Question

Determine the lattice energy of \(\mathrm{KF}(\mathrm{s})\) from the following data: \(\Delta \mathrm{H}_{\mathrm{f}}[\mathrm{KF}(\mathrm{s})]=-567.3 \mathrm{kJ} \mathrm{mol}^{-1} ;\) enthalpy of sub- limation of \(\mathrm{K}(\mathrm{s}), 89.24 \mathrm{kJ} \mathrm{mol}^{-1} ;\) enthalpy of dissociation of \(\mathrm{F}_{2}(\mathrm{g}), 159 \mathrm{kJ} \mathrm{mol}^{-1} \mathrm{F}_{2} ; I_{1}\) for \(\mathrm{K}(\mathrm{g}), 418.9 \mathrm{kJmol}^{-1}\) EA for \(\mathrm{F}(\mathrm{g}),-328 \mathrm{kJ} \mathrm{mol}^{-1}\)

Short answer

The lattice energy of KF is -985.94 kJ/mol.

Step-by-step solution

Understand Born-Haber Cycle

The Born-Haber cycle is a thermochemical cycle. It relates the lattice energy of an ionic compound to its enthalpy of formation and other atomic and molecular properties. The cycle for the formation of KF would include the sublimation of potassium, the dissociation of fluorine, the ionization of K and the electron affinity of F.

Sum Up Intermediate Steps' Energies

Sum up the energies of the intermediate steps: sublimation energy of K (89.24 kJ/mol), bond dissociation energy of F (half of 159 kJ/mol), Ionization energy of K (418.9 kJ/mol), and electron affinity of F (-328 kJ/mol). This gives 89.24 + (159/2) + 418.9 - 328 = 418.64 kJ/mol.

Applying Hess's law

Hess's law states that the enthalpy change for a reaction is the same, whether it happens in one step or in many. In other words, it should be the same whether KF forms directly from its elements or via a series of steps in a Born-Haber cycle. The enthalpy of formation of KF given (-567.3 kJ/mol). So, according to Hess's Law, the sum of enthalpies of intermediate steps is equal to the enthalpy of formation of KF.

Calculate Lattice Energy

So, the lattice energy of an ionic compound is defined as the energy released when the isolated gaseous ions combine to form an ionic solid. Therefore, to find the lattice energy, subtract the sum of the energies you found in Step 2 from the enthalpy formation of KF. So, -567.3 kJ/mol - 418.64 kJ/mol = -985.94 kJ/mol

Key concepts

Understanding the Born-Haber Cycle

The Born-Haber cycle is an essential concept in thermochemistry used to calculate the lattice energy of ionic compounds. It cleverly applies Hess's law, which states that the total enthalpy change for a reaction is the same, irrespective of the number of steps the reaction occurs in. Essentially, it provides a method to deconstruct the formation of ionic solid into various processes, each with its quantifiable enthalpy change.

Let's take a closer look at the cycle involved in the formation of potassium fluoride (KF). It includes several stages, starting with the sublimation of solid potassium to form gaseous K atoms, then the dissociation of F2 gas to form F atoms. Next, it considers the first ionization energy required to convert K atoms to K+ ions and the energy change associated with adding an electron to an F atom (electron affinity), forming an F- ion. Finally, these gaseous ions combine to form solid KF, releasing the lattice energy. By summing up all these individual energies, we can relate them back to the overall enthalpy of formation of the compound.

Applying Hess's Law to the Cycle

Hess's law is central to the Born-Haber cycle. It's a statement about conservation of energy, asserting that the total enthalpy change for a given chemical reaction is the same regardless of the pathway taken by the reaction. On the molecular level, this means that whether elements combine directly to form a compound or do so in a roundabout series of steps, the overall energy required or released will be equivalent.

So, when students are given various enthalpies like sublimation, dissociation, ionization, and electron affinity, they are not to be overwhelmed by the complexity but to remember that these values serve as pieces of a puzzle. According to Hess's law, when these pieces are correctly assembled, they must fit into the big picture – which is the enthalpy of formation of the ionic compound. This allows us to use the provided data to track the energy changes throughout the cycle, eventually leading us to calculate the elusive lattice energy.

Enthalpy of Formation and Its Connection

The enthalpy of formation is a measure of the energy change when one mole of a compound is formed from its elements in their standard states. It incorporates all aspects of the chemical changes, including bond making and breaking and the physical state changes. In the case of KF, its enthalpy of formation signifies the energy difference between the reactants (solid K and gaseous F2) and the product (solid KF).

For the KF lattice energy calculation, the enthalpy of formation is a crucial final comparison point. In the Born-Haber cycle, it effectively captures the sum of the individual processes leading up to the formation of the ionic lattice from isolated gaseous ions. Thus, by analyzing the enthalpy of formation in relation to the sum of intermediate steps, as provided in the exercise, students can deduce the lattice energy that has not been directly provided. This reinforces the application of Hess's law to the Born-Haber cycle, emphasizing the holistic view chemists must take when considering the energy aspects of chemical formation.