Acid1TA__TA - TA Version Acids and Bases I: Strong and Weak...

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Unformatted text preview: TA Version Acids and Bases I: Strong and Weak Acids and Bases I STRONG AND WEAK Common strong acids, such as HA (aq), are molecules that dissociate to give H+ and A- in water. The H+ binds to water to generate H3O+. Familiar strong bases, such as MOH (aq), are ionic solids that dissociate in water to generate OH-. The equilibrium constants for these reactions tend to be greater than 1. strong acid → HA (aq) + H2O (l) ← H3O+ (aq) + A- (aq) K>1 strong base → MOH (aq) + H2O (l) ← M+ (aq) + H2O (l) + OH- (aq) K>1 The common weak acids, also referred to as HA(aq), react with water in the same way as strong acids. However, the equilibrium constant for the reaction is small, and thus, only a small amount of H3O+ forms. The common weak bases, B(aq), at first seem different than strong bases because they do not contain OH -. However, B(aq) causes H 2O to dissociate to give OH-(aq). The H + binds to B to generate HB+. Like the weak acid, the equilibrium constant for the reaction is small, and thus only a small amount of OH- forms. weak acid → HA (aq) + H2O (l) ← weak base → B (aq) + H2O (l) ← H3O+ (aq) + A- (aq) HB (aq) + + OH- (aq) [H 3O+ ][A − ] < 1 Ka = [HA ] + - HB OH K= b B] [ <1 In this discussion sheet, we will explore visual, chemical, and mathematical representations for aqueous solutions of strong and weak acids, as well as for strong and weak bases. Thinking about it: • • How do weak acids and strong acids differ? How do weak bases and strong bases differ? INITIAL IDEAS (answer on your own, then participate in a class discussion) You have a 1.00 M solution of the weak acid, HF, and a 1.00 M solution of the strong acid, HCl. Which solution has the highest concentration of H3O+ and why? (A) (B) (C) (D) HF: It is a weak acid, and fully hydrolyzes to give more H3O+ in solution. HCl: It is a strong acid, and fully hydrolyzes to give more H3O+ in solution. Same concentration of H3O+: Both solutions have a concentration of 1.00 M H3O+. Same concentration of H3O+: The concentration of HF is equal to the concentration of HCl. Justify your choice. HCl (Ka ~ 107) is a very strong acid. HCl fully dissociates and a “1.00 M” solution of HCl will contain 1.00 M of H3O+ in solution. HF (Ka = 3.5 x 10-4) is a weaker acid than HCl. The HF does not fully dissociate and a “1.00 M” solution of HF will contain less than 1.00 M of H3O+. © Regents of the University of California, 2002. 1 Acids and Bases I: Strong and Weak TA Version EXPLORING (work in groups of 4) The views of the following two solutions are at the level of molecules and ions. The total volume for each view is 3.65 x 10-22 L. Flask I: 0.100 M HCl Ka ~ 107 Flask II: 0.100 M HF Ka = 3.5 x 10-4 Legend + I H Cl F 2O II (Note: White background for both views = ∼12,200 molecules of H2O) Flask II: Concentration of HF (to be completed by a pair of students) Flask II: Concentration of H3O+ (to be completed by 2nd pair of students) a(I) Calculate [HF] in Flask II by: • using the Ka expression AND • using the molecular scale view. Are your two values consistent? Using the Ka expression a(II) Calculate [H3O+] in Flask II by: • using the Ka expression AND • using the molecular scale view. Are your two values consistent? Using the Ka expression We know that HF dissocates according to: We know that HF dissocates according to: H3O+ + F- HF + H2O Assume that the amount of H3O+ dissociated is “x” M. At equilibrium we have, Ka [H 3O ][F ] = HF x 2 1x 3.5 10 H3O+ + F- HF + H2O 4 Assume that the amount of H 3O+ dissociated is “x” M. At equilibrium we have, Ka = [H 3O ][F ] HF x2 1x 3.5 10 4 Solve for x. Solve for x. x = 5.74 x 10-3 M x = 5.74 x 10-3 M [H3O+] = 5.74 x 10-3 M [HF] = 0.1 - x = 0.094 M Using the molecular scale view Using the molecular scale view 21 molecules [HF] 1 mole 23 6.02 10 molecules 3.65 10-22 L = 0.095 M 1 molecule + [H 3O ] 1 mole 23 6.02 10 molecules -22 3.65 10 L = 4.55 x 10-3 M Yes, the two values are consistent. Yes, the two values are consistent. 2 © Regents of the University of California, 2002. TA Version Acids and Bases I: Strong and Weak Comparisons (Work together with the other pair in your group of 4 to answer the following questions) b. The label on Flask II is 0.100 M HF. At equilibrium, which of the following equals 0.100 M? A) [HF] B) [H3O+] C) [HF] + [H3O+] c. For the strong acid solution in Flask I and the weak acid solution in Flask II, the total number of ions is different. Explain why. There are more ions present in Flask I. All of the HCl molecules dissociate to form H3O+ and Cl- ions. In Flask II, approximately 1 out of 22 HF molecules dissociates to form H3O+ and F- ions. d. The atomic scale view for 3.65 x 10-22 L of 0.100 M NaOH is shown below in Flask III. Draw an atomic scale view for 3.65 x 10 -22 L of 0.100 M of base, B, in the box to the right (Flask IV). Kb for B is 3.5 x 10-4. (Hint: the value of Kb for B is the same as the value of Ka for HF in Flask II.) Flask III: 0.100 M NaOH Flask IV: 0.100 M “B” Legend + III H + Na B 2O IV *Note: Students shouldn’t have to do any calculations to determine the atomic scale view for Flask IV. The Kb for the base, B, is the same magnitude as the Ka for HF. In the atomic scale view for HF, 1 out of 22 molecules of HF dissociated in H2O to form H3O+ and F-. Thus, for the atomic scale view for B, 1 out of 22 molecules of B will react with H2O to form HB+ and OH-. e. Flask III is labeled as 0.100 M NaOH, yet there are no NaOH species shown in the view for Flask III. Flask IV is labeled as 0.100 M B, and there are B species present. Explain why. NaOH is a strong base and will dissociate completely to generate Na+ and OH-. Thus, no NaOH species are present in Flask III. B is a weak base since its Kb is 3.5 x 10-4. A small amount of B molecules will hydrolyze to generate HB+ and OH-. However, most of the B molecules will not react and we find B species present in Flask IV. © Regents of the University of California, 2002. 3 Acids and Bases I: Strong and Weak TA Version CLOSURE (answer on your own, then participate in a class discussion) Complete the following statements below to summarize the main points of the discussion. • “An acid reacts with water to produce … H3O+, while a base reacts with water to produce…OH-” Complete the statement above and explain. H3O+ (aq) + A- (aq) strong acid strong base → MOH (aq) + H2O (l) ← M+ (aq) + H2O (l) + OH- (aq) weak acid → HA (aq) + H2O (l) ← weak base • HA (aq) + H2O (l) → B (aq) + H2O (l) ← H3O+ (aq) + A- (aq) HB+ (aq) + OH- (aq) K>1 Ka = K~1 [H 3O+ ][A − ] < 1 [HA ] + - HB OH K= b [B] <1 “Strong acids dissociate completely in water. Weak acids… dissociate partially in water.” Complete the statement above and explain. We define a weak acid as one that does not completely dissociate in water. A 1.00 M solution of a strong acid such as HCl will contain 1.00 M H3O+. While a 1.00 M solution of a weak acid, HA, contains less than 1.00 M of H3O+. • “Strong bases dissociate completely in water. Weak bases… dissociate partially in water.” Complete the statement above and explain. We define a weak base as one that does not completely dissociate in water. A 1.00 M solution of a strong base such as NaOH will contain 1.00 M OH-. A 1.00 M solution of a weak base, B, contains less than 1.00 M of OH-. 4 © Regents of the University of California, 2002. ...
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This note was uploaded on 10/12/2011 for the course CHEM 1A taught by Professor Nitsche during the Fall '08 term at University of California, Berkeley.

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