JEE Chemistry S Block Elements MCQs Set 26

Answer: (c) Francium

Explanation:
1. Francium is the last member of group 1 and has only short-lived radioactive isotopes.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) \( \ce{Na+}, \ce{K+}, \ce{Mg^{2+}}, \ce{Ca^{2+}} \)

Explanation:
1. Na+ and K+ maintain ionic balance and nerve impulses, while Mg2+ and Ca2+ occur in enzymes, bones, muscles and nerves.
2. Sr2+ and Ba2+ are not required by living systems.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) They exist in the free state in nature

Explanation:
1. Alkali metals are so reactive that they occur only as compounds, never free.
2. They do have the largest atoms in their periods, so (c) is true and the false statement is (b).

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (d) None of these

Explanation:
1. Li (0.53), Na (0.97) and K (0.86 g/cm³) are lighter than water; Rb (1.53) is the first alkali metal denser than water, but it is not given, so none of the listed ones.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (a) low ionisation enthalpy

Explanation:
1. The valence electron is easily excited by the flame's heat and emits visible light when it returns to the ground state.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) Rb

Explanation:
1. Rb gives a red-violet flame; Ba gives apple green.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (d) Cs

Explanation:
1. Caesium has the lowest melting point of the group (about 28.5 °C), so it melts at 30 °C.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) sodium

Explanation:
1. Li gives the normal oxide, Na the peroxide \( \ce{Na2O2} \), and K, Rb, Cs give superoxides.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) Li

Explanation:
1. Lithium is small and strongly polarising, so it forms \( \ce{Li2O} \) (and some nitride) only.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) \( \ce{KO2} \)

Explanation:
1. \( \ce{4KO2 + 2CO2 -> 2K2CO3 + 3O2} \) removes the exhaled \( \ce{CO2} \) and releases oxygen; it gives more oxygen per unit mass than the others.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (d) \( \ce{Rb+} \)

Explanation:
1. The least hydrated (largest) ion carries the smallest hydration shell and moves fastest, so ionic mobility increases from Li+ to Rb+.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (a) \( \ce{Li+} \)

Explanation:
1. Li+ is the most heavily hydrated, so the hydrated ion is the largest and moves slowest.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

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

Explanation:
1. Ca gives \( \ce{H2} \) and \( \ce{CaH2 + 2H2O -> Ca(OH)2 + 2H2} \) also gives \( \ce{H2} \).
2. Na gives \( \ce{H2} \) but \( \ce{Na2O2} \) gives (with cold water) \( \ce{H2O2} \)/\( \ce{O2} \); CaO gives no gas.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) BaO

Explanation:
1. Basic character of oxides increases down a group and with electropositivity; BaO is the most basic here. \( \ce{As2O3} \) and \( \ce{Al2O3} \) are amphoteric.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (d) LiH

Explanation:
1. The stability of alkali metal hydrides decreases down the group, as the small \( \ce{Li+} \) ion has the highest lattice energy with \( \ce{H-} \).

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (a) LiF

Explanation:
1. LiF has the highest lattice enthalpy because \( \ce{F-} \) is the smallest halide ion.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) blue

Explanation:
1. The ammoniated electrons absorb light in the red region, so the dilute solution is deep blue and paramagnetic.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) kerosene

Explanation:
1. Sodium is stored under kerosene because it reacts with air, moisture and alcohol.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) Sodium

Explanation:
1. Liquid sodium (often with potassium) has excellent heat conduction and is used as a coolant in fast breeder reactors.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (a) CsOH

Explanation:
1. Basic strength of the alkali metal hydroxides increases down the group as the M-OH bond becomes weaker.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) NaF

Explanation:
1. For the same anion, lattice energy is highest for the smallest cation, \( \ce{Na+} \) here.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) CsI

Explanation:
1. \( \ce{Cs+} \) and \( \ce{I-} \) are the largest cation and anion in the list, so their radii sum is the greatest.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (c) LiI

Explanation:
1. By Fajans' rules covalent character is greatest for a small, highly polarising cation with a large, easily polarised anion.
2. \( \ce{Li+} \) is the smallest cation and \( \ce{I-} \) is large, so LiI is the most covalent.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (d) no gas is evolved

Explanation:
1. Sodium carbonate is very stable and melts without decomposition, unlike \( \ce{Li2CO3} \).

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.

Answer: (b) Li is much softer than the other group 1 metals

Explanation:
1. Lithium is harder than the other alkali metals, so saying it is much softer is wrong; the other three are real anomalies.

Teacher's Note:
1. Identify the key concept in the question first, then check each option against it.
2. Learn the exceptions to general trends, as they are the most common source of questions.