Question

In the chemical process called electron transfer, an electron is transferred from one atom or molecule to another (We will talk about electron transfer extensively in Chapter 20.) A simple electron transfer reaction is $$ \mathrm{A}(g)+\mathrm{A}(g) \longrightarrow \mathrm{A}^{+}(g)+\mathrm{A}^{-}(g) $$ In terms of the ionization energy and electron affinity of atom A, what is the energy change for this reaction? For a representative nonmetal such as chlorine, is this process exothermic? For a representative metal such as sodium, is this process exothermic? [Sections \(7.4\) and \(7.51\)

Short answer

The energy change for the given electron transfer reaction can be represented as Energy Change = Ionization Energy (IE) - Electron Affinity (EA). For a representative nonmetal like chlorine, the energy change is +902.2 kJ/mol, making the process endothermic. Similarly, for a representative metal like sodium, the energy change is +443 kJ/mol, which is also endothermic. Therefore, this process is not exothermic for either chlorine or sodium.

Step-by-step solution

Understand the given reaction

The given electron transfer reaction: \( \mathrm{A}(g)+\mathrm{A}(g) \longrightarrow \mathrm{A}^{+}(g)+\mathrm{A}^{-}(g) . \) Here, two neutral atoms of A interact and transfer one electron, with one atom losing an electron (A+) and the other gaining an electron (A-).

Determine energy change in terms of ionization energy and electron affinity

When the first atom of A loses an electron to form A+ ion, the energy change involved is equal to its Ionization Energy (IE). For the second atom of A, when it gains an electron to form A- ion, the energy change involved is equal to its Electron Affinity (EA). So, the total energy change for the reaction can be written as: Energy Change = Ionization Energy (IE) - Electron Affinity (EA)

Determine if the process is exothermic for a representative nonmetal (Chlorine)

For a representative nonmetal, such as chlorine, we need to compare the Ionization Energy to the Electron Affinity. For chlorine, the Ionization Energy is 1251.2 kJ/mol, while the Electron Affinity is 349 kJ/mol. Comparing these values, we find: Energy Change = IE - EA = 1251.2 kJ/mol - 349 kJ/mol = 902.2 kJ/mol Since the energy change is positive (+902.2 kJ/mol), the process is endothermic, not exothermic, for chlorine.

Determine if the process is exothermic for a representative metal (Sodium)

For a representative metal, such as sodium, we again need to compare the Ionization Energy to the Electron Affinity. For sodium, the Ionization Energy is 495.8 kJ/mol, while the Electron Affinity is 52.8 kJ/mol. Comparing these values, we find: Energy Change = IE - EA = 495.8 kJ/mol - 52.8 kJ/mol = 443 kJ/mol Since the energy change is positive (+443 kJ/mol), the process is endothermic, not exothermic, for sodium as well.

Key concepts

Ionization Energy

Understanding ionization energy is critical when exploring electron transfer reactions. Ionization energy, often abbreviated as IE, refers to the amount of energy needed to remove an electron from an isolated atom or molecule in its gaseous state. It's a fundamental concept in chemistry as it provides insight into the reactivity and chemical bonding of an element.

For example, noble gases have high ionization energies because their electron shells are full, making them stable and less likely to lose an electron. In contrast, alkali metals have lower ionization energies, which indicates their readiness to give up electrons and form positive ions, a trait that defines their chemical behavior.

Ionization energy is usually measured in electron volts (eV) or kilojoules per mole (kJ/mol). Each subsequent electron removed from an atom requires more energy, hence the concept of successive ionization energies. This increase is due to the reduced electron shielding and increased nuclear charge experienced by the remaining electrons.

Electron Affinity

Electron affinity (EA) complements ionization energy in the study of chemical reactions. It signifies the amount of energy released when a neutral atom or molecule in a gas phase gains an electron to form a negative ion. While ionization energy gives us a view of the energy landscape for losing an electron, electron affinity maps out the energy contours for gaining one.

Typically, nonmetals have higher electron affinity because they are close to completing their electron shell and thus, release more energy when they gain an electron. Whereas metals, which have electrons to spare, tend to have a lower electron affinity. This property varies greatly among the elements and it even can be endothermic for certain atoms, meaning that energy must be supplied for the electron addition to occur.

Exothermic and Endothermic Processes

Exothermic and endothermic processes are two fundamental concepts used to describe the energy changes that occur during chemical reactions. An exothermic process releases energy to the surroundings, typically in the form of heat, leading to a rise in temperature around the system. The energy could manifest in other forms, such as light or electricity, depending on the reaction.

An endothermic process, on the other hand, absorbs energy from the surroundings, which causes a decrease in temperature around the system. In terms of reactions, for an exothermic reaction, the products are at a lower energy state than the reactants, hence the release of energy. For endothermic reactions, the opposite is true, whereby the products are at a higher energy state than the reactants and energy is absorbed to make up for the difference.

In the context of electron transfer reactions, whether the process is exothermic or endothermic depends on the balance between the ionization energy and the electron affinity. If the ionization energy exceeds the electron affinity, the net energy change is positive, and the process is endothermic. Conversely, if the electron affinity surpasses the ionization energy, the net energy change is negative, and the process is exothermic.