5._Chapter_18_acid_base - Common-Ion Effect in Acid-Base...

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Common-Ion Effect in Acid-Base Equilibria Common-Ion Effect: is the shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance. Consider the ionization of a weak acid, HA: HA(aq) + H 2 O(l) A¯(aq) + H 3 O + (aq) According to Le Chatelier’s principle, the addition of A¯ (e.g. in the form NaA) or H 3 O + (e.g. In the form HCl or HNO 3 ) causes the equilibrium to shift to the left. In the presence of excess strong acid or excess A¯, the ionization of a weak acid HA is significantly suppressed. Similarly, the ionization of a weak base (B) is suppressed significantly in the presence of excess HB + or strong base.
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Example Calculate the percent ionization of CH 3 COOH in each of the following solutions. a) 1.0L if 0.125 M CH 3 COOH (aq) (K a = 1.8 x 10 -5 ) b) 1.0L if 0.125 mol CH 3 COOH and 0.025 mol CH 3 COONa
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Acid-base Neutralization reactions In an acid-base neutralization reaction, an acid and a base react in aqueous solution to produce a salt. Sometimes water is also produced. HCl (aq) + NaOH (aq) HF (aq) + NaOH (aq) We are especially interested in neutralization reactions that involve a strong base, an strong acid, or both. If a least one of the reactants is a strong acid or a strong base, then we can safely assume (usually!) that the neutralization goes to completion. How do we know that the reaction between weak acid and a strong base goes to completion? Because the equilibrium constant for the neutralization reaction (K neutr ) is very large. HCl (aq) + NaOH (aq) H 2 O (l) + NaCl (aq) Because HCl (aq) + NaOH (aq) completely dissociates the net equation for the neutralization reaction is H 3 O + (aq) + OH¯ (aq) 2 H 2 O (l) If we mix equal moles of HCl (aq) and NaOH (aq) the concentration of H 3 O + and OH¯ in the remaining NaCl solution after neutralization will be the same as those of pure water. Thus the reaction is the reverse of the dissociation of water, K n (‘n’n for neutralization) is just the reciprocal of the ion product for water K n = 1/K w 14 14 w 3 1x10 1x10 1 K 1 ] ][OH O [H 1 = = = = - - + n K H 3 O + (aq) + OH¯ (aq) 2 H 2 O (l)
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HF (aq) + OH¯ (aq) A¯(aq) + H 2 O (l) HF (aq) + NH 3 (aq)
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When we mix a weak acid and a strong base, we know that the neutralization will go to completion. The pH of the solutions is determined by what’s left behind by the neutralization reaction. Similarly, weak base plus a strong acid goes to completion and the pH of the final solution depends on what’s left behind by the neutralization reaction.
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