General Principles of Chemistry CHEM110. Tutorial 8 9 th and 11 th April 2014 Inorganic Chemistry

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1 Chemistry CHEM110 Tutorial 8 9 th and 11 th April 2014 Inorganic Chemistry Atomic Radii 1. Arrange the following atoms in order of increasing atomic radius: Ca, Mg, Sr, Ba. Answer: Mg < Ca < Sr < Ba Atomic radius increases down a group. Ionic Radii 2. Arrange the ions N 3-, O 2-, Mg 2+, Na +, and F - in order of increasing ionic radius? The more + the charge, the smaller the ionic radius. Also, when an atom loses electrons, it clings ever more tightly to the ones it has left, further reducing the ionic radius. Note that charge means adding electrons. These electrons go in the outermost shells. Mg 2+ < Na + < F - < O 2- < N 3-3. Which of the following has the largest radius: Al 3+, Al, B? Answer: Al Al is below B in the periodic table, Al is larger than B. Cations are smaller than their parent atoms, and so the Al 3+ ion is smaller than the Al atom. 1

2 Ionic Radii in an Isoelectronic Series 4. Arrange the following isoelectronic series in order of decreasing size. Rb +, Sr 2+, Br -, As 3-, Se 2- It's based on the atomic number, the larger the atomic number the smaller the radius First, we note that this is an isoelectronic series of ions, with all ions having 36 electrons. In such a series, size decreases as the nuclear charge (atomic number) of the ion increases. The atomic numbers of the ions are As (33), Se (34), Br (35), Rb (37), and Sr (38). Reasoning : Consider the trends within ions For isoelectronic cations: the greater the positive charge, the smaller the ion For isoelectronic anions: the greater the negative charge, the larger the ion Thus, the ions decrease in size in the order As 3- > Se 2- > Br - > Rb + > Sr 2+ Largest: Smallest: Periodic Trends in Ionisation Energy 5. Referring to a periodic table, arrange the following atoms in order of increasing first ionisation energy: N, Be, Mg, Ar, Ne Ionisation energy increases as we move left to right across a row. It decreases as we move from the top of a group to the bottom. Because Be, N and Ne are in the same row of the periodic table, we expect IE to vary in the order Be < N < Ne. 2

3 Because Ne is above Ar in group 8A, we expect Ne to have the greater first ionization energy: Ar < Ne. Similarly, Mg is below Be in group 2A, and so we expect IE for Mg to be less than that of Be: Mg < Be. From these observations, we conclude that the ionization energies follow the order Mg < Be < N < Ar < Ne Oxides 6. Classify the oxides: BaO, CO 2, SnO, BeO, Li 2 O, GeO, and SO 2 into: (i) (ii) (iii) Acidic Basic and Amphoteric oxides: i) Acidic - CO 2 and SO 2 ii) Basic - : BaO and Li 2 O iii) Amphoteric BeO, SnO, GeO A resultant borderline between basic and acidic oxides occurs along a diagonal. Properties of s- and p-block Elements Li Be B C N O F Na Mg Al Si P S Cl K Ca Ga Ge As Se Br Rb Sr In Sn Sb Te I Cs Ba Tl Pb Bi Po At Basic Oxides Amphoteric Oxides Acidic Oxides 3

4 7. Complete and balance the following equations. Also identify the product as an oxide, superoxide, peroxide or hydroxide. Include the phases of the products. (i) K(s) + O 2 (g) (Excess) Heat KO 2 (s) (superoxide) Heat (ii) Sr(s) + 2H 2 O (g) Sr(OH) 2 (aq) + H 2 (g) (hydroxide) Heat (iii) 2Na + O 2 (g)(excess) Na 2 O 2 (s) (Peroxide) Additional Questions 8. Write chemical equations to illustrate the amphoteric nature of (i) H 2 O (ii) PbO (i) H 2 O is amphoteric as it reacts in both basic and acidic media. Reactions with acid: H 2 O + HCl H 3 O + + Cl - Reactions with base: H 2 O + NH 3 OH - + NH 4 +- (ii) PbO reacts with both acids and with bases: In acid: PbO + 2HCl PbCl 2 + H 2 O In base: PbO + 2NaOH + H 2 O Na 2 [Pb(OH) 4 ] 4

5 Electronegativity 9. Explain the trends in Electronegativity in the Periodic Table In general, electronegativities of the elements in the same Period increases as you go from left to right across the period. In general, the electronegativities of the elements in a Group decrease as you go down the group from top to bottom. 10. Put the following in order of INCREASING electronegativity: Cs, F, Na, K These elements are from different Groups and Periods. However, their electronegativity order can be deduced as follows: F is the most electronegative atom of all elements. Therefore it ranks highest. The remaining elements are all in the same Group (Group 1) and so their order will decrease down the Group - i.e. Na > K > Cs. Combining this with the knowledge that F is the most electronegative of all elements, the order is Cs < K < Na < F. Acid Strength 11. Determine which is the stronger acid in each of the following pairs? Give your reasoning. (i) H 3 PO 4 and H 3 AsO 4 (ii) H 2 SO 4 or H 2 SeO 4 i) H 3 PO 4 and H 3 AsO 4 Since electronegativity decreases as one goes down a group in the Periodic Table, the strengths of the oxoacids will decrease as you go down a group. H 3 PO 4 > H 3 AsO 4 5

6 ii) In case of oxoacids, for different central element, but same number of oxygen atoms, the more electronegative central atom stronger acid. H 2 SO 4 > H 2 SeO 4 6