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QLDChemistryQuick questions
Unit 2: Molecular interactions and reactions
Quick questions on Water as a solvent and the dissolution of ionic and polar substances (QCE Chemistry Unit 2)
11short 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 structure of water?Show answer
Water has the molecular formula H_2O. The O atom is sp3-like with two bond pairs (to H) and two lone pairs. VSEPR predicts a bent shape with H-O-H angle around 104.5 degrees.
What is hydrogen bonding in water?Show answer
H bonded to O (one of N, O, F) can hydrogen bond. Each water molecule has:
What is dissolution of ionic compounds (ion-dipole interaction)?Show answer
When an ionic solid such as NaCl is placed in water:
What is dissolution of polar molecular substances?Show answer
Polar molecules with hydrogen-bond donor or acceptor groups dissolve readily because they can substitute for water-water hydrogen bonds with comparable strength water-solute hydrogen bonds.
What is "Like dissolves like" as a heuristic?Show answer
Polar / hydrogen-bonding substances dissolve in polar / hydrogen-bonding solvents.
What is non-polar molecular substances and the hydrophobic effect?Show answer
Non-polar molecules placed in water disturb water-water hydrogen bonding without offering any replacement attraction. Water responds by forming ordered cages around the solute (a clathrate-like structure), which is entropically unfavourable. This drives non-polar molecules to clump together (or float as a separate phase), the hydrophobic effect. It is the basis of micelle and membrane formation, soap action and protein folding.
What is calling water "neutral" to mean "non-polar"?Show answer
Water is electrically neutral overall but highly polar. Confusing the two terms is a frequent EA mark-loss.
What is forgetting the orientation in ion-dipole solvation?Show answer
Cations attract the delta- O of water; anions attract the delta+ H. Reversing this loses the explanation marks.
What is using "ionic bond" for hydration interactions?Show answer
Ion-dipole interactions are not bonds; they are non-covalent attractions, weaker than ionic bonds but strong enough to overcome lattice energies for soluble salts.
What is ignoring entropy?Show answer
Dissolution can be slightly endothermic and still spontaneous if it increases disorder substantially (NaCl in water is a classic case).
What is applying "like dissolves like" without naming the force?Show answer
The principle is descriptive; the explanation must name the specific intermolecular forces involved (dispersion, dipole-dipole, hydrogen bonding, ion-dipole).