chem lecture 8

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

Info iconThis preview shows pages 1–9. Sign up to view the full content.

View Full Document Right Arrow Icon
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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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+
Background image of page 2
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
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 4
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
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
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
Background image of page 6
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.
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Based on Coulombic forces, which would you expect to have the strongest ionic bond? (a) CaO
Background image of page 8
Image of page 9
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 08/07/2009 for the course CHEM 6A taught by Professor Pomeroy during the Spring '08 term at UCSD.

Page1 / 36

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

This preview shows document pages 1 - 9. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online