Acids & Bases. Strong Acids. Weak Acids. Strong Bases. Acetic Acid. Arrhenius Definition: Classic Definition of Acids and Bases.

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1 Arrhenius Definition: Classic Definition of Acids and Bases Acid: A substance that increases the hydrogen ion concetration, [H ], (also thought of as hydronium ion, H O ) when dissolved in water. Acids & Bases HCl (aq) à H (aq) Cl (aq) Alternate view of acid dissociation: HCl (aq) H 2 O (l) à H O (aq) Cl (aq) Chapter 17 H ions do not exist free in solution, but are strongly coordinated to one or more water molecules. Base: A substance that contains increases the hydroxide ion concentration, [OH ], when dissolved in water. NaOH (aq) à Na (aq) OH (aq) Strong Acids " HNO, HCl, HBr, HI, H 2 SO 4 and HClO 4 are among the few known strong acids. " These acids completely ionize in water. " K a values for strong acids are LARGE. Weak Acids Weak acids are much less than 0% ionized in water. One of the best known is acetic acid = CH CO 2 H K a values for weak acids are SMALL. Acetic Acid Strong Bases Strong bases are 0% dissociated in water. NaOH (aq) à Na (aq) OH (aq) Alkali metal (Na, K, Li, etc.) hydroxides and alkaline earth metal (Mg, Ca, etc.) hydroxides are the common strong bases. Alkali metal hydroxides are highly soluble. Alkaline earth metal hydroxides are less soluble. 1

2 Weak Bases Weak bases are less than 0% ionized in water. One of the most common weak bases is ammonia, NH. BrønstedLowry Definition of Acids Acid: A substance that donates a proton (H ) to another substance. HNO (aq) H 2 O (l) à H O (aq) NO (aq) CH CO 2 H (aq) NH (aq) ó NH 4 CH CO 2 (aq) BrønstedLowry Definition of Bases Base: A substance that accepts a proton from another substance. BrønstedLowry Acids and Bases A) Hydrochloric acid B) Ammonia NH (aq) H 2 O (l) ó OH (aq) NH 4 (aq) (CH ) 2 NH (aq) HClO (aq) ó (CH ) 2 NH 2 (aq) ClO (aq) KOH (aq) à K (aq) OH (aq) KOH is still a base by this definition it contains OH, which is an excellent proton acceptor. Titrations Autoionization of Water H 2 O can function as both an ACID and a BASE. In pure water there can be autoionization: 2 H 2 O (l) ó H O (aq) OH (aq) H O H O H H H O H H O [ ] H 2

3 Autoionization of Water (cont.) Example: [H ] & [OH ] What is the concentration of OH ions in a HCl solution whose hydrogen ion concentration is 0.01 M? Equilibrium constant for autoionization (K w ) K w = [H O ] [OH ] = 1.00 x 14 at 25 o C In pure water, or in a neutral solution [H O ] = [OH ], so [H O ] = [OH ] = 1.00 x 7 M ph A Measure of Acidity poh A Measure of Basicity ph = log [H ] poh = log [OH ] [OH ] = (poh) ph is a logarithmic scale commonly used to measure the [H ] in solution. Because the ph scale is logarithmic, a change of one ph unit is equal to a x change in the [H ] concentration. If the ph of a solution is known, the [H ] can be easily determined by the following relationship, which is a rearrangement of the above equation: poh is a logarithmic scale used to measure the [OH ] in solution. poh is not used as commonly as ph, but can be used in a similar manner. [H ] = (ph) Relationship of ph & poh AcidBase Concentration Measurements If you know any one of the following: [H] [OH] ph poh you can calculate the other three. [H ] [OH ] ph poh Any one of these three pieces of information can be used to determine whether a solution is acidic, basic, or neutral. Solution Is At 25 0 C neutral [H ] = [OH ] [H ] = 1 x 7 ph = 7 poh = 7 acidic [H ] > [OH ] [H ] > 1 x 7 ph < 7 poh > 7 basic [H ] < [OH ] [H ] < 1 x 7 ph > 7 poh < 7 ph [H ] poh [OH ]

4 AcidBase Concentration Problems 1) What is the ph of a M solution of HCl? 2) What is the ph of a 5.8 x 4 M solution of KOH? ) What is the [H ] of a solution of NaOH with a poh of.2? 4) What is the ph of concentrated HCl (12 M)? 5) Calculate the ph, poh, [H ], and [OH ] of a solution prepared by dissolving 57 mg calcium hydroxide in enough water to make ml solution. ph poh [H ] [OH ] & powers of ph/poh scales logic problems A solution with a ph = 6 is x more than a solution with a ph = 2. A solution with a poh = is x more than a solution with a poh = 4. A solution with a ph = 6 is x more than a solution with a poh = 2. A solution with a poh = 12.7 is x more than a solution with a poh = 1.7. A solution with a poh = 0 is x more than a solution with a ph = 7. A solution with a ph =.45 is x more than a solution with a ph = ph scales Common Household Products Logarithm basics Logarithm basics A logarithm is an exponent. For common logs, is the base for the exponent. For instance, log (,000) = 4, because 4 =,000. For less simple numbers, the log includes a decimal portion: log (45,982) = (rounded), and = 45,982 (rounded) In the above example, for the log, , The 4 is known as the characteristic, and tells us the power of ten in the number The decimal portion,.66259, known as the mantissa, gives us the value of the number at that power of ten. Consider again: log (45,982) = , and = 45,982 If we change only the power of for the number we wish to take the log of, this will change only the characteristic: log (459.82) = , and =

5 Significant Figures & logs Because the characteristic only tells us the power of, the characteristic of is not considered a significant digit, just as the exponential portion of scientific notation does not affect the number of significant digits. For instance, 45,982 has 5 significant digits. Expressed as x 4, the number again has 5 significant digits. For the log of 45,982, , the 4 in the ones place of the log is not a significant digit. However, every digit of the mantissa, expressed as is significant there are 5 significant digitsin the log: Therefore, the log of 45,982, expressed as (rounded) expresses the same number of significant digits (5) as the number 45,982 itself. Significant Figures & logs In any log number, the characteristic (part of the log to the left of the decimal) is not significant; however, all digits of the mantissa (part of the log to the right of the decimal) that are expressed are significant. Therefore, the log of any number should be expressed to have the same number of decimal places as the number itself has total significant digits. Example: log ( ) =.2052 Example: (5.442) = 2.77 x 5 Significant Figures & logs: Application to ph Because the ph & poh are logs, the rules of counting significant digits in logs apply. Examples: ph of a x M HNO solution is Mixing a Strong Acid and a Strong Base Calculate the ph, the [H] and the [OH] of a solution prepared by mixing 0.0 ml of a 1. x M HCl solution with 50.0 ml of a 4.8 x M NaOH solution. ph of a 1.0 x M HNO solution is.00. ph of a.24 x 5 M HNO solution is The [H O ] of a ph = 4.22 solution is 6.0 x 5 M. The [H O ] of a ph = solution is??? Weak AcidBase Dissociations: Equilibrium Processes Weak acids and bases only partially dissociate in water. The equilibrium constant for the dissociation of an acid in water is known as the acid dissociation constant, K a. Consider a generic acid, HA: HA (aq) H 2 O (l) ó H O (aq) A (aq) [A ][H O ] = [HA] Ka Why doesn t water appear in the reaction quotient? Would the reaction quotient chage if the reaction were expressed as: HA (aq) ó H (aq) A (aq) Weak AcidBase Dissociations: Equilibrium Processes The equilibrium constant for the reaction of a weak base with water is known as the base ionization constant, K b. Consider the generic base, B: B (aq) H 2 O (l) ó BH (aq) OH (aq) K b [BH ][OH ] = [B] 5

6 Acid Dissociation Acetic acid, CH CO 2 H, only partially dissociates in water. It dissociates in a reactantfavored equilibrium (small K). [H O ] eq < [CH CO 2 H] o Base Ionization Methylamine, CH NH 2, only partially ionizes water. It ionizes water in a reactant favored equilibrium. [OH ] eq < [CH NH 2 ] o CH CO 2 H (aq) H 2 O (l) ó H O (aq) CH CO 2 (aq) [CHCO2][HO ] 5 Ka = = 1.8 [CH CO H] 2 CH NH 2(aq) H 2 O (l) ó CH NH (aq) OH (aq) K [CHNH ][OH ] 4 b = = 4.4 [CHNH2] Dissociation of HCN Because the ph of a solution is not directly calculated from the concentration of the acid, we must analyze the equilibrium to determine the ph. Consider the dissociation of HCN: HCN H 2 O ó H O CN Write the reaction quotient expression for K a, and look up the value. HCN (continued) We will need to consider the equilibrium condition. K a = 6.2 x (very small relative to 0.5 M) HCN H 2 O ó H O CN initial 0.50 N/A 0 0 change x N/A x x What is the [H ] concentration and ph of a 0.50 M solution of HCN? equilib x 0.50 N/A x x Dissociation of Acetic Acid What is the ph of a 0.50 M solution of acetic acid, CH CO 2 H? What is the percent of the original CH CO 2 H (0.50 M) dissociated? What would be the % dissociation of a 2.0 M acetic acid solution? What would be the % dissociation of a 0.0 M acetic acid solution? What is the percent dissociation of a strong acid like HCl at 2.0 M? At 0.50 M? At 0.0 M? Dissociation of NH What is the [OH ] and the ph for a solution that is 0.75 M in NH? NH H 2 O ó OH NH 4 initial 0.75 N/A 0 0 change x N/A x x equilib x 0.75 N/A x x 6

7 Hydroxylamine Hydroxylamine (HONH 2 ) is a weak base with a base ionization constant of 1.0 x 8. What is the ph of a 0. M solution of hydroxylamine? Monoprotic & Polyprotic Acids We have primarily studied monoprotic acids acids that can donate one proton (H ): HCl (aq) H 2 O (l) à H O (aq) Cl (aq) Polyprotic acids can donate more than one proton. However, they do not give all their protons at once. Rather, we must consider a stepwise ionization with different K a values at each step. H PO 4(aq) H 2 O (l) ó H O (aq) H 2 PO 4 (aq) K a1 = 7.2 x H 2 PO 4 (aq) H 2 O (l) ó H O (aq) HPO 4 2 (aq) K a2 = 6. x 8 HPO 4 2 (aq) H 2 O (l) ó H O (aq) PO 4 (aq) K a = 4.2 x 1 Dissociation of Phosphoric acid For a 0.0 M H PO 4 solution, calculate the ph and the concentration of all phosphoruscontaining species. Polybasic amine For a 0.0 M solution of ethylenediamine, calculate the ph of the solution and the concentration of all of the amine species. Ethylenediamine = NH 2 CH 2 CH 2 NH 2 K b1 = 8.5 x 5 K b2 = 7.1 x 8 BrønstedLowry AcidBase Definition: Review The Conjugate Pairs in Some AcidBase Reactions Conjugate Pair An acid is a proton donor, any species which donates an H. Acid Base Base Acid A base is a proton acceptor, any species which accepts an H. An acidbase reaction can be viewed from the standpoint of the reactants AND the products. Conjugate Pair Reaction 1 HF H 2 O F H O Reaction 2 HCOOH CN HCOO HCN Reaction NH 4 CO 2 NH HCO An acid reactant will produce a base product and the two will constitute an acidbase conjugate pair. Reaction 4 H 2 PO 4 OH Reaction 5 H 2 SO 4 N 2 H 5 Reaction 6 HPO 2 4 SO 2 HPO 4 2 H 2 O HSO 4 N 2 H 2 6 PO 4 HSO 7

8 Identifying Conjugate AcidBase Pairs Identify the species acting as the acids and those acting as the bases in the forward and reverse reactions represented by the following equations: Strengths of conjugate acidbase pairs HClO 2 NH ó NH 4 ClO 2 HONH 2 CH CO 2 H ó HONH CH CO 2 CO 2 H 2 O ó HCO OH What is the conjugate acid of OH? Conjugate Acid / Base Pairs and K a / K b Relationships Conjugate acid/base pairs A weak acid will have a weak conjugate base. Consider the HCN / CN conjugate pair: HCN H 2 O ó H O CN K a = 6.2 x CN H 2 O ó OH HCN To be presented in class as board notes. K K 1 14 W 5 b = = = 1.6 Ka 6.2 Conjugate acid/base pairs A weak base will have a weak conjugate acid. Consider the NH / NH 4 conjugate pair: NH H 2 O ó NH 4 OH K b = 1.8 x 5 NH 4 H 2 O ó NH H O K K 1 14 W a = = = Kb 1.8 Conjugate acid/base pairs A strong acid will have a very weak conjugate base. Consider the HNO / NO conjugate pair: HNO H 2 O à H O NO K a >>> 1 (cannot be measured in water). NO H 2 O OH HNO K b <<< 1 (cannot be measured in water) 8

9 Conjugate acid/base pairs A strong base will have a very weak conjugate acid. Consider the NaOH / Na conjugate pair: NaOH à Na OH K >>> 1 (0% ionization) Na H 2 O NaOH H K a <<< 1 (does not occur) Acid / Base Properties of Salts Soluble ionic salts produce may produce neutral, acidic, or basic solutions depending on the acidbase properties of the individual ions. Consider the salt sodium nitrate, NaNO. It dissociates into sodium and nitrate ions. Sodium nitrate produces a neutral solution because neither sodium nor nitrate reacts with water. Na H 2 O NaOH H NO H 2 O OH HNO Weak Base Salt When sodium cyanide, NaCN, dissolves in water, it will produce sodium and cyanide ions. As we have seen before, Na does not react with water. However, CN is the weak base conjugate of the acid, HCN. CN will react with water to produce a basic solution. What will be the ph of a 0.50 M solution of NaCN? Weak Acid Salt When ammonium nitrate, NH 4 NO, dissolves in water, it will produce NH 4 and NO ions. As we have seen before, NO does not react with water. However, NH 4 is the weak acid conjugate of the weak base, NH. NH 4 will dissociate in water to produce an acidic solution. What will be the ph of a 0.50 M solution of NH 4 NO? AcidBase Conjugates What is the ph of a 0.0 M pyridinium hydrochloride (C 5 H 5 NH Cl ) solution if the K b of pyridine (C 5 H 5 N) = 1.5 x 9? Salt with WA and WB Ions Sometimes salts have a weak acid cation and a weak base anion. In such a case, we must consider the relative strengths (K a vs. K b values) of the acid and base. Ammonium cyanide, NH 4 CN is such a salt: NH 4 H 2 O ó NH H O K a = 5.6 x CN H 2 O ó OH HCN K b = 2.5 x 5 The resulting solution would be basic because cyanide is a stronger base than ammonium is an acid. 9

10 Amphiprotic Species Compounds that can either accept or donate a proton are called amphiprotic. Salt with an Amphiprotic Ion What is the ph of a salt solution that is a 0.0 M solution of the salt potassium bisulfite? Water is amphiprotic: H 2 O (l) H 2 O (l) ó H O (aq) OH (aq) The bisulfite ion, HSO, is amphiprotic: HSO (aq) H 2 O (l) ó H O (aq) SO 2 (aq) HSO (aq) H 2 O (l) ó OH (aq) H 2 SO (aq) Salts with an Amphiprotic Ion Relative Strengths of Acids & Bases and K overall Would you predict the following salts to be acidic, basic, or neutral when dissolved in water? Na PO 4 Na 2 HPO 4 NaH 2 PO 4 Predicting the Net Direction of an AcidBase Reaction Based on the relative strengths of the acids for the forward & reverse reactions, you can predict whether an acidbase reaction will be productfavored or reactantfavored. Consider: CH CO 2 H NH ó NH 4 CH CO 2 stronger acid weaker acid Because acetic acid is a stronger acid than the ammonium ion, we would predict this reaction to be productfavored. Predicting the Net Direction of an AcidBase Reaction Predict whether the following reactions are reactantfavored or productfavored: HCN F ó HF CN CO 2 H 2 PO 4 ó HCO HPO 4 2

11 Acid Strength and Molecular Structure Acid Strength and bond energy follow opposite trends. Consider the halide series of acids: HF HCl HBr HI Increasing Acid Strength (Ka) Acid Strength and Molecular Structure Oxyacids Acid Strengths: HClO 4 > H 2 SO 4 > H PO 4 Cl is more electronegative than S and P. Cl better stabilizes the negative charge on the ClO 4 anion. This is called an inductive effect. Decreasing Bond Strength (bond dissociation energy) Acid Strength and Molecular Structure Acid Strengths: H 2 SO 4 > H 2 SO The sulfate ion has four oxygens which help to spread out the charge on the anion form of the molecule to more atoms resonance stabilization: Lewis Acids and Bases A broader concept of acids and bases than Arrhenius or BronstedLowry is the Lewis definition. A Lewis acid is a lone pair acceptor atoms, molecules, or ions with unfilled valence orbitals. A Lewis base is a lone pair donor atoms, molecules, or ions with valence shell lone pairs. The additional oxygen is also stabilizing through inductive effects. Lewis AcidBase Reactions Reactions of metal oxides to form bases Formation of metal complexes in solution: Na 2 O (s) H 2 O (l) à 2 NaOH (aq) Reactions of a nonmetal oxide with water to form an oxyacid: The reaction of ammonia with an H ion to make ammonium can be considered a Lewis acidbase reaction. MgO (s) H 2 O (l) à Mg(OH) 2(aq) The reaction creates a coordinate covalent bond a bond in which both electrons are provided by a single atom. 11