122.
What are successive ionisation enthalpies? Explain why the second ionisation enthalpy is higher than the first ionisation enthalpy?
Successive ionisation enthalpies : The enthalpy required to remove the first loosely bound electron from gaseous isolated atom is called the first ionisation enthalpy (IE1). The enthalpies required to remove second, third and fourth electrons are called second (IE2), third (IE3) or fourth (IE4) ionisation enthalpies respectively. The amounts of enthalpies required to remove first, second and subsequent electrons from the gaseous atom one after the other are collectively called successive ionisation enthalpies.
Theoretically speaking, there are as many ionisation enthalpies for an atom as there are electrons in it. In the above example
IE
3 > IE
2 > IE
1.
Second ionisation enthalpy is always greater than the first ionisation enthalpy. After the removal of first electron, the atom changes into monovalent positive ion (M
+). In the ion (M
+), the number of electrons decreases but the nuclear charge remains the same as the parent atom. As a result the attraction of the nuclear charge (protons) increases over the remaining electrons. Hence, more enthalpy is required to remove the second electron. In other words, the second ionisation enthalpy is greater than the first ionisation enthalpy (IE
2 > IE
1).
Similarly, the removal of second electron results in the formation of divalent positive ion (M
++) and the attraction between the nucleus and remaining electrons increases further. This accounts for the progressive increase in the value of ionisation enthalpies. For example, for aluminium,
IE, = 577.4 kJ mol
-1, IE
2 = 1816 kJ mol
-1 and IE
3 = 2744 kJ mol
-1
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