2015 Pearson Education, Inc. Chapter 17 Lecture presentation Acids and Bases Catherine E. MacGowan Armstrong Atlantic State University 2015 Pearson Education, Inc. General Properties of Acids Sour taste Ability to dissolve many metals Ability to neutralize bases Change blue litmus paper to red Acetic acid: vinegar 2015 Pearson Education, Inc. Table of Common Acids 2015 Pearson Education, Inc. Structure of Organic Acids: Carboxylic Acids Carboxylic acids have a COOH group. Acetic acid: HC 2 H 3 O 2 Citric acid: H 3 C 6 H 5 O 7 Only the H in the COOH (carboxylic acid) group is acidic. The H is on the COOH. HOC || O 2015 Pearson Education, Inc. General Properties of Bases Taste bitter Alkaloid = plant product that is alkaline Often poisonous Feel slippery to touch Ability to neutralize acids Change red litmus paper to blue 2015 Pearson Education, Inc. Table of Common Bases 2015 Pearson Education, Inc. Hydronium Ion: H 3 O + The H + ions (protons) produced by the acid are so reactive they cannot exist in water. Instead, they react with water molecules to produce complex ions, mainly hydronium ion, H 3 O +. H + + H 2 O H 3 O + 2015 Pearson Education, Inc. Definitions of Acids and Bases Arrhenius definition (simplest and most restrictive) Acids are substances that when dissolved in water produce a hydronium, H 3 O + (hydrogen ion H + ). Bases are substances that when dissolved in water produce a hydroxide ion, OH . BrnstedLowry definition (based on reactions in water) Acids are substances that when dissolved in water donate hydronium, H 3 O + (hydrogen ion H + ). Bases are substances that accept a hydronium, H 3 O + (hydrogen ion H + ). Lewis definition (most expansive and based on electron donors and acceptors) Acids are substances that accept or need an electron pair. Bases are substances that donate an electron pair to another substance. 2015 Pearson Education, Inc. Arrhenius Theory Acids produce H + ions in aqueous solution. HCl(aq) H + (aq) + Cl (aq) 2015 Pearson Education, Inc. Arrhenius Theory Bases produce OH ions in aqueous solution. NaOH(aq) Na + (aq) + OH(aq) 2015 Pearson Education, Inc. Arrhenius AcidBase Reactions The H + from the acid combines with the OH from the base to make a molecule of H 2 O. Think of H 2 O as HOH. The cation from the base combines with the anion from the acid to make a salt. acid + base salt + water Example: HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) (acid) (base) (salt) (water) 2015 Pearson Education, Inc. Problems with Arrhenius Theory: Too Narrow of a Definition It does not explain why molecular substances, such as ammonia, NH 3, dissolve in water to form basic solutions, even though they do not contain OH ions. It does not explain how some ionic compounds, such as sodium carbonate (washing soda), Na 2 CO 3, or sodium oxide, Na 2 O, dissolve in water to form basic solutions, even though they do not contain OH ions. It does not explain why molecular substances, such as CO 2, dissolve in water to form acidic solutions, even though they do not contain H + ions. It does not explain acidbase reactions that take place outside aqueous solution. 2015 Pearson Education, Inc. BrnstedLowry AcidBase Theory It defines acids and bases based on what happens in a reaction. Any reaction involving an H + (proton) that transfers from one molecule to another is an acidbase reaction, regardless of whether it occurs in aqueous solution or if there is OH present. All reactions that fit the Arrhenius definition also fit the BrnstedLowry definition. 2015 Pearson Education, Inc. BrnstedLowry Theory: Acid The acid is an H + donor. The base is an H + acceptor. Base structure must contain an atom with an unshared pair of electrons. In a BrnstedLowry acidbase reaction, the acid molecule donates an H + to the base molecule. HA + :B :A + HB + (acid) (base) (conjugate) (conjugate) base acid 2015 Pearson Education, Inc. BrnstedLowry Acids BrnstedLowry acids are H + donors. Any material that has H can potentially be a BrnstedLowry acid. Because of the molecular structure, often one H in the molecule is easier to transfer than others. When HCl dissolves in water, the HCl is the acid because HCl transfers an H + to H 2 O, forming H 3 O + ions. Water acts as base, accepting H +. HCl(aq) + H 2 O(l) Cl (aq) + H 3 O + (aq) (acid) (base) (conjugate) (conjugate) base acid 2015 Pearson Education, Inc. BrnstedLowry Bases BrnstedLowry bases are H + acceptors. Any material that has atoms with lone pairs can potentially be a BrnstedLowry base. Because of the molecular structure, often one atom in the molecule is more willing to accept H + transfer than others. When NH 3 dissolves in water, the NH 3 (aq) is the base because NH 3 accepts an H + from H 2 O, forming OH (aq). Water acts as acid, donating H +. NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) (base) (acid) (conjugate) (conjugate) acid base 2015 Pearson Education, Inc. BrnstedLowry AcidBase Reactions One of the advantages of BrnstedLowry theory is that it illustrates equilibrium reactions to be as follows: HA + :B :A + HB + (acid) (base) (conjugate) (conjugate) base acid The original base has an extra H + after the reaction, so it will act as an acid in the reverse process. And the original acid has a lone pair of electrons after the reaction, so it will act as a base in the reverse process: :A + HB + HA + :B (conjugate) (conjugate) (acid) (base) base acid 2015 Pearson Education, Inc. Conjugate AcidBase Pairs In a BrnstedLowry acidbase reaction, the original base becomes a conjugate acid in the reverse reaction; the original acid becomes a conjugate base in the reverse process. Each reactant and the product it becomes is called a conjugate pair. 2015 Pearson Education, Inc. Conjugate Pairs A base accepts a proton and becomes a conjugate acid. An acid donates a proton and becomes a conjugate base. 2015 Pearson Education, Inc. Practice Problem: Identifying BrnstedLowry Acids and Bases and Their Conjugates 2015 Pearson Education, Inc. Acid Strength and Molecular Structure of Acids Binary acids (HY) have acidic hydrogens attached to a nonmetal atom. Example: HCl and HF The more polarized the bond ( + HX ), the more acidic the bond. The stronger the HX bond, the weaker the acid. Binary acid strength increases to the right across a period. Acidity: HC < HN < HO < HF Binary acid strength increases down the column. Acidity: HF < HCl < HBr < HI 2015 Pearson Education, Inc. Relationship between Bond Strength and Acidity for Binary Acids 2015 Pearson Education, Inc. Strengths of Oxyacids, HOY The more electronegative the Y atom, the stronger the oxyacid. Example: HClO > HIO Acidity of oxyacids decreases down a group. Same trend as binary acids Helps weaken the HO bond The larger the oxidation number of the central atom, the stronger the oxyacid. Example: H 2 CO 3 > H 3 BO 3 Acidity of oxyacids increases to the right across a period. Opposite trend of binary acids The more oxygens attached to Y, the stronger the oxyacid. Further weakens and polarizes the HO bond HClO 3 > HClO 2 2015 Pearson Education, Inc. Structure of Oxyacids Oxyacids have acid hydrogens attached to an oxygen atom. 2015 Pearson Education, Inc. Relationship between Electronegativity and Acidity of Oxyacids The more electronegative Y is, the weaker and more polarized is the HO bond in the acid. The more electronegative Y is, the more acidic is the acid. 2015 Pearson Education, Inc. Relationship between Number of Oxygens on the Central Atom and Acidity 2015 Pearson Education, Inc. Strong Acid/Base versus Weak Acid/Base A strong acid is a strong electrolyte. Complete or near complete ionization of acid molecule in water A weak acid is a weak electrolyte. Only a small percentage or partial ionization of the acid molecule in water An equilibrium ( ) is established. A strong base is a strong electrolyte. Complete or near complete ionization of base molecule in water Produces OH ions, either through dissociation or reaction with water A weak base is a weak electrolyte. Only a small percentage or partial ionization of the base molecule in water Produces OH ions, either through dissociation or reaction with water An equilibrium ( ) is established. 2015 Pearson Education, Inc. General Trends in Acidity The stronger an acid is at donating H, the weaker the conjugate base is at accepting H. Higher oxidation number = stronger oxyacid H 2 SO 4 > H 2 SO 3 ; HNO 3 > HNO 2 Cation stronger acid than neutral molecule; neutral stronger acid than anion H 3 O + > H 2 O > OH ; NH 4 + > NH 3 > NH 2 Trend in base strength opposite 2015 Pearson Education, Inc. Strong Acids: K a > 1 Strong acids donate practically all their hydrogen atoms. Near 100% ionization of acid molecule occurs in water Strong electrolyte [H 3 O + ] = [strong acid] [HA] = [H + ] The brackets designate molarity There are six strong acids: HCl, HBr, HI, HNO 3, HClO 4, and H 2 SO 4 2015 Pearson Education, Inc. Weak Acids: K a < 1 Weak acids donate a small fraction (partial ionization) of their hydrogen atoms. Weak acid molecules do not donate much of their hydrogens to water. Much less than 1% ionized in water [H 3 O + ] 1.00 10 7 ; [OH ] < 1.00 10 7 Basic solutions have a larger [OH ] than [H 3 O + ]. [H 3 O + ] 1.00 10 7 2015 Pearson Education, Inc. Practice Problem: K a Calculations 2015 Pearson Education, Inc. Measuring Acidity: pH The acidity or basicity of a solution is often expressed as pH. pH = log[H 3 O + ] Exponent on 10 with a positive sign pH water = log[10 7 ] = 7 We need to know the [H 3 O + ] concentration to find the pH. pH 7 is basic. pH = 7 is neutral. If we know the pH, we can determine [H 3 O + ]. [H 3 O + ] = 10 pH 2015 Pearson Education, Inc. What Does the pH Number Imply? The lower the pH, the more acidic the solution; the higher the pH, the more basic the solution. One pH unit corresponds to a factor of 10 difference in acidity. Normal range of pH is 0 to 14. pH 0 is [H 3 O + ] = 1 M; pH 14 is [OH ] = 1 M. pH can be negative (very acidic) or larger than 14 (very alkaline). 2015 Pearson Education, Inc. Practice Problem: Calculating pH from [H 3 O + ] or [OH ] 2015 Pearson Education, Inc. Practice Problem: Calculating [H 3 O + ] from pH 2015 Pearson Education, Inc. What is pOH? Another way of expressing the acidity/basicity of a solution is pOH. pOH = log[OH ] If you know pOH, then you can determine [OH ]. [OH ] = 10 pOH pOH water = log[10 7 ] = 7 You need to know the [OH ] concentration to find pOH. pOH 7 is acidic; pOH = 7 is neutral. 2015 Pearson Education, Inc. Relationship between pH and pOH pH + pOH = at 25 C. If you know pOH, you can determine pH; if you know pH, you can determine pOH. 2015 Pearson Education, Inc. The pKs: pK a and pK b A way of expressing the strength of an acid or base is through its pK. pK a = log(K a ), K a = 10 pKa The stronger the acid, the smaller the pK a. Larger K a = smaller pK a Because pK a is log(K a ) pK b = log(K b ), K b = 10 pKb The stronger the base, the smaller the pK b. Larger K b = smaller pK b 2015 Pearson Education, Inc. [H 3 O + ] and [OH ] in a Strong Acid or Strong Base Solution There are two sources of H 3 O + in an aqueous solution of a strong acidthe acid and the water. There are two sources of OH in an aqueous solution of a strong acidthe base and the water. For a strong acid or base, the contribution of the water to the total [H 3 O + ] or [OH ] is negligible. The [H 3 O + ] acid shifts the K w equilibrium so far that [H 3 O + ] water is too small to be significant. Except in very dilute solutions, generally < 1 10 4 M 2015 Pearson Education, Inc. Finding pH of a Strong Acid There are six strong acids: Five are monoprotic and one is diprotic. Monoprotic: HCl, HBr, HI, HClO 4, and HNO 3 Diprotic: H 2 SO 4 For a monoprotic strong acid, the acid concentration equals the hydronium concentration. [H 3 O + ] = [HAcid] Example: 0.10 M HCl has [H 3 O + ] = 0.10 M and pH = 1.00 For H 2 SO 4, the second ionization can generally be ignored. However, you must account for the TWO H+ ions when determining the pH. Example: 0.10 M H 2 SO 4 has [H 3 O + ] = 0.20 M and pH = 0.70 2015 Pearson Education, Inc. Finding the pH of a Weak Acid: Using ICE There are also two sources of H 3 O + in an aqueous solution of a weak acidthe acid and the water. However, finding the [H 3 O + ] is complicated by the fact that the acid undergoes only partial ionization. Calculating the [H 3 O + ] requires solving an equilibrium problem (using an ICE format) for the reaction that defines the acidity of the acid. HAcid + H 2 O Acid + H 3 O + 2015 Pearson Education, Inc. Practice Problem: Calculating [H 3 O + ] for a Weak Acid using K a 2015 Pearson Education, Inc. Practice Problem continued 2015 Pearson Education, Inc. Practice Problem: Calculating pH for a Weak Acid 2015 Pearson Education, Inc. Practice Problem continued 2015 Pearson Education, Inc. Practice Problem: Calculating pH for a Weak Acid when 5% Approximation Does Not Work 2015 Pearson Education, Inc. Practice Problem continued 2015 Pearson Education, Inc. Practice Problem continued 2015 Pearson Education, Inc. Practice Problem: Finding Equilibrium Constant K a when pH Is Known 2015 Pearson Education, Inc. Percent Ionization Another way to measure the strength of an acid is to determine the percentage of acid molecules that ionize when dissolved in water; this is called the percent ionization. The higher the percent ionization, the stronger the acid. Because [ionized acid] equil = [H 3 O + ] equil 100 = percent ionization molarity of ionized acid initial molarity of weak acid 100 = percent ionization [H 3 O] + equil [HA] init 2015 Pearson Education, Inc. Relationship between [H 3 O + ] equil and [HA] init Increasing the initial concentration of acid results in increased [H 3 O + ] at equilibrium. Increasing the initial concentration of acid results in decreased percent ionization. This means that the increase in [H 3 O + ] concentration is slower than the increase in acid concentration. 100 = percent ionization [H 3 O] + equil [HA] init 2015 Pearson Education, Inc. Practice Problem: Percent Ionization of a Weak Acid 2015 Pearson Education, Inc. Why Doesnt the Increase in H 3 O + Keep Up with the Increase in HA? The reaction for ionization of a weak acid is as follows: HA(aq) + H 2 O(l) A (aq) + H 3 O + (aq) According to Le Chteliers principle, if we reduce the concentrations of all the (aq) components, the equilibrium should shift to the right to increase the total number of dissolved particles. The (aq) concentrations can be reduced by using a more dilute initial acid concentration. The result will be a larger [H 3 O + ] in the dilute solution compared to the initial acid concentration. This will result in a larger percent ionization. 2015 Pearson Education, Inc. Finding the pH of Mixtures of Acids Generally, you can ignore the contribution of the weaker acid to the [H 3 O + ] equil. For a mixture of a strong acid with a weak acid, the complete ionization of the strong acid provides more than enough [H 3 O + ] to shift the weak acid equilibrium to the left so far that the weak acids added [H 3 O + ] 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. 2015 Pearson Education, Inc. Practice Problem: Mixtures of Weak Acids 2015 Pearson Education, Inc. Strong Bases: K b > 1 Hydroxide compounds are strong bases. The stronger the base, the more willing it is to accept H in water (where water is acting as an acid). For ionic bases, practically all units are dissociated into OH or accept Hs. Strong electrolyte Multi-OH strong bases completely dissociated 2015 Pearson Education, Inc. Finding pOH and pH for a Strong Base Solution For a strong mono hydroxyl ionic base, the [BOH] = [OH ]. Example: 0.10 M KOH has [OH ] = 0.10 M To find the pH of a mono hydroxyl ionic base, first find the pOH and then determine the pH. Example: 0.10 M KOH has 0.10 M [OH ] ions. pOH = log [OH ] or pOH = log [0.10], so pOH is 1.0. pH + pOH = 14, so pH = 14; thus, the pH is 13. For strong poly hydroxyl ionic base compounds, the [OH ] is equal to number of OH ions in the base. Example: 0.10 M Ca(OH) 2 has [OH ] = 0.20 M and pH = pOH = log [OH ] or pOH = log [0.20], so pOH is pH + pOH = 14, so pH = 14; thus, the pH is 13. 2015 Pearson Education, Inc. Practice Problem: Finding pOH and pH of a Strong Base 2015 Pearson Education, Inc. Weak Bases: K b < 1 In weak bases, only a small fraction of molecules accept Hs. Weak electrolyte Most of the weak base molecules do not take H from water. Much less than 1% ionization in water [OH ] K a2 > K a3 Generally, the difference in K a values is great enough so that the second ionization does not happen to a large enough extent to affect the pH. Most pH problems just do first ionization. [A 2 ] = K a2 as long as the second ionization is negligible. 2015 Pearson Education, Inc. Table of Polyprotic Acids and Their K a Values 2015 Pearson Education, Inc. Practice Problem: Determining the pH of Polyprotic Acids 2015 Pearson Education, Inc. Practice Problem: Determining the pH of Polyprotic Acids 2015 Pearson Education, Inc. Practice Problem continued 2015 Pearson Education, Inc. Practice Problem: Finding the Concentration of Anions in a Weak Diprotic Acid 2015 Pearson Education, Inc. Lewis AcidBase Theory Lewis acidbase theory focuses on transferring an electron pair. Lone pair bond Bond lone pair It does NOT require H atoms to be classified as an acid. The electron donor is called the Lewis base. It is electron rich; therefore, it is a nucleophile. The electron acceptor is called the Lewis acid. It is electron deficient; therefore, it is an electrophile. 2015 Pearson Education, Inc. Lewis Acids: Electron Pair Acceptors They are electron deficient, either from being attached to electronegative atom(s) or as a result of not having an octet. They must have an empty orbital willing to accept the electron pair. H + has an empty 1s orbital. B in BF 3 has an empty 2p orbital and an incomplete octet. Many small, highly charged metal cations have empty orbitals they can use to accept electrons. Atoms that are attached to highly electronegative atoms and have multiple bonds can be Lewis acids. 2015 Pearson Education, Inc. Lewis Bases: Electron Pair Donors The Lewis base has electrons it is willing to give away to or share with another atom. The Lewis base must have a lone pair of electrons on it that it can donate. Anions are better Lewis bases than neutral atoms or molecules. N: < N: Generally, the more electronegative an atom, the less willing it is to be a Lewis base. O: < S: 2015 Pearson Education, Inc. Lewis AcidBase Reactions The base donates a pair of electrons to the acid. It generally results in a covalent bond forming. H 3 N: + BF 3 H 3 NBF 3 The product that forms is called an adduct. Arrhenius and BrnstedLowry acidbase reactions are also Lewis acidbase reactions. 2015 Pearson Education, Inc. Examples of Lewis AcidBase Reactions