VJC 2021 Prelim H2P2 Ans [Final]

1 Ó VJC 2021 9729/02/PRELIM [Turn over Victoria Junior College 2021 H2 Chemistry Prelim Exam 9729/2 Suggested Answers 1 The table below shows the fifth to eighth ionisation energies of two consecutive elements, X and Y in the second period of the Periodic Table. successive ionisation energies / kJ mol–1 5th 6th 7th 8th X 10990 13330 71330 84080 Y 11020 15160 17870 92040 (a) (i) With the aid of an equation, define the term first ionisation energy with reference to X. [2] First ionisation energy of X is the amount of energy required to remove one mole of electrons from one mole of X gaseous atoms, producing one mole of gaseous X+ ions. X(g) ® X+(g) + e– (ii) State and explain the group number of X. [1] Group 16. There is a big jump from 6th to 7th IE, indicating that the 7th electron is removed from the inner principal quantum shell [OR there are 6 valence electrons]. (b) (i) Write down the full electronic configurations of X+ and Y+. [1] X+: 1s22s22p3 Y+: 1s22s22p4 (ii) Hence, explain which element has a less endothermic second ionisation energy. [2] Y has a less endothermic second ionisation energy. It is easier to remove a paired 2p electron in Y+ due to the presence of interelectronic repulsion between electrons in the same orbital. (c) Boron is another Period 2 element which reacts vigorously with fluorine to form boron trifluoride, BF3, an important reactant in organic syntheses. Boron trifluoride is a very reactive gas and it is hard to handle at room temperature. It can be converted to a liquid compound which is easily stored by reacting it with diethyl ether in the mole ratio of 1:1. Diethyl ether can be represented by the formula ROR, where –R represents the ethyl group. (i) Explain why boron trifluoride can form a compound with diethyl ether. [2] ROR has a lone pair of electrons on O while the B in BF3 has a vacant orbital, hence, B can accept lone pair of electrons to form a stable octet structure via dative bond formation.

2 Ó VJC 2021 9729/02/PRELIM [Turn over (ii) Draw the structure of the compound formed, indicating clearly the shape and bond angle around each central atom. [2]