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Thermochemistry & Structure · Part 2 of 3
6 exam-style questions with model answers, plus 8 quick multi-choice questions — every question on this part of the standard, grouped by the 2 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
Explain what must happen to the particles of a molecular substance when it boils, and state what is NOT broken.
The boiling points of the group 17 hydrides are HCl −85 °C, HBr −67 °C and HI −35 °C. Explain this trend.
Water (M = 18) boils at 100 °C while hydrogen sulfide (M = 34) boils at −60 °C. Ethanol (M = 46) boils at 78 °C while dimethyl ether, CH3OCH3 (M = 46), boils at −24 °C. Compare and contrast the reasons for these two large differences, and explain what the two comparisons together show about the relative importance of molecular size and hydrogen bonding.
State whether each of ammonia, iodine and methanol is likely to be soluble in water, and give the reason in each case.
Explain why methanol, CH3OH, is completely miscible with water but octan-1-ol, CH3(CH2)7OH, is essentially insoluble.
Propanone, CH3COCH3, is completely miscible with water. Butane, CH3CH2CH2CH3, of almost identical molar mass, is insoluble. Silver chloride, AgCl, is also insoluble in water despite being ionic. Justify all three observations, and explain why the reason for AgCl's insolubility is fundamentally different from the reason for butane's.