Chemistry MCQs for NEET — Practice Questions with Answers

Key Idea In peroxides, the oxidation state of O is -1 and they give H₂O₂, with dilute acids, and have peroxide linkage.

In K0₂,

           +1 + (x $\times$ 2)=0

           x = -1/2 (thus, it is a superoxide, not a peroxide.)

 In BaO₂,

           +2 + (x $\times$2) = 0

           x = -1

Thus, it is a peroxide. Only it gives H₂O₂ when reacts with dilute acids and has peroxide linkage as

                    Ba²⁺ [O - O]^2-

                 (peroxide linkage)

 In ${\mathrm{MnO}}_2$ and ${\mathrm{NO}}_2$, Mn and N exhibit variable oxidation states, thus, the oxidation state of O in these is - 2. Hence, these are not peroxides. Thus, it is clear, that among the given molecules only ${\mathrm{BaO}}_2$ is a peroxide.

(a) Key Idea : The amount of energy required to remove an electron from a unipositive ion is called second IP which generally increases in a period from left to right.

The second ionisation energies of the first transition series increase almost regularly with increase in atomic number. However, the value for Cr is sufficiently higher than those of its neighbour, i.e., (Mn). This is due to stable configuration of Cr⁺ (3d⁵ exactly half filled).

Note: The half-filled and completely filled configurations are more stable.

Key Idea: The smaller the size of ion, the greater the degree of hydration and

The mobility of ion in aqueous solution $\propto$ 1/size of ion in aqueous solution

Generally in a group size or radii of ions increases, so Li has smallest size thus bind maximum number of water molecules with itself and becomes largest in size (in aqueous solution). Hence, the order of ionic radii (size) of alkali metal ions in aqueous solution is

     Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺

Thus, the order of mobility

      Cs⁺>Rb⁺>K⁺>Na⁺>Li⁺

(c) Key Idea : Generally bond dissociation energies decreases in a group. Bond dissociation energy also decreases with repulsion.
X-X Bond                F-F         Cl-Cl         Br-Br           I-I
Bond length ($\stackrel{\circ }{\mathrm{A}}$)     1.42        1.99          2.28           2.67
Bond dissociation    38            57           45.5           35.6
energy (kcal/mol)

In general the bond dissociation energy decreases as the bond length increases, but the
bond dissociation energy of F₂ is less than that of Cl₂ is due to greater interelectronic repulsions between the lone pair of electrons on the two bonded fluorine atoms. Hence, the order of bond dissociation energy is not as: F₂ > Cl₂ > Br₂ > I₂ 

(b) Thermal stability of carbonates increases in a group as we move from top to bottom and decreases in a period as we move from left to right. So, the correct order of thermal stability of given carbonates is:

(b) BeCO₃ < MgCO₃ < CaCO₃ < K₂CO₃

Be, Mg and Ca present in second group and K present in first group.

it is because of polarizability and energetics..generally larger cation form stable compound with larger anion ,or their stability increases with increase in ionic character. 

Hydration energy of sulphate decreases from top to bottom in a IInd group. Mg²⁺ is smaller than other ions of that group so Mg²⁺ is readily hydrated. MgSO₄ has higher hydration energy than lattice energy.

(c) The electronic configuration  1s²2s²2p⁵3s¹ shows lowest ionisation energy because this configuration is unstable due to the presence of one electron in s-orbital. Hence, less energy is required to remove the electron.

A cation has always the lesser ionic size than a metal atom due to loss of electron and an anion has always the greater size than metal atom due to gain of electrons. So, the correct order of size is as

          Ca²⁺<K⁺<Ar<Cl<S^2-

In NH3, the electronegativity difference between N and H causes the electron cloud to be displaced towards the more electronegative H atoms. This results in a bond dipole moment in the same direction as the atomic dipole moment of N. In NF3, the highly electronegative F atoms displace the electron cloud away from N, causing the bond dipole moments to be in the opposite direction to the atomic dipole moment of N. This cancellation effect in NF3 results in a smaller net dipole moment compared to NH3.

(d) The correct order of the mobility of the alkali metal ions in aqueous solutions. Rb⁺ > K⁺ > Na⁺ > Li⁺ due to following order of hydration energy of these ions Li⁺ > Na⁺ > K⁺ > Rb⁺ and due to hydration of ion, mobility decreases.