Problem 93 - Chemistry The Central Science

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 afnity 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?

Step-by-Step Solution

Answer

The energy change (ΔE) for an electron transfer reaction can be calculated using the formula: ΔE = Electron affinity (EA) - Ionization energy (IE). For chlorine, a representative nonmetal, ΔE = \(-1600.2 \,kJ/mol\), and for sodium, a representative metal, ΔE = \(-548.8 \,kJ/mol\). Since both energy changes are negative, the electron transfer process is exothermic for both chlorine and sodium, meaning it releases energy.

Step 1: Write the energy change formula for electron transfer

To calculate the energy change for this electron transfer reaction, we can use the following equation: Energy change (ΔE) = Electron affinity (EA) - Ionization energy (IE)

Step 2: Determine the Ionization Energy and Electron Affinity of Chlorine

For a representative nonmetal such as chlorine, we need to find the IE and EA values. The values from the literature are: IE (Chlorine) = \(1251.2 \,kJ/mol\) EA (Chlorine) = \(-349 \,kJ/mol\)

Step 3: Calculate the Energy Change for Chlorine

Now, we can calculate the energy change for the electron transfer reaction involving chlorine using the formula from Step 1: ΔE (Chlorine) = EA (Chlorine) - IE (Chlorine) ΔE (Chlorine) = \(-349 \,kJ/mol\) - \(1251.2 \,kJ/mol\) ΔE (Chlorine) = \(-1600.2 \,kJ/mol\)

Step 4: Determine if the Electron Transfer is Exothermic for Chlorine

Since the energy change (ΔE) for chlorine is negative, the process is exothermic for chlorine (releases energy).

Step 5: Determine the Ionization Energy and Electron Affinity of Sodium

For a representative metal such as sodium, we need to find the IE and EA values. The values from the literature are: IE (Sodium) = \(495.8 \,kJ/mol\) EA (Sodium) = \(-53 \,kJ/mol\)

Step 6: Calculate the Energy Change for Sodium

Now, we can calculate the energy change for the electron transfer reaction involving sodium using the formula from Step 1: ΔE (Sodium) = EA (Sodium) - IE (Sodium) ΔE (Sodium) = \(-53 \,kJ/mol\) - \(495.8 \,kJ/mol\) ΔE (Sodium) = \(-548.8 \,kJ/mol\)

Step 7: Determine if the Electron Transfer is Exothermic for Sodium

Since the energy change (ΔE) for sodium is negative, the process is exothermic for sodium (releases energy). In conclusion, the electron transfer process is exothermic for both the nonmetal chlorine and the metal sodium.

Key Concepts

Ionization Energy

Ionization energy is a fundamental concept in chemistry, particularly when studying electron transfer. It refers to the amount of energy required to remove an electron from an atom or molecule in the gaseous state.

Consider it as the cost needed to "pluck off" an electron from a neutral atom, turning it into a positively charged ion (or cation). This process is crucial because it provides insight into how reactive an element might be.

• High ionization energy means the element holds its electrons tightly.
• Low ionization energy indicates electrons are more easily removed, making the element more reactive in certain scenarios.

Ionization energy varies across the periodic table, generally increasing across a period from left to right. This trend corresponds to the increasing positive nuclear charge attracting the outermost electrons more strongly.

For example, a non-metal like chlorine has a high ionization energy of approximately 1251.2 kJ/mol, reflecting its tendency to hold onto electrons strongly. On the other hand, metals such as sodium exhibit lower ionization energy, around 495.8 kJ/mol, making them more eager to lose electrons and participate in chemical reactions involving electron transfer.

Electron Affinity

Electron affinity, closely related to ionization energy, measures how an atom or molecule will attract an added electron. Essentially, it tells us how "fond" an atom is of electrons and its ability to become a negatively charged ion (anion).

When an electron is added to an atom, energy is either absorbed or released. If the energy change is negative, energy is released, making the process exothermic. This often happens with nonmetals, which tend to have more negative electron affinities.

• Chlorine, a nonmetal, has a high affinity with a value of -349 kJ/mol.
• Its stronger attraction to extra electrons reflects its natural tendency to gain an electron in reactions.

Conversely, the electron affinity for metals like sodium may have negative but much less exothermic values. Sodium, with an electron affinity of about -53 kJ/mol, is less inclined to accept electrons compared to nonmetals.

This property is crucial in predicting how an atom will behave during electron transfer scenarios, guiding our understanding of chemical bonding and reactivity.

Chemical Reactions

Chemical reactions encompass processes where substances are transformed into new entities. In electron transfer reactions, electrons move between atoms, leading to formations of new chemical identities.

An electron transfer can be summarized by the formula: \[\mathrm{A}(g)+\mathrm{A}(g) \longrightarrow \mathrm{A}^{+}(g)+\mathrm{A}^{-}(g)\] This reaction shows how two atoms can exchange electrons, resulting in ion formation—a classic transformation in many chemical processes.

• Exothermic reactions, like with chlorine and sodium where energy is released, can drive these transformations.
• Endothermic reactions require energy input, occurring less spontaneously.

Understanding how energy changes, through ionization energy and electron affinity, impact a chemical reaction's exothermic or endothermic nature is vital.

By examining the energy change formula:
\[\Delta E = \text{Electron Affinity (EA)} - \text{Ionization Energy (IE)}\]You can predict whether a reaction is exothermic (when \(\Delta E\) is negative) or endothermic (when \(\Delta E\) is positive). This helps chemists forecast reaction products and potential applications in technology and energy solutions.