2023 VJC H2 Chem Prelim P2 (Ans)

1 2023 VJC H2 Chemistry Prelim Paper 2 Answers 1 (a) State three ways in which an ideal gas differs from a real gas. Molecules/particles of an ideal gas have zero/negligible volume. There are negligible (or NO) intermolecular forces of attraction between ideal gas molecules/particles Collisions between ideal gas molecules are perfectly elastic (i.e. no loss of energy during collision) [3] (b) (i) Draw a labelled diagram to show the significant force of attraction between two molecules of hydrogen fluoride. Include the name of the attraction in your diagram. Label as hydrogen bond Lone pair and dipole [2] (ii) The value of pV/RT is plotted against p for 1 mol of an ideal gas and 1 mol of fluorine gas at 300 K, where p is the pressure and V is the volume of the gas. On the same axes, sketch the variation of pV/RT against p for one mole of hydrogen fluoride, HF, at the same temperature of 300 K. Briefly explain your answer. Correct sketch HF deviates more from ideal behaviour because it experiences stronger intermolecular hydrogen bonding between the molecules compared to F2 which has only instantaneous dipole−induced dipole interaction between its molecules. [2] [Total: 7] pV/RT p 1.0 ideal gas fluorine HF

2 2 (a) The type of bonding present in a binary compound can be predicted from the electronegativities of the elements involved. This can be shown on a van Arkel - Ketelaar triangle in Figure 2.1, which plots the difference in electronegativity, ∆𝜒, on the y-axis against the average electronegativity of the two elements, 𝜒̅, on the x-axis. In this triangle, the three corners represent the extremes of metallic, ionic, and covalent bonding, with caesium (Cs), caesium fluoride (CsF) and fluorine (F2) at these corners. Figure 2.1 (answer for (a)(iii) Position in the triangle NaF any point below and to the right of CsF SiF4 any point below and to the right of NaF AND higher and to the left of F2. (i) Describe and explain the variation in electronegativity across the third period of the Periodic Table. Electronegativity increases across a period. Nuclear charge increases (OR number of protons increases ) while shielding effect is relatively constant (OR number of inner shell electrons is the same). Hence there is an increase in effective nuclear charge leading to stronger attraction between the nucleus and the electrons, and so ability of the atom to attract electrons to itself increases. [2] ionic metallic covalent × Cs F2 Difference in electronegativity ∆𝜒 Average electronegativity 𝜒̅ CsF × × × × NaF SiF4

3 (ii) State the type of bonding present in sodium fluoride (NaF), silicon tetrafluoride (SiF4) and the magnesium-aluminium