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QLDChemistryQuick questions
Unit 4: Structure, synthesis and design
Quick questions on Physical properties of organic compounds (QCE Chemistry Unit 4)
7short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the three intermolecular forces in organic chemistry?Show answer
These ranges overlap by design: a very large non-polar molecule (e.g. long alkane) can have stronger total dispersion than a small hydrogen-bonded molecule (e.g. methanol). The dot point asks for relative reasoning, not numerical force calculation.
What is boiling point trends across homologous series?Show answer
Within a series, boiling point rises with chain length because dispersion forces scale with surface area and electron count. Each added CH2 raises boiling point by roughly 20 to 30 degrees C in the short-chain region, less above C10.
What is branching lowers boiling point?Show answer
Branched isomers boil lower than straight-chain isomers. Branching reduces molecular surface area and the number of contact points for dispersion. Compare:
What is aqueous solubility depends on whether the molecule can hydrogen-bond with water?Show answer
A compound is highly soluble in water if it can hydrogen-bond with water and its non-polar region is small. Two competing factors:
What is melting point?Show answer
Melting points follow similar IMF logic, but with an extra crystal-packing factor. Symmetric molecules pack tightly and melt higher than less symmetric isomers of the same Mr. For example, 2,2-dimethylpropane (very symmetric) melts at -17 degrees C, far higher than n-pentane (-130 degrees C), despite n-pentane boiling 27 degrees higher. QCAA EA questions on melting point are less common than on boiling point but follow the same logic plus this symmetry caveat.
What is building a model answer?Show answer
When asked "explain the difference in boiling point between A and B":
What is why this matters in IA3 and the EA?Show answer
IA3 research investigations often compare organic compounds in food chemistry, pharmaceutical design, or polymer applications, where solubility and volatility set the practical context. The EA Paper 2 short response set includes one or two property-based reasoning items most years. Memorising the boiling-point order is insufficient; QCAA marks for the IMF-based reasoning behind it.
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