Acids and Bases  Arrhenius ◦ Acid:Substance that, when dissolved in water, increases the concentration of hydrogen ions. ◦ Base:Substance that, when dissolved in water, increases the concentration of hydroxide ions. ◦ Limitation: Restricted to aqueous system!

Acids and Bases  Brønsted–Lowry ◦ Broader definition! ◦ Acid:Proton (H + ) donor ◦ Base:Proton(H + ) acceptor

Acids and Bases A Brønsted–Lowry acid… …must have a removable (acidic) proton. A Brønsted–Lowry base… …must have a pair of nonbonding electrons.

Acids and Bases...it is amphiprotic. HCO 3 − HSO 4 − H2OH2O

Acids and Bases  Water acts as a Brønsted–Lowry base and abstracts a proton (H + ) from the acid.  As a result, the conjugate base of the acid and a hydronium ion are formed.

Acids and Bases  Reactions between acids and bases always yield their conjugate bases and acids.

Acids and Bases  a)HNO 3 + OH - 

Acids and Bases  CH 3 NH 2 + H 2 O 

Acids and Bases   c)OH - + HPO 4 -2 

Acids and Bases  NH 3 + H 2 O 

Acids and Bases  Strong acids are completely dissociated in water.  Have large K values!! ◦ Their conjugate bases are quite weak.  Weak acids only dissociate partially in water. ◦ Their conjugate bases are weak bases. ◦ Small K values

Acids and Bases

Acids and Bases  HCl  HBr  HI  (not HF)  HNO 3  H 2 SO 4  HClO 4 

Acids and Bases  Substances with negligible acidity do not dissociate in water. ◦ Their conjugate bases are exceedingly strong.

Acids and Bases In any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl − (aq) H 2 O is a much stronger base than Cl −, so the equilibrium lies so far to the right K is not measured (K>>1).

Acids and Bases Acetate is a stronger base than H 2 O, so the equilibrium favors the left side (K<1). HC 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 − (aq)

Acids and Bases  As we have seen, water is amphoteric.  In pure water, a few molecules act as bases and a few act as acids.  This is referred to as autoionization. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH − (aq)

Acids and Bases  The equilibrium expression for this process is K c = [H 3 O + ] [OH − ]  This special equilibrium constant is referred to as the ion-product constant for water, K w.  At 25°C, K w = 1.0  10 −14

Acids and Bases pH is defined as the negative base-10 logarithm of the hydronium ion concentration. pH = −log [H 3 O + ]

Acids and Bases  In pure water, K w = [H 3 O + ] [OH − ] = 1.0  10 −14  Because in pure water [H 3 O + ] = [OH − ], [H 3 O + ] = (1.0  10 −14 ) 1/2 = 1.0  10 −7

Acids and Bases  Therefore, in pure water, pH = −log (1.0  10 −7 ) = 7.00  An acid has a higher [H 3 O + ] than pure water, so its pH is <7  A base has a lower [H 3 O + ] than pure water, so its pH is >7.

Acids and Bases These are the pH values for several common substances.

Acids and Bases  The “p” in pH tells us to take the negative log of the quantity (in this case, hydrogen ions).  Some similar examples are ◦ pOH −log [OH − ] ◦ pK w −log K w

Acids and Bases Because [H 3 O + ] [OH − ] = K w = 1.0  10 −14, we know that −log [H 3 O + ] + −log [OH − ] = −log K w = or, in other words, pH + pOH = pK w = 14.00

Acids and Bases  For less accurate measurements, one can use ◦ Litmus paper  “Red” paper turns blue above ~pH = 8  “Blue” paper turns red below ~pH = 5 ◦ An indicator How Do We Measure pH?

Acids and Bases For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

Acids and Bases  You will recall that the six strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, and HClO 4.  These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.  For the monoprotic strong acids, [H + ] = [acid].

Acids and Bases  What is the Hydrogen ion concentrations of.013 M HCl?  What is the Hydrogen ion concentrations of.0125 M HNO 3 ?  HClO 4 has a [H + ] of 2.33 x M.. What is the concentration of the acid?

Acids and Bases  Strong bases are the soluble hydroxides, which are the alkali metal (LiOH, NaOH, KOH, RbOH, CsOH)and heavier alkaline earth metal hydroxides –  Ca(OH ) 2, Sr(OH ) 2 Ba(OH) 2  Again, these substances dissociate completely in aqueous solution.

Acids and Bases  They do not ionize much  Equilibium lies far to the left! the equilibrium expression would be  This equilibrium constant is called the acid-dissociation constant, K a. [H 3 O + ] [A − ] [HA] K c = HA (aq) + H 2 O (l) A − (aq) + H 3 O + (aq)

Acids and Bases The greater the value of K a, the stronger the acid.

Acids and Bases  Write the K a expression for acetic acid:

Acids and Bases  Write the K a expression for acetic acid:  H C 2 H 3 O 2 (aq)+H 2 O(l)  C 2 H 3 O 2 − (aq)+H 3 O + (aq)

Acids and Bases  Weak bases without OH react with water to produce hydroxide ion.  Examples: NH 3 CH 3 NH 2 (methylamine)  lone pair of electrons forms a bond with H +

Acids and Bases  B + H 2 O(l)  HB + OH - K b= [HB] [OH − ] [B]

Acids and Bases K b can be used to find [OH − ] and, through it, pH.

Acids and Bases Calculate the pH of a 0.30 M solution of acetic acid, HC 2 H 3 O 2, at 25°C. HC 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 − (aq) K a for acetic acid at 25°C is 1.8  10 −5.

Acids and Bases The equilibrium constant expression is [H 3 O + ] [C 2 H 3 O 2 − ] [HC 2 H 3 O 2 ] K a =

Acids and Bases [C 2 H 3 O 2 ], M[H 3 O + ], M[C 2 H 3 O 2 − ], M I C−x−x+x+x+x+x E 0.30 − x  0.30 xx We next set up a table… We are assuming that x will be very small compared to 0.30 and can, therefore, be ignored.

Acids and Bases  When the initial concnetration is very large compared to 100 K a. If initial concentration is large by comparison, neglect x.  Quantitatively, if the quantity x is more than about 5% of the initial value, it is better to use the quadratic formula

Acids and Bases Now, (x) 2 (0.30) 1.8  10 −5 = (1.8  10 −5 ) (0.30) = x  10 −6 = x  10 −3 = x

Acids and Bases pH = −log [H 3 O + ] pH = −log (2.3  10 −3 ) pH = 2.64

Acids and Bases Calculate the pH of a 1.00 x 10-4 M solution of acetic acid, HC 2 H 3 O 2, at 25°C. HC 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 − (aq) K a for acetic acid at 25°C is 1.8  10 −5.

Acids and Bases [C 2 H 3 O 2 ], M[H 3 O + ], M[C 2 H 3 O 2 − ], M I1.00 x C−x−x+x+x+x+x E1.00 x − xxx We next set up a table… x will is not very small compared to 1.00 x and cannot be ignored.

Acids and Bases  X 2 = 1.8 x x x Or  X x x x = 0  X = 3.44 x = [H + ]

Acids and Bases  pH = - log (3.44 x ) = 3.46

Acids and Bases  Only the acid in the mixture with the larger K a will donate an appreciable [H + ].  Determine pH Based on this acid, and ignore all others!

Acids and Bases  The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is Calculate K a for formic acid at this temperature.  We know that [H 3 O + ] [COO − ] [HCOOH] K a =

Acids and Bases  The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is Calculate K a for formic acid at this temperature.  We know that [H 3 O + ] [COO − ] [HCOOH] K a =

Acids and Bases  The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is Calculate K a for formic acid at this temperature.  To calculate K a, we need the equilibrium concentrations of all three things.  We can find [H 3 O + ], which is the same as [HCOO − ], from the pH.

Acids and Bases pH = −log [H 3 O + ] 2.38 = −log [H 3 O + ] −2.38 = log [H 3 O + ] 10 −2.38 = 10 log [H 3 O + ] = [H 3 O + ] 4.2  10 −3 = [H 3 O + ] = [HCOO − ]

Acids and Bases [HCOOH], M[H 3 O + ], M[HCOO − ], M Initially Change −4.2    10 −3 At Equilibrium 0.10 − 4.2  10 −3 = =  10 −3 Now we can set up a table…

Acids and Bases [4.2  10 −3 ] [0.10] K a = = 1.8  10 −4

Acids and Bases  Have more than one acidic proton.  If the difference between the K a for the first dissociation and subsequent K a values is 10 3 or more, the pH generally depends only on the first dissociation.

Acids and Bases Bases react with water to produce hydroxide ion.

Acids and Bases The equilibrium constant expression for this reaction is [HB] [OH − ] [B − ] K b = where K b is the base-dissociation constant.

Acids and Bases What is the pH of a 0.15 M solution of NH 3 ? [NH 4 + ] [OH − ] [NH 3 ] K b = = 1.8  10 −5 NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH − (aq)

Acids and Bases [NH 3 ], M[NH 4 + ], M[OH − ], M Initially At Equilibrium x  0.15 xx Tabulate the data.

Acids and Bases (1.8  10 −5 ) (0.15) = x  10 −6 = x  10 −3 = x 2 (x) 2 (0.15) 1.8  10 −5 =

Acids and Bases Therefore, [OH − ] = 1.6  10 −3 M pOH = −log (1.6  10 −3 ) pOH = 2.80 pH = − 2.80 pH = 11.20

Acids and Bases K a and K b are related in this way: K a  K b = K w Therefore, if you know one of them, you can calculate the other.

Acids and Bases  Percent Ionization =  100  In this example [H 3 O + ] eq = 4.2  10 −3 M [HCOOH] initial = 0.10 M [H 3 O + ] eq [HA] initial

Acids and Bases Percent Ionization =   10 − = 4.2%

Acids and Bases  Calculate the percent dissociation of acetic acid (K a = 1.8 x ) in each of the following solutions:  1.00 M HC 2 H 3 O 2

Acids and Bases  Anions are bases.  As such, they can react with water in a hydrolysis reaction to form OH − and the conjugate acid: X − (aq) + H 2 O (l) HX (aq) + OH − (aq)

Acids and Bases  Cations with acidic protons (like NH 4 + ) will lower the pH of a solution.  Most metal cations that are hydrated in solution also lower the pH of the solution.

Acids and Bases  Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water.  This makes the O-H bond more polar and the water more acidic.  Greater charge and smaller size make a cation more acidic.

Acids and Bases 1. An anion that is the conjugate base of a strong acid will not affect the pH. 2. An anion that is the conjugate base of a weak acid will increase the pH. 3. A cation that is the conjugate acid of a weak base will decrease the pH.

Acids and Bases 4. Cations of the strong Arrhenius bases will not affect the pH. 5. Other metal ions will cause a decrease in pH. 6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the K a and K b values.

Acids and Bases  The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.  Acidity increases from left to right across a row and from top to bottom down a group.

Acids and Bases In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.

Acids and Bases For a series of oxyacids, acidity increases with the number of oxygens.

Acids and Bases Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.

Acids and Bases  Lewis acids are defined as electron-pair acceptors.  Atoms with an empty valence orbital can be Lewis acids.

Acids and Bases  Lewis bases are defined as electron-pair donors.  Anything that could be a Brønsted–Lowry base is a Lewis base.  Lewis bases can interact with things other than protons, however.