Acid Base Equilibria

Types of Acids

  H + vs H 3 O + Arrhenius Definition   Acids – Increase [H+] when dissolved in water  HCl Bases – Increase [OH-] when dissolved in water   NaOH Br ønsted-Lowry Definition   Acids – Donates protons Bases – Accepts protons   HCl(g) + H 2 O(l) → H 3 O + (aq) + Cl (aq) HCl(g) + NH 3 (g) → NH 4 Cl(s) NH (aq) + H O(l) ↔ NH (aq) + OH (aq)

Conjugate Acid Base Pairs

  HX(aq) + H 2 O(l) ↔ X (aq) + H 3 O + (aq)  HNO 2 (aq) + H 2 O(l) ↔ NO 2 (aq) + H 3 O + (aq) What is the conjugate base of each of the following acids?

  HClO 4 , H 2 S, HCO 3 , NH 4 + What is the conjugate acid of each of the following bases?

 CN , SO 4 2 , H 2 O, HCO 3 -

Practice

 There are two possible reactions that HSO 4 can have with water. Write the reaction in which it acts as an acid and another where it acts as a base.

 When lithium oxide is dissolved in water, the solution turns basic from the reaction of the oxide ion(O 2 ) with water. Write the reaction and identify the conjugate acid base pairs.

Acid Base Strength

 Acid strength depends on how easily it gives up a proton  What are the strong acids?

 Bases strength depends on how readily it accepts one.

 What are the strong bases?

Acid Base Strength

 HX + H 2 O ↔ H 3 O + X +  Equilibrium lies on the side with weaker base  HCl + H 2 O ↔ H 3 O + Cl +  HC 2 H H 3 O + 3 O 2 + H 2 O ↔ + C 2 H 3 O 2 -

Practice

 Identify whether equilibrium lies predominantly to the left or right.

   HSO 4 SO 4 2 + CO 3 2 + HCO 3 ↔ HPO 4 2 H 2 PO 4 + H 2 O ↔ + OH NH 4 H 2 O + + OH ↔ NH 3 + Ans: R, L, R

Autoionization of Water

     H 2 O + H 2 O ↔ H 3 O + + OH What is the K e q At 25 °C K e q of this reaction = 1x10 1 4 What are the concentrations of each ion if each the concentration of [H 3 O + ]=[OH ]?

How can we use this relationship to identify if a solution is acidic, basic, or neutral?

pH Scale

 pH scale goes from 0 to 14  pH = - log[H + ]  pOH = -log [OH ]  pH + pOH = 14

Strong Acids and Bases

 For strong acids and bases no equilibrium is reached.

 For monoprotic strong acids: [H+] = [HA]   What is the pH of a 0.040M solution of HClO 4  pH = -log(0.040) = 1.40

For strong bases: [OH ] = to # of OH 's x [Base]  What is the pH of a 0.028M solution of NaOH and a 0.0011M solution of Ca(OH) 2

Practice

 An aqueous solution of HNO 3 has a pH of 2.34. What is the concentration of the acid?

 What is the concentration of a solution of KOH for which the pH is 11.89; Ca(OH) pH is 11.68?

2 for which the  Ans: #1 0.0046M, #2 7.8x10

- 3 M, 2.4x10

- 3 M

Weak Acids

 Weak acids only partially ionize and therefore reach equilibrium.

 K e q can be used to tell what extent it ionizes, how?

 HA(aq) + H 2 O(l) ↔ A (aq) + H 3 O + (aq) HA(aq) ↔ H + (aq) + A (aq)  What is the equilibrium expression for this reaction?

 K a = acid-dissociation constant  How does K a relate to acid strength?

Weak Acids

Using pH to find K

a  A Student prepared a 0.10M solution of formic acid (HCHO 2 ) and measured its pH using a pH meter. The pH was found to be 2.38. Calculate the K a for formic acid at this temperature. What percentage of the acid is ionized?

 Ans: 1.8x10

- 4 , 4.2%

Practice

 Niacin, a type of B vitamin has the formula C 5 H 4 NCO 2 H. A 0.020M solution of the vitamin has a pH of 3.26. What percentage of the acid is ionized in this solution? What is the acid dissociation constant Ka for niacin?

 Ans: 2.7%, 1.6x10

- 5

Using K

a

to find pH

 We need to know Ka and the initial concentration of the acid. Ka of acetic acid = 1.8x10

- 5 , [HC 2 H 3 O 2 ] = 0.30M

1) Write the ionization equilibrium for the acid HC 2 H 3 O 2 (aq) ↔ H + (aq) + C 2 H 3 O 2 (aq) 1) Write the equilibrium constant expression and value of K a K a = [H + ][C 2 H 3 O 2 ]/[HC 2 H 3 O 2 ] 1) Find equilibrium concentrations

Using K

a

to find pH

4) Substitute the equilibrium concentrations into the equilibrium expression and solve for x.

** x may be disregarded as long as it is less than 5% of the initial [ ] x = 2.3x10

- 3 = [H + ] pH = 2.64

Practice

 Calculate the pH of a 0.20M solution of HCN. K a for HCN is 4.9x10

- 1 0 .

Ans: pH = 5.00

Calculating Percent Ionization

 Calculate the percentage of HF molecules ionized in (a) 0.10M HF solution; (b) 0.010M HF solution.

Find x then divide that by initial concentration.

Ans: a) 7.9%, 23%

Polyprotic Acids

 Polyprotic acids have more than one ionizable H atoms and thus multiple K a 's.

 Easier to remove 1 s t proton than the next ones.

 Finding pH for polyprotic acids  Compare size of K a 's  7 H 2 CO 3 (aq) ↔ H + (aq) + HCO 3 (aq) K a 1 = 4.3x10

  HCO 3 (aq) ↔ H + (aq) + CO 3 2 (aq) For most acids only K a 1 is important K a 2 = 5.6x10

- 1 1

Polyprotic Acids

pH of Polyprotic Acids

 What is the pH of a 0.0037M solution of 1 H 2 CO 3 ? What is the [CO 3 2 ] in the solution? K a = 4.3x10

-7 , K a 2 = 5.6x10

- 1 1 Ans: pH = 4.40, [CO 3 2 ] = 5.6x10

- 1 1 M

Practice

 Calculate the pH and concentration of oxalate ion [C 2 O 4 2 ], in a 0.020M solution of oxalic acid (H 2 C 2 O 4 ). K a 1 = 5.9x10

- 2 , K a 2 = 6.4x10

- 5 Ans: pH= 1.80, [C 2 O 4 2 ]= 6.4x10

- 5

Weak Bases

    B(aq) + H 2 O(l) ↔ HB + (aq) + OH (aq) NH 3 (aq) + H 2 O(l) ↔ NH 4 + (aq) + OH (aq) K b = base-dissociation constant What is the K b expression for NH 3 ?

Calculate the [OH ] in a 0.15M solution of NH 3 . K b = 1.8x10

- 5 Ans: 1.6x10

- 3 M

Weak Bases

K

a

and K

b    K a x K b = K w The larger K a lower the K b the Calculate the K a HF if the K b of F 1.5x10

- 1 1 for is

pH of Salt Solutions

1) An anion that is the conjugate base of a strong acid will not affect the pH of a solution. ex. Br 2) An anion that is the conjugate base of a weak acid will cause an increase in pH. ex. CN 3) A cation that is the conjugate acid of a weak base will cause a decrease in pH. ex. NH 4 + 4) With the exception of ions of group 1A and heavier members of group 2A, metal ions will cause a decrease in pH.

5) When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the ion with the largest ionization

Practice

 Predict whether the salt Na 2 HPO 4 will form an acidic or basic solution on dissolving in water.

 Predict whether the K 2 HC 6 H 5 O 7 will form an acidic or basic solution in water. (look at citric acid)

Acid Strength

 Molecules only donate protons if the H —X bond is polar, where the anion X is more electronegative than the H.

 As you move from left to right X becomes more electronegative and acid strength increases.

  CH 4 < NH 3 << H 2 O < HF Strong H —X bonds are harder to break than weak ones.

 The strength of the H —X bond decreases as the size of X increases.

  HF versus HCl, H 2 S versus H 2 O The more stable the resulting anion, X , the

Binary Acid Strength

Oxyacid Strength

 Acids with OH groups and additional oxygen atoms bound to a central atom are called oxyacids.

H 2 SO 4 1) For oxyacids that have the same number of OH groups and the same number of O atoms, acid strength increases with increasing electronegativity of the central atom.

2) For oxyacids that have the same central atom, acid strength increases as the number of oxygen

Lewis Acids and Bases

 Lewis acids – electron pair acceptor     Increases the types of compounds that we can consider acids Lewis bases – electron pair donor  All bases that are Br ønsted-Lowry bases are Lewis bases.

NH 3 + BF 3 → NH 3 BF 3 Metal ions reacting with water act as Lewis acids

Homework