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Unformatted text preview: Ch. 18 Acid-Base Equilibria 18.1 Acids and Bases in Water Water is a product in all reactions btw strong acids and strong bases. Strong acids and strong bases = those that dissociate (into ions) completely in water. Water molecules surround the proton to form H-bonded species. b/c proton is so small, its charge density is very high, so its attraction to water is especially strong. The protons bonds covalently to one of the lone e-pairs of a water molecules O atom to form a hydronium ion, H 3 O + (H + (aq)), which forms H bonds to several other water molecules. Arrhenius acid-base definition: an acid is a substance that has H in its formula and dissociates in water to yield H 3 O + . a base is a substance that has OH in its formula and dissociates in water to yield OH- . Monoprotic acid: those w/one ionizable proton Neutralization: (when an acid and base react) when the H + ion from the acid and the OH- ion from the base combine to form H 2 O. Acid dissociation: strong acids dissociate completely into ions in water; weak acids dissociate very slightly into ions in water, great majority are undissociated. Acid-dissociation constant, K a : magnitude is temp. dependent and tells how far to the right reaction has proceeded to reach equilibrium. Stronger acid higher [H 3 O + ] at equilibrium, more dissociated larger K a Smaller K a lower % dissociation weaker acid Strengths of Acids and Bases: Strong Acids: 1. Hydrohalic acids HCL, HBr, HI 2. Oxoacids in which the # of O atoms exceeds the # H (ionizable protons) by two or more. HNO 3 , H 2 SO 4 , HClO 4 Weak Acids: 1. Hydrohalic acid HF 2. Acids in which H is not bonded to O or to a halogen. HCN and H 2 S 3. Oxoacids in which # of O atoms equals # H (ionizable protons) or exceeds by only one. HClO, HNO 2 4. Carboylic acids RCOO H , CH3COO H , C6H5COO H ( ionizable proton is bolded) Strong Bases: water-soluble compounds containing O 2- or OH- ions 1. M 2 O or MOH, where M=Group 1A (1) metal (Li, Na, K, Rb, Cs) 2. MO or M(OH) 2 , where M=Group 2A (2) metal (Ca, Sr, Ba) Weak Bases: N atom w/ lone e- pair 1. Ammonia (NH 3 ) lone e- pair on N 2. Amines (RNH 2 , R 2 NH, R 3 N). CH 3 Ch 2 NH 2 , (CH 3 ) 2 NH, and (C 3 H 7 ) 3 N 18.2 Autoionization of Water and the pH Scale Strong electrolytes dissociate completely, and weak electrolytes dissociate partially. Water is an extremely weak electrolyte b/c water itself dissociates into ions very slightly in an equilibrium process known as autoionization (self-ionization): H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH- (aq) K w : the ion-product constant for water ex. K c = [H 3 O + ][ OH- ] / [H 2 O] 2 include constant [H 2 O] 2 w/ term K c to get K c [H 2 O] 2 = K w = [H 3 O + ][ OH- ] Autoionization of water has 2 major consequences for aqueous acid-base chemistry: 1. A change in [H 3 O + ] causes an inverse change in [ OH...
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