Water molecules in the atmosphere can form hydrogenbonded dimers, \(\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} .\) The presence of these dimers is thought to be important in the nucleation of ice crystals in the atmosphere and in the formation of acid rain. (a) Using VSF.PR theory, draw the structure of a water dimer, using dashed lines to indicate intermolecular interactions. (b) What kind of intermolecular forces are involved in water dimer formation? (c) The \(K_{p}\) for water dimer formation in the gas phase is 0.050 at $300 \mathrm{~K}\( and 0.020 at \)350 \mathrm{~K}$. Is water dimer formation endothermic or exothermic?

: VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict the geometric structure of molecules. Water has the molecular formula H2O, and according to VSEPR theory, it exhibits a bent or angular structure because of the 2 lone pairs present on the oxygen atom. In the case of a water dimer, two water molecules are interacting via intermolecular forces. To show these interactions, draw two separate water molecules with their respective bent structures. Then, connect the hydrogen of one water molecule to the oxygen of the other water molecule with a dashed line to represent the hydrogen bond. Similarly, do this for the other hydrogen in the same water molecule as well.

: The main intermolecular force in water dimer formation is hydrogen bonding. This interaction arises because of the large difference in electronegativity between hydrogen and oxygen, which leads to an electrostatic attraction between the two. Hydrogen bonding is a strong intermolecular force and plays a major role in determining the properties of water, such as its high boiling point and surface tension.

: We are given the equilibrium constant (\(K_p\)) values for water dimer formation at two different temperatures. At \(300 \ \mathrm{K}\), \(K_p = 0.050\), and at \(350 \ \mathrm{K}\), \(K_p = 0.020\). Notice that the equilibrium constant decreases as the temperature increases, which implies that the reaction favors the reactants (individual water molecules) at higher temperatures and the products (dimers) at lower temperatures. According to Le Châtelier's principle, this is indicative of an exothermic process, as increasing the temperature leads to a shift of the equilibrium towards the reactants, offsetting the temperature increase and releasing heat. Therefore, water dimer formation is an exothermic process.