A student tries to illustrate, step-by-step, the dissolution of liquid ethanol (CH3CH2OH) into water. The student’s three sequential diagrams are described below.
Diagram 1 ("Before mixing"): Two separate regions are shown. On the left are 25 ethanol molecules, each drawn as a connected cluster of atoms with an explicit O–H group. On the right are 25 water molecules. No interactions are depicted between the two regions.
Diagram 2 ("Immediately after mixing"): Ethanol molecules are shown fully dissociated into CH3CH2O⁻ ions and H⁺ ions. The CH3CH2O⁻ ions are surrounded by water molecules with their hydrogen atoms pointing toward the negatively charged oxygen. The free H⁺ ions are surrounded by water molecules oriented with their oxygen atoms toward H⁺. Water molecules themselves retain their usual bent geometry.
Diagram 3 ("Final solution"): The CH3CH2O⁻ ions and H⁺ ions recombine to form neutral ethanol molecules. These ethanol molecules are uniformly dispersed among water molecules. Every water molecule is oriented so that one of its hydrogen atoms points toward the carbon backbone of ethanol.
(a) Diagram 2 contains TWO fundamental errors about the dissolution of ethanol in water. Identify both errors clearly.
(b) For EACH error identified in part (a), explain in detail why the representation is inconsistent with the nature of ethanol as a solute and with the types of intermolecular forces involved.
(c) Evaluate the orientation of water molecules around ethanol in Diagram 3. Is the depicted alignment appropriate for the dominant intermolecular interaction between ethanol and water? Justify your answer.
(d) Describe how a scientifically accurate final diagram of ethanol fully dissolved in water should appear. Your description should include (i) whether ethanol should be dissociated or molecular, (ii) how many ethanol molecules relative to water molecules are reasonable for an ethanol-in-water mixture containing 25 molecules of each substance before mixing, and (iii) the correct orientation of water molecules involved in hydrogen bonding with ethanol.