chem lecture 8

chem lecture 8 - Are there any conceptual or problem-based...

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Chemistry 6A Are there any conceptual or problem-based areas from Exam 1 which you are still ‘fuzzy’ for you? (a) Matter, Energy (b) Development of Quantum Mechanics (c) Atomic orbitals (d) Multi-electron configurations (e) Periodic trends 1
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Chemistry 6A 2 Which of the following metal ions has the ground-state electron configuration [Ar]3 d 6? (a) Ni 3+ (b) Fe 2+ (c) Mn 2+ (d) Cu + (e) Ca 2+
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Chemistry 6A Let’s consider the formation of an ionic bond from two neutral gas-phase atoms, sodium and chlorine: Step 1: Ionization of sodium. Na(g) Na + (g) + e- Δ E = IE 1 = 494 kJ mol -1 Step 2: Electron gain by chlorine. Cl(g) + e- Cl - (g) Δ E = -EA = -349 kJ mol -1 The total energy for the creation of the gaseous ‘ion pair’ is 145 kJ mol -1 – it costs energy to transfer an electron from a Na to Cl atom! So, what interaction leads to the reduction in potential energy of the system required for the spontaneous formation of an ionic bond? 3
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Chemistry 6A If the Coulombic attraction energy (for oppositely charged ions) exceeds the energy required to make the ions themselves, then an ionic bond may form. Step 3: Formation of the crystalline solid. Na + (g) + Cl - (g) NaCl(s) Δ E = Lattice energy = -787 kJ mol -1 Net change for the overall process: Na(g) + Cl(g) NaCl(s) Δ E =145 - 787 = -642 kJ mol -1 A solid composed of Na+ and Cl- ions has a lower energy than does a collection of a collection of widely separated Na and Cl atoms. An ionic bond is a ‘global’ characteristic of the entire crystal. How do we assess the total potential energy for the entire crystal? 4
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Chemistry 6A The Coulomb potential energy (per ion pair) is: where z 1 and z 2 are the charge numbers of the two ions, and r 12 is the internuclear distance. Each ion in a solid is attracted to all other oppositely charged ions and repulsed by all like-charged ions, but these energy-lowering and energy -raising contributions become progressively weaker as the separation, d , increases. Overall, the nearest neighbors of an ion give rise to a strong attraction, such that the net outcome of all these contributions is energy lowering. 5 E P 1,2 = ( z 1 e )( z 2 e ) 4 πε 0 r 12
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Chemistry 6A The total potential energy (per mole of ions, N A ) is the sum of attractive and repulsive terms, with the minimum given by the Born-Meyer equation: 6 E P ,min = N A z 1 z 2 e 2 4 πε 0 d 1 d * d A
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Chemistry 6A 7 Consist of cations & anions held together by electrostatic forces of attraction, which are non-directional; each ion is bound to all its neighbors Under normal conditions, most compounds are crystalline solids in which the ions form clusters or continuous arrays High melting and boiling points; molten salts conduct electricity NaCl: m.p. 801°C KMnO 4 : m.p. 270°C decomp.
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Chemistry 6A Based on Coulombic forces, which would you expect to have the strongest ionic bond?
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