P a g e 1 Chapter 16 ACID-BASE EQUILIBRIA Nature of Acids and Bases Before we formally define acids and bases, let s examine their properties. Properties of Acids Sour taste Ability to dissolve many metals Ability to turn blue litmus paper red Ability to neutralize bases. Some Common Acids Structure of Acids Acids can be binary acids, oxyacids, or carboxylic acids. Binary Acids have hydrogen ion (H + ) bonded to a nonmetal element. For example, HCl, HBr, HI, and so on. Oxyacids (also oxoacids) have the acid hydrogen attached to an oxygen atom.
P a g e 2 Carboxylic Acids They are a subset of oxyacids. Carboxylic acids have a COOH group. For example, HC2H3O2, H3C6H5O7 Only the first H in the formula is acidic. The H is on the COOH. We will explore carboxylic acids in Organic and Biochemistry. CHEMISTRY II HONORS. Properties of Bases Bitter taste. Slippery feel. o Bases feel slippery because they react with oils on the skin to form soap-like substances. Ability to turn red litmus blue. Ability to neutralize acids. Some Common Bases Definitions of Acids and Bases Over time, there has been several definitions of acids and bases put forth by chemists according to their observations and what was currently accepted. This course will only explore the Arrhenius and Brønsted- Lowry definitions. Arrhenius Definitions Proposed by Svante Arrhenius in the 1880 s. Acid: a substance that produces H + ions in aqueous solutions. o The H + ions produced by the acid are so reactive that they cannot exist in water. Instead, they react with water molecules to produce complex ions, mainly hydronium ion, H3O +. H + + H2O H3O + Base: a substance that produces OH - ions in aqueous solutions. Acid-Base Reactions: Acid + Base Salt + Water HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
P a g e 3 Example 1 Complete and balance the following acid-base reactions a. H3PO4(aq) + Ba(OH)2(aq) b. H2CO3(aq) + KOH(aq) c. HClO2(aq) + Mg(OH)2(aq) d. HIO3(aq) + NaOH(aq) Brønsted -Lowry Definition Proposed in 1923 by Johannes Brønsted and Thomas Lowry. It focuses on transfer of protons (H + ) and is widely applicable. Acid: proton (H + ion) donor. Base: proton (H + ion) acceptor. It expands our knowledge of compounds that are acids and bases applies nicely to bases such as NH3 that do not inherently contain OH - ions but still produce OH - ions in solution. All reactions that fit the Arrhenius definition also fit the Brønsted Lowry definition. According to this definition, HCl is an acid because, in solution, it donates a proton to water: HCl(aq) + H2O(l) H3O + (aq) + Cl - (aq) When NH3 dissolves in water, the NH3(aq) is the base because NH3 accepts an H + from H2O, forming OH (aq). NH3(aq) + H2O(l) NH4 + (aq) + OH - (aq) Some substances such as water in the previous two equations can act as acids or bases. Substances that can act as acids or bases are amphoteric. Conjugate Acid-Base Pairs In a Brønsted Lowry acid base reaction, the original base becomes an acid in the reverse reaction. the original acid becomes a base in the reverse process. Each reactant and the product it becomes is called a conjugate pair. A base accepts a proton and becomes a conjugate acid. An acid donates a proton and becomes a conjugate base.
P a g e 4 Example 2 Complete the following table: Acid Base Conjugate Acid Conjugate Base Equation NH4 + H3O + NH4 + + H2O NH3 + H3O + H2O F - OH - NH3 + HCN NH4 + + CN - H2O ClO3 - S 2- + H2O OH - + HS - HCO2H OH - OH - CO3 2- HBr + OH - H2O + Br - SeO4 2- NH3 C6H5COOH HNO2 H2O SO3 2- H2O C5H5NH + Example 3 What is the conjugate base of each of the following acids: HClO4, H2S, PH4 +, HCO3 -? What is the conjugate acid of each of the following bases: CN -, SO4 2-, H2O, HCO3 -? Example 4 Hydrogen sulfite ion (HSO3 - ) is amphiprotic. (a) write an equation for the reaction of HSO3 - with water, in which the ion acts as an acid. (b) write an equation for the reaction of HSO3 - with water, in which the ion acts as a base. In both cases, identify the conjugate acid-base pairs.
P a g e 5 Example 5 When lithium oxide (Li2O) is dissolved in water, the solution turns basic from the reaction of the oxide ion (O 2- ) with water. Write the reaction that occurs, and identify the conjugate acid-base pairs. Acid Strength and the Acid Ionization Constant (Ka) Acids can be classified as either strong or weak, based on the extent of ionization. A strong acid completely ionizes in solution, whereas a weak acid only partially ionizes. Let s consider the ionization of the generic acid, HA: HA(aq) + H2O(l) H3O + (aq) + A - (aq) If the equilibrium lies far to the right, the acid is strong it completely ionizes. If the equilibrium lies to the left, the acid is weak only a small percentage of the acid molecules ionizes. Strong Acids HCl is an example of strong acid. HCl(aq) + H2O(l) H3O + (aq) + Cl - (aq) The single arrow indicates complete ionization. A 1.0 M HCl solution has an H3O + concentration of 1.0 M. There are six important strong acids (MUST MEMORIZE!!!!) Hydrochloric acid (HCl) Hydrobromic acid (HBr) Hydroiodic acid (HI) Nitric acid (HNO3) Perchloric acid (HClO4) Sulfuric acid (H2SO4) The first five are monoprotic containing one ionizable proton. The last one (H2SO4) is diprotic contains two ionizable protons. Weak Acids They only partially ionize in aqueous solutions. Let s consider HF, a weak acid: HF(aq) + H2O(l) H3O + (aq) + F - (aq) An HF solution will contain a large number of un-ionized HF molecules; it also contains some HF and F -. A 1.0 M HF solution will have an H3O + concentration less than 1.0 M. The degree to which an acid is strong or weak depends on the attraction between the anion of the acid (the conjugate base) and the hydrogen ion, relative to the attraction of these ions to water. In general, the stronger the acid, the weaker its conjugate base, and vice versa.
P a g e 6 Some Weak Acids The Acid Ionization Constant, Ka We can quantify the strength of a weak acid using the acid ionization constant, Ka. Let s consider two equivalent reactions: Recall that [H3O + ] and [H + ] are equivalent. The larger the Ka, the stronger the acid. Ka Values for Some Monoprotic Acids at 25 C
P a g e 7 Autoionization (Autoprotolysis) of Water and ph Recall that H2O is amphoteric; therefore, it can react with itself in a process known as autoionization to produce H + and OH -. H2O(l) + H2O(l) H3O + (aq) + OH - (aq) Which can also be written as, H2O(l) H + (aq) + OH - (aq) The equilibrium constant, Kw, can be written as Kw = [H3O + ] [OH - ] = [H + ] [OH - ] Kw is known as the ion product constant for water. At 25 C, Kw= 1.0 x 10-14. In pure water, [H3O + ] and [OH - ] are equal; therefore, neutral. [H3O + ] = [OH - ] = K w = 1.0 10 7 The product of [H3O + ] and [OH - ] always equals 1.0 x 10-14. If [H3O + ] increases, then [OH - ] must decrease for the ion product constant to remain 1.0 x 10-14. An acidic solution contains [H3O + ] > [OH - ] A basic solution contains [OH - ] > [H3O + ] Example 6 Calculate [OH ] at 25 C for each solution and determine if the solution is acidic, basic, or neutral. a. [H3O + ] = 7.5 10 5 M b. [H3O + ] = 1.5 10 9 M c. [H3O + ] = 1.0 10 7 M Example 7 Calculate [H3O + ] at 25 C for each solution and determine if the solution is acidic, basic, or neutral. a. [OH ] = 1.5 10 2 M b. [OH ] = 1.0 10 7 M c. [OH ] = 8.2 10 10 M
P a g e 8 The ph Scale The ph scale is a convenient way of quantifying acidity and basicity. We define ph as: ph= -log[h3o + ] Also, ph= -log[h + ] A solution with [H3O + ] = 1.0 x 10-3 M, has a ph of: ph= -log[h3o + ] = -log (1.0 x 10-3 ) = -(-3.00) = 3.00 In general, at 25 C: ph < 7 The solution is acidic. ph > 7 The solution is basic. ph = 7 The solution is neutral. o 1 ph unit corresponds to a factor of 10 difference in acidity. o For example, a lime with a ph of 2.0 is 10 times more acidic than a plum with a ph of 3.0 and 100 times more acidic than a cherry with a ph of 4.0. Example 8 Calculate the ph of each solution at 25 C and indicate whether the solution is acidic or basic. a. [H3O + ] = 1.8 10 4 M b. [OH ] = 1.3 10 2 M Example 9 a. Calculate [H3O + ] for a solution with a ph of 4.80. b. Calculate [H3O + ] for a solution with a ph of 8.37.
P a g e 9 The poh Scale The poh scale is analogous to the ph scale, but is defined with respect to [OH - ] instead of [H + ]. poh = -log[oh - ] In general, at 25 C: poh < 7 The solution is basic. poh > 7 The solution is acidic. poh = 7 The solution is neutral. Example 10 Calculate the poh of each solution at 25 C and indicate whether the solution is acidic or basic. a. [H3O + ] = 1.8 10 4 M b. [OH ] = 1.3 10 2 M c. [H3O + ] = 1.5 10 9 M d. [H3O + ] = 1.0 10 7 M Relationship Between ph and poh [H 3 O + ][OH ] = 1.0 10 14 Take log of both sides log([h 3 O + ][OH ]) = log(1.0 10 14 ) log[h 3 O + ] log[oh ] = 14.00 ph + poh = 14.00 Example 11 Repeat example 10 using the relationship between ph and poh.
P a g e 10 pk a pk a is a convenient way of quantifying the relative strengths of acids. We define pk a as follows: pk a = logk a Larger pk a = weaker acid = stronger conjugate base. Writing K a Expressions Example 12 Write the balanced chemical equation and the corresponding Ka equilibrium expression for each of the following substances (acting as acids in water) a. HPO4 2- b. HCN c. NH4 + Finding the ph of Strong Weak Acid Solutions Strong Acids ionize completely in solution the concentration of H3O + in a strong acid solution is equal to the concentration of the strong acid. Example 13 Calculate the [H3O + ], [OH - ], ph, and poh of a 0.25 M HCl solution. Example 14 What mass of HClO4 should be present in 0.500 L solution to obtain a solution with ph = 1.50?
P a g e 11 Weak Acids partially ionize in solution. concentration of H3O + is not equal to the concentration of the weak acid. we have to solve an equilibrium problem. Let s consider the ionization of a 0.10 M generic weak acid, HA: We have to set up an equilibrium table (ICE table) as follows: Features of the ICE table o ICE Initial, Change, Equilibrium. o The initial concentration of H3O + is approximately zero; that of the conjugate base, A -, is zero. o For change, the reactant gets x, while the products get +x. o For equilibrium, sum the initial and change for each column. o Write the equilibrium constant expression for the weak acid. o Algebraically solve for the x quadratic equation. o Find ph. x = b ± b2 4ac 2a Example 15 Find the ph of a 0.100 M HCN solution. Solve using the x is small approximation. = x 2 0.10 x
P a g e 12 Example 16 Find the ph of a 0.200 M HNO2 solution. Solve using the x is small approximation. Example 17 Find the ph of a 0.100 M HClO2 solution. Solve using the x is small approximation; see if it works!
P a g e 13 Example 18 Calculate the ph and poh of a 0.180 M solution of benzoic acid. Example 19 A 0.100 M weak acid (HA) solution has ph of 4.25. Find Ka for the acid.
P a g e 14 Example 20 Niacin, one of the B vitamins, has the molecular structure shown in the figure. A 0.020 M solution of niacin has a ph of 3.26. What is the acid-dissociation constant, Ka, for niacin? [Recall Question] What kind of acid structure is niacin? Percent Ionization This is another measure of acid strength. The higher the percent ionization, the stronger the acid. Concentration of ionized acid = [H3O + ]equil Example 21 Calculate the percent ionization of niacin in example 20.
P a g e 15 Example 22 Calculate the percentage of HF molecules ionized in (a) a 0.10 M HF solution, (b) a 0.010 M HF solution. Example 23 A 0.148 M solution of a monoprotic acid has a percent ionization of 1.55%. Determine the Ka of the acid.
P a g e 16 Effect of Concentration on Ionization of a Weak Acid Increasing the concentration of the weak acid has the following effects: i. [H 3 O + ] equil increases ii. ph decreases (more acidic) iii. Percent ionization decreases. Overall, the increase in [H3O + ] concentration is slower than the increase in acid concentration. Acetic Acid CAUTION! The effect of the acid ionization constant, K a, takes priority over the effect of concentration. The effect of concentration on ph and percent ionization is used when comparing the same weak acid. Remember, higher K a means lower ph and higher percent ionization. Higher concentration of weak acid means lower ph and lower percent ionization. Guided Example Which of the following acids will have the lowest ph? Rank the acids in decreasing order of percent ionization. A. 0.025 M HNO2 B. 0.05 M HNO2 C. 0.025 M HClO2 D. 0.05 M HClO2 Mixtures of Acids For a mixture of a strong acid with a weak acid, the complete ionization of the strong acid provides more than enough [H3O + ] to shift the weak acid equilibrium to the left so far that the weak acid s added [H3O + ] is negligible. For mixtures of weak acids, you generally need to consider only the stronger for the same reasons, as long as one is significantly stronger than the other and their concentrations are similar. Guided Example Strong Acid and Weak Acid Mixture Consider a mixture that is 0.10 M in HCl and 0.10 M in HCHO2. Find the ph.
P a g e 17 Guided Example Two Weak Acids Find the ph of a mixture that is 0.150 M HF and 0.100 M in HClO.
P a g e 18 Base Solutions Strong Bases Completely dissociate in solution. Concentration of a strong base is equal to the concentration of OH -. Most strong bases are group 1A or 2A metal hydroxides. NaOH (aq) Na + (aq) + OH - (aq) 1.0 M NaOH has [OH - ] = 1.0 M. Sr(OH)2 (aq) Sr 2+ (aq) + 2 OH - (aq) Important: the two OH - ions dissociate in one step. Example 24 Determine the OH concentration and ph in each solution. a. 0.225 M KOH b. 0.0015 M Sr(OH)2 Weak Bases Most weak bases produce OH - by accepting a proton from water. Let s consider the ionization of a generic weak base, B: B (aq) + H2O (l) BH + (aq) + OH - (aq) For example, NH3 (aq) + H2O (l) NH4 + (aq) + OH - (aq) A 1.0 M NH3 solution will have [OH - ] less than 1.0 M.
P a g e 19 Kb and pkb The extent of ionization of a weak base is quantified using the base ionization constant, Kb. We define Kb as follows: B (aq) + H2O (l) BH + (aq) + OH - (aq) larger Kb = stronger base. pkb = -logkb larger pkb = weaker base = stronger conjugate acid. Recall that bases are proton acceptors. They do this by having lone pairs of electrons.
P a g e 20 Example 25 Write the reaction and the corresponding Kb equilibrium expression for each of the following substances (acting as bases in water) a. PO4 3- b. CN - c. pyridine, C5H5N Example 26 Pick the stronger base from each pair and explain your reasoning: a. ClO4 - or ClO2 - b. CN - or ClO - c. Cl - or H2O Example 27 Find the [OH ] and ph of a 0.100 M NH3 solution.
P a g e 21 Example 28 Find the [OH ] and ph of a 0.33 M methylamine solution. Example 29 A solution of NH3 in water has a ph of 11.17. What is the molarity of the solution?
P a g e 22 Example 30 A solution made by adding solid sodium hypochlorite (NaClO) to enough water to make 2.00 L of solution has a ph of 10.50. Calculate the mass of NaClO that were added to the water. [pkb of NaClO = 6.48]. Example 31 Morphine is a weak base. A 0.150 M solution of morpine has a ph of 10.5. What is the Kb of morphine?
P a g e 23 The Acid-Base Properties of Ions and Salts anions tend to form either basic or neutral solutions. cations tend to form either acidic or neutral solutions. Anions as Weak Bases We can think of any anion, A-, as the conjugate base of an acid, HA. Cl - HCl F - HF NO3 - HNO3 HC2H3O2 Not every anion can act as a base. o an anion that is the conjugate base of a weak acid is a weak base. o an anion that is the conjugate base of a strong acid is ph-neutral. The stronger the acid, the weaker the conjugate base. HF(aq) + H2O(l) H3O + (aq) + F (aq) HCl (aq) + H2O(l) H3O + (aq) + Cl - (aq) C2H3O2 -
P a g e 24 Example 31 Classify each anion as a weak base or ph-neutral. a. NO3 b. NO2 c. C2H3O2 d. CHO2 e. ClO4 Relationship between Ka and Kb HA and A are conjugate acid-base pairs. HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) K a = [H 3O + ][A ] [HA] A (aq) + H 2 O(l) HA(aq) + OH (aq) K b = [HA][OH ] [A ] K a K b = [H 3O + ][A ] [HA][OH ] [HA] [A = [H ] 3 O + ][OH ] = K w = 1.0 10 14 K a K b = K w = 1. 0 10 14 Important Note! This relationship is only valid for conjugate acid-base pairs and the value of Kw changes with temperature. However, for this course and the AP exam, assume that the temperature is 25 C. Example 33 Find the ph of a 0.100 M NaCHO2 solution. The salt completely dissociates into Na + (aq) and CHO2 (aq), and the Na + ion has no acid or base properties.
P a g e 25 Example 34 Find the ph of a 0.250 M NaC2H3O2 solution. Relationship between pka and pkb K a K b = K w = 1.0 10 14 Take log of both sides log(k a K b ) = log(1.0 10 14 ) logk a logk b = 14.00 pk a + pk b = 14. 00 Again, only valid for conjugate acid-base pairs. Cations as Weak Acids The stronger the base, the weaker the conjugate acid. A cation that is the counterion of a strong base is ph neutral. o Na +, K +, Ca 2+, Sr 2+ A cation that is the conjugate acid of a weak base is weakly acidic. o NH4 + (aq) + H2O(l) NH3(aq) + H3O + (aq) A cation that is a small, highly charged metal ion forms weakly acidic solution. o Al 3+, Fe 3+ Example 35 Classify each cation as a weak acid or ph-neutral. a. C5H5NH + b. Ca 2+ c. Cr 3+ d. Li + e. CH3NH3 + f. Fe 3+
P a g e 26 Classifying Salt Solutions as Acidic, Basic, or Neutral If the salt cation is the counterion of a strong base and the anion is the conjugate base of a strong acid, it will form a neutral solution. NaCl, Ca(NO3)2, KBr If the salt cation is the counterion of a strong base and the anion is the conjugate base of a weak acid, it will form a basic solution. NaF, Ca(C2H3O2)2, KNO2 If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a strong acid, it will form an acidic solution. NH4Cl If the salt cation is a highly charged metal ion and the anion is the conjugate base of a strong acid, it will form an acidic solution. Al(NO3)3 If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a weak acid, the ph of the solution depends on the relative strengths of the acid and base. NH4F is acidic because HF is a stronger acid than NH4 +, and Ka of NH4 + is larger than Kb of the F. Example 35 Determine if the solution formed by each salt is acidic, basic, or neutral. a. SrCl2 b. AlBr3 c. CH3NH3NO3 d. NaCHO2 e. NH4F f. NaHCO3 g. CH3CH2NH3Cl h. KNO3 i. Fe(NO3)3 j. RbI Polyprotic Acids Contain more than one ionizable proton. o diprotic (2 H + ), triprotic (3 H + ) Ionizes in successive steps, each with its own Ka. Ka1 > Ka2 > Ka3 Generally, the difference in Ka values is great enough so that the second ionization does not happen to a large enough extent to affect the ph. o For most ph problems, we just use the first ionization.
P a g e 27 Example 37 Find the ph of a 0.100 M ascorbic acid (H2C6H6O6) solution. Example 38 Find the ph of a 0.0100 M sulfuric acid (H2SO4) solution.
P a g e 28 Dissociation of a Polyprotic Acid Example 39 Find the [C6H6O6 2 ] of the 0.100 M ascorbic acid (H2C6H6O6) solution in Example 37.
P a g e 29 Acid Strength and Molecular Structure The structure of an acid affects its acidity. We will examine binary acids and oxyacids. Binary Acids Factors affecting the relative strengths of binary acids (HX) are bond polarity and bond strength. The more polarized the bond, the more acidic. The more electronegative the atom bonded to H, the more acidic. Electronegativity increases as atoms get closer to fluorine on the periodic table. Binary acid strength increases to the right across a period. Acidity: H C < H N < H O < H F The stronger the H X bond, the weaker the acid. Because atomic size increases down the column, bond strengths decreases while acidity increases. Acidity: H F < H Cl < H Br < H I Oxyacids Two factors affecting the strength of oxyacids are electronegativity of Y and the number of oxygen atoms bonded to Y. The more electronegative the Y atom, the stronger the oxyacid. HClO > HIO Acidity of oxyacids decreases down a group. Same trend as binary acids Helps weaken the H O bond
P a g e 30 The more oxygens attached to Y, the stronger the oxyacid. Further weakens and polarizes the H O bond via inductive effect. HClO4 > HClO3 > HClO2 > HClO Example 40 Based on their molecular structure, pick the stronger acid from each pair of binary acids. Explain your choice. HF or HCl H2O or HF H2Se or H2S Example 41 Based on molecular structure, arrange the binary compounds in order of increasing acid strength. Explain your choice. H2Te, HI, H2S, NaH Example 42 Based on their molecular structure, pick the stronger acid from each pair of oxyacids. Explain your choice. H2SO4 or H2SO3 HClO2 or HClO HClO or HBrO CCl3COOH or CH3COOH
P a g e 31 Example 43 Which is a stronger base? Explain your choice. S 2 or Se 2 PO 4 3 or AsO 4 3