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 $$\mathrm{A}(g)+\mathrm{A}(g)⟶{\mathrm{A}}^{+}(g)+{\mathrm{A}}^{-}(g)$$ 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 7.5$]$

Short answer

The energy change for the electron transfer process can be expressed as: $$\mathrm{\Delta }E=-\mathrm{IE}+\mathrm{EA}$$ For chlorine (nonmetal), the energy change is -902 kJ/mol, and for sodium (metal), it is -443 kJ/mol. Since the energy changes are negative, the electron transfer process is exothermic for both chlorine and sodium.

Step-by-step solution

Write the reaction formula in terms of ionization energy and electron affinity

We can write the energy change for the given reaction as follows: $$\mathrm{\Delta }E=-\mathrm{IE}+\mathrm{EA}$$ Where: - ΔE = energy change for the reaction - IE = ionization energy of atom A - EA = electron affinity of atom A Expressing the energy change of the electron transfer in terms of ionization energy and electron affinity is crucial to determine if the process is exothermic or not.

Analyze the energy change for chlorine, a nonmetal

For chlorine (a nonmetal), we must find the values for ionization energy (IE) and electron affinity (EA) and apply the formula from Step 1 to calculate the energy change of the reaction. The ionization energy for chlorine is 1,251 kJ/mol, and its electron affinity is 349 kJ/mol. Plugging these values into the formula, we get: $$\mathrm{\Delta }{E}_{\mathrm{Cl}}=-1,251\text{kJ/mol}+349\text{kJ/mol}=-902\text{kJ/mol}$$ Since the energy change is negative, the electron transfer process between two chlorine atoms is exothermic.

Analyze the energy change for sodium, a metal

Similarly, for sodium (a metal), we must find the values for ionization energy (IE) and electron affinity (EA) and apply the formula from Step 1 to calculate the energy change of the reaction. The ionization energy for sodium is 496 kJ/mol, and its electron affinity is approximately 53 kJ/mol. Plugging these values into the formula, we get: $$\mathrm{\Delta }{E}_{\mathrm{Na}}=-496\text{kJ/mol}+53\text{kJ/mol}=-443\text{kJ/mol}$$ Again, since the energy change is negative, the electron transfer process between two sodium atoms is also exothermic. In conclusion, the energy change for the process of electron transfer can be expressed in terms of ionization energy and electron affinity as: $$\mathrm{\Delta }E=-\mathrm{IE}+\mathrm{EA}$$ The process is exothermic for both chlorine (nonmetal) and sodium (metal) as the energy changes are negative.

Key concepts

Ionization Energy

Ionization energy is the energy needed to remove an electron from a neutral atom in its gaseous state. It's an essential concept in understanding how atoms interact with each other.

• The higher the ionization energy, the more difficult it is to remove an electron from the atom. This means atoms with high ionization energy hold onto their electrons tightly.
• Ions are formed when the electron is successfully removed, and this process can influence reaction dynamics.

In the context of electron transfer, ionization energy plays a crucial role. It determines how much energy is required to start the electron transfer process. For instance, in our exercise, chlorine has a high ionization energy (1251 kJ/mol), indicating that it strongly resists losing an electron. On the other hand, sodium has a lower ionization energy (496 kJ/mol) making it easier to lose an electron.
The influence of ionization energy can significantly affect whether a reaction is endothermic or exothermic.

Electron Affinity

Electron affinity refers to the change in energy when an atom gains an electron. It indicates how strongly an atom can attract and hold onto an extra electron.

• Atoms with high electron affinity tend to accept electrons more readily. This often leads to the release of energy.
• Electron affinity is measured in terms of energy released, so it often has a negative value, showing that energy is released during the process.

With respect to the exercise provided, chlorine has a significant electron affinity (349 kJ/mol), meaning it easily attracts additional electrons, usually resulting in an exothermic release of energy. Conversely, sodium has a comparatively low electron affinity (53 kJ/mol), indicating a weaker attraction for gaining electrons. This difference in electron affinity between atom types helps predict reaction behavior and energy changes experienced during electron transfer.

Exothermic Reactions

Exothermic reactions are chemical reactions that release energy to the surroundings, often in the form of heat. These reactions are generally more favorable because they result in a lower energy state after completion.

• The sign of the energy change $\mathrm{\Delta }E$ is negative in exothermic reactions, indicating that energy is released.
• Exothermic reactions are commonplace in both natural processes (such as combustion) and industrial applications.

In our electron transfer example, both chlorine and sodium undergo exothermic reactions when transferring electrons. For both cases, the calculated $\mathrm{\Delta }E$ values were negative, with chlorine at -902 kJ/mol and sodium at -443 kJ/mol. This negative change confirms that both the processes are exothermic, effectively releasing energy. Identifying whether a reaction is exothermic can help predict the type of products formed and how substances will interact in various environments.