23.1 Lattice Energy and Born-Haber Cycles
It is the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. It represents the energy required to break the bonds within one mole of gaseous atoms.
It is the enthalpy change when one mole of an ionic compound is formed from its constituent ions in the gas phase.
First electron affinity, EA:
It is the enthalpy change when one mole of gaseous atoms accepts an electron to form one mole of gaseous ions with a single negative charge.
(b) Factors affecting electron affinities: The factors influencing electron affinities include:
Atomic size: Smaller atoms have higher electron affinities because the incoming electron is closer to the nucleus, resulting in a stronger attractive force.
Nuclear charge: Higher nuclear charge increases the electron affinity since the attractive force between the electron and the nucleus is stronger.
Electron shielding: Increased electron shielding reduces the effective nuclear charge experienced by the incoming electron, resulting in a lower electron affinity.
(c) Trends in electron affinities:
Group 16 elements (Chalcogens): The electron affinities generally become more negative (increasing) as you move down the group. This is because the atomic radius increases, and the effective nuclear charge experienced by the incoming electron decreases.
Group 17 elements (Halogens): The electron affinities become more negative (increasing) as you move up the group. This is because the atomic radius decreases, and the effective nuclear charge experienced by the incoming electron increases.
Born-Haber cycles for ionic solids: Born-Haber cycles are energy cycles that show the various steps involved in the formation of an ionic solid from its constituent elements. The steps typically include atomization, ionization, electron affinity, lattice energy, and formation of the solid.
Calculations involving Born-Haber cycles: Born-Haber cycles can be used to calculate various energy changes involved in the formation of ionic solids. By summing up the energy changes along a cycle, you can determine unknown values such as lattice energy or enthalpy change of formation.
Effect of ionic charge and radius on lattice energy:
Ionic charge: Higher charges on ions lead to stronger electrostatic attractions, resulting in higher lattice energies.
Ionic radius: Smaller ionic radii allow ions to come closer together, increasing the strength of the electrostatic attractions and thus increasing the lattice energy.
23.2 Enthalpies of Solution and Hydration
Enthalpy change of hydration, ΔHhyd: It is the enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions.
Enthalpy change of solution, ΔHsol: It is the enthalpy change when one mole of solute is dissolved in a specified amount of solvent to form a solution.
Energy cycle involving enthalpy change of solution, lattice energy, and enthalpy change of hydration: This energy cycle represents the steps involved in the dissolution of an ionic solid. It includes the enthalpy change of solution, the lattice energy, and the enthalpy change of hydration.
Calculations involving energy cycles: Energy cycles can be used to calculate unknown values, such as enthalpy change of solution or enthalpy change of hydration, by summing up the energy changes along the cycle.
Effect of ionic charge and radius on enthalpy change of hydration:
Ionic charge: Higher charges on ions result in stronger attractions with water molecules, leading to more exothermic enthalpy changes of hydration.
Ionic radius: Smaller ionic radii allow ions to approach water molecules more closely, resulting in stronger attractions and more exothermic enthalpy changes of hydration.
23.3 Entropy Change, ΔS
Entropy, S: Entropy is a measure of the degree of randomness or disorder in a system. It represents the number of possible arrangements of particles and their energy in a given system.
Predicting and explaining entropy changes: (a) Change in state: Entropy generally increases when a substance changes from a more ordered state (solid) to a more disordered state (liquid or gas). (b) Temperature change: Entropy generally increases with an increase in temperature, as particles have more energy and can explore more arrangements. (c) Change in the number of gaseous molecules: Increasing the number of gaseous molecules usually increases entropy, as there are more possible arrangements of particles.
Calculating entropy change for a reaction, ΔS:
ΔS = ΣS(products) - ΣS(reactants)
This equation calculates the overall entropy change based on the standard entropies of the reactants and products.
23.4 Gibbs Free Energy Change, ΔG
Gibbs equation:
ΔG⦵ = ΔH⦵ - TΔS⦵
The Gibbs equation relates the standard Gibbs free energy change (ΔG⦵) of a reaction to the standard enthalpy change (ΔH⦵), the standard entropy change (ΔS⦵), and Temperature.
Calculations using the Gibbs equation: The Gibbs equation allows the calculation of ΔG⦵ at a specific temperature, given the values of ΔH⦵ and ΔS⦵.
Feasibility of a reaction: If ΔG⦵ is negative, the reaction is thermodynamically feasible, indicating that it is spontaneous under standard conditions. If ΔG⦵ is positive, the reaction is non-spontaneous under standard conditions.
Effect of temperature change: Changes in temperature can affect the feasibility of a reaction. Increasing the temperature can make a non-spontaneous reaction spontaneous if the increase in entropy (TΔS) dominates over the increase in enthalpy (ΔH).