Unit 17 Acids and Bases Chapter 14

What is the Arrhenius concept? Acids produce H ions in aqueous solution while bases produce hydroxide ions Originally important but limited because it only applies to aqueous solutions and allows for only hydroxide ion bases) Bronsted-Lowry Model acid is a proton (H+) donor. Base is proton acceptor Show how water can act as an acid or a base Hydronium ion- H 3 O + Section 14.1: The Nature of Acids and Bases

Figure 14.1 The Reaction of HCl and H 2 O Figure 14.3 The reaction of NH 3 with HCl to form NH 4 + and Cl -.

Conjugate base- everything that remains of an acid after a proton is lost Conjugate acid- formed when the proton is transferred to the base Conjugate acid/base pair- two substances related to each other by the donating and accepting of a single proton You can think of the reaction between an acid and a base as competition for the proton between the two bases (the original base and the conjugate base of the acid) Acid dissociation constant: K a = [H 3 O + ][A - ]/[HA] or [H + ][A - ]/[HA] Remember that pure solids or pure liquids are not included in equilibrium expressions- this is the reason that water is not present in the equation Although water is not included in the equilibrium expression, it does play a role in ionization of the acid

Strong acid- almost all the original HA is dissociated (ionized) at equilibrium- strong acid yields a weak conjugate base (weak conjugate base has a low affinity for a proton) acid which yields a conjugate base that is much weaker than water HA + H 2 O  H 3 O + + A - The equilibrium lies far to the right for a strong acid K a is large because the equilibrium lies to the right Some common strong acids- sulfuric, hydrochloric acid, nitric acid, and perchloric acid Weak acid- acid for which the equilibrium lies far to the left most of the acid originally present is still present as HA at equilibrium dissociates only to a small extent- conjugate base is much stronger than water- strong conjugate base K a is small because the equilibrium position lies far to the left Section 14.2: Acid Strength

Diprotic acid- acid that has 2 acidic protons sulfuric acid- in terms of the first proton, it is very strong acid. HSO 4 - is a weak acid and does not dissociate greatly Oxyacids- acidic proton is attached to an oxygen atom- most acids Organic Acids- acids with a carbon atom backbone- usually contain the carboxyl group- usually these acids are weak- acetic acid, benzoic acid- the rest of the H atoms are not acidic Monoprotic acids- acids with one proton Amphoteric- substances that behave either as acids or as bases- water is the most common Figure 14.7 Two Water Molecules React to Form H 3 O + and OH -

Autoionization- transfer of a proton from one water molecule to another to produce a hydroxide ion and a hydronium ion H 2 O + H 2 O  H 3 O + + OH - One water molecule acts as an acid by providing a proton while the other acts as base by accepting the proton- occurs in water and in ammonia Ion-product constant- dissociation constant- K w = [H + ][OH - ]

In ANY aqueous solution at 25 degrees Celsius, no matter what it contains, the value of K w is always 1.0 x There are three possibilities: Neutral solution where [H + ] = [OH - ] Acidic Solution where [H + ] > [OH - ] Basic Solution where [H + ] < [OH - ]

Describe how to find significant figures for logarithms- the number of decimal places in the log is equal to the number of significant figures in the original number Equations: pH = -log[H + ] the pH changes by 1 for every power of 10 change in [H + ] because the pH is a log scale based on 10 Also, the pH decreases as [H + ] increases pOH = -log[OH-] pK = -log K Section 14.3: The pH Scale

What are two things you must focus on in order to perform acid-base calculations effectively? solution components and their chemistry. For example HCl is not actually units of HCl. It is dissociated into hydrogen ions and chloride ions Then you need to determine which components are significant and which can be ignored Major species- solution components present in relatively large amounts What is the first step in any acid-base problem? writing the major species in the solution Section 14.4: Calculating the pH of Strong Acid Solutions

What are the steps for calculating the pH of a weak acid solution? Find the major species in the solution. Determine which of the major species supplies the hydrogen ions. List the initial concentrations. Determine the change required to reach equilibrium. Let x be the change in the concentration of the substance required to reach equilibrium. Solve Use the 5% rule to verify whether approximations are valid Section 14.5: Calculating the pH of Weak Acid Solutions

Percent dissociation- also percent ionization amount dissociated initial concentration x 100% What happens to percent dissociation when a weak acid is diluted? the percent dissociation increases Explain the following statement: For solutions of any weak acid, the concentration of hydrogen ions decreases as the concentration of the original acid decreases, but the percent dissociation increases as the concentration of the original acid decreases. When water is added, Q is less than Ka, so the system must adjust to the right to reach the new equilibrium position, so percent dissociation increases when the acid is diluted.

Strong bases- complete dissociation of bases- NaOH and KOH Other strong bases- Calcium hydroxide and barium hydroxide Alkaline earth hydroxides are not very soluble, so they are used when solubility is not important- this is useful for neutralizing stomach acid- it prevents too much hydroxide from forming. More forms as the stomach acid reacts with the hydroxide, but there’s never too much hydroxide formed this way Many bases do not contain the hydroxide ion. How do these substances act as bases? they increase the concentration of hydroxide ion because of their reaction with water. NH 3 +H 2 O  NH OH - Section 14.6: Bases

Equations: K b = [BH + ][OH - ] / [B] What do many of these bases without hydroxide ions have in common? A lone pair of electrons is located on a nitrogen atom in many cases K b – refers to the reaction of a base with water to form the conjugate acid and the hydroxide ion Weak bases- bases with small K b values- where the base competes with the OH - for the H + ion

Polyprotic acids- acids that furnish more than one proton- dissociates in a stepwise manner- one proton at a time Triprotic acids- acids with 3 protons- like phosphoric acid. Describe the relative sizes of dissociation constants for polyprotic acids. (K a 1 > K a 2> K a 3 This means that the first proton is easiest to remove and that it is more difficult to remove successive protons- this happens because the negative charge on the acid increases, making it more difficult to remove a positively charged proton Section 14.7 Polyprotic Acids

In most acids, successive K a values are very different. Why is this important? only the first dissociation step makes an important contribution to [H+] This simplifies pH calculations Why is sulfuric acid unique among acids? it is a strong acid in its first dissociation step and a weak acid in its second step. General Rules: For concentrated solutions, 1M or higher, the large concentration of hydrogen ions from the first dissociation step represses the second step, which can be neglected as a contributor of hydrogen ions. For dilute concentrations- the second step does make a significant contribution and the quadratic equation needs to be used

Salt- an ionic compound What type of salts produce neutral solutions when dissolved in water? salts that consist of the cations of strong bases and the anions of strong acids the cations of strong bases have no affinity for hydrogen ions and cannot produce hydrogen ions. The anions do not combine with hydrogen ions and have no effect on the pH Section 14.8: Acid-Base Properties of Salts

A solution of sodium acetate contains sodium and acetate and water. NaC 2 H 3 O 2 + H 2 O  Na + + C 2 H 3 O 2 - (H + ) + OH - Sodium is neither acidic nor basic. The acetate ion is the conjugate base of acetic acid, which is a weak acid, so acetate is a strong base- it has a high affinity for a proton. The pH of the solutions will be determined by the acetate ion. Because it is a base, it will react with water, creating hydroxide ion. Therefore a basic solution is produced. Basic solutions are produced by bases reacting with water to produce hydroxide ions and conjugate acids. For any salt whose cation has neutral properties ad whose anion is the conjugate base of a weak acid, the aqueous solution will be based. What kind of salt yields a basic solution?

Conjugate bases of weak acids may not always be strong bases. (a) HCN + H 2 O  CN - + H 3 O + (b) CN - + H 2 O  HCN + OH - For example, when hydrocyanic acid dissolves in water (a), the cyanide ion appears to be a strong base compared to water, but when cyanide reacts with water (b), hydroxide ions are produced. The Kb value is much smaller than predicted because cyanide is competing with hydroxide for the hydrogen ion rather than competing with water

Salts in which the anion is not a base and the cation is the conjugate acid of a weak base For example: ammonium chloride dissolves to produce ammonia and hydrogen ions. NH 4 Cl + H 2 O  NH 3 + Cl - + H + The chloride ion really has no affinity for the hydrogen ion, so it does not influence the pH. Salts with highly charged metal ions also produce acidic solutions. The high charge on the metal ion polarizes the O-H bonds in the attached water molecules, making the hydrogens in these water molecules more acidic than those in free water molecules. The higher the charge on the metal ion, the stronger the acidity of the hydrated ion. What kind of salt produces acidic solutions?

How do you determine whether a solution will be acidic, basic, or neutral when there is a salt made up of 2 ions that can affect the pH of aqueous solutions? Compare the Ka value for the acidic ion with the Kb value for the basic ion. If the Ka value for the acidic ion is larger than the Kb value for the basic ion, the solution will be acidic. If the Kb value is larger than the Ka value, the solution will be basic. Equal Ka and Kb values mean that the solution is neutral Equations: Ka + Kb = Kw

Many substances that contain hydrogen atoms are potentially acids but do not actually behave as acids. C-H bonds are strong and nonpolar, so there is no tendency to donate protons. What are the two main factors that determine whether a molecule containing a bond between an atom and hydrogen will behave as a Bronsted-Lowry acid? 1. strength of the bond 2. polarity of the bond What is the trend in strength of oxyacids? The strength of the acid grows as the number of oxygen atoms increases. This occurs because very electronegative oxygen atoms are used to draw electrons away from the chlorine atom and the O-H bond, which polarizes and weakens the O-H bond A proton is easiest to remove from a molecule with the largest number of attached oxygen atoms. This is the same reason that hydrated metal ions become acidic. The greater the electronegativity of the attached ion, the stronger the acid. Section 14.9: The effect of Structure on Acid-Base Properties

Substances that contain H-O-X bonds can behave as acids or bases. Is a hydroxide ion instead of a proton is produced. What determines which behavior will occur? If X has a high electronegativity, the O-X bond will be covalent and strong, meaning that a proton will be released. If X has a low electronegativity, the O-X bond will be ionic and can be broken in polar water Acidic oxides: when a covalent oxide dissolves in water, an acidic solution forms Basic oxides: when an ionic oxide dissolves in water and a basic solution results Section 14.10: Acid-Base Properties of Oxides

Table 14.9 Comparison of Electronegativity of X and Ka Value for a Series of Oxyacids

Lewis acid- electron-pair acceptor OR it has an empty atomic orbital that it can use to accept or share an electron pair from a molecule that has a lone pair of electrons Lewis base- electron-pair donor OR molecule that has a lone pair of electrons Lewis model for acids and bases is beneficial because it covers many reactions that don’t involve Bronsted-Lowry aids Section 14.11: The Lewis Acid-Base Model