2021_VJC_Prelim_P2_Ans

1  VJC 2021 8873/02/PRELIM/21 [Turn over Victoria Junior College 2021 H1 Chemistry Prelim Exam 8873/2 Suggested Answers Section A Answer all the questions in this section in the spaces provided. 1 (a ) The nth (n = 1, 2, 3, 4 or 5) ionisation energies of consecutive elements Q to Z, with atomic number below 20, are shown in Fig. 1.1. Fig. 1.1 (i) Define the nth ionisation energy of element T. The nth ionisation energy of T is the amount of energy required to remove one mole of electrons from one mole of T (n-1)+ gaseous ions, producing one mole of Tn+ gaseous ions. T(n-1)+(g)  Tn+(g) + e– [1] (ii) Element U is identified as Al. State the value of n. n = 3 or 3rd ionisation energy [Since U is a Group 13 element and T is a Group 2. There is a large decrease from T to U indicating that electrons is removed from a higher principle quantum shell in U2+.] [1] (b ) Aluminium chloride has a wide number of uses in the chemical industry. Anhydrous AlCl3 adopts different structures depending on the temperature and the state it is in. In the liquid state, it exists as the dimer Al2Cl6 while a mixture of monomer and dimer can be found in the gaseous phase.

2  VJC 2021 8873/02/PRELIM/21 [Turn over (i) State the type of interactions between two molecules of AlCl3. instantaneous dipole–induced dipole interactions [1] (ii) Draw a diagram to show the bonding in an Al2Cl6 dimer, and state the bond angle about the Al atom. Bond angle: 109.5o [2] (c ) Beryllium chloride, BeCl2, has properties similar to those of AlCl3. (i) Explain why BeCl2 behaves similarly to AlCl3. Both BeCl2 and AlCl3 are Lewis acids, i.e. they are electron deficient and act as an electron pair acceptor. [OR Both are covalent compounds due to similar charge to size ratio.] [1] (ii) BeCl2 and dimethylamine, (CH3)2NH reacts in the molar ratio 1:2. By considering the number of bond pairs and lone pairs around the central atoms of each molecule, explain the molar ratio and draw a diagram to show the bonding in the product. BeCl2: 2 bond pairs, 0 lone pair (CH3)2NH: 3 bond pairs, 1 lone pair BeCl2 can accept 1 lone pair of electrons from 2 molecules of (CH3)2NH to achieve octet configuration. [2] [Total: 8]

3  VJC 2021 8873/02/PRELIM/21 [Turn over 2 (a ) Fig. 2.1 shows the trend in melting points of some Period 3 elements: Fig. 2.1 (i) Complete the trend in melting point of the Period 3 elements for aluminium, silicon, chlorine and argon on Fig 2.1. [1] (ii) Explain, with reference