JEE Chemistry S Block Elements MCQs Set 09

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Multiple Choice Questions

Question 1: The order of thermal stability of carbonates of group 2 is
(a) \(\ce{CaCO3 > SrCO3 > BaCO3 > MgCO3}\)
(b) \(\ce{BaCO3 > SrCO3 > CaCO3 > MgCO3}\)
(c) \(\ce{MgCO3 > CaCO3 > SrCO3 > BaCO3}\)
(d) \(\ce{MgCO3 = CaCO3 > SrCO3 = BaCO3}\)
Show Answer & Explanation

Answer: (b) \(\ce{BaCO3 > SrCO3 > CaCO3 > MgCO3}\)

Explanation:
1. Stability increases as cation size increases because polarisation of the carbonate ion decreases.

Teacher's Note:
1. Same trend as nitrates and sulphates.

Question 2: A pair of substances which gives the same products on reaction with water is
(a) Mg and MgO
(b) Sr and SrO
(c) Be and BeO
(d) Ca and \(\ce{CaH2}\)
Show Answer & Explanation

Answer: (d) Ca and \(\ce{CaH2}\)

Explanation:
1. \(\ce{Ca + 2H2O -> Ca(OH)2 + H2}\) and \(\ce{CaH2 + 2H2O -> Ca(OH)2 + 2H2}\) give the same products.
2. Oxides give only the hydroxide; Be does not react.

Teacher's Note:
1. Check the products, not the ratios.

Question 3: The incorrect statement is
(a) Mg forms an alkaline hydroxide while Be forms amphoteric oxides
(b) the first ionisation potential of Be is higher than that of Mg
(c) Be can form complexes due to its very small atomic size
(d) Mg cannot form complexes
Show Answer & Explanation

Answer: (d) Mg cannot form complexes

Explanation:
1. Mg forms complexes, such as chlorophyll.
2. The other statements are correct.

Teacher's Note:
1. Be forms \(\ce{BeF4^2-}\) and \(\ce{Be(OH)4^2-}\).

Question 4: The commercial method for preparation of potassium by reduction of molten KCl with metallic sodium at 850 C is based on the fact that
(a) sodium is more reactive than potassium at 850 C
(b) potassium is solid and sodium distils off at 850 C
(c) potassium, being more volatile, distils off, thus shifting the reaction forward
(d) sodium has less affinity to chloride ions in the presence of potassium ion
Show Answer & Explanation

Answer: (c) potassium, being more volatile, distils off, thus shifting the reaction forward

Explanation:
1. \(\ce{Na + KCl -> NaCl + K}\).
2. K is more volatile and distils out of the mixture, shifting the equilibrium forward (Le Chatelier).

Teacher's Note:
1. This works by removing a product, not by reactivity.

Question 5: \(\ce{Be2C + H2O -> BeO + X}\) and \(\ce{CaC2 + H2O -> Ca(OH)2 + Y}\). X and Y are respectively
(a) \(\ce{CH4}\), \(\ce{C2H6}\)
(b) \(\ce{CH4}\), \(\ce{C2H2}\)
(c) \(\ce{CH4}\), \(\ce{CH4}\)
(d) \(\ce{C2H2}\), \(\ce{CH4}\)
Show Answer & Explanation

Answer: (b) \(\ce{CH4}\), \(\ce{C2H2}\)

Explanation:
1. Be2C is a methanide and gives methane.
2. CaC2 contains the \(\ce{C2^2-}\) ion and gives ethyne.

Teacher's Note:
1. Methanides give methane, acetylides give acetylene.

Question 6: \(\ce{MgBr2}\) and \(\ce{MgI2}\) are soluble in acetone because of their
(a) metallic nature
(b) coordinate nature
(c) covalent nature
(d) ionic nature
Show Answer & Explanation

Answer: (c) covalent nature

Explanation:
1. The polarisable bromide and iodide ions make the Mg compounds appreciably covalent.
2. So they dissolve in organic solvents like acetone.

Teacher's Note:
1. Covalent halides dissolve in organic solvents.

Question 7: Which of the following compounds of alkaline earth metals are amphoteric in nature? Pick the correct set from: 1) BeO 2) MgO 3) \(\ce{Be(OH)2}\) 4) \(\ce{Mg(OH)2}\)
(a) 1 and 3
(b) 2 and 4
(c) 1 and 2
(d) 3 and 4
Show Answer & Explanation

Answer: (a) 1 and 3

Explanation:
1. Be is the only group 2 element with amphoteric oxide and hydroxide.
2. MgO and Mg(OH)2 are basic.

Teacher's Note:
1. BeO and Be(OH)2 react with acids and bases.

Question 8: \(\ce{KO2}\) is used in oxygen cylinders for space and submarines. The facts related to this use are: 1) it produces \(\ce{O2}\) 2) it produces \(\ce{O3}\) 3) it absorbs \(\ce{CO2}\) 4) it absorbs both CO and \(\ce{CO2}\). The correct choice is
(a) 1 and 3
(b) 2 and 3
(c) 1 and 4
(d) 2 and 4
Show Answer & Explanation

Answer: (a) 1 and 3

Explanation:
1. \(\ce{4KO2 + 2CO2 -> 2K2CO3 + 3O2}\).
2. It gives oxygen and removes carbon dioxide.
3. It does not give ozone and does not absorb carbon monoxide.

Teacher's Note:
1. KO2 refreshes air in closed spaces.

Question 9: A highly pure dilute solution of sodium in ammonia: 1) shows blue colour due to solvated electrons; 2) shows electrical conductivity due to both solvated electrons and solvated sodium ions; 3) shows red colour due to solvated electrons but is a bad conductor; 4) produces hydrogen gas or carbonate. The correct statements are
(a) 1 and 2
(b) 1 and 3
(c) 2 and 4
(d) 3 and 4
Show Answer & Explanation

Answer: (a) 1 and 2

Explanation:
1. Pure dilute solutions are blue and conduct because of solvated ions and electrons.
2. They are stable unless a catalyst is present, so hydrogen is not produced.

Teacher's Note:
1. Blue colour is due to solvated electrons.

Question 10: Which of the following compounds are paramagnetic? 1) \(\ce{KO2}\) 2) \(\ce{K2O2}\) 3) \(\ce{Na2O2}\) 4) \(\ce{RbO2}\)
(a) 1 and 4
(b) 2 and 3
(c) 1 and 2
(d) 3 and 4
Show Answer & Explanation

Answer: (a) 1 and 4

Explanation:
1. Superoxides contain \(\ce{O2-}\) with an unpaired electron.
2. Peroxides contain the paired \(\ce{O2^2-}\).

Teacher's Note:
1. Superoxides are paramagnetic.

Question 11: Which of the following substances are used in laboratory for drying purposes? 1) anhydrous \(\ce{P2O5}\) 2) graphite 3) anhydrous \(\ce{CaCl2}\) 4) \(\ce{Na3PO4}\)
(a) 1 and 3
(b) 2 and 4
(c) 1 and 2
(d) 3 and 4
Show Answer & Explanation

Answer: (a) 1 and 3

Explanation:
1. P2O5 and anhydrous CaCl2 are strong dehydrating agents.
2. Graphite and Na3PO4 are not drying agents.

Teacher's Note:
1. Common desiccants: P2O5, CaCl2, conc. H2SO4, silica gel.

Question 12: Which of the following statements are false? 1) \(\ce{BeCl2}\) is linear in the vapour state and polymeric in the solid. 2) Calcium hydride is called hydrolith. 3) Carbides of both Be and Ca react with water to form acetylene. 4) Oxides of both Be and Ca are amphoteric.
(a) 1 and 2
(b) 3 and 4
(c) 1 and 3
(d) 2 and 4
Show Answer & Explanation

Answer: (b) 3 and 4

Explanation:
1. 1 is true.
2. 2 is true.
3. Be2C gives methane, so 3 is false.
4. CaO is basic, so 4 is false.

Teacher's Note:
1. Be2C is a methanide.

Question 13: Which of the following are ionic carbides? 1) \(\ce{CaC2}\) 2) \(\ce{Al4C3}\) 3) SiC 4) \(\ce{Be2C}\)
(a) 1, 2 and 4
(b) 1 and 3
(c) 2, 3 and 4
(d) 3 only
Show Answer & Explanation

Answer: (a) 1, 2 and 4

Explanation:
1. CaC2, Al4C3 and Be2C are saline (ionic) carbides that hydrolyse.
2. SiC is a covalent carbide with a giant structure.

Teacher's Note:
1. Silicon carbide is the covalent giant-structure example.

Question 14: Gypsum is heated at 120 C to form a monoclinic to orthorhombic plaster of Paris, which is a compound formed by loss of three fourths of its water. What will be the product if gypsum is heated at 200 C?
(a) \(\ce{CaSO4}\)
(b) \(\ce{CaSO4.H2O}\)
(c) \(\ce{CaO}\), \(\ce{SO2}\) and \(\ce{O2}\)
(d) \(\ce{2CaSO4.H2O}\)
Show Answer & Explanation

Answer: (a) \(\ce{CaSO4}\)

Explanation:
1. At 200 C all the water of crystallisation is lost to give anhydrous \(\ce{CaSO4}\) (dead burnt plaster).
2. Decomposition to CaO needs a very high temperature.

Teacher's Note:
1. 120 C gives hemihydrate, 200 C gives anhydrous salt.

Question 15: Sodium reacts with excess oxygen at 350 C (passage: peroxide and some superoxide form from alkali metals in excess air). The oxidation number of oxygen per atom changes from
(a) 0 to -1
(b) +1 to -1
(c) 0 to -1/2
(d) -1 to -2
Show Answer & Explanation

Answer: (a) 0 to -1

Explanation:
1. Na forms mainly the peroxide \(\ce{Na2O2}\), in which oxygen is -1.
2. Elemental oxygen is 0.

Teacher's Note:
1. Peroxide oxygen has oxidation number -1.

Question 16: Which of the following compounds is used in emergency breathing apparatus?
(a) soda lime (\(\ce{NaOH + CaO}\))
(b) sodium monoxide, \(\ce{Na2O}\)
(c) hydrated calcium sulphate
(d) potassium superoxide, \(\ce{KO2}\)
Show Answer & Explanation

Answer: (d) potassium superoxide, \(\ce{KO2}\)

Explanation:
1. \(\ce{KO2}\) absorbs CO2 and moisture and releases oxygen, so it is used in breathing masks.

Teacher's Note:
1. Superoxides refresh air.

Question 17: The stability of alkali metal superoxides increases in the order
(a) \(\ce{KO2 < NaO2 < RbO2 < CsO2}\)
(b) \(\ce{RbO2 < CsO2 < NaO2 < KO2}\)
(c) \(\ce{NaO2 < KO2 < RbO2 < CsO2}\)
(d) \(\ce{CsO2 < RbO2 < KO2 < NaO2}\)
Show Answer & Explanation

Answer: (c) \(\ce{NaO2 < KO2 < RbO2 < CsO2}\)

Explanation:
1. Large anions are stabilised by large cations through lattice energy.
2. So stability increases down the group.

Teacher's Note:
1. Larger cation, more stable superoxide.

Question 18: Match column I with column II. a) \(\ce{NaO2}\) b) \(\ce{KO2}\) c) \(\ce{RbO2}\) d) \(\ce{CsO2}\) with P) paramagnetic Q) yellow, pyrites structure R) orange S) brown. Choose the matching.
(a) a-P,Q; b-P,R; c-P,S; d-P
(b) a-Q; b-P,R; c-P,Q; d-R,S
(c) a-P; b-R,S; c-P,R; d-P
(d) a-R; b-Q; c-S; d-P
Show Answer & Explanation

Answer: (a) a-P,Q; b-P,R; c-P,S; d-P

Explanation:
1. All superoxides contain \(\ce{O2-}\) with an unpaired electron, so all are paramagnetic (P).
2. NaO2 is yellow (pyrites structure), KO2 is orange, RbO2 is brown.
3. Only the listed matching 'NaO2 - P,Q; KO2 - P,R; RbO2 - P,S; CsO2 - P' agrees with this.

Teacher's Note:
1. Paramagnetism follows from the unpaired electron of the superoxide ion.

Question 19: Assertion (A): Beryllium does not impart any characteristic colour to the Bunsen flame.
Reason (R): Due to its very high ionisation energy, beryllium requires a large amount of energy for excitation of the electrons.
(a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
Show Answer & Explanation

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Be gives no flame colour.
2. Its excitation energy is very high, more than a Bunsen flame can supply.
3. So R explains A.

Teacher's Note:
1. Same reason holds for Mg.

Question 20: Assertion (A): The best diagonal relationship is shown between Be and Al.
Reason (R): The ionisation energy of Be is almost the same as that of Al.
(a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
Show Answer & Explanation

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. Be and Al have similar charge to radius ratio, so the relationship is close (A true).
2. IE of Be (899 kJ per mol) is clearly different from Al (577 kJ per mol), so R is false.

Teacher's Note:
1. Diagonal relationship rests on similar polarising power.

Question 21: Assertion (A): Beryllium halides dissolve in organic solvents.
Reason (R): Beryllium halides are ionic in character.
(a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
Show Answer & Explanation

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. Be halides are covalent, so they dissolve in organic solvents (A true).
2. R is false because the halides are covalent.

Teacher's Note:
1. Covalent compounds dissolve in non-polar solvents.

Question 22: Assertion (A): \(\ce{BeCl2}\) fumes in moist air.
Reason (R): \(\ce{BeCl2}\) reacts with moisture to form HCl gas.
(a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
Show Answer & Explanation

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. \(\ce{BeCl2 + 2H2O -> Be(OH)2 + 2HCl}\).
2. The HCl fumes are the cause of fuming.

Teacher's Note:
1. Hydrolysis explains fuming.

Question 23: Assertion (A): Calcium carbide on hydrolysis gives methane.
Reason (R): Calcium carbide contains \(\ce{C2^2-}\) anion.
(a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
Show Answer & Explanation

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. Calcium carbide gives ethyne (acetylene), not methane, so A is false.
2. It contains the \(\ce{C2^2-}\) ion, so R is true.

Teacher's Note:
1. Acetylide ion gives acetylene.

Question 24: Calcium oxalate, \(\ce{CaC2O4}\), is heated gently (about 500 C) so that it loses only one gaseous product and becomes \(\ce{CaCO3}\). How many gaseous products are evolved in this step?
(a) 1
(b) 2
(c) 3
(d) 4
Show Answer & Explanation

Answer: (a) 1

Explanation:
1. \(\ce{CaC2O4 -> CaCO3 + CO}\).
2. Only carbon monoxide is evolved at this stage.

Teacher's Note:
1. Further heating of CaCO3 gives CO2 and CaO.

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