Chemistry
Chemistry
Problem
Substances A and B each have a molar mass of 58 g mol⁻¹. Representative molecular structures (not shown) indicate that substance A is non-polar, whereas substance B can engage in hydrogen bonding. At 1 atm the measured phase transition temperatures are: • Substance A: m.p. –95 °C b.p. –1 °C • Substance B: m.p. –60 °C b.p. unknown (student prediction = 56 °C) Respond to the prompts below. (a) Rank the relative strengths of the intermolecular forces operating in the solid phase for substances A and B. Provide a clear justification based on molecular structure. (b) Explain why substance B melts at a higher temperature than substance A, even though the two substances have identical molar masses. (c) Predict which substance has the larger molar enthalpy of fusion (ΔH_fus). Justify your prediction in terms of the energy needed to increase particle freedom of motion. (d) Evaluate the student’s predicted boiling point for substance B (56 °C). State whether the prediction is likely too low, too high, or reasonable, and justify your answer by reference to the types and strengths of intermolecular forces that must be overcome during vaporization.Intuitive explanation
The key insight is that intermolecular forces determine boiling and melting points, not molar mass alone. Since A and B have identical molar masses, their very different phase transition temperatures reveal the dramatic effect of hydrogen bonding in B. Substance B's higher melting point and expected boiling point reflect the much stronger intermolecular attractions from hydrogen bonding compared to A's weak dispersion forces—even though both molecules are the same size. Use this pattern to evaluate whether the student's prediction is reasonable.
Intuitive explanation
The key insight is that intermolecular forces determine boiling and melting points, not molar mass alone. Since A and B have identical molar masses, their very different phase transition temperatures reveal the dramatic effect of hydrogen bonding in B. Substance B's higher melting point and expected boiling point reflect the much stronger intermolecular attractions from hydrogen bonding compared to A's weak dispersion forces—even though both molecules are the same size. Use this pattern to evaluate whether the student's prediction is reasonable.