Aspects of Bonding & Acid Strength

Transcription

1 Aspects of Bonding & Acid Strength CHEM 110/ 2014 Slide 1 of 35

2 Intramolecular Bonding The bonding between molecules/atoms in the solid state Ionic bonding Covalent bonding Metallic bonding e.g. sodium chloride e.g. Graphite or diamond e.g. copper/gold etc. Van der Waals bonding e.g. iodine or benzene Hydrogen bonding e.g. water and alcohols Slide 2 of 23

3 Typically ionic solids. Have high melting and boiling points They are brittle. They form electrolyte solutions if they dissolve in water. Ionic Bonding Ions stack together in regular crystalline structures Slide 3 of 23

4 Ionic Solids The attractive force of attraction between a pair of oppositely charged ions which increases with increased charge on the ions & with decrease in ionic sizes requires less energy to break up requires more energy to break up Na + = 99 pm Slide 4 of 23

5 Covalent Bonding Sharing of a pair of Electrons in the area between the nuclei. Complete their octets by sharing electron pairs E.g. Consider fluorine, F 2 F F The electron configuration of F is: 1s 2 2s 2 2p 5 F F These are combined to form F 2. Slide 5 of 23

6 It is a special case The more valence electrons, the stronger the bond. This gives rise to their properties such as malleability or ductility. Good thermal and electrical conductors Metallic Bonding The electrons are considered to be delocalized Slide 6 of 23

7 Van der Waals Bonding Due to instantaneous dipoles non-conducting species Slide 7 of 23

8 Compounds of xygen (xides) Li 2 Be B 2 3 C C 2 N 2, N N 2 Na 2 Mg Al 2 3 Si 2 P 2 3 P 2 5 S 2 S 3 K 2 Ca Ga 2 3 Ge 2 As 2 3 As 2 5 Se 2 Se 3 Cl 2 Cl 2 Cl 2 7 Br 2 Rb 2 Sr In 2 3 Sn Sn 2 Sb 2 3 Sb 2 5 Te 2 Te 3 I 2 5 Xe 3 Xe 4 Cs 2 Ba Basic xides (Metals form basic Ionic oxides ) Tl 2 3 Tl 2 Pb Pb 2 Bi 2 5 Po 2 Amphoteric xides (Metalloids) Acidic xides (Nonmetals form acidic covalent oxides) Acidity increases from left to right Slide 8 of 23

9 Acids & Bases Arrhenius: An acid provides H + ions (ionizable hydrogen); and A base produces H - ions in an aqueous solution Brønsted-Lowry: An acid is a proton donor, and A base is a proton acceptor Lewis: An acid is a species that accepts an e - -pair electron deficient (incomplete octet), and A base is an e - -pair donor electron rich (has lone pair(s)) Lewis acid-base reaction forms an adduct via a Slide 9 of 23 coordinate covalent bond

10 Basic xides Base Anhydride Anhydride means without water. Metal oxides are base anhydrides & react with water to form a basic solution: Ca(s) + H 2 Ca 2+ (aq) + 2H - (aq) basic oxide calcium hydroxide The oxide ion is protonated to produce the hydroxide: 2- + H 2 2H - (aq) (100%) A basic oxide reacts with acids to produce water Ca(s) + 2H + (aq) Ca 2+ (aq) + H 2 (l) Slide 10 of 23

11 Acidic xides Acid Anhydride All nonmetal oxides react with water to form oxoacids. xoacids contain H, and one other atom. Dissociation of an oxoacid produces H + ions. Reaction of sulfur trioxide, S 3, with water: S 3 (g) + H 2 (l) H 2 S 4 (aq) Sulfuric H 2 S 4 (aq) 2H + (aq) + S 2-4 (aq) acid Reaction of sulfur dioxide, S 2, with water: S 2 (g) + H 2 (l) H 2 S 3 (aq) Sulfurous H 2 S 3 (aq) 2H + (aq) + S 2-3 (aq) acid Slide 11 of 23

12 Amphoteric xides Metal/metalloid oxides with both acidic and basic properties They are generally insoluble in water. Reaction as a Base: Al 2 3 (s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 (l) Base Acid Reaction as an Acid: Al 2 3 (s) + 3H 2 (l) + 2H - (aq) 2[Al(H) 4 ] - (aq) Acid Base Under basic conditions, the soluble complex ion: tetra(hydroxo)aluminate(iii) ion, [Al(H) 4 ] - (aq) is formed. With excess H -, the octahedral soluble [Al(H) 6 ] 3- complex ion is formed. Slide 12 of 23

13 Exercise 1. Write a balanced equation for Zn(H) 2 with: a) an acid b) a base 2. Do the following species act as acids, bases or amphoteric species? a) P 4 3- b) CH 3 NH 3 + Slide 13 of 23

14 Neutral oxides Relatively few exist; Insoluble in water; & Neutral xides Do not reaction with either acids or bases. Examples include: Carbon monoxide, C Dinitrogen monoxide, N 2 Slide 14 of 23

15 xoacids xoacids are ternary compounds containing hydrogen, oxygen and one other element. Halogens, include chlorine, bromine and iodine: eg HCl 4 - Perchloric acid HCl 3 - Chloric acid HCl 2 - Chlorous acid HCl - Hypochlorous acid Group 6 elements including S, Se and Te*: eg H 2 S 4 - Sulfuric acid H 2 S 3 - Sulfurous acid Group 5 elements including N and P: eg HN 3 - Nitric acid Slide 15 of 23 HN 2 - Nitrous acid H 3 P 4 - Phosphoric acid (rthophosphoric acid)

16 Halogen xoacids HCl - Hypochlorous acid (Weak acid) HCl 2 - Chlorous acid HCl 3 - Chloric acid HCl 4 - Perchloric acid (Strong acid) The strength of the oxoacid increases with increasing Slide 16 of 23 number of terminal oxygen atoms

17 Halogen xoacids as xidising Agents Halogen oxides and oxoacids are strong oxidising agent, especially in acidic medium, due to formation of the stable halide anion Cl(aq) + 2H + (aq) + 2e - Cl - (aq) + H 2 (l) E o Cell = V This strong oxidizing ability makes them effective bleaches and disinfectants Most common are Ca(Cl) 2, and NaCl Slide 17 of 23

18 Polyprotic acids Polyprotic acids contain more than one ionizable hydrogen atom per acid molecule Examples include: Phosphoric acid H 3 P 4 Triprotic Sulfuric acid H 2 S 4 Sulfurous acid H 2 S 3 Diprotic Carbonic acid H 2 C 3 Dissociate in a stepwise manner: H 2 S 4 (aq) H + (aq) + HS 4- (aq) Strong HS 4- (aq) H + (aq) + S 2-4 (aq) Weak H 3 P 4, H 2 S 3, & H 2 C 3 - all dissociations are weak K a1 > K a2 > K a3 Slide 18 of 23

19 Strength of xoacids The acid strength of an oxoacid is dependent upon two factors 1. The electronegativity of the central atom 2. The number of terminal atoms in the acid molecule Slide 19 of 23

20 Strength of xoacids 1. The Electronegativity of the Central Atom H Cl H Br E Neg (Cl) = 3.0 K a = 2.9 x 10-8 E Neg (Cl) = 2.8 K a = 2.1 x 10-9 HCl is stronger acid than HBr because Cl is more electronegative than Br, stronger pull on the electrons towards Cl Slide 20 of 23

21 Acid Strength of xoacids 2. The Number of Terminal xygen Atoms in the Acid Molecule H S H H S H K a = 10 3 K a = 1.3 x 10-3 The acid strength of an oxoacid increases with: increasing number of terminal oxygen atoms, due to the stability of the conjugate base as a result of resonance delocalisation Slide 21 of 23

22 Acid Strength of xoacids 2. The Number of Terminal xygen Atoms in the Acid Molecule Explanation -2 S S -2 In S 4 2- the negative charge is spread over more oxygen atoms than in S 3 2-, reducing the partial negative charge on each of the oxygen atoms. The lower the partial negative charge, the more stable the anion, the stronger the acid! Slide 22 of 35

23 Exercise Which acid is the stronger acid? Give reasons. Cl H I H Slide 23 of 23