Factors Affecting Lattice Energy
Lattice energy is the energy released when one mole of an ionic compound is formed from its gaseous ions. The main factors influencing lattice energy are:
* Charge of the ions: Higher charges lead to stronger electrostatic attractions and thus higher lattice energy.
* Size of the ions: Smaller ions have a higher charge density, leading to stronger attractions and higher lattice energy.
* Distance between the ions: Smaller distances between ions result in stronger attractions and higher lattice energy.
Comparing KBr and RbI
* Charge: Both KBr and RbI have +1 and -1 charges on their ions, so this factor is the same.
* Size: Potassium (K) is smaller than rubidium (Rb), and bromide (Br) is smaller than iodide (I).
* Smaller cation (K+): Smaller cations exert a stronger electrostatic attraction on the anion.
* Smaller anion (Br-): Smaller anions also contribute to stronger attractions.
Conclusion
Since both the cation and anion in KBr are smaller than those in RbI, the electrostatic attractions are stronger in KBr. This results in a higher (more exothermic) lattice energy for potassium bromide compared to rubidium iodide.
In summary: Smaller ionic radii lead to stronger electrostatic forces and, therefore, a more exothermic lattice energy.
