9-10 Bond Energies And you

Remember: it requires energy to break a bond, whereas energy is released when a bond is formed. SO HOW DO WE KNOW THE ENERGY CHANGE ASSOCIATED WITH CHEMICAL REACTIONS ????????????????????????????? Method 1 Bond energies have been tabulated, and represent averages over the different molecules atom pairs measures.

Calculating ∆H from Bond Energies for Covalent Bonds Set up rxn. For 1 mole of product Add all energies on the reactant side; add all energies on the product side. If more energy is on the reactant side, then then overall reaction requires energy (endothermic). If more energy is on the product side, then the overall reaction releases energy (exothermic).

H = -91 kJ/mol (exothermic) notice – sign = gives off E = exothermic Example 1/2 H2(g) + 1/2 Cl2(g) → HCl 216 120 → 427 336 → 427 net = → 91 Therefore: H = -91 kJ/mol (exothermic)  notice – sign = gives off E = exothermic

Calculating ∆H for the Formation of Ionic Bonds in a Lattice The calculation for the enthalpy change in producing a crystal lattice is a bit more complicated and can be done by summing five steps: Just think “SIBEL”

Li(s) + 1/2F2(g) → LiF(s) 758 → 1375 net → 617 Li(s) + 161 → Li(g) sublimation Li(g) + 520 → Li+(g) + e- ionization energy 1/2F2(g) + 77 → F(g) bond energy F(g) + e- → F-(g) + 328 electron affinity Li+(g) + F-(g) → LiF(s) + 1047 lattice energy 758 → 1375 net → 617 H = -617 kJ/mol (exothermic) WOW!!!! 

9-11 Summary Fundamentally, chemical bonding affects the properties of a substance. An excellent example of this is the bonding in two form of pure carbon (allotropes): diamond and graphite.

Graphite (above) has C atoms that are sp2 hybridized Graphite (above) has C atoms that are sp2 hybridized. The C atoms for six membered rings within “layers” of graphite. These layers are then held together by weak dispersion forces. This structure is soft, black, and conducts.

Diamond, in contrast, has C atoms that are sp3 hybridized into a tetrahedral network, giving diamond properties of hardness, colorless, and a nonconductor WOW so hybridization does have an impact and important to know? You said it!!!!