Acid-Base Chemistry: Strengths, Ionization, and Buffers

HA(aq) ⇄ H+(aq) + A-(aq)

What is the equilibrium expression for the dissociation of a weak acid? (generic acid equation)

Ka = [H+][A-]/[HA]

What equation is used to solve for the acid ionization constant (Ka)?

HCO2 is the stronger acid because its Ka value is greater

Determine which acid is stronger:

HCO2 (Ka = 1.8 x 10^-4)

HOCl (Ka = 2.9 x 10^-8)

1) HCl

2) HBr

3) HI

4) HNO3

5) H2SO4

6) HClO4

7) HClO3

What were the 7 strong acids discussed in lecture?

- strong

- completely ionized

If K >> 1, this indicates that the acid is ________________ and _________________

1. Binary acids (HxAy)

2. Oxoacids (HxAyOz)

3. Carboxylic acids

O

II

C - O - H

I

R

The structure of an acid influences its acidity. What structures are acids seen as?

Binary acids (HxAy)

What structure is represented in the acids below:

HCl

H2O

The electronegativity and bond polarity increases as the atomic size of A decreases

Binary acids: HxAy

Why does the acid get stronger as A changes from L → R?

HF is stronger because the H-F bond is more polar than the H-O bond, allowing H+ to ionize more easily

Which acid is stronger and why?

HF

H2O

The electronegativity decreases as the atomic size of A increases

Binary acids: HxAy

Why does the acid get stronger as A changes from T → B?

oxygen atoms

Oxoacids: HxAyOz

An oxoacid with more ______________ than another oxoacid with the same A is the stronger acid

- electronegative

- polar

As A becomes more _____________, the acid becomes stronger and more _____________

HClO3 is the stronger acid because it has more oxygen atoms, which increases the polarity of the H-O bond

Which acid is stronger and why?

HClO2

HClO3

HClO2

Which acid is stronger?

HClO2

HBr2O2

- R (another group)

- Increases

Carboxylic acids:

As ____________ becomes more electronegative, acid strength _______________, making the bond more polar

CF3COOH is the stronger acid because CF3 is more electronegative than CH3

Which acid is stronger?

CH3COOH

CF3COOH

- 100%

- Groups 1 and 2 hydroxides

Strong bases are __________% ionized found in groups ________ and _________

B(aq) + H2O(l) ⇄ BH+(aq) + OH-(aq)

What is the equilibrium expression for the dissociation of a weak base? (generic base equation)

Kb = [BH+][OH-]/[B]

What is the base ionization constant (Kb)?

strength of weak bases

The base ionization constant (Kb) is used to measure the _________________

Kb = [NH4+][OH-]/[NH3]

Identify the base ionization constant:

NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH-(aq)

stronger

The greater the Kb value, the _________________ the base

Dimethylamine is the stronger base because its Kb value is larger

Which is the stronger base? Methylamine CH3NH2

Kb = 4.4 x 10^-4

Dimethylamine (CH3)2NH

Kb = 5.9 x 10^-4

Conjugate pairs

____________________ consist of an acid and its corresponding base that differ by a proton (H+)

1) differs by a H+ → conjugate acid pair

2) not a conjugate acid pair

3) differs by a H+ → conjugate acid pair

Identify which is a conjugate acid pair and which is not:

1) H3PO4/H2PO4-

2) HI/H+

3) NH3/ NH4+

- donated (removed)

- accepted (added)

To form a conjugate base, a proton (H+) is _______________

To form a conjugate acid, a proton (H+) is _______________

Conjugate base

- Take away a H+ → SO4 2-

Conjugate acid

- Add an H+ → H2SO4

Identify the conjugate base and conjugate acid of the compound below:

HSO4

amphoteric substance

An ______________________ can act as either an acid or a base

Water (H2O)

Give an example of an amphoteric substance

leveling effect

The _______________ states that in water, the strongest acid is H3O+ and the strongest base is OH-

weaker

Reciprocal relationship:

The stronger the acid, the _______________ the conjugate base

The side with the weaker acid and base

In an acid-base equilibrium, which side does the equilibrium favor?

Kw = [H3O+][OH-]

What is the equilibrium constant for the autoionization of water?

H2O(l) + H2O(l) ⇄ H3O+(aq) + OH-(aq)

1.0 x 10^-14

What is the value of Kw?

- neutral

- acidic

- basic

The Autoionization of H2O:

If [H3O+] = [OH-] → the solution is ______________

If [H3O+] > [OH-] → the solution is ______________

If [H3O+] < [OH-] → the solution is ______________

- neutral

- acidic

- basic

If pH = 7.00 → the solution is _______________

If pH < 7.00 → the solution is _______________

If pH > 7.00 → the solution is _______________

pH = - log[H+]

How is pH calculated from [H+]?

[H+] = 10^-pH

How is [H+] calculated from pH?

pKa + pKb = pKw = 14.00

pKa + pKb = ___________ = ____________

pKa = -log(Ka)

pKb = -log(Kb)

What equations are used to solve for pKa and pKb?

Ka x Kb = Kw = 1.0 x 10^-14

Ka x Kb = ____________ = _______________

pH = - log (2.5 x 10-12) = 11.60

pH + pOH = 14.00

11.60 + pOH = 14.00

pOH =2.40

Solve for pH and pOH with whats given:

[H3O+] = 2.5 x 10^-12M

Ka x Kb = Kw = 1.0 x 10^-14

(1.8 x 10^-5) x Kb = 1.0 x 10^-14

Kb = 5.6 x 10^-10

Calculate Kb with whats given:

Ka = 1.8 x 10^-5

1) solve for [H+]

[H+] = 10-pH

10-2.44 = 0.0036M

2) ICE

HA(aq) ⇄ H+(aq) + A-(aq)

I(M) 0.100 0 0

C(M) - 0.0036 + 0.0036 + 0.0036

E(M) 0.096 0.0036 0.0036

3) solve for Ka

Ka = [H+][C3H5O3-]/[HC3H5O3]

(0.0036)(0.0036)/0.096 = 1.4 x 10^-14

4) solve for pKa

pKa = - log(1.4 x 10^-14) = 3.85

If 0.100M lactic acid (HC3H5O3) has a pH of 2.44 at 25 degrees celsius, what is the acidic pKa?

1) solve for [H+]

[H+] = 10^-2.40 - 0.0040

2) ICE

HA(aq) ⇄ H+(aq) + A-(aq)

I(M) 0.954 0 0

C(M) - 0.0040 +0.0040 +0.0040

E(M) 0.950 0.0040 0.0040

3) solve for Ka

Ka = [H+][C2H3O2-] / [HC2H3O4]

(0.0040)^2/0.950 = 1.68 x 10^-5

4) solve for pKa

pKa = - log (1.68x 10^-5) = 4.77

A sample of vinegar has a pH of 2.40, it was determined that the initial concentration of acetic acid in vinegar was 0.954M. Solve for pKa

The pH decreases as the concentration of H+ ions increases

What happens to the pH of a solution when a strong acid is added?

- Increases pH

- Decreases acidity

Dilution on a strong acid generally ______________pH and _____________ acidity as the concentration of H+ decreases

- strong

- weak

___________ acids completely ionize in solution, while __________ acids do not

1) ICE

HA(aq) ⇄ H+(aq) + A-(aq)

I(M) 1.0 0 0

C(M) -x +x +x

E(M) 1.0 - x x x

2) solve for [H+]

1.8 x 10-5 = x^2/1.0

x = 0.0042

3) solve for pH

pH = -log (0.0042) = 2.38

2.38 < 7 (weak acid)

Determine the pH of a 1.0M HC2H3O2 solution with whats given:

weak base or weak acid

Ka = 1.8 x 10-5

1) ICE

B(aq) + H2O(l) ⇄ BH+(aq) + OH-(aq)

I(M) 0.020 0 0

C(M) - x + x + x

E(M) 0.020 - x x x

2) solve for [OH-]

1.8 x 10-9 = x^2/ 0.020

x = 6.0 x 10^-6

3) solve for pOH

pOH = -log (6.0 x 10-6) = 5.22

4) solve for pH

pH + 5.22 = 14.00 pH = 8.78

Calculate the pH of a 0.020M solution of base pyridine (C5H5N) with whats given:

Kb = 1.8 x 10-9

1) ICE

HA(aq) ⇄ H+(aq) + A-(aq)

I(M) 0.025 0 0

C(M) - x + x + x

E(M) 0.025 - x x x

2) solve for [H+]

1.3 x 10-10 = x2/0.025

x = 1.8 x 10^-6

3) solve for pH

pH = -log (1.8 x 10-6) = 5.74

Determine the pH with whats given:

Ka = 1.3 x 10-10

0.025M C8H5OH

2.38 < 7 (weak acid)

8.78 > 7 (weak base)

5.74 < 7 (weak acid)

Weak base or weak acid?

pH = 2.38

pH = 8.78

pH = 5.74

% Ionization = ([H+] equilibrium / [HA] initial) x 100%

What is the formula for percent ionization of a weak acid?

% Ionization = ([OH-] equilibrium/ [B] initial) x 100%

What is the formula for percent ionization of a weak base?

% Ionization = ([H+] equilibrium / [HA] initial) x 100%

(1.8 x 10-6M / 0.025M) x 100% = 0.0072%

Determine the % Ionization with whats given:

[H+] = 1.8 x 10^-6

[HA] = 0.025M

Acidic cation

An __________________ is a conjugate acid of a weak base

Basic anion

A _______________ is a conjugate base of a weak acid

Cation - negative

Anion - positive

An acidic cation comes from the _______________ ion in an ionic compound

A basic anion comes from the ________________ ion in an ionic compound

1) Basic, NO3- behaves as a base

2) Basic, F- behaves as a base

3) Neutral

4) Acidic, NH4+ behaves as an acid

Basic, Acidic or Neutral solution?

1) NaNO3

2) KF

3) KCl

4) NH4Br

Acidic, Ka > Kb

Basic, Acidic or Neutral solution?

NH4CHO2(aq) → NH4+(aq) + CHO2-(aq)

NH4+

- Acidic

- Ka = 5.6 x 10^-10

CHO2-

- Basic

- Kb = 5.6 x 10^-11

Neutral, Ka = Kb

Basic, Acidic or Neutral solution?

NH4C2H3O2(aq) → NH4+(aq) + C2H3O2-(aq)

NH4+

- Acidic

- Ka = 5.6 x 10^-10

C2H3O2-(aq)

- Basic

- Kb = 5.6 x 10^-10

- pH increases

- more basic (because base was added)

Le Chateliers principle:

If C2H3O2- is added, the pH will __________, causing the solution to become more ______________

HC2H3O2(aq) ⇄ H+(aq) + C2H3O2-(aq)

adding a common ion

Common ion effect:

The ionization of an acid can be suppressed by _______________________

Henderson-Hasselbalch equation

Identify the equation below:

pH = pKa + log ([A-]/[HA])

It is used to calculate the pH of a buffer solution

What is the Henderson-Hasselbalch equation used for?

The pH changes to 4.74, which is a decrease of 0.090.

How does adding 0.010 M HCl to a buffer affect the pH?

pKa

The ___________ value in buffer solutions indicates the strength of the acid

True

True or False:

The pKa value in buffer solutions should be within ±1 of the desired pH

1) Mol

50.0g HC2H3O2 x (1 mol/ 60.05g) = 0.833 mol

50.0g NaC2H3O2 x (1 mol/ 82.03g) = 0.610 mol

2) Molarity

0.833 mol/1.00 L solution = 0.833 M

0.610 mol/1.00 L solution = 0.610M

3) pKa

pKa = -log (1.8x10-5) =4.745

4) pH

pH = 4.745 + log (0.610/0.833) = 4.61

Identify the pH with whats given:

1.00L buffer

50.0g HC2H3O2 (60.05 g/mol) 50.0g NaC2H3O2 (82.03 g/mol)

Ka = 1.8 x 10-5

1.00L solution

The buffer consumes H+ ions, shifting the equilibrium to the reactants and increasing the pH

What are the effects of adding a weak base (C2H3O2-) to a buffer solution?

HC2H3O2(aq) ⇄ H+(aq) + C2H3O2-(aq)

adding C2H3O2-