Table 1 shows the experimental values of some standard enthalpy changes for potassium iodide, KIKIKI.
| Name of enthalpy change | ΔH⊖/kJ mol−1\Delta H^\ominus / \text{kJ mol}^{-1}ΔH⊖/kJ mol−1 |
|---|---|
| Enthalpy of atomisation of potassium | +89+89+89 |
| First ionisation energy of potassium | +419+419+419 |
| Electron affinity of iodine | −295-295−295 |
| Enthalpy of lattice formation of potassium iodide | −648-648−648 |
| Enthalpy of formation of potassium iodide | −328-328−328 |
Calculate the standard enthalpy of atomisation of iodine, ΔHat⊖(I)\Delta H_{at}^\ominus(\text{I})ΔHat⊖(I), using the data in Table 1.
The experimental value for the enthalpy of lattice formation of potassium iodide is −648 kJ mol−1-648\text{ kJ mol}^{-1}−648 kJ mol−1. A theoretical calculation based on the perfect ionic model yields −636 kJ mol−1-636\text{ kJ mol}^{-1}−636 kJ mol−1. Deduce what the comparison of these two values indicates about the bonding in potassium iodide.
Use the standard entropy data in Table 2 to calculate the standard Gibbs free energy change (ΔG⊖\Delta G^\ominusΔG⊖) at 298 K298\text{ K}298 K for the decomposition of potassium iodide into its elements:
KI(s)→K(s)+12I2(s) KI(s) \rightarrow K(s) + \frac{1}{2}I_2(s) KI(s)→K(s)+21I2(s)and explain whether this reaction is feasible at 298 K298\text{ K}298 K.
| Substance | S⊖/J K−1 mol−1S^\ominus / \text{J K}^{-1}\text{ mol}^{-1}S⊖/J K−1 mol−1 |
|---|---|
| KI(s)KI(s)KI(s) | 104.3104.3104.3 |
| K(s)K(s)K(s) | 64.764.764.7 |
| I2(s)I_2(s)I2(s) | 116.1116.1116.1 |
