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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 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 q1?Show answer
State and justify the trend in boiling points for the homologous series methane to butane. [3 marks]
What is q2?Show answer
Compare boiling points of butan-1-ol () and pentane (). Account for the difference. [3 marks]
What is q3?Show answer
Three liquids of equal molar mass: pentane, butan-1-ol, butanoic acid. (a) Rank by boiling point. (b) Justify with intermolecular force comparison.
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