12th Class Chemistry Short Questions Preparation for Pakistani Students

Question # 1

During S_n1 reaction,What is the significance of first step?

Ans 1: The first step involved the breakage of a covalent bond so it is slow step as compared to the second step which involve the energetically favourable combination of ions,the first step is therefore called the rate determining step.The mechanism step is called unimolecular because only one molecule take part in the rate determining step.

Question # 2

Why group II-A elements are called Alkaline Earth Metals.

Ans 1: Group IIA elements are called alkaline earth metals because they produce alkalies in water and are widely distributed in earth's crusts.

Question # 3

Mention two major problems that may arise in Nelson's cell.

Ans 1: i) Chlorine produced can react with hydroxide ions in cold giving hypochlorite ions.
ii) Hydroxide ions may be attracted toward anode.where they can be discharged releasing oxygen gas.This oxygen gas may contaminate the chlorine and renders it impure.

Question # 4

What are the following
1.Isoenzyme
2.Nucleotide

Ans 1: Isoenzyme:These are the enzymes from the same organisms which catalyze same reaction but are chemically and physically distinct from each other.
Nucleotide:Both DNA and RNA are formed by joining together a large number of Nucleotide units ,each of which is a nitrogenous base sugar phosphoric acid complex.

Question # 5

In which way Al becomes water soluble by using NaOH?

Ans 1: Aluminium dissolves in sodium hydroxide to form a soluble aluminate,with the evolution of hydrogen.
2Al_(s)+ 2NaOH_(aq)+ 6H₂O(I)---->2NaAI(OH)_4(aq)+ 3H_2(g)

Question # 6

What is Tin Plating ?

Ans 1: The process of tin plating consists of dipping the clean sheet of iron in a bath of molten tin and then passing it through hot pair of roller.

Question # 7

What is iodize salt?

Ans 1: When sodium or potassium iodine is added to the common salt,then the common salt is called iodized salt.

Question # 8

Define Primary Alkyl Halides and secondary alkyl Halides.

Ans 1: The alkyl halides in which halogen atom halogen atom is attached with carbon which is further atached to one or no carbon atom is called primary alkyl halide.
Secondry alkyl Halide:The alkyl halides in which halogen atom is attached with a carbon which is durther attached to two carbon atom is called secondry alkyl halide.

Question # 9

What is effect of aeration on quality of raw water?

Ans 1: The quality of raw water is improved by aeration.In this process air is passed through water to remove the dissolved gasses such as foul smelling,organo-sulphur compound and volatile organic compound,Some of the organic material in the raw water which could be easily oxidized with air produce in the aeration process,.

Question # 10

Why SO₃is dissolved in H₂SO₄and not in water?

Ans 1: SO₃is not directly dissolved in water,since absorption is incomplete and mist of SO₃and H₂SO₄fills the factory,which causes great causes great inconvenience the workers.Therefor,SO₃is absorbed in concentrated H₂SO₄and Oleum(H₂S₂O₇) formed can be converted to sulphuric acid of any strength by mixing adequate quantities of water.

Question # 11

Compare halogen acids in their reducing properties?

Ans 1: HF,HCI,HBr and HI acts as reducing agents in the following order:
HF<HCI < HBr<HI

Question # 12

Give the name and formula for common minerals of Be.

Ans 1: Name Formula
Beryl Be₃Al₂(SiO₃)₂
Chrysoberyl Al₂BeO₄

Question # 13

Write the behavior of Al with conc. HNO₃

Ans 1: Aluminium does not react with nitric acid at any concentration,probably because of the formation of protective layer of aluminium oxide.

Question # 14

Water has higher boiling point that ethanol.Explain.

Ans 1: Melting and boiling point of alcohol are higher than corresponding alkanes,Methyl alcohol and ethyl alcohol are liquids while methane and ethane are gases,This is also due to hydrogen bonding which is present in alcohols but absent in alkanes.

Question # 15

What is the action of litmus with aqueous solution of Na₂CO₃?

Ans 1: The solution of Na₂CO₃in water is basic due to hydrolysis of carbonate ion.So,it will turn red litmus to blue.
Na₂CO₃+2H₂O------------>2NaOH + H₂CO₃

Question # 16

Define two reaction which show that Benzene is an unsaturated hydrocarbons?

Ans 1: Benzene is reduced to cyclohexane on heating at high temperature with hydrocarbons in the presence if Pt in an acidic solvent or Ni at 200 Degree as a catalyst,This reaction show that benzene is Un-saturated compound.

Halogenation: Benzene reacts with chlorine and bromine in the presence of sunlight to give addition products ,hexechlorobenezene or hexebromobenzene.

Question # 17

Write importance of cracking.

Ans 1: Besides increasing the yield of gasolines,cracking has also produced large amount of useful by products,such as 60 ethane,propane,butane and benzene.These are used for manufacturing drugs,plastic ,detergents ,synthetic fibres,fertilizers weed killer and important chemical like ethanol,phenol and acetone.

Question # 18

What is Vital force of theory?

Ans 1: Early scientist believe that the organic compound could be manufactured only by and within living thing and these compounds could be synthesized from inorganic material this theory was referred to as Vital force theory.

Question # 19

How phenol can be converted into benzene?

Ans 1: Benzene is prepared by the reduction of phenol in the presence of Zn.

Question # 20

Define functional group isomerism .

Ans 1: An atom or group of atoms or a double bond or a triple bond whose presence imparts specific properties to organic compounds is called a functional group because they are chemically functional part of molecules.

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