CBSE 12th Exam 2021 : Chemistry Case Study Problems

CBSE 12th Exam 2021 : Chemistry Case Study Problems

In Coming Exams, CBSE will ask two Case Study Questions in the CBSE class 12 Chemistry questions paper. Each theme will have five questions and students will have a choice to attempt any four of them.

Here are some example questions Based On Case Study Problems:

Question-1

Read the passage given below and answer any four out of the following questions:
Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea. On a large scale, ammonia is manufactured by Haber’s process. In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states, it is associated with hydrogen bonds as in the case of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH ions. The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base.

The following questions are multiple-choice questions. Choose the most appropriate choice 

  1. On a small scale, ammonia is obtained from ammonium salts which decompose when treated with
    1. caustic soda
    2. calcium chloride
    3. sodium hydroxide
    4. sodium chloride
  2. The optimum conditions for the production of ammonia are a pressure of
    1. ​200*105 Pa
    2. 400*105 Pa
    3. 100*105 Pa
    4. 300*105 Pa
  3. The catalyst which is used in the preparation of NH3 by Haber’s process
    1. Mg2O3 + K2O
    2. Al2O+ K2O
    3. NaO+ K2O
    4. None of these
  4. The ammonium molecule has:
    1. five bond pair and two lone pair
    2. four lone pair and one bond pair
    3. three bond pair and one lone pair
    4. three bond pair and two lone pair
  5. A compound reacts with ammonia to form deep colour solution, identify the compound
    1. Au2+
    2. Cu2+
    3. Al3+
    4. Mg2+

Questions-2

Read the passage and answer any four out of the following questions:
Colloidal particles always carry an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either positive or negative. The charge on the sol particles is due to one or more reasons, viz., due to electron capture by sol particles during electrodispersion of metals. When two or more ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle usually takes place. When silver nitrate solution is added to the potassium iodide solution, the precipitated silver iodide adsorbs iodide ions from the dispersion medium, and negatively charged colloidal solution results. acquired a positive or a negative charge by selective adsorption on the surface of a colloidal particle The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer. The presence of equal and similar charges on colloidal particles is largely responsible for providing stability to the colloidal solution.

In these questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices. 

  1. Assertion and reason both are correct statements and reason is correct explanation for assertion
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion
  3. Assertion is correct statement and reason is wrong statement
  4. Assertion is wrong statement but reason is correct statement
  1. Assertion: The presence of equal and similar charges on colloidal particles is largely responsible in providing stability to the colloidal solution.
    Reason: The repulsive forces between charged particles having the same charge prevent them from aggregating and provide stability.
  2. Assertion: The first layer is mobile in Helmholtz electrical double layer.
    Reason: The potential difference between the fixed layer and the diffused layer of opposite charges is called zeta potential.
  3. Assertion: The sol particle in the colloid has a charge.
    Reason: The charge in sol is due to electron capture by sol particles during the electrodispersion of metals.
  4. Assertion: Methylene blue sol is a negatively charged sol.
    Reason: When KI solution is added to AgNO3 solution, positively charged sol formed.
  5. Assertion: If FeCl3 is added to an excess of hot water, a positively charged sol of hydrated ferric oxide is formed.
    Reason: When ferric chloride is added to NaOH a negatively charged sol is obtained with adsorption of OH- ions.

Question-3

Read the passage and answer the following questions:
The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionically arising purely from electrostatic interactions between the metal ion and the ligand. Ligands are treated as point charges in case of anions or point dipoles in case of neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion have the same energy, i.e., they are degenerate. In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands. This splitting of the degenerate levels due to the presence of ligands in a definite geometry is termed crystal field splitting and the energy separation is denoted by Δ0. The colour in the coordination compounds can be readily explained in terms of the crystal field theory. 

In these questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.

  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4.  Assertion is wrong statement but reason is correct statement.
  1. Assertion: The dx2-y2 and dz2 orbitals which point towards the axes along the direction of the ligand will experience more repulsion. 
    Reason: The dxy, dyz and dxz orbitals which are directed between the axes will be lowered in energy.
  2. Assertion: The complex [Ti(H2O)6]3+, which is red in colour.
    Reason: The crystal field theory attributes the colour of the coordination compounds to d-d transition of the electron.
  3. Assertion: Ligands for which Δ0Δ0 < P are known as weak field ligands and form high spin complexes.
    Reason: If Δ0 > P, then the fourth electron enters one of the e​​​​​​g orbitals giving the configuration t2g3 eg1
  4. Assertion: In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting.
    Reason: Spectrochemical series is based on the absorption of light by complexes with different ligands. 
  5. Assertion: The crystal field model is successful in explaining the formation, structures, colour and magnetic properties of coordination compounds.
    Reason: The anionic ligands are found at the low end of the spectrochemical series.

Answer Key:

  1. (b) Assertion and reason both are correct statements and reason is not correct explanation for assertion.
  2. (c) Assertion is correct statement but reason is wrong statement.
  3. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
  4. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
  5. (d) Assertion is wrong statement but reason is correct statement.

Question-4

Read the passage and answer any four out of the following question
Alfred Werner (1866-1919), a Swiss chemist was the first to formulate his ideas about the structures of coordination compounds. Werner proposed the concept of a primary valence and a secondary valence for a metal. The coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it is called the central atom or ion. The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. Ligands may be simple ions such as Cl-, small molecules such as H2O or NH3, larger molecules such as H2NCH2CH2NH2 or N(CH2CH2NH2)3 or even macromolecules, such as protein. Ligands are unidentate, bidentate and polydentate. The coordination number (CN) of a metal ion in a complex is the number of ligand donor atoms to which the metal is directly bonded. 

In these questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.

  1. Assertion and reason both are correct statements and reason is correct explanation for  assertion
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion
  3. Assertion is correct statement but reason is wrong statement
  4.  Assertion is wrong statement but reason is correct statement
  1. Assertion: Binary compounds such as CrCl3, have a primary valence of 3.
    Reason: Coordinate compound metals show only one type of linkage that is primary linkage.
  2. Assertion: CoCl3(NH3)3 is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ion.
    Reason: The central atom/ion in the coordination entities: [NiCl2(H2O)4] is Ni2+.
  3. Assertion: H2NCH2CH2NH2 (ethane-1,2-diamine) ligand is said to be didentate.
    Reason: Didentate ligands are bind through two donor atoms.
  4. Assertion: The complex ions, [PtCl6]2- the coordination number of Pt is 4.
    Reason: Ligand which can ligate through two different atoms is called ambidentate ligand.
  5. Assertion: EDTA can bind through two nitrogen and four oxygen atoms to a central metal ion.
    Reason: The number of ligating groups attach to an atom is called the denticity of the ligand.

Answer Key:

  1. (c) Assertion is correct statement but reason is wrong statement
  2. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion
  3. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion
  4. (d) Assertion is wrong statement but reason is correct statement
  5. (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion

Question-5

Read the passage given below and answer any four out of the following questions:
Nitrogen differs from the rest of the members of group 15 due to its smaller size, high electronegativity, high ionisation enthalpy, and non-availability of d orbitals. Nitrogen has a unique ability to form pπ-pπ multiple bonds with itself. Nitrogen exists as a diatomic molecule with a triple bond one s and two p between the two atoms. Phosphorus, arsenic and antimony from single bonds as P–P, As–As and Sb–Sb while bismuth forms metallic bonds in an elemental state. Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K). In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite. Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. It has two stable isotopes 14N and 15N. It has very low solubility in water. The main use of dinitrogen is in the manufacture of ammonia and other industrial chemicals containing nitrogen.

The following questions are multiple-choice questions. choose the most appropriate answer

  1. N–N bond is weaker than the single P–P bond because 
    1. high interelectronic repulsion of the bonding electrons
    2. high interelectronic repulsion of the non-bonding electrons
    3. no repulsion between bonding electrons
    4. no repulsion between non-bonding electrons
  2. Very pure nitrogen can be obtained by the
    1. thermal decomposition of sodium
    2. thermal decomposition of barium azide
    3. thermal decomposition of ammonium dichromate
    4. both (a) and (b)
  3. Dinitrogen is rather inert at room temperature because of
    1. low bond enthalpy of N≡≡N bond
    2. high bond enthalpy of N≡≡N bond
    3. low freezing point
    4. low boiling point
  4. Dinitrogen combines with dioxygen only at very high temperature (at about 2000 K) to form
    1. nitric oxide
    2. nitrate
    3. nitrites
    4. nitric acid
  5.  Liquid dinitrogen is used as a refrigerant to
    1. preserve biological materials 
    2. preserve food items  
    3. in cryosurgery
    4. all of these

Answer Key:

  1. (b) high interelectronic repulsion of the non-bonding electrons
  2. (d) both (a) and (b)
  3. (b) high bond enthalpy of N≡≡N bond
  4. (a) nitric oxide
  5. (d) all of these

Question-6

Read the following passage and answer any four out of the following questions:
Transition metal oxides are generally formed by the reaction of metals with oxygen at high temperatures. The highest oxidation number in the oxides coincides with the group number. In vanadium, there is a gradual change from the basic V2O3 to less basic V2O4 and to amphoteric V2O5. V2O4 dissolves in acids to give VO​​​​​2+ salts. Potassium dichromate is a very important chemical used in the leather industry and as an oxidant for the preparation of many azo compounds. Dichromates are generally prepared from chromate. Sodium dichromate is more soluble than potassium dichromate. The latter is, therefore, prepared by treating the solution of sodium dichromate with potassium chloride. Sodium and potassium dichromates are strong oxidising agents; sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis.

The following questions are multiple-choice questions. Choose the most appropriate answer.

  1. All transition metal reacts with oxygen to form MO oxide except
    1. scandium
    2. vanadium
    3. cupper
    4. zinc
  2. As the oxidation number of a metal increases, ionic character
    1. increases
    2. decreases
    3. remain the same
    4. none of these
  3. The shape of chromate ion is
    1. tetrahedral
    2. pyramidal
    3. square planer
    4. triangular
  4. Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of 
    1. FeCr2O
    2. FeCr2O4
    3. Na2CrO4
    4. Na2Cr2O7
  5. The oxo cations stabilise V​​​​​IV
    1. VO
    2. VO​​​​​4+
    3. VO​​​​​2+
    4. all of these

Answer Key:

  1. (a) scandium
  2. (b) decreases
  3. (a) tetrahedral
  4. (b) FeCr2O4
  5. (c) VO​​​​​2+

 

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