5. Surface energy (2 pts). Let’s use the concept of surfacetension as surface energy per unit area to see if we can estimate,at least to the correct order of magnitude, the surface tension ofwater.
a) Water has a molar mass of 18 g/mol and a density of 1000kg/m3 (or 1 g/cm3). Based on this data, estimate the number ofwater molecules per unit surface area of water.
b) The coordination number of water (i.e., the average number of“neighbors†each water molecule has) in the liquid state is 4.Neighboring water molecules attract each other via hydrogen bonds,each of which has a binding energy of roughly 10–20 J (althoughthis number depends relatively strongly on temperature). Use thisinformation to estimate the surface tension of water. How does yourestimate compare to the observed figure ( γ water = 0.072 N/m )(hints: Keep in mind that we can think of surface tension assurface energy per unit area, and consider the energy needed tobring a molecule from the bulk to the surface)?
