Question

Match each of the following energy changes with one of the processes given: ionization energy, electron affinity, bond enthalpy, and standard enthalpy of formation. (a) \(\mathrm{F}(g)+e^{-} \longrightarrow \mathrm{F}^{-}(g)\) (b) \(\mathrm{F}_{2}(g) \longrightarrow 2 \mathrm{~F}(g)\) (c) \(\mathrm{Na}(g) \longrightarrow \mathrm{Na}^{+}(g)+e^{-}\) (d) \(\mathrm{Na}(s)+\frac{1}{2} \mathrm{~F}_{2}(g) \longrightarrow \mathrm{NaF}(s)\)

Short answer

(a) Electron affinity, (b) Bond enthalpy, (c) Ionization energy, (d) Standard enthalpy of formation.

Step-by-step solution

Identify Electron Affinity

Process (a) is \( \mathrm{F}(g) + e^{-} \rightarrow \mathrm{F}^{-}(g) \). Electron affinity refers to the energy change when an electron is added to a gaseous atom, forming an anion. Hence, energy change for process (a) is electron affinity.

Identify Bond Enthalpy

Process (b) is \( \mathrm{F}_{2}(g) \rightarrow 2 \mathrm{~F}(g) \). Bond enthalpy is the energy required to break one mole of a bond in a molecule in the gas phase, resulting in atoms from a diatomic molecule. Hence, energy change for process (b) is bond enthalpy.

Identify Ionization Energy

Process (c) is \( \mathrm{Na}(g) \rightarrow \mathrm{Na}^{+}(g)+e^{-} \). Ionization energy refers to the energy required to remove an electron from a gaseous atom, forming a cation. Hence, energy change for process (c) is ionization energy.

Identify Standard Enthalpy of Formation

Process (d) is \( \mathrm{Na}(s)+\frac{1}{2} \mathrm{~F}_{2}(g) \rightarrow \mathrm{NaF}(s) \). The standard enthalpy of formation is the energy change that occurs when one mole of a compound is formed from its elements in their standard states. Hence, energy change for process (d) is the standard enthalpy of formation.

Key concepts

Ionization Energy

Ionization energy is a fundamental concept in thermochemistry that involves the amount of energy necessary to remove an electron from a gaseous atom or ion. It is concerned with transforming a neutral atom into a positively charged ion, known as a cation.
For instance, in process (c), \( \mathrm{Na}(g) \rightarrow \mathrm{Na}^{+}(g)+e^{-} \), the energy required to remove an electron from sodium in its gaseous state represents the ionization energy. Several factors influence ionization energy, including:

• Atomic size: Larger atoms generally have lower ionization energy because the outer electrons are further from the nucleus and less tightly held.
• Nuclear charge: A higher positive charge in the nucleus increases ionization energy, making it more difficult to remove an electron.
• Electron shielding: Inner electrons can shield outer electrons from the nucleus's pull, affecting the ionization energy.

Understanding ionization energy helps in predicting how elements react and form bonds.

Electron Affinity

Electron affinity measures the change in energy that occurs when an electron is added to a neutral atom in the gaseous state, forming a negative ion, also known as an anion.
In the context of process (a), when fluorine accepts an electron to become \( \mathrm{F}^{-}(g) \), energy is released or absorbed. This concept is crucial for understanding how atoms gain electrons and how they might interact with other elements.

• Sign: Unlike ionization energy, which is always positive, electron affinity can be either negative or positive based on whether energy is released (exothermic) or absorbed (endothermic).
• Factors: Exceptional cases, like elements with full electron shells, may have different electron affinities due to stability concerns.

Understanding electron affinity is vital for predicting the formation of anions and chemical bonding behaviors.

Bond Enthalpy

Bond enthalpy, often referred to as bond energy, is a measure of the strength of a chemical bond. It denotes the energy required to break one mole of bonds in a molecule, giving separate atoms in the gas phase.Consider process (b), which involves breaking the bond in \( \mathrm{F}_{2}(g) \rightarrow 2 \mathrm{~F}(g) \), illustrating the energy transformation involved when the bond between two fluorine atoms is broken.
Higher bond enthalpy values indicate stronger bonds.Key points about bond enthalpy include:

• Average Values: Bond enthalpies are often averaged over similar compounds because the bond energy can change based on the molecular environment.
• Predicting Reaction Heat: Sum of bond enthalpies of reactants and products helps predict whether a reaction is exothermic or endothermic.

Analyzing bond enthalpy provides insight into the stability and reactivity of molecules in chemical reactions.

Standard Enthalpy of Formation

Standard enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states. The concept is crucial for understanding the energetic changes associated with forming compounds from basic elements.
Taking process (d) as an example,\( \mathrm{Na}(s)+\frac{1}{2} \mathrm{~F}_{2}(g) \rightarrow \mathrm{NaF}(s) \), we observe the specific energy changes involved in forming sodium fluoride from sodium and fluorine gas in their most stable forms.Insights into standard enthalpy of formation include:

• Standard Conditions: Measurements taken under specific conditions (usually 298 K and 1 atm) ensure consistency and comparability.
• Significance: Provides a basis for calculating the enthalpy changes of broader chemical reactions using Hess's law.
• Thermodynamic Reference: Often used as a reference to calculate other reaction enthalpies.

Understanding this concept is essential for comprehensively predicting the energy transformations within chemical reactions.