Question

The first $({{\Delta }_{i}}{{H}_{1}})$ and the second $({{\Delta }_{i}}{{H}_{2}})$ ionization enthalpies $(in\,kJ\,mo{{l}^{-1}})$ and the $({{\Delta}_{eg}}H)$ electron gain enthalpy $(in\,kJ\,mo{{l}^{-1}})$ of a few elements are given below:
Elements $\Delta {{H}_{1}}\Delta {{H}_{2}}{{\Delta }_{eg}}H$
I 520, 7300, -60
II 419, 3051, -48
III 168, 13374, -328
IV 1008, 1846, -295
V 2372, 5251, +48
VI 738, 1451, -40

Which of the above elements is likely to be :
(a) the least reactive element.
(b) the most reactive metal.
(c) the most reactive non-metal.
(d) the least reactive non-metal.
(e) the metal which can form a stable binary halide of the formula $M{{X}_{2}}$ (X=halogen)
(f) the metal which can form a predominantly stable covalent halide of the formula MX (X=halogen)?

Answer 1

The element to be reactive it should have highest first ionization enthalpy and positive electron gain enthalpy and vice versa. Most reactive non- metals have high first ionization enthalpy and highest negative electron gain enthalpy and vice versa. An element to be metal should have negative electron gain enthalpy.

Complete step by step answer:
From your chemistry lessons you have learned about the ionization enthalpy and the electron gain enthalpy and their reactivity with metals and non metals.

Ionization enthalpy$({{\Delta }_{i}}H)$ is the energy required to remove an electron from the outermost orbital and the electron gain enthalpy $({{\Delta }_{eg}}H)$ is defined as the energy released during the addition of electron in the outermost shell of an atom. Metals have the tendency to lose electrons whereas non-metals have the tendency to gain electrons.
Metals with low ionization enthalpy and negative electron gain enthalpy will be more reactive whereas non- metals with high ionization enthalpy and negative electron gain enthalpy will be more reactive.
The first ionization enthalpy $({{\Delta }_{i}}{{H}_{1}})$ is defined as the energy required to remove an electron from the neutral atom whereas second ionization enthalpy $({{\Delta }_{i}}{{H}_{2}})$ is the amount of energy remove an electron from a +1 ion. Elements in its second ionization enthalpy state are more reactive than the first ionization enthalpy.
Elements with negative electron gain enthalpy will be more reactive.
So, the element which has the highest first ionization enthalpy and positive electron gain enthalpy will be least reactive.
let us answer the question one by one:
(a) The least reactive element will be V because it has the highest first ionization enthalpy and positive electron gain enthalpy so the addition and removal of electrons from the outermost orbital will be tough.

(b) The most reactive metal will be element II because it has the lowest first ionization enthalpy and the low negative electron gain enthalpy so the removal of electrons from the outermost orbital will be easy and hence it will be reactive.

(c) The most reactive non-metal will be element III because it has high first ionization enthalpy and highest negative electron gain enthalpy. So, the addition of electrons in the outermost orbital will be easy.

(d) the least reactive non-metal will be element V because it has the highest first ionization enthalpy and positive electron gain enthalpy.

(e) The low negative electron gain enthalpy element is VI, so it is a metal. And the second ionization enthalpy of this element is also low. Therefore it can form a stable binary halide with the formula $M{{X}_{2}}$(X = halogen).

(f) To form a predominantly stable covalent halide of the formula MX (X = Halogen) element should have low first ionization enthalpy and high second ionization enthalpy. Thus the element I will be the answer.
So, the correct answer is “Option C”.

Note: Electron gain enthalpy is of two types positive electron gain enthalpy and negative electron gain enthalpy. If an atom have a positive tendency to gain an electron or accept the electron easily then its electron gain enthalpy is said to be negative electron gain enthalpy whereas if an atom have negative tendency or it should be forced to accept the electron then its electron gain enthalpy is said to be positive electron gain enthalpy for example all the noble gases.