Ionisation energy and electron affinity are defined as what?

Answer: The correct answer is (C) Enthalpy. Ionisation energy (IE) and electron affinity (EA) are both defined in terms of enthalpy. Enthalpy means the heat energy change that happens during a chemical reaction at constant pressure. When we remove an electron from a gaseous atom, energy is absorbed, and this is called ionisation energy. When an atom in the gaseous state gains an electron, energy is usually released, and that is called electron affinity. In both cases, energy is either taken in or given out, which is measured as enthalpy change.

Complete Answer:

Ionisation energy (IE) and electron affinity (EA) are both defined in terms of enthalpy (ΔH). Enthalpy refers to the heat energy change that occurs during a chemical process at constant pressure. When an electron is removed from a gaseous atom, energy is absorbed, and this process is called ionisation energy. For example:

X(g)→X⁺(g)+e^–  ΔH=+IE

Here, a neutral atom X(g) loses one electron to form a positive ion X⁺(g), and the energy absorbed is the ionisation energy.

On the other hand, when a gaseous atom gains an electron, energy is usually released, and this process is called electron affinity. For example:

X(g)+e^–→X^–(g)  ΔH=−EA

In this case, a neutral atom X(g) gains an electron to form a negative ion X^–(g), and the energy released is the electron affinity. In both cases, the energy absorbed or released is measured as a change in enthalpy (ΔH). That’s why ionisation energy and electron affinity are both types of enthalpy changes, making (C) Enthalpy the correct answer.

Let’s understand why other options are incorrect:

(A) Equilibrium: Equilibrium is a state where the rate of the forward reaction equals the rate of the backward reaction in a chemical system. It refers to the balance in a reaction, not to the energy involved. Ionisation energy and electron affinity deal with the energy change when electrons are removed or added, not with the balance of chemical reactions. So, equilibrium does not define them and is not the correct answer.

(B) Spontaneity: Spontaneity refers to whether a reaction happens on its own without needing external energy. It depends on Gibbs free energy (ΔG), which considers both enthalpy (ΔH) and entropy (ΔS). While IE and EA involve enthalpy, they do not directly determine whether the reaction is spontaneous. A reaction can have high ionisation energy or low electron affinity and still be spontaneous or non-spontaneous depending on other factors. So, spontaneity is not the defining concept here.

(D) None: This option means that none of the choices are correct, but that is not true. As clearly explained, both ionisation energy and electron affinity involve enthalpy changes because they measure how much heat energy is absorbed or released when an electron is removed or added to a gaseous atom. Hence, (C) is definitely correct, and selecting “None” would be incorrect.

So, the correct and most suitable answer is (C) Enthalpy, because both ionisation energy (IE) and electron affinity (EA) are energy changes that are measured in terms of enthalpy (ΔH). These processes involve adding or removing electrons, with heat energy either absorbed or released, making enthalpy the key concept that defines them.

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