Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry, 2007 (John Wiley) ISBN:

Slide 2/16 e CHEM1002 [Part 2] A/Prof Adam Bridgeman (Series 1) Dr Feike Dijkstra (Series 2) Weeks 8 – 13 Office Hours: Monday 2-3, Friday 1-2 Room: 543a

Slide 3/16 e Lecture 4: Titrations Lecture 5: Periodic Trends Oxides and Hydroxides Strengths of Acids and Bases Blackman, Chapter 4, Section Periodic Trends

Slide 4/16 e Periodicity of Atomic Radii Li Na K Rb Cs He Ne Ar Kr Xe

Slide 5/16 e Periodicity of Atomic Radii Across each period  Increase in nuclear charge Z  Increase in e - /e - repulsion Electrons move very fast and can avoid each other  Increase in effective nuclear charge: Z eff  Z eff increases across period  Size decreases across period

Slide 6/16 e Periodicity of Atomic Radii Increase in size down each group  Increase in principal quantum number, n  Increase in shell size Elements in a group have similar properties Electrons in inner shells shield charge Core charge = number of protons – number of core electrons Z eff (Li) ≈ = 1 Z eff (Na) ≈ = 1 Z eff (K) ≈ = 1

Slide 7/16 e Trends in Two Atomic Properties Trends in atomic properties are usually either similar to the trend in the atomic radii or the exact opposite Diagonally related elements often have similar properties: diagonal relationship atomic size decreases ionization energy increases decreases

Slide 8/16 e Ionic and Atomic Radii Ionization decreases e - /e - repulsion  Z eff increases  Cations are smaller than the respective atom  e.g. Ba pm but Ba pm Adding an electron increases e - /e - repulsion  Z eff decreases  Anions are larger than the respective atom  e.g. O - 73 pm but O pm, F 72 pm but F pm Anions are often much larger than cations  e.g. Ba pm (row 6) but O pm (row 2)

Slide 9/16 e Tendency of an atom to attract electrons towards itself in a molecule  increases with Z eff  decreases with atomic size Electronegativity electronegavity increases decreases

Slide 10/16 e The Pure Elements Bonding ranges from metallic for metals (bottom left of the periodic table) to covalent for non-metals (top right), with semimetals (metalloids) lying between. tend to lose electrons to form cations tend to gain electrons to form anions

Slide 11/16 e Oxides and Hydroxides I Metal oxides and hydroxides are basic  Metal oxides and hydroxides are ionic due to large difference in electronegativity (  EN ) between O and metal Metal oxides (e.g. Na 2 O,  EN = 2.6) contain the O 2- ion, which combines with H + to form H 2 O: Na 2 O(s) + H 2 O(l)  2OH - (aq) + 2 Na + (aq) Metal hydroxides (e.g. NaOH) contain the OH - ion OH - and (especially) O 2- have high affinity for H + ion

Slide 12/16 e Oxides and Hydroxides II Non-metal oxides and ‘hydroxides’ are acidic  non-metals form covalent bonds to oxygen due to low electronegativity difference  they do not contain O 2- or OH - Non-metal oxides dissolve in OH - to form a salt + H 2 O: e.g. CO 2 (g) + 2OH - (aq)  CO 3 2- (aq) + H 2 O(l) Non-metal hydroxo compounds (e.g. HNO 3, H 2 SO 4 ) react with OH - as follows: e.g. X-O-H(aq) + OH - (aq)  X-O - (aq) + H 2 O(l)

Slide 13/16 e Oxides and Hydroxides III Semi-metal (metalloid) oxides and ‘hydroxides’ are amphoteric - they can act both as acids and bases)  electronegativity is intermediate between metal and non-metal  As a base: Al 2 O 3 (s) + 6 HCl(aq)  Al 2 Cl 6 (aq) + 3 H 2 O(l)  As an acid: Al 2 O 3 (s) + 2 OH - (aq) + 3 H 2 O(l)  2 Al(OH) 4 - (aq)

Slide 14/16 e electronegativity increases Acid-Base Behaviour basic oxide (ionic) + H 2 O → OH - + acid → salt + water amphoteric oxide + acid → salt + water + base → oxyanion + water acidic oxide (covalent) + base → oxyanion + water + acid → H +

Slide 15/16 e Strongest acids lose their protons easily  more polarised the H-X bond is, the stronger the acid  the larger the X is, the weaker the bond, the stronger the acid  the more X=O groups, the greater the acid strength H2OH2OHF H2SH2SHCl H 2 SeHBr H 2 TeHI increasing electronegativity, increasing acidity increasing bond length, increasing acidity Relative Strengths of Acids and Bases

Slide 16/16 x Practice Examples 1. Rank the following series of atoms in order of increasing electronegativity. N O F P As (a) N < O < F < P < As (b) F < O < N < P < As (c) As < P < N < O < F (d) P < As < N < O < F (e) F < N < O < As < P 2. Arrange the following atoms in order of increasing atomic radius. N F Si P (a) F < N < P < Si (b) N < F < P < Si (c) N < F < Si <P (d) F < N < Si < P (e) P < Si < N <F 3. Rank H 2 O(l), H 2 S(aq) and HF(aq) in order of their Brønsted acid strengths. Explain your reasoning (2007-N-3)