Ionisation Enthalpy - Practice Questions & MCQ

Ionisation Enthalpy

Ionisation enthalpy may be defined as the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom to convert it into gaseous monovalent positive ion. 

IE1 is ionisation enthalpy or also known as first ionisation enthalpy.

Ionisation Potential

Ionisation enthalpy is also known as ionisation potential. It is the minimum potential difference required to remove the outermost electron from a gaseous atom to form a cation.

Factors Affecting Ionisation Enthalpy

The ionisation enthalpy of any atom is affected by the following factors.

• Size of the atom: The larger the size of an atom, the lower is the ionisation enthalpy. As the atomic size increases, the distance between the outermost electrons and the nucleus increases due to which the force of attraction between the nucleus and these outermost electrons decreases, thus it becomes easy to remove an electron from the atom and hence the ionisation enthalpy decreases. Thus, 

Ionisation enthalpy ∝ 1/Atomic size
 

• Screening effect: The higher is the value of screening effect, the lower is the ionisation enthalpy. As the screening effect increases, the repulsion between the electrons increases and thus the removal of electron from the atom becomes easier. Thus, 

Ionisation enthalpy ∝ 1/Screening effect
 

• Nuclear charge: As the nuclear charge increases, the force of attraction between the nucleus and electrons also increases and thus the removal of electron from the atom becomes difficult and hence the ionisation enthalpy increases. Thus,

Ionisation enthalpy ∝ Nuclear charge
 

• Half filled and fully filled orbitals: The atoms with half filled and fully filled orbitals are more stable than other atoms. Thus removing an electron from these atoms requires a little more energy. Thus for these atoms with half filled and fully filled orbitals, the ionisation enthalpy is higher than others.
   

• Shape of orbital: The ionisation enthalpy also depends on the shape of orbital in which the last electron enters. The more the orbital is close to the nucleus, the more energy is required to remove the electron in the same orbit. Thus, the ionisation enthalpy for the orbitals from the same orbit follows the given order:

s > p > d > f

Variation of Ionisation Enthalpy

• In moving down the group, the ionisation enthalpy decreases. As we move down in the group, the number of shells increases due to which the force of attraction between the nucleus and the outer electrons decreases, thus removing an electron from the atom becomes easy and hence ionisation enthalpy decreases. There are some exceptions after the element with atomic number 72. The elements with atomic number from 73 to 82 have higher ionisation enthalpy than the earlier elements in their respective group. This deviation of behaviour is because of the lanthanide contraction.

• In moving from left to right in a period, the ionisation enthalpy increases. In period, the nuclear charge increases but the number of shells remain the same, thus the force of attraction between the nucleus and the outer electrons increases and hence the ionisation enthalpy increases. In a period, some elements like Be, Mg, N and P have exceptionally higher ionisation enthalpies than expected. This is because of their half filled or fully filled outer orbitals. 

Importance of Ionisation Enthalpy

Ionisation enthalpy is an important factor for determining the nature of an element. The elements with low ionisation enthalpies are metals while the elements with higher ionisation enthalpies are non-metals. 

The stability of oxidation states of an element can also be determined on the basis of the value of ionisation enthalpies.

(i) When the difference between two successive ionisation enthalpies for an atom is approximately 965 - 1450kJ/mol or less, then higher oxidation state will be more stable.

For example,  Li has IE₁ = 520kJ/mol and IE₂ = 7298.1kJ/mol.

Thus, IE₂ - IE₁ = (7298.1 - 520.3)
= 6777.8kJ/mol

Therefore, Li⁺ or first oxidation state of Li is more stable.

(ii) When the difference between two successive ionisation enthalpies for an atom is higher than 1450kJ/mol, then lower oxidation state will be more stable.

For example, Ca has IE₁ = 589.8kJ/mol and IE₂ = 1145.4kJ/mol.

Thus,  IE₂ - IE₁ = (1145.4 - 589.8)

= 555.6kJ/mol

Therefore, Ca²⁺ or second oxidation state of Ca is more stable.

• Comparison of IE₁ and IE₂ of oxygen and nitrogen

Oxygen has electronic configuration as 1s²2s²2p⁴. After IE¹, its electronic configuration becomes 1s²2s²2p³. Now nitrogen has electronic configuration as 1s²2s²2p³. After IE₁, its electronic configuration becomes 1s²2s²2p².

Thus in case of oxygen, after IE₁, O⁺ has achieved the stable half filled electronic configuration and hence more energy is required in IE₂ to remove an electron further. Similarly, nitrogen already has a stable half filled electronic configuration, thus more energy is required for IE₁ to remove the first electron.

Therefore, the order of different ionisation enthalpies is followed as mentioned below: 

(i) N(IE₁) > O(IE₁)            (ii) O(IE₂) > N(IE₂)

• Comparison of IE₁ and IE₂ of chromium and manganese

Chromium has electronic configuration as [Ar]3d⁵4s¹. After IE₁, its electronic configuration becomes [Ar]3d⁵. Now manganese has electronic configuration as [Ar]3d⁵4s². After IE₁, its electronic configuration becomes [Ar]3d⁵4s¹. Thus in case of chromium, after IE₁, Cr⁺ has achieved the stable half filled electronic configuration and hence more energy is required in IE₂ to remove an electron further. Similarly, manganese already has a stable half filled d-orbitals and fully filled 4s orbitals, thus more energy is required for IE₁ to remove the first electron.

Therefore, the order of different ionisation enthalpies is followed as mentioned below: 

(i) Mn(IE₁) > Cr(IE₁)            (ii) Cr(IE₂) > Mn(IE₂)

• Comparison of different ionisation enthalpies of N and N⁺

Nitrogen has electronic configuration as 1s²2s²2p³. After IE₁, nitrogen becomes N⁺ and has the electronic configuration as 1s²2s²2p². Everytime some amount of energy has to supply to remove the electron. But the nuclear charge remains the same, thus removing the second and third electron from the atom becomes very difficult. Thus for any atom, multiple ionisation enthalpies follow the order given below:

IE₃ > IE₂ > IE₁

• Group exception

In moving from top to bottom in a group, the ionisation enthalpy decreases but there are some exceptions as mentioned below.

(i) In group 13, the expected ionisation enthalpy is as follows:

B > Al > Ga > In > Tl

But thallium has inner f-orbitals due to which it has the lanthanoid contraction and thus its size reduces and ionisation enthalpy increases. Thus the real order of ionisation enthalpy is:

B > Tl > Ga > Al > In

(ii) In group 14, the expected ionisation enthalpy is as follows:

C > Si > Ge > Sn > Pb

But lead(Pb) has inner f-orbitals due to which it has the lanthanoid contraction and thus its size reduces and ionisation enthalpy increases. Thus the real order of ionisation enthalpy is:

C > Si > Ge > Pb > Sn