2025 SAJC H1 Chem Prelims P2_Answers

1 ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS HIGHER 1 (STUDENT REVIEW) CANDIDATE NAME CLASS 2 4 S CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper. Additional Materials: Data Booklet 8873/02 02 September 2025 2 hours READ THESE INSTRUCTIONS FIRST Write your name and class on all the work that you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions in the spaces provided on the Question Paper. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 25 printed pages (including this cover page). For Examiner’s Use Section A Q1 25 Q2 12 Q3 12 Q4 11 Section B Q5 20 Q6 20 Total 80

2 1 (a) Aluminium oxide , Al2O3, i s a white, crystalline solid with high thermal stability and hardness. (i) Complete the electronic configuration of an aluminium atom. 1s2 …………………………………… [1] 1s2 2s2 2p6 3s2 3p1 (ii) Explain why the first ionisation energy of Al is lower than the first ionisation energy of Mg. [2] In Mg, the first electron is removed from the 3s orbital, whereas for Al, the first electron is removed from the 3p orbital (√). In Al, the valence electron in the 3p orbital experiences additional shielding effect (√) by the 2s electrons. This is so as valence electron of Al in the 3p orbital is further away (√) from the nucleus than the electrons in the 3s orbital. This factor outweighs the effect of increase in nuclear charge from Mg to Al. The valence electron in 3p orbital of Al experiences weaker attraction(√) to the nucleus and requires less energy to be removed. (iii) Draw the ‘dot-and-cross’ diagram of Al2O3. [1] 3+ O x 2- xAl2 3 (iv) The ionic radius of Al3+ is 0.050 nm and Ga3+ is 0.062 nm. Explain the difference in ionic radii between Al3+ and Ga3+. [2] Ga3+ has a higher nuclear charge than A l3+. Ga3+ also has one more filled electronic/electron shells / inner shells and the valence electrons are further away from the nucleus and shielding effect increases. The valence electrons are less strongly attracted to the nucleus . Ga3+ has a larger ionic size than Al3+.

3 (v) The melting point of Al2O3 is 2072 °C while the melting point of Ga 2O3 is 1720 °C. Explain the difference in melting points of these two oxides. [2] Al2O3 & Ga2O3 have giant ionic lattice structure , with strong electrostatic forces of attraction between oppositely charged ions. The charges of the cations and anions of both oxides are similar. The ionic radius of O2– is the same. |L