Most Important Scientific Reasoning Questions - Class 12 Chemistry

 Q.1 Why d- Block elements are called outer transition elements and f- Block elements are called inner transition elements.   

d-Block elements are called outer transition elements because they have partially filled d-orbitals in their outermost energy level. These elements undergo transition in their valence electron configuration during chemical reactions, which is why they are referred to as transition elements.

On the other hand, f-Block elements are called inner transition elements because they have partially filled f-orbitals in their penultimate energy level, which is considered "inner" compared to the outermost energy level. These elements are also referred to as "rare earth elements." Their valence electrons are shielded by the filled 4f orbitals, which makes them less reactive than the d-block elements.

Therefore, the main difference between the two groups of elements is the location of their partially filled orbitals in relation to the outermost energy level.

Q.2 Metallic hydrides form interstitial hydrides.

Metallic hydrides form interstitial hydrides because metals have a close-packed structure, with small interstitial spaces between metal atoms. When hydrogen atoms are introduced into these interstitial spaces, they form interstitial hydrides, where hydrogen atoms occupy the interstitial sites without disrupting the metal lattice. The interstitial hydrides have different physical and chemical properties compared to the metallic hydrides.

Q.3 Atomic or Nascent hydrogen is more reactive than molecular hydrogen.

Atomic hydrogen is obtained by dissociation of a hydrogen molecule using an electric arc. This form of hydrogen is extremely reactive as it has very little life time. Nascent hydrogen is obtained when zinc reacts with sulphuric acid or by reducing agent.

The two major reasons for higher reactivity is associated with the release of high energy.

(1) This production of nascent hydrogen is associated with the release of high energy. This released energy activates the nascent hydrogen and makes it more energy rich than that of ordinary occurring molecular hydrogen. Because more energy means more reactivity, nascent hydrogen is more reactive than molecular hydrogen.

(2) The next reason for higher reactivity of nascent hydrogen is related to the high internal pressure. In the time of formation nascent hydrogen is in the form of minute bubbles with high internal pressure which makes it more reactive.

Q.4  Alkali and Alkaline earth metal hydrides are called salt like hydrides

Alkali and alkaline earth metal hydrides are called salt-like hydrides because they have a similar ionic structure to ionic salts. These hydrides have a high melting point and are insoluble in most solvents due to their strong ionic bonding. They also readily dissociate in water to form hydroxides and hydrogen gas. The ionic nature of these hydrides is due to the large difference in electronegativity between the metal and hydrogen atoms, resulting in the transfer of electrons from metal to hydrogen, and the formation of ionic bonds.

Q.5  Li + ions are more readily hydrated than K+ ions.

Li+ ions are more readily hydrated than K+ ions because Li+ ions have a smaller ionic radius than K+ ions, which means they have a higher charge density. The higher charge density of Li+ ions makes them more attractive to water molecules, which leads to a stronger hydration shell around the Li+ ion compared to the K+ ion.

Q.6 Li+ / Li couple has exponentially high negative electrode potential.

The Li+/Li couple has an exponentially high negative electrode potential because lithium is the most electropositive element. As a result, the reduction potential for the Li+/Li couple is very low, meaning it takes a lot of energy to reduce Li+ ions to metallic lithium. This makes the Li+/Li couple a very strong reducing agent and gives it a very negative electrode potential.

Q.7  Alkali metals are highly reactive.

Alkali metals are highly reactive because they have a single valence electron in their outermost shell, which makes them very unstable and prone to losing that electron to form a cation. This property makes them highly reactive towards other elements and compounds, particularly towards water, oxygen, and halogens.

Q.8 Alkaline earth metals ions are more strongly hydrated than alkali metal ions.

Alkaline earth metal ions are more strongly hydrated than alkali metal ions because they have a higher charge density than alkali metal ions. The higher charge density makes them more attractive to water molecules, which leads to a stronger hydration shell around the alkaline earth metal ion compared to the alkali metal ion.

  Q.9 Lithium and berylium markedly differ from other members of their respective group.

Lithium and beryllium markedly differ from other members of their respective groups due to their small size and high charge density. Lithium, for example, has a much smaller atomic radius than other alkali metals, which makes it more similar in size to magnesium than the other alkali metals. Similarly, beryllium has a much higher charge density than other alkaline earth metals, which affects its chemical behavior and makes it behave more like aluminum than the other alkaline earth metals.

Q.10  Ionization potential decreases from top to bottom in a group

Ionization potential decreases from top to bottom in a group because as we move down the group, the number of electron shells increases, and the atomic radius of the elements increases. This results in a weaker attraction between the valence electrons and the positively charged nucleus, making it easier to remove the outermost electron. Additionally, the shielding effect of the inner electrons increases, reducing the effective nuclear charge experienced by the outermost electron, which also makes it easier to remove.

Q.11 Electropositivity increases from top to bottom in group.

Electropositivity increases from top to bottom in a group because as we move down the group, the number of electron shells increases, and the atomic radius of the elements increases. This results in a weaker attraction between the valence electrons and the positively charged nucleus, making it easier for the atoms to lose electrons and become positively charged cations. Additionally, the shielding effect of the inner electrons increases, reducing the effective nuclear charge experienced by the outermost electrons, which also makes it easier for the atoms to lose electrons. Therefore, the electropositivity of elements increases as we move down a group.

Q.12 Transition elements form complex compounds.

Transition elements form complex compounds because they have partially filled d-orbitals that can form coordinate covalent bonds with other atoms or ions. These d-orbitals can accept electron pairs from ligands, which are molecules or ions that bond to the central metal atom or ion. This ability to form coordinate covalent bonds with ligands results in the formation of complex compounds.

Q.13 Transition elements are good catalyst.

Transition elements are good catalysts because they can undergo changes in oxidation state during a chemical reaction, which makes them capable of catalyzing a wide range of reactions. Additionally, the partially filled d-orbitals in transition metals can accept and donate electrons, allowing them to interact with other molecules and facilitate chemical reactions.

Q.14 Transition elements form coloured compound.

Transition elements form colored compounds because of the presence of partially filled d-orbitals that can absorb visible light. When light is absorbed, electrons are promoted to higher energy levels, and the color of the compound is determined by the wavelength of light that is absorbed. The energy levels of the d-orbitals are dependent on the oxidation state and the coordination environment of the metal ion, leading to a wide range of colors in transition metal compounds.

Q.15 Most of the transition elements are para magnetic.

Most of the transition elements are paramagnetic because they have unpaired electrons in their partially filled d-orbitals. These unpaired electrons create a magnetic moment, which aligns with an external magnetic field, making the compound paramagnetic. The degree of paramagnetism depends on the number of unpaired electrons present in the d-orbitals of the transition metal ion.

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