CBSE Chemistry Sample Paper for Class 12

Sep 8 • Board Sample Papers • 11664 Views • 53 Comments on CBSE Chemistry Sample Paper for Class 12

Central Board of Secondary education, New Delhi is acronym as CBSE. There are large  number of schools in India which are affiliated by this board. This board is responsible for  producing great scholars for over years  by developing  educational policies in India for fruitful educations. For years many CBSE schools have been identified as the best education provider in the secondary and higher secondary levels. Many students are now preferring for CBSE board as for years CBSE board students are performance has enhanced. Given sample papers to help the students to prepare for exam.

As a Engineer I have prepared Chemistry sample paper for class 12th students  based on CBSE board syllabus  . So here is the cbse chemistry sample paper for class 12 :-

1) Give the IUPAC name of the compound (CH3)3 CCH2Br. [1 mark]

Ans : The IUPAC name  is 1-bromo-2, 2 – dimethyl propane .

2) What is meant by this statement ‘Crystalline solids are anisotropic in nature’ ? [1 mark]

Ans : The statement means that some of their physical properties show different values when measured along different directions in the same crystal.

3) What is meant by biodegradable polymers?  [1 mark]

Ans : Biodegradable polymers  are polymers which undergo bacterial degradation in the environment and are thus eco-friendly.

4) Define rate constant of a reaction. [1 mark]

Ans : The rate constant is defined as the rate of reaction when the concentration of the reactants is unity.

5) What is electrophoresis ? [1 mark]

Ans : The movement of colloidal particles under an applied electric potential towards oppositely charged electrodes is known as electrophoresis.

6) State the principle behind  Vapour phase refining of a metal processes. [1 mark]

Ans : For vapour phase refining of a metal processes the metal is converted into its volatile compound which is then decomposed to give pure metal.

7) State reason why Sulphur has greater tendency for catenation than oxygen. [1 mark]

Ans : Sulphur has greater tendency for catenation than oxygen because S-S single bond is stronger than O-O single bond

8) State reason why Copper (I) ion is not known in aqueous solution. [1 mark]

Ans : Copper (I) ion is not known in aqueous solution because copper(I) ion is unstable in aqueous solution and undergoes disproportionation.

9) Define Peptization. [1 mark]

Ans : Peptization is defined as the procedure to convert a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte .

10) State the recovery of silver after silver ore was leached with NaCN. [1 mark]

Ans : More basic and cheaper zinc can displace silver from the complex and silver metal can be recovered.

11) State how the phenomenon of adsorption finds application in Production of vacuum and Froth Floatation  processes. [2 mark]

Ans : For Production of vacuum process the remaining traces of air can be adsorbed by charcoal from a vessel to create a vacuum and for  Froth Floatation process  sulphide ore is concentrated by using pine oil which adsorbs the ore particles and impurities are wetted by water which settle at the bottom.

12) What you understand by primary structure and secondary structure of proteins. [2 mark]

Ans : Primary structure of proteins is the protein in which amino acids are linked with each other in a specific sequence .

Secondary structure of proteins indicates the shape in which a long polypeptide chain can exist that is a – helix and b-pleated structure.

13) Define rate expression of a reaction. [2 mark]

Ans : Rate Law is the expression in which reaction rate is presented by means of molar concentration of reactants with each term raised to some power which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.

14) State the principle behind  Electrolytic refining of a metal. [2 mark]

Ans : For Electrolytic refining of a metal the impure metal is made to act as anode and metal in pure form is used as cathode. They are put in a suitable electrolytic bath that contain soluble salt of the same metal. The more basic metal goes to the anode and the less basic metal gets deposited at the cathode.

15) State two vital uses Bakelite and Nylon 6. [2 mark]

Ans: Bakelite is used for making combs,  handles of utensils and Nylon6 is used for making tyre cords and ropes .

16)  (a)State  why Haloalkanes easily dissolve in organic solvents? [2 marks]

(b) State why an alkylamine is more basic than ammonia.

Ans : (a) Halo alkanes easily dissolve in organic solvents  because the new intermolecular attractions between haloalkanes and solvent molecules have about the same strength or stronger than the existing ones in the molecules.

Ans : (b) An alkylamine is more basic than ammonia because of  electron donating character of alkyl group.

17) (a)Define racemic mixture.                     [2 marks]

(b) Give reason why  NF3 is an exothermic compound whereas NCl is not.

Ans : (a) Racemic mixture  is defined as a mixture containing two enantiomers in equal proportions .

Ans : (b) NF3 is an exothermic compound whereas NCl is not because of increase in bond dissociation enthalpy from H-Cl bond to H-P bond.

18)  Distinguish between molarity and molality for a solution. [2 marks]

Ans : Molality  is the no. of moles of the solute per kilogram  of the solvent while Molarity is the no. of moles of solute present in one litre of solution at a particular temperature.

Molality is not dependant on temperature whereas Molarity is function of temperature because volume depends on temperature and the mass does not .

19) Give reasons why   [2 marks]

(a) SFis kinetically an inert substance.
(b) F2 is most reactive of all the four common halogens.

Ans: (a) SFis kinetically an inert substance  because SF6 is sterically protected by six F atoms / co-ordinatively saturated.

Ans : (b) F2 is most reactive of all the four common halogens because of the energy factor  , stability of + 3 oxidation state increases than that of +5 oxidation state.

20) What do you mean by the terms osmosis and osmotic pressure?  [2 marks]

Ans : Osmosis  is defined as the flow of solvent molecules from solution of low concentration to higher concentration through semipermeable membrane  and  Osmotic Pressure is defined as the hydrostatic pressure that has to be applied on the solution to restrict the entry of the solvent into the solution through the semipermeable membrane .

21) Describe about  Detergents and Antacids with examples. [3marks]

Ans : Detergents are sodium salts of long chain alkyl sulphonates or benzene sulphonates. eg: Sodium Lauryl sulphate and  Antacids are the drugs used to prevent the overproduction of acid in the stomach. e.g. Sodium hydrogen carbonate.

22)  Amino acids may be acidic, alkaline or neutral state how does this happen? Mention the essential and non-essential amino acids also name one of each type. [3 marks]

Ans : Acidic amino acids contain more number of carboxyl groups than amino groups. Basic amino acids consists of more number of amino groups than carboxyl groups. Neutral amino acids consists of equal number of amino acids and carboxyl groups. Those amino acids which must be supplied in our diet are called essential amino acids and those which can be made by our bodies and not required in our diet are called non-essential amino acids.

Essential amino acids:  Valine,  isoleucine, argenine, leucine.

Non Essential amino acids:  Glycine, alanine

23) Explain each terms with one example : [3marks]

(i) Food preservatives
(ii) Enzymes
(iii) Detergents

Ans : (i) Food preservatives: are the compounds which prevent spoilage of food due to microbial growth. eg:  vinegar
(ii) Enzymes are the biological catalysts which increase the rate of metabolism. eg:  Zymase.
(iii) Detergents are sodium salts of long chain alkyl sulphonates or benzene sulphonates.eg: Sodium Lauryl sulphate.

24) Give reason  for the following: [3 marks]

(i) NCl3-  endothermic compound while NF3 is an exothermic one.
(ii) XeF2-  Linear molecule without a bend.
(iii) The electron gain enthalpy with negative sign for fluorine is less than that for chlorine, still fluorine is a stronger oxidising agent than chlorine.

Ans: (i) Because  bond dissociation enthalpy of F2  is lower than that of Cl
(ii) Because it has sp3d hybridization with 3 lone pairs.
(iii) Because of high hydration enthalpy of F

25) (a) Explain the following terms: [3 marks]

(i) Rate of a reaction
(ii) Activation energy of a reaction

(b) State what is order of a reaction ?

Ans: (a) (i) Rate of a reaction- Rate of Change of concentration of reactant or product with time is called rate of reaction

(ii) Activation Energy – Minimum energy which the reacting molecules should acquire so that they react to give product is called activation energy.

Ans : (b) Order of a reaction is defined as the sum of powers of the concentration terms of the reactants in the rate law expression

26) Answer the following : [3marks]

(a) What do you mean by lanthanoid  contraction ?
(b) What do you mean by  shape selective catalysis ?
(c) State what is pyranose structure of glucose ?

Ans : (a)The regular decrease in the atomic and ionic radii of Lathanoids with increasing atomic number is called as Lanthanoid contraction.

Ans :(b) The catalytic reaction in which the pore structure of the catalyst and the size of the reactant and product molecules are comparable is known as shape selective catalysis.

Ans : (c)The six membered cyclic structure of glucose is called pyranose structure in analogy with pyran heterocyclic structure.

27) Mention the chemical test to distinguish between : [3marks]

(i) Propanal and propanone,
(ii) Benzaldehyde and acetophenone.

(b) Tendency to form pentahalides decreases down the group in group 15 of the periodic table, state the reason.

Ans: (a) (i) Propanal and Propanone

Iodoform lest. Warm each compound with iodine and sodium hydroxide on a water bath With.
Propanal : No yellow ppt is formed with
Propanone : Yellow crystals of Iodoform are formed.

(ii) Benzaldehyde and Actopbenone

Iodoform test. Warm each organic compound with I3
and NaOH solution. Acetophenone  Yellow precipitates of iodoform are formed with Benzaldehyde does not respond to this test.

Ans: (b) Because of the energy factor, stability of + 3 oxidation state increases than that of +5 oxidation state.

28) (a) Complete the  chemical reaction equations: [5 marks]

(i) P4 + SO2Cl2   ->
(ii) XeF4  + H2O->

(b) Explain the  giving appropriate reasons:

(i) The stability of + 5 oxidation state decreases down the group in group 15 of the periodic table.
(ii) Solid phosphorus pentachloride behaves as an ionic compound.
(iii) Halogens are strong oxidizing agents.

Ans: (a) (i) P4 + 10 SO2Cl2   ->   4PCl5 + 10SO2
(ii) 6XeF4  + 12H2O->  2XeO3   +  4Xe O3 + 24HF  + 3O2

Ans: (b) (i) Because down the group, +3 oxidation state becomes more & more stable due to                     higher energy involved to un pair the s electrons / due to inert pair effect.
(ii) Due to the formation of [PCL4]+ [PCL6]
(iii) Because they readily accept an electron.

29) Describe the principle behind each of the following processes: [5 marks]

(i) Vapour phase refining of a metal.
(ii) Electrolytic refining of a metal.
(iii) Recovery of silver after silver ore was leached with NaCN.

(b) Explain the following terms :

(i) Ambident ligand
(ii) Denticity of a ligand

Ans : (a) i) Vapour phase refining of a metal is a method in which the metal is converted into its volatile compound which is then decomposed to give pure metal.

ii) Electrolytic refining of a metal as is a  method in which the impure metal is made to act as anode and metal in pure form is used as cathode. They are put in a suitable electrolytic bath containing soluble salt of the same metal. The more basic metal goes to the anode and the less basic metal gets deposited at the cathode.

iii) Recovery of silver after silver ore was leached with NaCN as More basic and cheaper zinc can displace silver from the complex and silver  metal can be recovered.

(b) i) Ambident ligand is a unidentate ligand which can co-ordinate to the central metal atom through more than one co-ordinating bond.

ii) The number of donor atoms in ligating groups is identified as denticity of that ligand.

30) (a)Classify colloids where the dispersion medium is water also State their characteristics  and write an example of each of these classes.                                                             [5 marks]
(b)State what is observed when

(i) an electric current is passed through a sol
(ii) a beam of light is being passed through a solution
(iii) an electrolyte say NaCl will be added to ferric hydroxide sol

Ans: (a)They are of two types i) Hydrophilic ii)   hydrophobic

  • The hydrophile sol is more stable and reversible while hydrophobic sol is less stable and is irreversible.
  • Hydrophilic sol e.g. Starch, gelatin, gum etc.
  • Hydrobhobic sol e.g metal hydroxide , metal sulphide, 

(b) i) Electrophoresis takes place when sol particles move towards opposite electrodes due to attraction.
ii) Tyndall effect will be observed due to scattering of light by colloidal particles.
iii) Coagulation takes place

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