Completely solved paper for Class 12 Chemistry board exam 2018 is available here. This paper is based on latest CBSE Class 12 Chemistry Syllabus and most of the questions given in this paper are from NCERT Textbook. There is a high probability that these questions might be asked in Chemistry board exam 2018. This paper is scheduled to be held on 13 March 2018.
General Instructions for the paper:
(i) All questions are compulsory.
(ii) Questions number 1to 5 are very short answer questions and carry 1 mark each.
(iii) Questions number 6 to 10 are very short answer question and carry 2 marks each.
(iv) Questions number 11 to 22 are also short answer question and carry 3 marks each.
(v) Question number 23 is a value based question and carry 4 marks.
(vi) Questions number 24 to 26 are long answer question and carry 5 marks each.
(vii) Use log tables, if necessary. Use of calculators is not allowed
Some solved questions from the paper are given below
Question: Name a substance that can be used as an antiseptic as well as a disinfectant.
(0.2% solution of phenol acts as antiseptic whereas 1% solution acts as disinfectant)
Question: What is the coordination number of each type of ions in a rock-salt type crystal structure?
Coordination number of Na+ ion = 6
Coordination number of Cl‒ ion = 6.
Question: Why does bleeding stop by rubbing alum?
Bleeding stops due to coagulation of blood.
Question: CuSO4. 5H2O is blue in colour while CuSO4 is colourless. Give reason?
In CuSO4.5H2O, water acts as ligand and causes crystal field splitting. So, d-d transition is possible hence, CuSO4. 5H2O is coloured. Now, in the anhydrous CuSO4 due to the absence of water (ligand), crystal field splitting is not possible and hence, it is colourless.
Question: Define Kraft temperature
The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC).
Question: What are biodegradable and non-biodegradable detergents? Give one example of each type
Biodegradable detergents: Detergents having straight hydrocarbon chains are easily degraded by micro-organism and hence called biodegradable detergents, e.g., sodium–4–(1-dodecyl) benzene sulphonate.
Non-biodegradable detergents: Detergents having branched hydrocarbon chains are not easily degraded by the micro-organism and hence are called non-biodegradable detergents, e.g., sodium-4-(1, 3, 5, 7-tetramethyl octyl) benzenesulphonate.
Question: State Henry’s law correlating the pressure of a gas and its solubility in a solvent and mention two applications of the law.
It states that at constant temperature the mass of a gas(m) dissolved in a given volume of the liquid is directly proportional to the pressure of the gas (P) present in equilibrium with the liquid.
Mathematically, m ∝ Por m = KH P where KH is the Henry’s law constant.
• To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.
• To minimize the painful effects accompanying the decompression of deep sea divers, oxygen
Question: The formula of a coordination compound is CoCl3.4NH3. It does not liberate ammonia but precipitates chloride ions as silver chloride.
(i) Write the structural formula of the complex?
(ii) What is the IUPAC name of the complex?
(i) The structural formula of the complex will be [Co(NH3)4Cl2]Cl.
(ii) The IUPAC name of the complex is tetraamminedichlorocobalt (III) chloride.
Question: Give an example of a reaction in which molecularly and order of a reaction are equal.
2 HI (g) → H2 (g) + I2 (g)
In this reaction order and molecularity both are equal to 2.
Question: (i) Why salt bridge is essential in a galvanic cell?
(ii) What is the role of ZnCl2 in a dry cell?
(i) Salt bridge completes the inner circuit and also maintains electrical neutrality of the electrolytic solutions of the half cells.
(ii) ZnCl2 combines with NH3 produced to form the complex salt [Zn (NH3)2 Cl2] as otherwise the pressure developed due to NH would crack the seal of the cell.
Question: (i) Write the structural difference between starch and cellulose.
(ii) What type of linkage is present in nucleic acids?
(iii) Give an example of each fibrous protein and globular protein.
(i) Starch is branched homopolysaccharide of D-glucose and contains two types of glucose polymer, amylase and amylopectin. The long, unbranched chains of D-glucose residues connected by alpha1, 4 linkages are termed as amylase. The branched polymer of glucose wherein alpha 1, 4 glycosidic linkages join successive glucose with branch points having alpha 1, 6 linkages. Cellulose is linear unbranched polymer of glucose residues linked together by beta 1, 4 linkage.
(ii) Phosphodiester linkage in which the 5’-phosphate group of one nucleotide unit is joined to the 3’-hydroxyl group of the next nucleotide (a covalent bond).
(iii) Fibrous protein: silk; globular protein: egg albumin.
Question: Give reasons for the following
(i) Nickel does not form low spin octahedral complexes.
(ii) The p-complexes are known for the transition metals only.
(iii) Co2+ is easily oxidised to Co3+ in the presence of a strong ligand.
(i) Nickel does not form low spin octahedral complexes because ‘Ni’ has electronic configuration 3d8 4s2, in which two inner d-orbitals are not available which are required to form d2sp3 hybridization.
(ii) The p-complexes are known for the transition metals only because they have ‘sandwich’ structure in which the metal ion lies between two planar C5H5 rings. The bonding involves overlap of p-electrons of the C5H5 rings with unfilled d-orbital of the metal. So, that all M–C bonds are identical for their stability.
(iii) Co2+ is easily oxidised to Co3+ in the presence of a strong ligand because in presence of strong ligand, the 3d electrons pair up leaving two orbitals empty to be involved in d2sp3 hybridisation.
Question: (i) What are azo dyes?
(ii) What do you understand by coupling reaction?
(i) Dyes containing ‒N=N‒ bonds are called azo dyes.
(ii) Reaction of diazonium salts with phenols and amines to produce compounds of the type Ar‒N=N‒Ar is called coupling reaction.
Question: Name the reagents which are used in the following conversions:
(i) A primary alcohol to an aldehyde
(ii) Butan-2-one to butan-2-ol
(iii) Phenol to 2, 4, 6-tribromophenol
(i) Pyridinium chlorochromate C2H5NH+CrO3Cl‒
Question: Define the following terms:
(b) Tyndall Effect
(c) Lyophobic colloids
(a) Emulsion: It is a colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids.
(b) Tyndall Effect: It is defined as the scattering of light by the colloidal particles present in colloidal solution.
(c) Autocatalysis: It is reaction in which one of the product acts as a catalyst then, it is called autocatalysis.