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Studio_9_Chemical_Periodicity_FINAL_F2006 - Chem 25...

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Chem. 25: Studio #9 ___ Chemical Periodicity Chemical Periodicity: Hydrolysis of Metal and Nonmetal Oxides The modern periodic chart contains a wealth of information. The table was originally constructed on the basis of patterns in the chemical reactivity of the elements. One of the greatest triumphs of the quantum mechanical model of matter is its ability to account for this arrangement. In this studio, we examine periodic trends in the acid-base behavior of the oxides of some main group elements. A useful application of chemical periodicity is that it enables us to predict behavior of elements in terms of their reactivity and the stoichiometry of their compounds. Elements within a given family or group (a column on the chart) have the same number of valence electrons and hence tend to exhibit the same oxidation numbers. Furthermore, the behavior of elements and their compounds tends to vary in predictable ways as one moves across a row on the chart. Nearly all elements (E) form oxides. Typical examples are lithium forming lithium oxide, Li 2 O, and sulfur, forming sulfur trioxide, SO 3 . These oxides undergo hydrolysis reactions when treated with water. In a hydrolysis reaction, the oxide dissolves in the water and a new compound forms that has the elements of water added to its formula. Two types of hydrolysis reactions occur: (1) metal oxides react with water to form bases; (2) nonmetal oxides react with water to form acids. (1) Li 2 O (s) + H 2 O (l) 2 LiOH (aq) (2) SO 3 (g) + H 2 O (l) H 2 SO 4 (aq) The reverse reaction – the dehydration of a metal hydroxide or an oxoacid to the corresponding oxide and water occurs at elevated temperatures: (1) 2 LiOH (s) Li 2 O (s) + H 2 O (g) (229 H 2 SO 4 (l) SO 3 (g) + H 2 O (g) The structures of the acids and bases produced in the hydrolysis reactions reveal a similar subunit – an E-O-H moiety: Li O H and S O O O O H H 1
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Whether the compound behaves as an acid or base depends on the character of the –O-H bond. When E is a metal, E is ionic, and when the compound dissolves in water, the metal ion and the hydroxide ion are formed; when E is a nonmetal, the E-O bond is covalent, and the hydrogen ion dissociates, leaving the –E-O- unit intact. LiOH(s) in water Li + (aq) + OH - (aq) H 2 SO 4 (l) in water 2 H + (aq) + SO 4 2- (aq) The changing nature of the E-O-H bond as one goes across a row on the periodic chart gives rise to the periodic variability in the character of the hydrolysis products of the oxides of the elements.
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Studio_9_Chemical_Periodicity_FINAL_F2006 - Chem 25...

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