Chap 14, Aqueous Equilibria, Acids & Bases This chapter is an extension of the equilibrium chapter primarily to rxns. involving the transfer of H + ions in aqueous soln. I. Acid-Base Concepts: Brønsted-Lowry A. Chapter 4: Arrhenius acid-base ideas: 1. Acids dissociate, produce H + in H 2 O: HA (aq) H + (aq) + A (aq) 2. Bases too, produce OH in H 2 O: MOH (aq) M + (aq) + OH (aq) 3. Arrhenius theory doesn t always work (NH 3 is basic) 1
B. Brønsted-Lowry Theory 1. An acid is a substance that can donate a H +. 2. A base is a substance that can accept a H +. 3. A general expression of an acid-base rxn.: HA + B BH + + A acid base acid base 4. Conjugates: a) The conjugate base is what is leftover after the acid has donated its H +. b) The conjugate acid is what is leftover after the base has accepted its H +. (Probs 14.1-2) 2
II. Acid Strength & Base Strength (context dependent!!!) A. For now we take a qualitative approach to this subject. Later, we will do a quantitative approach. 1. Think about H 2 O functioning as a base: HA (aq) + H 2 O (l) H 3 O +( aq) + A (aq) acid base acid base The 2 bases are competing for the H + ion. Will the H + ion spend most of its time associated with the weaker of the 2 bases or the stronger? 3
2. For pictures, look at Conceptual Problem 14.3 & Table 14.1. 4
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3. You must become adept at this. Try Prob. 14.5 Alternatively, if HA is a strong acid, is its conjugate base strong or weak? Prob 14.5 6
III. Factors That Affect Acid Strength (comparative) A. Bond strength (within hydrohaloacid series) B. Electronegativity (within Period 2 elements) C. For oxoacids 1. Increasing electonegativity correlates with increasing acid strength. 2. Increasing oxidation number correlates with increasing acid strength. 7
IV. Dissociation of Water A. Water is interesting. It can be both an acid & a base: H 2 O + H 2 O H 3 O + + OH Lewis acid base acid base dot? 8
B. Can we write a K c expression for the above? 1. K c = 2. Can we simplify this? K w = 3. In pure water at 25 C, [H 3 O + ] = 1.0 x 10 7 M. 4. In pure water at 25 C, [OH ] =? 5. What is [H 2 O] =? 6. Numerical value for K w : K w = [H 3 O + ] [OH ] Prob. 14.8-10 9
V. The ph Scale (power of Hydrogen) There are a bunch of reasons for using the ph, as opposed to [H 3 O + ]. Convenience in expressing the numbers is probably most important to you. A. Definition: ph = log [H 3 O + ], so: [H 3 O + ] = 10 ph 1. Let s start with an example of ph. Prob. 14.11 2. Make sure you can also find [H 3 O + ]. Prob. 14.12 B. Note that as [H 3 O + ] increases, ph decreases because ph is the negative log of [H 3 O + ]. 10
C. What about rounding? Essentially the x / rule plus one. log (1.0 x 10 9 ) = 9.00 log (3.28 x 10 5 ) = 4.484 126 156 11
VI. Measuring ph A. Indicators 1. ph indicators have an acid form and a conjugate base form. The two forms are different colors: HInd (aq) + OH (aq) H 2 O (l) + Ind (aq) acid base acid base Color 1 Color 2 12
2. Example: Methyl orange (MeO) HMeO + OH H O + MeO (aq) (aq) 2 (l) (aq) acid base acid base red (< ph 3) orange (> ph 4.5) 3. ph paper has indicator linked to the paper B. ph meters have H + selective membranes/electrodes 13
VII. ph in Solns. of Strong Acids & Bases A. By definition, a strong acid or a strong base dissociates essentially completely in water: 1. HCl(aq) H 3 O + (aq) + Cl (aq) If [HCl] total = 0.5 M, at equilibrium: [H 3 O + ] = [Cl ] = 0.5 M, & [HCl] 0 M Aside on kinetics: Generally, acid-base rxns. in aqueous systems have extremely rapid kinetics. Does this mean it takes a long or a short time to reach equilibrium? 14
2. NaOH(aq) Na + (aq) + OH (aq) If [NaOH] total = 0.5 M, at equilibrium: [Na + ] = [OH ] = M, & [NaOH] M Try Prob. 14.13 (d), p. 566. B. If you dissolve H 2 SO 4 in water so [H 2 SO 4 ] total = 0.5 M, can you predict [H 2 SO 4 ],[HSO 4 ], [SO 4 2 ], & [H 3 O + ] =? 15
VIII. Equilibria in Solutions of Weak Acids A. We can write an equilibrium constant relationship for acid-base rxns. just as we did in Chapter 13. 1. HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) [H 3 O + ] [A ] K c = [HA] [H2 O] 16
2. [H 2 O] is so large compared to the acid component, it is essentially constant. Therefore: [H 3 O + ] [A ] [H 2 O] x K c = If [H 2 O] x K c = K a [HA] K a = [H 3 O + ] [A ] [HA] B. The value of K a gives us an absolute (not relative) measure of acid strength. 1. Is it clear that stronger acids have larger K a s? 2. Refer to Table 14.2, p. 567. 3. The acids that we previously described as strong acids have K a >1. (Often >>>1) Try Prob. 14.15, p. 568. 17
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Conceptual Problem 14.15 (not 13), p. 568. IX. Equilibrium Calculations: Weak Acid Solns. A. We will now use the same approach (0.1 M x) for calculating concentrations of components at equilibrium that we used in Chapter 13. (Summary in Fig. 14.7.) 1. List the system components. 2. Write equations & examine K a values for the rxns. 3. Identify the principal reaction by its larger K a value. 19
The other rxns. are subsidiary reactions, and we can worry about them later. 4. Make a table like we did in Chap. 13: [Reactant] [H 3 O + ] [A ] [Initial] M 0.050 0 0 [Change] M x +x +x [Equilibrium] M 0.050 x x x 5. Now shift to your algebra mode: a) Substitute the bottom row into the K a expression. b) Solve for x. 20
6. Use the value of x to solve for all [equilibrium]. 7. If you wish, check your assumption re. Item #3. B. Practice this thoroughly, so you will be comfortable doing it on an exam. Try Prob. 14.17 (a), p. 573. 21
X. % Dissociation in Solns. of Weak Acids. A. This is another way of comparing [A ]/[HA]. By definition: [A ] % dissociation = x 100% [HA] total B. If you compare equal [HA] total s of a strong acid & a weak one, strong acid will obviously have a higher % dissociation. (Check this on your own.) C. What happens when you look at % dissociation as a function of [HA] total? 22
1. The % dissociation increases as you dilute the acid. 2. This should seem logical if you consider collision theory and if you examine the forward and backward rate laws. (See Fig. 14.8, p. 573) 3. Do prob. 14.19, p. 573 on your own. 23
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XI. Polyprotic Acids A. Acids that donate > 1 H + are polyprotic. Ex.: H SO, H PO, H 2 CO 4, & citric acid 3 4 2 3 B. If the 2 (or more) acidic H are near each other, K ~10 4 10 6 > than K. (Electrostatics.) a1 a2 1. Let s look at the dissociation of H 2 CO 3. 1 st dissociation K a1 = 2 nd dissociation K a2 = 25
2. Look at Table 14.3, p. 574, for summary of values. 3. Try Prob. 14.20, p. 577 on your own.. XII. Equilibria in Solutions of Weak Bases. A. Bases accept H + : H 2 O + B BH + + OH K b = [BH + ] [OH ] [B] B. This is directly analogous to what we have done with acids. (See Table 14.4, p. 577, for a comparison of K & K values for bases and their b a conjugate acids.) Transition! 26
XIII. Relationship Between K a and K b. A. Sect II, we concluded that a strong acid must have a weak conjugate base. Now, quant. B. Consider the loss of H + by the acid HF, and the gain of H + by the base F : 27
1. Reactions: Acid rxn. HF(aq) + H 2 O(l) H 3 O + (aq) + F (aq) Base rxn. F (aq) + H 2 O(l) HF(aq) + OH (aq) Sum: HF + H 2 O + F + H 2 O H 3 O + + F + HF + OH (states omitted to save space) 2. A K c expression for the summed rxn. would be: [H 3 O + ] [F ] [HF] [OH ] K c = [HF] [H2 O] [F ] [H 2 O] which simplifies to: K c = [H 3 O + ] [OH ] = K w 3. The same result is obtained by multiplying the K c 28
expressions for the individual rxns.: [H 3 O + ] [F ] [HF] [OH ] K a x K b = x = [H 3 O + ][OH ] = K w [HF] [F ] Aside: We have just proven for this case something that is true generally. When 2 or more rxns are added together, the K c for the summed rxn. = the product of the K c values for the individual rxns. C. Finally, if K a x K b = K w, that means that K a and K b must be inversely proportional. (If K a is big, K b must be small.) This supports our intuitive comments on the strength of acids & their conjugate bases earlier in Section II. 29
XIV. Acid-Base Properties of Salts A. Ionic compounds are also called salts. 1. One way to make salt is: acid + base salt + water. 2. Understand salt s acid-base properties by looking at the strengths of the acid & base used to prepare it. B. Some guidelines: 1. If both acid & base were strong, salt will be neutral. 2. If acid was strong & base weak, salt will be acidic. 3. If base was strong & acid weak, salt will be basic. 4. If both acid and base were weak, the outcome is decided by which is less weak (compare K a & K b ). Try prob. 14.25, p. 583. 30
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XV. Lewis Acids and Bases A. By definition: 1. A Lewis acid is an e pair acceptor. 2. A Lewis base is an e pair donor. B. Lewis acid-base concepts are extremely useful for analyzing some types of chemical rxns./processes. 1. Organic chemistry 2. Metal ion-ligand interactions (hemoglobin?) 3. Some branches of inorganic chemistry (boron) 32
FYI: Acid Rain and its Effects A. Sulfur and nitrogen oxides + H 2 O form oxoacids. B. SO x & NO x are formed in combustion processes. C. Oxo acids can kill creatures like fish and dissolve marble statues. See prob. 14.31 & 32, p. 589. 33