2024 Zhenghua Prelim MS

ZHENGHUA SECONDARY SCHOOL PRELIMINARY EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY 6092/02 MARKS SCHEME 1 a combustion b precipitation c electrolysis d rusting 1 1 1 1 2ai in correct order: methane nitrogen oxygen argon 1 2aii gases with lowest atomic/molecular mass diffuse fastest. (OR gases with highest atomic/molecular mass diffuse slowest) argon will diffuse the slowest since its Ar is the lowest at 40, followed by oxygen (Mr = 32) and nitrogen (Mr = 28) and methane will diffuse the fastest since it has the lowest M r of 16 (A: shows trend without stating Ar or Mr) 1 1 2b when temperature is constant, particles of all gases have the same average kinetic energy (OR at different temperatures, particles will have different average kinetic energy) constant temperature ensures results of the experiment is dependent solely on the molecular mass of the gases. 1 1 2c helium / hydrogen 1 2d ammmonia is very soluble in water, hence would dissolve instead of diffuse towards the diffusion plug. 1 3a carbon can reduce silicon oxide to silicon, hence, silicon is less reactive than carbon. aluminium oxide cannot be reduced to aluminium by carbon, hence aluminium is more reactive than carbon hence aluminium is more reactive than silicon. 1 1 3b make references about the (correct) type of bonding & structure present in each molecule link to amt of energy needed and hence, the melting point make references about presence of (correct) mobile charged particles link to ability to conduct electricity 1 1 1 1

CO is a covalent compound with simple molecular structure. smaller amount of energy is needed to overcome the weak intermolecular forces of attraction between the molecules, this its low melting point. SiO2 is a covalent compound with giant molecular / covalent structure. large amount of energy is needed to overcome the strong covalent bonds between silicon atoms and oxygen atoms thus, its very high melting point. Al2O3 is an ionic compound with giant ionic / crystal lattice structure. Large amount of energy is needed to overcome strong electrostatic forces of attraction between aluminium cations and oxide anions. Thus, its high melting point. Both CO and SiO2 cannot conduct electricity as both have no mobile charged particles as all the valence electrons are used up for bonding. In molten state, the giant ionic / crystal lattice structure of Al 2O3 breaks down, hence there are free moving aluminium cations and oxide anions to conduct electricity. 4a calculate no. of moles of K, Cu and Cl no. of mole K = 31.5 ÷ 39 no. of mole Cu = 25.6 ÷ 64 no. of mole Cl = 42.9 ÷ 35.5 calculate simplest ratio by dividing with smallest no. of moles (Cu) empirical formula: K2CuCl3 1 1 1 4b 27; 29; 37 1 4c F2 + CuCl2 → CuF2 + Cl2 fluorine is more reactive than chlorine; displa