12th Class Chemistry Short Questions Preparation for Pakistani Students

Question # 1

CO₂is a gas while SiO₂is solid at room temperature.Justify?

Ans 1: Silicon atoms are much larger than carbon atoms and thus tend to surround themselves with more oxygen neighbours. Silicon forms only single bonds to oxygen atoms whereas carbon may form double bonds.Carbon,in fact forms double bond to each of the two oxygen atoms to produce a small,symmetrical,linear molecules CO₂,which is volatile and reasonably reactive.

Question # 2

Define and differentiate between essential and non essential amino acids.

Ans 1: The ten amino acids which body can be synthesized called non-essential amino acids,The remaining ten amino acids when the body is not able to synthesize are called essential amino acids,The essential amino acids must be supplied to our body through our diet because they are require for proper health and growth,The deficient of essential amino acids may cause disease.

Question # 3

Write structural formulas for two compound;i) Vinyl acetylene ii) But-3-en-yne

Ans 1: i) Vinyl acetylene: CH₂=CH-C=CH
Ans 2: ii) 1-Butene-3-yne: CH₂=CH-C=CH

Question # 4

What is difference between clinker and cement?

Ans 1: Clinker : The resulting product obtained from the kiln is known as cement clinker,This has the appearance of greenish black or grey coloured balls varying in size from small nuts to paes.
Cement: Cement is the material obtained by burning an intimate mixture of calcareous and argillaceous material at sufficiently high temperature to produce clinker.

Question # 5

Aluminium sheets are said to be corrosion free.Comment.

Ans 1: When a sheet of aluminium is exposed to moist air it acquires a thin,continuous coating of aluminium oxide,which prevents further attack on the metal by atmospheric oxygen and water under normal conditions.Because of this aluminium sheets are said to be corrosion-free.

Question # 6

How PVC is prepared?

Ans 1: It is an addition polymer obtained by polymerizing vinyl chloride at 52 Degree and 9 atmospheric pressure.

Question # 7

Write the advantages of Nelson's cell.

Ans 1: i) By using this method sodium hydroxide is manufactured on large scale.
ii) Sodium hydroxide is product of this process but some by products are also achieved like hydrogen gas and chlorine gas.
iii) It is very cheap process because its raw material is sodium chloride which is not costly.

Question # 8

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 # 9

Why alkyl halides are more reactive than alkanes?

Ans 1: An alkyl halides molecule consists of two parts,an alkyl group with a partial positive charge on the carbon atom attached to halogen atom and the halide atom with partial negative charge,While alkanes have no partial position or negative charged sites in their molecules.Halogens atom attached to alkyl groups is more electronegative which makes the alkyl and halide weak bonds.

Question # 10

What is Wurtz -Fritting reaction?

Ans 1: The Wurtz reaction for the synthesis of alkanes was extended by Fitting in 1864 to the synthesis of alyl aromatic hydrocarbons.

Question # 11

What are elimination reaction?

Ans 1: When nucleophile attacks on hydrogen atom attached to the b carbon of the alkyl halide,we get an alkene such type of reaction are called elimination reaction.

Question # 12

Why aqueous solution of Na₂Co₃is alkaline in nature?

Ans 1: The solution of Na₂CO₃in water is alkaline due to hydrolysis of carbonate ion.So it will turn red litmus to blue.It produces an acid and base,but the base is stronger than that of acid.So the aqueous solution of Na₂CO₃is alkaline in nature.
Na₂CO₃+ 2H₂O -----------> 2NaOH + H₂CO₃

Question # 13

What is the role of potassium in growth of plants?

Ans 1: Potassium is required for the formation of starch sugar and the fibrous material of the planet.They increase resistance to disease and make the plant strong by helping in healthy root development.Potassium fertilizers are especially used for tobacco,coffee and corn.

Question # 14

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 # 15

Alkanes are less reactive than alkenes. Comment?

Ans 1: The alkanes or paraffins(Latin: parum=little,affins=affinity) under ordinary condition are inert towards acids,alkalis,oxidizing and reducing agents.The un-reactivity of alkanes can also be explained on the basis of inertness of a o-bond.In a o-bond the electrons are very tightly held between the nuclei which makes it is very stable bond.A lot of energy is required to break it.
Ans 2: On the other hand alkenes contains sigma-bond,having partially filled p-orbitals overlap in a parallel fashion.sigma-electron are less firmly held between the nulei. In alkenes a sigma-bond is,therefore,a weak bond as compared to a o-bond.

Question # 16

How Aluminium reacts with aqueous sodium hydroxide?

Ans 1: Aluminium dissolves in sodium hydroxide to form a soluble aluminate,with the evolution of hydrogen.
2AI_(s)+ 2NaOH_aq)+ 6H₂O_(I)------->2NaAI(OH)_4(aq)+3H₂(g)

Question # 17

Alkali and alkaline earth metals are reactive elements of periodic table.Justify it.

Ans 1: Alkali and alkaline earth metals are reactive elements of periodic table because,these elements are most electropositive elements.Alkali metals have only one electron in their valence shell.Ionization energy values of alkali metals are very low.
Ans 2: Alkaline earth metals have two electrons in their valence shell.These also have very low ionization values.So both alkali and alkaline earth metals are reactive.

Question # 18

Define condensation polymerization.

Ans 1: The polymerization results from the mutual reaction of two functional groups .The reaction usually involves the removal of a water molecule or methanol molecule,It take place at both end of the growing chain.

Question # 19

Why Alkynes are less reactive than alkenes towards Electrophilic Regents?

Ans 1: A sigma-bond in alkenes is not only weak but its electrons are more expose to an attack by an electrophilic reagent.Both these acts make the alkenes a very reactive class of compounds. Alkynes although contain two sigma-bond are less reactive than alkenes towards electrophilic reagents this is because the bond distance between the two triple bonded carbon atoms is very short and hence the sigma-electrons are not available to be attacked by electrophilic reagents.

Question # 20

What is Tollen's Test?

Ans 1: Aldehydes form silver mirror with Tollen's reagent (ammonical silver nitrate solution).Add tollen's reagent to an aldehyde solution in a test tube and warm,A silver mirror is formed on the inside of the test tube.

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