ASRJC 2021 J2Prelims H2Chem P2 soln

1 ASRJC JC2 PRELIMS 2021 9729/02/H2 [Turn over Anderson Serangoon Junior College 2021 JC2 Preliminary Examination H2 Chemistry (9729) Paper 2 Suggested Solutions 1 (a) Describe and explain the trend in the electronegativity of elements across Period 3 from sodium to chlorine. [2] Electronegativity increases across Period 3. • Nuclear charge increases across Period 3. • Shielding effect between nucleus and valence electrons are similar since successive elements in the period have an additional electron in the same valence shell/same no. of inner shells. • There are stronger (electrostatic) forces of attraction between the nucleus and the electron pair in a covalent bond. (b) Some ionic radii are listed in the Data Booklet. (i) Explain the trend in ionic radius down Group 2. [2] Ionic radius increases down the group. • Nuclear charge increase but number of (inner) electronic shells increases. • Outer electrons experienced greater shielding effect and are further from the nucleus. • there are weaker electrostatic forces of attraction between the nucleus and the outer electrons. (ii) Explain the differences between the ionic radii of P3–, Cl– and Ca2+. [1] Ionic radius decreases from P3– to Cl– to Ca2+. • P3–, Cl– and Ca2+ are isoelectronic/same number of electrons and • number of protons / nuclear charge increases from P3– to Ca2+ while the shielding effect remains the same, • resulting in an increasing attraction between the nucleus and outer electrons. (accept there is an increasing attraction between the increasing number of protons and the same number of electrons.) (iii) Hence, suggest a value for the ionic radius of a potassium ion, K+. [1] 0.138 nm (any value between 0.099 and 0.181) K+ is isoelectronic with the ions in (b)(ii) and its nuclear charge is between that of Cl– and Ca2+. Hence, its ionic radius will be between 0.099 and 0.181 nm.

2 ASRJC JC2 PRELIMS 2021 9729/02/H2 (iv) Explain the difference in size between the radius of potassium ion and the radius of a potassium atom. [1] Radius of K+ ion is smaller than that of K atom • nuclear charge remains the same. • the cation has one less electronic shell than the atom. • the outer electrons in the cation are less shielded and closer to the nucleus. • there are stronger electrostatic forces of attraction between the nucleus and the outer electrons in cation. (c) Anodisation of aluminium is a process which coats an oxide layer on aluminium objects. (i) Draw a labelled diagram of the electrolysis cell used to anodise a small piece of aluminium object. Include details of the cathode, anode