chapter 7

chapter 7 - Formation of NaCl Ionic Solid Energetics of...

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Formation of NaCl Ionic Solid
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Energetics of Formation of NaCl Ionic Solid from Elements Na(s) Na(g) 107.3 kJ/mol Na(g) Na + (g) + e - 495.8 kJ/mol ½ Cl 2 (g) Cl(g) 122 kJ/mol Cl(g) + e - Cl - (g) -348.6 kJ/mol Na + (g) + Cl - (g) NaCl (s) -787 kJ/mol NET: Na(s) + ½ Cl 2 (g) NaCl (s) -411 kJ/mol
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Energetics of Ionic Bond Formation Lattice Energy (U)  of the solid refers to the breakup of  the crystal into ions.  It is the sum of the electrostatic  interaction energies betweens ions in a crystal Example: NaCl(s)   Na + (g)   +   Cl - (g) U=+787 kJ/mol Na + (g)   +   Cl - (g)    NaCl(s) U=-787 kJ/mol Force that results from interaction of electrical energies: Coulumb’s Law 2 2 1 d z z k F = d z z k Fd U 2 1 = = - Lattice energies are largest when  distance d between ions is small and  charges z 1  and z 2  are large
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  For compounds with the  same anion  but  different cation as the  cation  becomes  smaller , the  lattice energy increases   For compounds with the  same cation , but  different anion , As the  anion  becomes  smaller,  the  lattice energy increases   Compounds with higher charges have larger lattice energies    than compounds of ions with lower charges
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Formation of Ions In a solid lattice, any given ion has a large number of attractive and repulsive interactions.
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Covalent bonds: formed by sharing pairs of electrons Length and strength of bond results form interplay of attraction between opposite charges and repulsion between like charges Net attractive forces hold atoms together
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Bond Length of Covalent Bond Covalent bond has length that leads to maximum stability. This internuclear distance is the bond length Bond lengths can be estimated from atomic radii.
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Figure 7.4
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Chemical Bonds and Energy When two atoms approach one another, the negatively charged electron clouds are attracted to the other atom’s positively charged nucleus. Figure 7.5
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Bond Dissociation Energy Bond Dissociation Energy (D): Amount of energy required to break a chemical bond Value is positive because energy is required to break a bond Bonds between similar atoms have similar dissociation energies
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Average Bond Dissociation Energies
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Chemical Bonds and Reactions
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During ionic bond formation , the cations and anions achieve np 6 electronic configurations (noble gas configuration). Metals lose electrons. Nonmetals gain electrons. During covalent bond formation , the bonding nonmetals achieve np 6 electronic configurations by sharing electrons. Shared electrons available to both bonding atoms.
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chapter 7 - Formation of NaCl Ionic Solid Energetics of...

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