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Lecture+9sf - Acids and Bases Arrhenius Definition Acid...

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Acids and Bases Arrhenius Definition Acid: produces H + ions Base: produces OH - ions Bronsted-Lowry Definition Acid: Proton Donor Base: Proton Acceptor In context of acid-base, a proton is represented as H + Since an H atom has 1 proton and 1 electron, when that electron is removed, what remains is a proton (H + ion). In the Bronsted-Lowry concept, acids and bases are complementary—one doesn’t exist without the other. Consider hydrochloric acid, HCl. Pure HCl is a gas. In order to function as an acid (produce and donate H + ions), there must be an acceptor for these H + ions. This will occur in water. HCl + H 2 O → H 3 O + + Cl - acid base acid base Water accepts the proton donated by HCl, becoming a hydronium ion, represented as H 3 O + . We will mostly deal with aqueous solutions, where water is the acceptor of the H + ion donated by the acid. For generalized acid HX: HX + H 2 O → H 3 O + + X - Acid base acid base X - is called the conjugate base of HX (what is left over after the acid donates its H + ion). H 3 O + is the conjugate acid of water (what is obtained by adding an H + to water) The strength of an acid is measured by the acid dissociation constant, K a , which represents the equilibrium constant for the reaction of the acid with water. HX + H 2 O H 3 O + + X - K a = ] HX [ ] X ][ O H [ 3 - +
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The value of K a measure the strength of the acid, meaning the ability of the acid to donate a proton to water. Water is always the base (proton acceptor) in all reactions denoted by K a . K a HCl + H 2 O → H 3 O + + Cl - large H 2 SO 4 + H 2 O → H 3 O + + HSO 4 - large HNO 3 + H 2 O → H 3 O + + NO 3 - large H 3 O + + H 2 O → H 3 O + + H 2 O 1 HSO 4 - + H 2 O → H 3 O + + SO 4 2- 1.2 x 10 -2 CH 3 COOH + H 2 O → H 3 O + + CH 3 COO - 1.8 x 10 -5 HCN + H 2 O → H 3 O + + CN - 6.2 x 10 -10 H 2 O + H 2 O → H 3 O + + OH - K w = 1.0 x 10 -14 Acids with “large” K a values are called strong acids . They are essentially 100% ionized in water. Almost none of the unionized acid exists in water. Using HCl as an example: HCl + H 2 O H 3 O + + Cl - K a is large stronger acid stronger base weaker acid weaker base This means that at equilibrium, there is almost no molecular HCl left. This implies that HCl is a stronger acid than H 3 O + and that its conjugate base, Cl - , is a weaker base than water. Acids which are stronger than H 3 O + are all strong acids. In water solutions, they get converted to H 3 O + by completely donating a proton to water. For these strong acids, water is considered to be a leveling solvent . All the strong acids appear equal in strength (in water), and are 100% ionized. These acids could be differentiated by adding them to a base which was weaker than water. Weak acids, on the other hand, are differentiated in water, with a wide variety of K a values. A acid, CH 3 COOH, is a weak acid with K a = 1.8 x 10 -5 . CH 3 COOH + H 2 O H 3 O + + CH 3 COO - K a = 1.8 x 10 -5 weaker acid weaker base stronger acid stronger base Acetic acid, CH 3 COOH, is only partially ionized in water. It is a weaker acid than H 3 O + , and its conjugate base, CH 3 COO - , is a stronger base than H 2 O.
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