The correct option is **B. Ionization enthalpy**. ### Detailed Explanation: 1. **Definition of Ionization Enthalpy**: - Ionization enthalpy (or ionization energy) is defined as the amount of energy required to remove an electron from a gaseous atom or ion in its ground state. This process can be represented by the following equation: \[ \text{X(g)} \rightarrow \text{X}^+(g) + e^- \] - Here, X represents the atom, and the process involves converting a neutral atom (X) into a positively charged ion (X⁺) by removing one electron (e⁻). 2. **Why Option B is Correct**: - The question specifically asks for the enthalpy change associated with the removal of an electron from a gas-phase atom or ion. This is precisely what ionization enthalpy measures. It quantifies the energy needed to overcome the attraction between the negatively charged electron and the positively charged nucleus of the atom. 3. **Analysis of Other Options**: - **A. Electron gain enthalpy**: - This term refers to the energy change that occurs when an electron is added to a neutral atom in the gas phase. It is the opposite of ionization enthalpy. The process can be represented as: \[ \text{X(g)} + e^- \rightarrow \text{X}^-(g) \] - Since the question is about removing an electron, this option is incorrect. - **C. Enthalpy change**: - This is a very general term that refers to any change in enthalpy, which could involve various processes (not just the removal of electrons). It does not specifically define the process of removing an electron from an atom. Therefore, it is too vague and not the correct answer. - **D. Enthalpic gas removal**: - This term is not standard in chemistry and does not specifically refer to the process of removing an electron from an atom. It could imply a variety of processes involving gases but lacks the specificity needed to describe ionization. Thus, it is not a correct option. ### Summary of Key Points: - **Ionization enthalpy** is the energy required to remove an electron from a gaseous atom or ion. - It is a specific term that quantifies the energy change during the ionization process. - Other options either refer to different processes (like electron gain enthalpy) or are too vague (like enthalpy change and enthalpic gas removal). ### Revision Summary: - Ionization enthalpy measures the energy needed to remove an electron from a gaseous atom. - It is represented by the equation: \(\text{X(g)} \rightarrow \text{X}^+(g) + e^-\). - Electron gain enthalpy refers to adding an electron, not removing one. - General terms like "enthalpy change" do not specify the process and are not suitable answers.