Lecture 18 The Network and the Chemistry of Hydrogen and its Compounds

Transcription

1 2P32 Principles of Inorganic Chemistry Dr. M.Pilkington Lecture 18 The Network and the Chemistry of Hydrogen and its Compounds 1. Inert Pair Effect 2. Division of Elements into Metals and Non Metals 3. Hydrogen and its Compounds 1. Inert Pair Effect - also known as the 6s inert- pair effect and the inert s-pair effect. Seen in groups 3A and 4A especially (5s and 6s electrons). Metallic elements, particularly those with 5s 2 and 6s 2 pairs that follow the second and third-row transition metals are less reactive than we would expect on the basis of trends in effective nuclear charge, atomic sizes and ionization energies. See Figure 9.17 pg 234 Rodgers. 1

2 Group3A Most stable oxidation state Sum of IE 2 and IE 3 B Al Ga In Tl This translates to the fact that In, Tl, Pb, Sb and Bi do not always show their maximum oxidation states, but sometimes form compounds where the oxidation state is 2 less than the expected group valence. For example: GaCl versus GaCl 3. We cannot fully rationalize this, but there are two partial explanations. How can we explain or partially rationalize this effect? We can partially rationalize this effect with two explanations. 1. The trend in ionization energies going down a group. A general decrease is expected due to the increase in atomic size. The table shows the sum of the 2 nd and 3 rd ionization energies for the Group 3A elements. Note that the expected decrease from B to Al but Ga and Tl have higher values than expected. 2

3 The best explanation is that the 4s, 5s and 6s electrons in Ga, In and Tl are not shielded as effectively from the nucleus by the intervening d and f subshells. We see this also in the lanthanides, although there is a general decrease in size of the elements as we go from left to right this is very small for the lanthanides. Cd = 1.54 A and Hg = 1.57 A. These similarities are evidence that the nd and nf electrons do not shield one another or succeeding electrons from the nucleus particularly well (otherwise we would expect a more dramatic decrease in size as we move from left to right along the periodic table). This means that the 4s, 5s and 6s electrons experience a larger effective nuclear charge than expected and that means they are more difficult to ionize. This makes them less likely to form +3 oxidation states. 2. The trends in bond energies going down a group, e.g. Group 3A. For example, the bond energies of the chlorides we expect a decrease in bond energy as we go down a group due to the increase in atomic size and therefore the bond distance. The bonding electrons in the region of overlap of the valence orbitals of these larger atoms are farther from the nuclei of the atoms and have less ability to hold the two nuclei together. 3

4 Conclusions the inert pair effect Combination of these two explanations, (1) the higher than expected IE s and (2) the lower bond energies (as expected) for compounds involving these elements are partially responsible for the inert pair effect. In other words, for these elements, more energy is needed to get the 4s, 5s and 6s electrons to form bonds, but not enough energy is recovered upon bond formation. Therefore for example, GaCl compounds are more stable over GaCl 3 and thallium(1) compounds are more important than one would normally expect for a group 3A element. 2. Metal, Nonmetal and Metalloid Regions of the Periodic Table The periodic table is divided into the metals at the bottom left, the nonmetals at the top right and the semimetals or metalloids, in between. Metals - (low IE s, low electron affinities) tend to loose electrons to form positive ions. Nonmetals - (high IE s, high electron affinities) tend to gain electrons to form negative ions. Metals high melting and boiling pt s, lustrous, and exist in close-packed structures of cations surrounded by a sea of electrons. Nonmetals low melting and boiling, non-lustrous, and exist in chains, rings and diatomic molecules. Metalloids or Semimetals The elements in between- have both metallic and non metallic characteristics. Since metals are generally conductors of electricity and nonmetals are not, then semimetals are generally semiconductors. 4

5 3. Hydrogen The timeline After the big bang billion years ago. Hydrogen was the first element to form. It is the principle component of our sun synthesized almost 5 billion years ago and a primary component of life Man evolved 100,000 years ago. We have only been aware of H for 300 years! In 1671 Robert Boyle prepared a gas, later identified as H by dissolving Iron in dilute hydrochloric or sulfuric acid. We still use this procedure for the laboratory preparation of the gas. A balloon filled with hydrogen - Lemery thought he had discovered the origin of thunder and lightening! In 1766 Henry Cavendish reported to the RSC in London the detailed properties of hydrogen gas. Cavendish is given the credit for the discovery of hydrogen. He showed hydrogen is lighter than air and that water is produced when hydrogen and oxygen are reacted together. Hydrogen (water producer) and oxygen (acid producer) were named by Lavoisier in the late 1770 s Cavendish described the properties of a gas that formed when metals were treated with acids. Zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) (metal) (acid) (salt) (hydrogen gas) The Industrial Preparation of H 2 The catalytic steam hydrocarbon reforming process: Treats a mixture of hydrocarbons (propane, for example) from natural gas or crude oil with steam ( C) over a Ni catalyst and reforms it into a mixture of CO and H 2 gases called synthesis gas or syngas. C 3 H 8 (g) +3H 2 O(g) 3CO(g) +7H 2 (g) Ni heat synthesis gas 5

6 The second reaction known as the water-gas shift reaction, because it shifts oxygen from one reactant to another and thereby adjusts the composition of the syngas. It is carried out at elevated temperatures ( C) over an iron(iii)oxide catalyst. CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) heat, Fe 2 O 3 The largest single use of hydrogen is for the production of ammonia by the Haber process: N 2 (g) +3H 2 (g) 2NH 3 (g) High T and P, Fe This in turn provides the starting product for a number of useful nitrogen compounds such as in fertilizers and explosives (ammonium nitrate). i.e 3H 2 + N 2 2NH 3 HNO 3 NH 4 NO 3 Isotopes of Hydrogen Hydrogen forms three isotopes, Hydrogen (H) (or protium), Deuterium (D) and Tritium (T). The differences in atomic properties carry over to hydrogen compounds. In D 2 O, the molecules move slowly at a given temp than H 2 O molecules. For example, heavy water or deuterium oxide D 2 O, can be separated from ordinary water by electrolysis. One reason for this is that the hydrogen ions H + move to the negative electrode more rapidly than the twice as heavy deuterium ions D +. It follows that ordinary hydrogen gas H 2, is the preferred product and the concentration of D 2 0 in the water left behind increases as the electrolysis is carried out. The remaining water becomes heavier and heavier and is called heavy water. 6

7 H 2 O and D 2 O can also be separated by fractional distillation, H 2 O boils off first, then D 2 O. D 2 O is poisonous when given to mice they showed signs of extreme thirst and died. Why?- D 2 O it has a lower mean velocity. Kinetic energy = 1/2mv 2 (increase the mass, decrease the velocity at a given temperature). Hence D 2 O has a lower rate of diffusion into cells. Another contributing factor that the rate of transfer of D +, catalyzed by various enzymes is slower than the lighter H +. D 2 O does not support life since the biological properties of D 2 O and H 2 O are extremely different! Carbonic anhydrases are enzymes that catalyze the hydration of carbon dioxide and the dehydration of bicarbonate: CO 2 + H 2 O <-----> HCO 3- + H + These carbonic anhydrase-driven reactions are of great importance in a number of tissues. Examples include: Parietal cells in the stomach secrete massive amounts of acid (i.e. hydrogen ions or protons) into the lumen and a corresponding amount of bicarbonate ion into blood. Pancreatic duct cells do essentially the opposite, with bicarbonate as their main secretory product. 7

8 These series of reactions are 3 times slower when run in D 2 O Deuterium Labelling Both deuterium and tritium can be incorporated into a variety of hydrogen containing compounds and used to follow (trace) the course of reactions involving these compounds. When various hydrogen-containing compounds are dissolved in heavy water, the hydrogen s bound to electronegative atoms such as oxygen, nitrogen, sulfur or one of the halogens are replaced with deuterium s, whereas hydrogen s bound to the carbon are not. For example: H-X + D 2 O D-X + D-O-H HCH 3 + D 2 O no reaction Why is this the case? - D 2 O like ordinary water, contains polar O-D bonds that can interact via dipole-dipole forces with the polar H-X. C-H bonds however are not polar and so do not interact with the polar O-D bonds of the D 2 O. 8

9 The exchange of deuterium for hydrogen (a) in compounds containing a polar covalent H-X bond, where X = O, N, S, F, Cl, Br, I and (b) in compounds containing essentially nonpolar H-C bonds. 9