Question

For sulfur, write an equation for the change associated with (a) its first electron affinity and (b) its second electron affinity. How should they compare?

Short answer

First electron affinity: \(S(g) + e^- \rightarrow S^{-}(g)\), usually exothermic. Second electron affinity: \(S^-(g) + e^- \rightarrow S^{2-}(g)\), generally endothermic. The first is typically less energy-intensive compared to the second.

Step-by-step solution

Writing the Equation for the First Electron Affinity

The first electron affinity refers to the energy change when an electron is added to a neutral atom in the gaseous state to form a negative ion. For sulfur (S), the equation can be represented as: \(S(g) + e^- \rightarrow S^{-}(g)\). This process is typically exothermic for nonmetals.

Writing the Equation for the Second Electron Affinity

The second electron affinity is the energy change when an electron is added to a negative ion to form a doubly charged negative ion. For sulfur, the equation is: \(S^-(g) + e^- \rightarrow S^{2-}(g)\). This process requires energy input, hence it is endothermic.

Comparing the First and Second Electron Affinities

First electron affinities are typically exothermic because adding one electron to a neutral atom results in a configuration closer to noble gas stability. Second electron affinities are endothermic because adding an electron to an already negative ion means the electron is entering a repulsive environment.

Key concepts

First Electron Affinity

First electron affinity is an important concept in chemistry, especially when discussing the reactivity of elements. It denotes the amount of energy released when an electron is added to a neutral atom to form a negatively charged ion. This process involves an atom in its gaseous state, making it easier for an electron to join its electron cloud.

For instance, sulfur's first electron affinity is illustrated by the process: \( S(g) + e^- \rightarrow S^{-}(g) \). This reaction is exothermic, meaning it releases energy—this occurs because the electron is attracted to the nucleus, releasing energy as it 'falls' into the electron cloud. Nonmetals like sulfur often have higher first electron affinities as their atoms are eager to gain electrons to achieve a configuration similar to noble gases, which are highly stable.

The exothermic nature of first electron affinities in nonmetals can be quite significant, leading to a variety of chemical reactions and compounds formed as a result of this electron gain.

Second Electron Affinity

When extending our understanding to the second electron affinity, the concept takes a different turn. This time it relates to the energy change when a second electron is added to an already negatively charged ion. For sulfur, the process can be expressed as: \( S^-(g) + e^- \rightarrow S^{2-}(g) \). Unlike the first electron affinity, the second electron affinity is an endothermic process.

This difference arises because the second electron is entering an environment that already holds a negative charge due to the first electron. The repulsion between the negatively charged ion and the incoming electron means energy must be inputted into the system to ‘force’ the electron in. This explains why the adding of a second electron, in general, needs more energy and thus, the process ends up consuming energy rather than releasing it.

Exothermic Process

An exothermic process is a fundamental chemical occurrence in which energy is released into the surroundings. This can come in the form of heat, light, or sound, but it typically involves the release of heat. In the context of electron affinities, the addition of the first electron to a neutral atom is a classic example of an exothermic process.

Energy is released during this process because the electron experiences a force of attraction to the positively charged nucleus. As the electron affixes to the atom, the energy this attraction releases is dissipated into the environment. Exothermic reactions are essential for a variety of applications, ranging from biological processes to industrial reactions, as they are often used to generate heat and other forms of energy.

Endothermic Process

Conversely, an endothermic process requires energy to be absorbed from the surroundings for the reaction to occur. This is what takes place during a second electron affinity reaction. Given the negative charge of the ion, incoming electrons are naturally repelled, necessitating external energy to integrate the electron into the ion's electron cloud.

Endothermic reactions are less intuitive than their exothermic counterparts because they involve the absorption rather than the release of energy. Examples of endothermic processes, beyond the addition of a second electron, include the melting of ice into water or the conversion of carbonate compounds into oxides and carbon dioxide upon heating. These are important for understanding chemical thermodynamics, which explores the energy exchanges in chemical reactions.