VJC 2021 Prelim H2P3 Ans [Final]

1 Ó VJC 2021 9729/03/PRELIM/21 [Turn over Victoria Junior College 2021 H2 Chemistry Prelim Exam 9729/3 Suggested Answers Section A Answer all the questions in this section. 1 (a) Elements in the same group tend to form products with similar physical properties. However, the oxides of carbon and silicon have very different physical structures. While CO2 exists as a gas at room temperature, SiO2 is a solid. (i) Explain, in terms of structure and bonding, the difference in the physical states of CO2 and SiO2. SiO2 has a giant molecular structure where each Si atom is covalently bonded to 4 other O atoms. CO2 has simple molecular structure with weak instantaneous dipole-induced dipole interactions (id-id) between the CO2 molecules. Much more energy is required to break the strong covalent bonds in SiO2 as compared to the weak id-id for CO2. Hence, SiO2 is a solid while CO2 is a gas at room temperature. [2] (ii) Describe the bonding in a single CO2 molecule. Include a fully labelled diagram showing how the different types of bonds are formed in terms of orbital overlap. C–O σ bond is formed via head-on overlap between sp hybridized orbital of C and sp2 hybridised orbital of O (accept p orbital of O). C–O π bond is formed via side-ways overlap between unhybridized p orbital of C and unhybridised p orbital of O. [2] (iii) By considering the extent of overlap of the orbitals, suggest a reason why SiO2 does not form bonds similar to that of CO2. Si atom has a larger atomic radius than C. This leads to less effective overlap of the p orbitals between Si and O. Thus, a π bond formed between S and O would be much weaker and less likely to be formed. [1] σ bond π bond sp2 orbital of O sp orbital of C p orbital of C p orbital of O

2 Ó VJC 2021 9729/03/PRELIM/21 [Turn over (b) The extraction of iron from its ores typically involves the use of a Blast Furnace. One of the most common impurities in iron ore is silicon dioxide. To remove silicon dioxide, limestone is added. The calcium carbonate in limestone decomposes due to the high temperatures in the Blast Furnace to form calcium oxide. The calcium oxide then reacts with silicon dioxide to form calcium silicate, Ca2SiO4, which runs to the bottom of the Blast Furnace to be removed easily. (i) State the type of reaction between silicon dioxide and calcium oxide. acid-base [OR neutralization] [1] (ii) Define the term lattice energy. Lattice energy is energy released when one mole of solid ionic solid is formed from its isolated gaseous ions. [1] (iii) Using the inform