12th Class Chemistry Short Questions Preparation for Pakistani Students

Question # 1

How the classification of elements in different blocks help in understanding their chemistry?

Ans 1: Classification of elements in different blocks help in understanding chemistry of elements and predicting their properties especially the concept of valency or oxidation state.
Ans 2: According to this classification elements of IA and IIA subgroups are called s-block elements,because their valence electrons available in s orbital.The elements for IIIA to VIIIA subgroups are known as p block elements as their valence electrons are present in p orbital.Similarly in transition elements,electrons are in d orbital are responsible for their valency hence they are called d-block elements.For Lanthanides and Actinides valence electrons are present in f orbital hence these elements are called f-block elements.

Question # 2

Why the metals are good conductors?

Ans 1: Metal are good conductors due to the presence of relatively loose electrons in the outermost shell of the element and ease of their movement in the solid lattice.

Question # 3

Justify that Ethene is more reactive than C₆H_6.

Ans 1: The highly sable ,delocalized electron of benzene ring not readily available for the nucleophilic attack like the electron of alkenes,therefore the electrons of benzune ring do not assist in the attack of weak electrophiles.

Question # 4

Give four used of acetic acid?

Ans 1: Acetic acid is used:
1.as a coagulant for latex in rubber industry.
2.in the manufacturing of plastic rayon and silk.
3.in medicine as a local irritant.
4.in the manufacturing of pickles.

Question # 5

Why Fluorine acts as a strong oxidation agent?

Ans 1: Oxidizing power of fluorine is higher,because it has low energy of dissociation and higher hydration energy of its ions,Due to the relative strength as oxidizing agents,it is possible for each free halogen to oxidize the ions of other halogens next to it in the family .Fluorine can oxidize all the halide ions molecule halogens.

Question # 6

Distinguish ethanol and tertiary butyl alcohol by Lucas test?

Ans 1: Primary,Secondary and tertiary alcohol are identified and distinguished by reacting them with con,HCI in anhydrous ZnCI2,An oily layer of alkyl halides separated out in these reaction:
1.Ethanol is a primary alchohol which form an oily layer only on heating.
2.Tertiary butyl alcohol is a tertiary alcohol which form an oil layer immediately.

Question # 7

Write a note on blocks in periodic table.

Ans 1: Blocks in periodic table:Elements in the periodic table can be classified into four blocks,this classification is based upon the valence orbital of the elements involved in chemical bonding.According to this classification,Elements of IA and IIA subgroup are called s-block elements.The elements for IIA to VIIIA subgroup are known as p-block elements as their valence electrons are present in p orbital.Similarly in transition elements,electrons are in d orbital are responsible for their valency hence they are called d-block elements.For Lanthanides and Actinides valence electrons are present in f orbital hence these elements are f-block elements.

Question # 8

Write down mechanism of Cannizzaro reaction?

Ans 1: The hydroxid ion act as a nucleophile. It attacks ion the electrophile carbonyl carbons to form a complex anion.

Question # 9

What are sulphate aerosols?

Ans 1: SO₂,SO₃because of their pungent odour are very irritated and uccocating through various reaction in the atmosphere they form sulphate aerosols,These aerosol cause sever respiratory troubles particularly among older people.

Question # 10

How gases in the atmosphere absorb?

Ans 1: The gases in the atmosphere absorbs most of the cosmic rays and the major portion of the harmful electromagnetic radiation coming from the sun,The absorption of these harmful radiation protects the life on the earth.

Question # 11

Give two application of Radon gas.

Ans 1: 1.Radon is being radioactive is used in earthquake prediction.
2.Radon being radioactive is used in radiotherapy for cancer.

Question # 12

Addition of HCN on aldehydes and ketones is abase catalyzed reaction?

Ans 1: Hydrogen cyanide adds to aldehydes and ketones to form cyanohydrines. The reaction is carried out by adding slowly a mineral acid to an aqueous solution of sodium cyanide,The acid generates HCN from sodium cyanide in SItu.

Question # 13

What are Chelates?

Ans 1: When all the donor atoms of a polydentate ligand get coordinated with the same metal ion,a complex is formed which contain one or more rings in its structure and hence is called Chelate,

Question # 14

Give idea about knocking in the internal combustion engine.

Ans 1: The gasoline fraction present in petroleum in generally not of good quality.When it burn in an automobile engine ,combustion can be initiated before the spark plug fire ,This produced a sharp metallic sound called knocking which greatly reduced the efficiency of an engine.

Question # 15

What is meant by Polypeptide and protein?

Ans 1: The formation of peptide bonds can continue until a molecule containing a several hundred thousand amino acid is formed,Such a molecule is called polyptide or protein.By convention a peptide having molecule is called a polypeptide which peptide having molecular mass more than 10,000 is called protein.

Question # 16

How does an aldehyde react with
1.Hydroxylamine 2.Hydrazine

Ans 1: 1.Hydroxylamine:Aldehyde reacts with hydroxylamine to form oxims in the presence of an acid.
2.Hydrazine : Aldehyde reacts with hydrazinetoform hydrazone in the presence of acid.

Question # 17

HF is a weak acid while HCI is strong acid.Give reason.

Ans 1: The strength hydrogen halogen bond is very high in HF,The bond strength is reflected in the case of dissociation of hydrogen and halides,Hydrofluoric acid is a week acid due to limited ionization than hydrochloric acid.

Question # 18

What is the difference between aldhydes an ketones?

Ans 1: Aldehydes: In aldehydes,the cabonyl group is bonded to at leat one hydrogen atom,and so occurs at the end or a chain,An aldehyde can be represented by the general formula.
Ans 2: Ketones: In ketones ,the carbonyl group is bonded to two carbons atoms and so it occurs within a chain,A ketone may be represented by the general formula.

Question # 19

Define Ramy nickel.Give its use.

Ans 1: Rany nickel is a catalyst which is prepared by treating a Ni-Al alloy with caustic soda.Most alkanes are hydrogenated over Raney nickel at about 100 degree and upto 3-atmosphere pressure.

Question # 20

What do you mean by Ligand and Coordination sphere?

Ans 1: Ligand:The atom or ions or neutral molecules,which surrounded the central metal ion and donate electron pair to it called ligands.They must be anions or neutral molecules.
Coordination sphere: The central metal atom or ion along with ligand is called the coordination sphere,It is usually placed in square brackets.It may be anionic cationic or neutral.

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