CBSE Sample Papers for Class 12 Chemistry – CBSE Chemistry Questions and Answers
CBSE Chemistry Sample Paper
1. Write the dispersed phase and dispersion medium of butter.
Butter is a gel colloid. Thus, Dispersed phase is liquid Dispersion medium is solid
2. Define rate of reaction? Write two factors that affect the rate of reaction.
Rate of reaction is defined as the change in concentration of reactant or product per unit time. It is always a positive quantity.
Factors affecting rate of reaction.
(a) Concentration of reactants
3. Why does a solution containing non-volatile solute have higher boiling point than the pure solvent? Why is elevation of boiling point a colligative property?
Boiling point is a temperature at which vapour pressure of the substance becomes equal to atmospheric pressure. As the vapour pressure of the solution containing non-volatile solute is lower than that of the pure solvent and vapour pressure increases with increase in temperature. Hence, the solution has to be heated more to make its vapour pressure equal to the atmospheric pressure so that it starts boiling. Elevation of boiling point is a colligative property because, it depends upon the number of particles of solute dissolved in solution.
4. (i) What is the principle behind the zone refining of metals? (ii) What is the role of silica in the extraction of copper? (iii) How is ‘cast iron’ different from ‘pig iron’?
(i) Zone refining is based on the fact that the impurities are more soluble in the melt than in the solid state of the metal. In the process, one end of the impure metal rod is heated by means of a movable heater. The molten zone carrying impurities moves forward. In this way impurities are concentrated at the other end of the rod which is cut off. The process is repeated several times to obtain ultrapure metals (Ge, Si, Ga and In) used as semiconductors.
(ii) SiO2 acts as an acidic flux. Copper ore is heated with silica in a reverberatory furnance when FeO is removed as FeSiO3 (slag) and copper is produced in the form of copper matte.
(iii) The iron obtained from blast furnace is called pig iron. It contains about 4% carbon and many other impurities in smaller amount (eg., S, P, Si and Mn).
Cast iron is made by melting pig iron with scrap iron and coke using hot air blast. It has slightly lower carbon content (about 3%) and is extremely hard and brittle.
5. Define the following terms. (i) Primitive unit cells (ii) Schottky defect (iii) Ferromagnetism
(i) Primitive unit cells are defined as the unit cells in which the constituent particles are present only at the corner positions.
(ii) Schottky defect: It occurs when equal number of cations and anions are missing from the lattice sites. It decreases the density of the solid. For example: NaCl, KCl.
(iii) Ferromagnetism is the phenomenon shown by substances which are strongly attracted by magnetic field. These substances show magnetism even in the absence of a magnetic field.
Examples are Fe, Co, Ni and CrO2.
Class 12th CBSE Chemistry Solved Sample Papers
6. Give reasons for the following observations:
(i) Leather gets hardened after tanning.
(ii) Lyophilic sol is more stable than lyophobic sol.
(iii) It is necessary to remove CO when ammonia is prepared by Haber’s process.
(i) Tannin, which is obtained from plants, is a mixture of derivatives of polyhydroxy benzoic acids. It contains negatively charged colloidal particles. Animal hides are also colloidal in nature and contain positively charged particles. When they are soaked in tannin, their mutual coagulation takes place and leather becomes hard.
(ii) The strong forces of attraction between dispersed phase (colloidal particles) and the dispersion medium (solvent) make lyophilic colloid more stable. Lyophobic sols are electrically charged and repulsion between dispersed phase particles causes them to coagulate slowly and thus make them unstable.
(iii) CO acts as a poison for the catalyst used in the manufacture of ammonia by Haber’s process. Hence, it is necessary to remove it.
7. (i) Which one of the following is a disaccharide: Starch, Maltose, Fructose, Glucose?
(ii) What is the difference between fibrous protein and globular protein?
(iii) Write the name of vitamin whose deficiency causes bone deformities in children.
(ii) Fibrous proteins: These have thread like molecules which lie side by side to form fibres. The various molecules are held together by hydrogen bonds. These are insoluble in water but soluble in concentrated acids and alkalies.
Examples are keratin of hair, nails, wood, feathers and horn, myosin of muscles, and fibroin of silk. Globular proteins: This type of protein has molecules folded into compact units which often acquire spheroidal shape. Such proteins are soluble in water, dilute acids and alkalies. Examples are insulin, haemoglobin, albumin, etc.
(iii) Vitamin D
8. Why are haloalkanes insoluble in water but soluble in benzene?
Haloalkanes donot dissolve in water due to the lack of hydrogen bonding in the molecules of solute and solvent. However, they dissolve in benzene (organic solvent) as the solubility is based on the principle that ‘like dissolve like.’
9. Why is cellulose in our diet not nourishing?
Our body does not produce an enzyme to help in the metabolism of cellulose.
10. In the preparation of aldehydes from primary alcohols, aldehydes formed must be distilled as soon as they are formed. Why?
Aldehydes formed as a result of the oxidation of primary alcohols are prone to further oxidation and are expected to form acids. Therefore, it is advisable to remove them from the reaction mixture by distillation in case these are low boiling. Otherwise, other methods must be used to convert primary alcohols into aldehydes.
Cbse Class 12 Chemistry Sample Paper 2018 Questions and Answers
11. What are fuel cells? How do they resemble and differ from galvanic cells?
Fuel cells are those cells in which chemical energy of fuel is converted into electrical energy. In both cells, chemical energy is converted into electrical energy. The efficiency of fuel cell is higher than that of galvanic cell. Fuel cells are non-polluting.
12. (a) In some reactions, the energy possessed by colliding molecules is more than the threshold energy, yet the reaction is slow. Why? (b) State one condition in which a bimolecular reaction may be kinetically of the first order.
(a) Though the energy possessed by molecules is more than the threshold energy and the reaction should proceed at a reasonable rate yet in some cases the reaction is slow. It is due to the fact that in such cases the reacting molecules are not properly oriented. Due to this the number of effective collision decreases and so the reaction is slower than expected rate.
(b) By taking one of the reactants in large excess so that it may not contribute towards the order.
13. D (+) glucose gives most of the characteristics of the aldehydic (–CHO) group but it does not react with sodium bisulphite, ammonia and also does not restore the pink colour of Schiff’s reagent. Explain.
To explain the above, it has been suggested that glucose does not have an open chain structure rather it has a cyclic structure in which the aldehydic (–CHO) group is a part of the six membered oxide ring(G-oxide ring).
Sodium bisulphite, ammonia etc. fail to cleave the ring but Tollen’s reagent and Fehlings solutions etc. are strong enough to cleave the ring and then –CHO group is free to give its characteristic reactions.
14. (a) What is the difference between bathing soap and washing soaps? (b) Aspartame is unstable at cooking temperature, where would you suggest aspartame to be used for sweetening?
(a) Bathing soaps are potassium salts of long chain fatty acids while washing soaps are sodium salts of long chain fatty acids.
(b) In cold foods and soft drinks.
15. Name the alkyl halide which can be used to prepare methane and ethane in single step.
Methyl halide gives methane by reduction and ethane as a result of Wurtz reacton.