AP Chemistry: Unit 8

Autoionization

Autoionization

pure water has an equilibrium in the production of H3O+ and OH- ions (It will act as a Bronsted-Lowry acid and base)

The equilibrium constant for the autoionization of water

Kw=[H3O+][OH-] Kw= 1 × 10-14

[H3O+]>[OH-]

acidic

[H3O+]<[OH-]

Basic (Alkaline)

[H3O+]=[OH-]

Neutral

What does “p” direct you to do?

to the usage of -log of the measurement we’re using

pH=

-log[H3O+]

[H3O+][OH-]=

1 × 10-14

[H3O+]=

10 TO THE -pH

[OH-]=

10 TO THE -pOH

pOH+pH=

14

pOH=

-log[OH-]

Ka*Kb=

1 × 10-14

As temperature increases, what happens to Kw?

It increases

Strong acids have

weak H-X bonds (HCl)

Weak acids have

strong H-X bonds (HF); do not experience a strong induced dipole force.

Oxoacids

contain an atom bonded to one or more oxygen atoms, sometimes with hydrogens attached.

Inductive effect

when electrons in adjacent bonds are attracted to more electronegative atoms. A stronger inductive effect creates more polarity in the molecule as e- move away to the electronegative atoms and makes it more ionizable (a stronger acid)

Weak acids in terms of polarity

weak acids will have a less inductive effect, which leads to less attraction of e- to electronegative atoms, making it less ionizable.

A strong acid will produce a

weak conjugate base

A weak acid will produce a

strong conjugate base

Strong bases

attract more protons

Factors that affect base strength

1. How easily the lone pair picks up a hydrogen ion

2. Stability of the ions formed

Strong base will produce a

weak conjugate base

Weak base will produce

a strong conjugate acid

As Kb or Ka increases

the strength increases

Relative concentrations of an acid and its conjugate base can be predicted by comparing

the pH of a solution to the pKa of the acid in that solution

pH<pKa

the acid form is greater in the buffer solution

pH>pKa

the conjugate base is greater in the buffer solution

Equivalence point

the number of moles of titrant added is exactly sufficient to react completely with the number of moles of the titrated species in the sample.

Endpoint

point at which the indicator undergoes a color change in a titration

Buffers

a solution that is resistant to changes in pH when a strong acid/base is added because it is a solution of a strong acid + conjugate base or strong base + conjugate acid

pH of a buffer

includes the pKa of the acid an

Key assumptions of the Henderson-Hasselbalch Equation

Equil. concentrations of acid and conjugate base are approx equal to the initial concentrations

pH (according to the Henderson-Hasselbalch Equation)

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

The higher the buffer concentration

the greater the amount of acid or base it can neutralize - pH doesn’t change

What should you do in order to have the buffer solution be able to control the pH at the desired value?

Choosing an acid with a Ka close to the intended value of [H3O+] - Finding the ideal ratio of acid : conjugate base

Common strong acids

HCl, HNO3, HClO4, H2SO4, HBr, HI

Common strong bases

LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2, Sr(OH)2

What can you use to find pOH and pH of strong acids/bases?

Basic stoichiometric ratios to find the concentration of H+ or OH- when the strong base/acid dissociates

% ionization

[H+]equilibrium concentration/[Acid]initial concentration * 100

[OH-]equilibrium concentration/[Base]initial concentration * 100

Net ionic equation for strong base + strong acid

H+ + OH- → H2O(l)

Weak acid + Strong base

Weak acid + Strong base → Water + Conjugate base

Strong acid + Weak base

Strong acid + Weak base → Water + conjugate acid

Weak acid + Weak base

Check your Ka and Kb values (Ka>Kb - ACIDIC)

pH=pKa at the

half-equivalence point where the concentration of the weak acid/weak base is = to the conjugate acid/base

Strength of an acid

ELECTRONEGATIVE ATOMS - # OF OXYGENS

Common weak bases have

nitrogen and hydrogen (NH3)

An indicator should have a pKa value close to

the pH value of the equivalence point

What kind of titration is this?

Strong acid titrated with strong base - Equivalence point is 7 pH

What kind of titration is this?

Weak acid titrated with strong base - Equivalence point is higher than 7 pH

What kind of titration is this?

A strong acid titrated with weak base - Equivalence point is lower than 7 pH