Question

Born-Haber cycle is used to determine: (a) Electron affinity (b) Lattice energy (c) Crystal energy (d) All the above

Short answer

(b) Lattice energy

Step-by-step solution

Understanding Born-Haber Cycle

The Born-Haber cycle is a thermochemical cycle that relates the lattice energy of an ionic solid to other energies involved in its formation. The cycle is primarily used to calculate the lattice energy of ionic compounds, which is the energy required to form a solid ionic compound from gaseous ions.

Components of Born-Haber Cycle

The Born-Haber cycle includes several components: ionization energy, electron affinity, sublimation energy, bond dissociation energy, and lattice energy. It uses Hess's law to determine the enthalpy change for the formation of the compound from its elements.

Focus on Lattice Energy

Given the components involved in the Born-Haber cycle, the key purpose is to determine the lattice energy. Lattice energy is crucial because it directly relates to the stability of that ionic solid.

Answering the Question

While the Born-Haber cycle involves ionization energies and electron affinities, its main purpose is to calculate the lattice energy. The initial confusion might arise due to the inclusion of electron affinity, but the cycle is built primarily for lattice energy determination.

Key concepts

Lattice Energy

Lattice energy is a key concept in understanding the stability of ionic compounds. It refers to the energy released when gaseous ions come together to form a solid ionic lattice.
It is a measure of the strength of the bonds in an ionic solid.

Think of lattice energy as the glue that holds the ions in the crystal structure together. The more negative the lattice energy, the stronger the attraction between the ions, leading to a more stable compound. This energy impacts properties such as melting point, solubility, and hardness.

• High lattice energy: Greater bond strength, higher melting point.
• Low lattice energy: Weaker bond strength, lower melting point.

The Born-Haber cycle is often used to calculate the lattice energy because it can't be measured directly. This process considers various energy changes and uses them to determine how tightly the ions are packed in the crystal lattice.

Thermochemical Cycle

A thermochemical cycle is a sequence of chemical reactions that represent the formation of a compound.
These cycles help in calculating unknown thermochemical data by breaking down processes into smaller, manageable steps.

In the context of the Born-Haber cycle, a thermochemical cycle includes several key energy changes related to the formation of an ionic compound. The steps involve converting elements to gaseous ions and then forming the ionic solid. It is a powerful tool to understand formation enthalpies and how various energies interact during chemical formation processes.

These cycles use multiple energy terms:

• Ionization energy: Energy to remove an electron from an atom.
• Electron affinity: Energy change when an electron is added to an atom.
• Sublimation energy: Energy required to convert a solid element to gas.

Using a thermochemical cycle like the Born-Haber cycle allows us to apply Hess's Law in a systematic way, leading to a deeper understanding of the thermodynamics of compound formation.

Hess's Law

Hess's Law states that the total enthalpy change of a chemical reaction is the same, regardless of the route by which the chemical reaction takes place.
It's a fundamental principle used extensively in thermochemistry and in calculating lattice energy within the Born-Haber cycle.

This law hinges on the concept that enthalpy is a state function, meaning it depends only on the initial and final states of a system, not on the path taken. Therefore, whether you reach a final product through one step or multiple intermediate steps, the total enthalpy change remains constant.

By utilizing Hess's Law in the Born-Haber cycle:

• We can break down complex reactions into simpler steps.
• Calculate energies like lattice energy using known values of other thermochemical changes.
• Build on the principle that energy conservation holds true irrespective of the complexity of the reaction pathway.

Hess's Law makes it possible to tackle complex problems by breaking them into manageable calculations, making the calculation of lattice energies accurate and reliable.

Ionic Compounds

Ionic compounds are chemical compounds composed of ions held together by electrostatic forces of attraction, known as ionic bonds.
These compounds usually form when metals react with nonmetals, often resulting in the transfer of electrons from one element to another.

The formation of ionic compounds involves the formation of a stable electron configuration. Metals lose electrons to become positively charged cations, while nonmetals gain electrons to become negatively charged anions.

Key characteristics include:

• High melting and boiling points due to strong ionic bonds.
• Typically solid at room temperature.
• Conduct electricity when dissolved in water or molten.

The lattice energy plays an essential role in these compounds, determining their stability and physical properties. It correlates with how tightly the ions are packed, influencing the strength of the ionic bonds and the energy required to break the lattice apart.

Electron Affinity

Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom in the gaseous state.
It indicates how much an atom wants to gain an electron.

While not the primary focus of the Born-Haber cycle, electron affinity is still an important component. It provides insight into the energetics of forming an anion from a neutral atom, which is a part of forming ionic compounds.

Understanding electron affinity can help in:

• Predicting the likelihood of an atom to accept an electron.
• Comparing the reactivity and chemical behavior of different elements.
• Visualizing why certain elements, like halogens, form ions easily.

Negative electron affinities imply an exothermic process, where energy is released when an electron is added, resulting in more stable anions.