This defect arises in metal oxide in which the metal can exist in two different oxidation state.
eg : Iron oxide(Fe2O) is often fine to have a composition of Fe0.95 O1.00 . In this crystal some Fe+ ions are missing and the overall change is balance by the presence of required number of Fe3+ ion.

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