# unpaired electrons = #electrons alone in orbital C has 2 unpaired electrons K has 1, Ag has 1, Pb has 2, Mn has 5, Mg has 0 Unpaired electrons make element paramagnetic. Drawing Lewis Structures for COVALENT compounds: The atom with lowest EN is the central atom. Count the total number of valence electrons. MAGIC NUMBER! (adjust for ions) Place one sigma bond (a pair of electrons) between each pair of bonded atoms. Subtract the total number of valence electrons used for bonds from the MAGIC NUMBER. Place lone pairs about each terminal atom except for hydrogen atoms. Subtract the number of lone pairs from the sum. If e- are still left at this point, assign them to the central atom. If the central atom is from the third or a higher period, it can accommodate more than four pairs of electrons. If the central atom is not yet surrounded by four electron pairs, convert one or more terminal atom lone pairs to pi bond pairs. COVALENT Metal + nonmetal Metal +polyatomic ion Poly ion + nonmetal Poly ion + poly ion Nonmetal + nonmetal Within poly ion Strongest type of bond Weaker bond Electrostatic attraction between oppositely charged ions Electrostatic attraction between nuclei and shared electrons Electrons transferred, creating ions Electrons shared – NO IONS! Forms crystal lattice And thus crystals A few form covalent networks (SiO2, , most form individual molecules Formula units molecules Higher MP, Higher BP (must break bond to change phase) Lower MP, lower BP (only break IMFS NOT bonds to change phase) Lattice Energy Bond energy Higher charge, smaller size= stronger bond Higher bond smaller atom = stronger bond Forms polar and nonpolar bonds EN difference Medium (polar) or Small (nonpolar) EN difference If soluble, forms electrolytes when dissolved Nonelectrolytes, may dissolve but won’t ionize! Electrolytic when molten, not solid Nonelectrolyte High % ionic character Low % ionic character Show resonance! Total number of sigma bonds and lone pairs on central atom Shape Atoms bonded to central atom (# of bonds) Lone on central atom Molecular Picture And bond angles Hybridization Symmetrical in 3d Example of a Lewis structure that is this shape. (Yes, draw the Lewis structure!) 2 Linear 2 0 sp yes 3 Triangular Planar 3 0 sp2 yes 3 Bent 2 1 sp2 no
Titrations Strong acid/strong base Strong acid/weak base Strong base/ Strong acid Strong base/weak acid Starting point high not as high low not as low Midpoint Not relevant pH= pKa Not relevant pH = pKa Equivalence point Exactly 7 Below 7 (conjugate acid present) Exactly 7 Above 7 (conjugate base present) Vertical region Very long Not as long Very long Not as long Always pick an indicator whose pKa = pH at the equivalence point of the titration. Various Ways to Describe Acid Strength Property Strong Acid Weak Acid Ka Too large to measure << 1 Position of the dissociation equilibrium right left Equilibrium concentration of H+ compared to the original concentration of HA [H+ ] = [HA]o [H+ ] << [HA]o Equilibrium concentration of HA compared to the original concentration of HA [HA] ~ 0 M [HA] << [HA]o [HA] ~ [HA]o Percent Dissociation 100% < 5% Strength of conjugate base Conjugate base is nonexistent Conjugate base is also weak, but inversely proportional in strength to the acid (Kb = Kw/Ka). How to represent in a net ionic equation H+ + A- HA Sign Of H0 System Entropy Change Sign ofS0 Sign of G0 Sign of E0 (if a K Spontaneous? (Thermodynamically favorable?) 1. Exothermic - Increasing + - + >1 always 2. Exothermic - Decreasing - + or - - or + Don’t know Low T 3. Endothermic + Increasing + + or - - or + Don’t know High T 4. Endothermic + decreasing - + - < 1 Galvanic Cell/ Voltaic Cell/ Electrochemical Cell Spontaneous Ex: battery! Chemical energy electrical energy E = + ΔG = - anode= oxidation Cathode = reduction
Electrolytic Cell Nonspontaneous Ex: electrolysis Electrical energy chemical energy E = - ΔG = + Anode= oxidation Cathode= reduction As reaction progresses, K doesn’t change BUT When rxn is shifting right When rxn is shifting left When a reaction is at equilibrium Q approaches K Q < K Q > K Q = K G approaches 0 G = - G = + G = 0 E approaches 0 E = + E = - E = 0