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Unformatted text preview: en The the ionic character of a covalent bond and the electronegativity difference between the bonded atoms between Ionic Bond e.g., LiF, NaCl
12/04/09 Zumdahl Chapter 13 19 Covalent Bond Energy and Enthalpy
Bond Enthalpy (ΔH) is the enthalpy change in a reaction Bond in which a chemical bond is broken in the gas phase. Bond Energy (ΔE) is the energy needed to break a chemical bond.
– Energy is released when bonds are formed ( – Energy must be supplied when bonds are broken Energy ( ) ) Gas Phase: ΔH = Condensed Phase: ΔH = Condensed
12/04/09 Zumdahl Chapter 13 20 The breaking of chemical bonds in stable substances often generates highly reactive products (or often intermediates, i.e., radicals) intermediates,
4 3 CH → ·CH + ·H (ΔH° = + 439 kJ mol-1)
Bond Enthalpy 12/04/09 Zumdahl Chapter 13 21 Average Bond Enthalpies C2H6 → ·C2H5 + ·H ·C
-1 ΔH° = + 410 kJ H° mol mol
-1 CHF3 → ·CF3 + ·H ·CF ΔH° = + 429 kJ mol
-1 CHCl3 → ·CCl3 + ·H ΔH° = + 380 kJ mol ·CCl CHBr3 → ·CBr3 + ·H ·CBr
12/04/09 ΔH° = + 377 kJ H°
22 mol mol Zumdahl Chapter 13 12/04/09 Zumdahl Chapter 13 23 Estimate the Standard Enthalpy of Reaction for Estimate the gas-phase reaction that forms methanol from methane and water methane
4 2 3 2 CH (g) + H O(g) → CH OH(g) + H (g) CH Approach:
(1) Break all bonds in reactants to generate C+O+6H (1) (2) Reassemble the atoms into products 12/04/09 Zumdahl Chapter 13 24 4 2 3 2 CH (g) + H O(g) → CH OH(g) + H (g) CH
H C H H H + H H O H H C H O H + H H Broken 4 C−H = 4 x 413 2 O−H = 2 x 467 Formed 3 C−H or 3 x (−413) 1 O−H or 1 x (−467) 1 C−O or 1 x (−358) 1 H−H or 1 x (−432)
-1 −2496 kJ mol H(broken) ΔH = ΔH(broken) + ΔH(formed)Exothermic +2586 kJ mol-1 Endothermic = 12/04/09 +2586+(-2496)= Zumdahl Chapter 13 25 CCl2F2 + 2H2 → CH2Cl2 + 2HF ∆ Hr = ? = ΔHbond breaking – ΔHbond bond making making
-1 -1 = –114 kJ mol –114 12/04/09 Zumdahl Chapter 13 26 12/04/09 Zumdahl Chapter 13 27 Lewis Structures to Represent Covalent Bonding
Lewis structures (also called Lewis dot diagrams) are diagrams Lewis that show the bonding of a molecule and the lone pairs of electrons that bonding lone that may exist in the molecule. A Lewis structure can be drawn for any covalently-bonded molecule. any The Lewis structure is named after Gilbert N. Lewis, who Lewis Gilbert introduced it in his 1916 article entitled The Atom and the Molecule introduced The [Journal of the American Chemical Society 1916, Vol. 38, p. 762]. Journal 1916 Vol. 38 12/04/09 Zumdahl Chapter 13 28 Drawing Lewis Structures
1. Count up the total number of valence electrons available (A) by first adding Count total valence available by the group numbers of all the atoms present. If the species is a negative ion, add the absolute value of the total charge; if it is a positive ion, subtract it. add subtract 1. Calculate the total number of electrons needed (N) for each atom to have its Calculate total needed for own noble-gas set of electrons around it (two for...
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