Acid-Base Titrations

Acid-Base Titrations

Acid-Base Equilibria Chapter 16

Review of Acids and BasesAn acid is a substance that when dissolved in

water increases the concentration of H+

A base is a substance that when dissolved in water increases the concentration of OH-

Bronsted-Lowry Acids and BasesThese types of acid base reactions have to do with

the transfer of a proton.

Conjugate Acid/Base pairs

The Relative Strengths of Acids and BasesWe can predict the acid or basic properties of

conjugate pairs.A strong acid produces a conjugate base that will

be very unwilling to accept a proton.A weak acid produces a conjugate base that has

some tendency to accept a proton.A substance that contains hydrogen but has

negligible acidity will produce a conjugate base that is a very strong base.

The Autoionization of Water

Calculate the concentration of H+ in a solution in which [OH-] is 0.01 M.

The pH scale

Calculate the [H+] in a sample of freshly squeezed apple juice that has a pH of 3.76.

Strong AcidsThe seven most common strong acids are:

Strong BasesThe most common strong bases are the

hydroxides of the alkali metals.

Weak AcidsMost acidic substances are weak acids and are

therefore only partially ionized.

Calculate Ka from pHA student prepared a 0.10 M solution of formic

acid (HCOOH) and measured its pH. The pH at 25oC was found to be 2.38. Calculate Ka for formic acid.

Calculating pH from Ka

Polyprotic Acids

Weak Bases Many substances behave as weak bases when

mixed with water by removing a hydrogen from the water producing OH-.

Relationship Between Ka and Kb

Acid-Base Properties of Salt SolutionsSolutions made from soluble salts (ionic

compounds) can be acidic or basic. We know these solutions produce anions and

cations.Many of these ions are able to react with water to

generate H+(aq) or OH-(aq). These types of reactions are often called hydrolysis.

Anions Reacting with WaterOften times an anion is the conjugate base of an

acid:

If this anion is the conjugate base of a strong acid it will have negligible tendency to remove a proton from water.

Cations Reacting with Water Polyatomic cations whose formulas contain one or

more protons are usually the conjugate acids of weak bases.

Combined Effect of Cations and Anions in SoluitonIf a solution contains two different ions that can

effect the pH differently.

Summary An anion that is the conjugate base of a strong acid (example

Br-) will not affect the pH of a solution. An anion that is the conjugate base of a weak acid (example

CN-) will cause and increase in pH. An cation that is the conjugate acid of a weak base (example

CH3NH3+) will cause a decrease in pH.

Cations of group 1A metals have no affect on pH other metals will cause a decrease in pH when a solution contains both the conjugate base of a weak acid

and the conjugate acid of a weak base the ion with the larger equilibrium constant will have the greater influence on pH.

Acid-Base Behavior and Chemical StructureWhat makes an acid or a base strong or weak?It’s chemical structure.

Binary AcidsBinary acids are ones that contain only 2 atoms

(one being H).The strength of binary acids where elements X are

in the same group decreases as the size of X increases.

OxyacidsOxyacids are ones where OH groups are attached

to a central atom.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.

For oxyacids that have the same central atom, acid strength increases as the number of oxygen atoms attached to that central atom increases.

Carboxylic Acids

Lewis Acids and BasesA Lewis base is a substance that has a free pair of

electrons.A Lewis acid is a substance that can accept that

free pair of electrons

Additional Aspects of Aqueous Equilibria

Chapter 17

The Common Ion EffectThe common ion effect describes what happens

when we add a strong electrolyte to a weak electrolyte solution and both compound share a common ion

ExampleWhat is the pH of a solution made by adding 0.3

mol of acetic acid and 0.3 moles of sodium acetate to enough water to make 1.0 L of solution? (Ka = 1.8 x 10-5)

Buffer SolutionsBuffer solutions are ones that resist drastic pH

changes when small amounts of strong acids or bases are added.

These solutions contain significant enough amounts of H+ and OH- that they are able to neutralize any acid or base we might add.

Buffers are usually prepared by mixing a weak acid or base with a salt of that acid or base.

Calculating the pH of a BufferWhat is the pH of a buffer that is 0.12 M lactic acid

(HC3H5O3) and 0.1 M sodium lactate (NaC3H5O)? (Ka = 1.4x10-4)

Buffer Capacity and pH RangeBuffer capacity refers to the amount of strong acid

or strong base that can be added before the pH begins to change by an appreciable amount.

pH range refers to the range of pH over which the buffer best resists pH change.

Addition of Strong Acids or Bases to BuffersAdding a strong acid or a strong base cause

opposite neutralization reactions to occur in the buffer.

ExampleA buffer is made by adding 0.3 mol of acetic acid

and 0.3 mol of sodium acetate to enough water to make 1 L of solution. the pH of the buffer is 4.74. (a) Calculate the pH of the solution after 0.02 mol of NaOH are added. (Assume that volume remains constant.)

Titration Calculations Calculate the pH when the following quantities of

0.1 M NaOH solution have been added to 50.0 mL of 0.1 M HCl

(a) 49.0 mL

(b) 51.0 mL

Weak Acid-Strong Base TitrationsConsider titrating 50.0 mL of 0.1 M acetic acid

with 0.1 M NaOH.

ExampleCalculate the pH of a solution that is formed when

45.0 mL of 0.1 M NaOH is added to 50.0 mL of 0.1 M CH3COOH. (Ka = 1.8 x 10-5)

Effect of Ka on Titration Curves

Titrations of Polyprotic Acids

Solubility EquilibriaWe have seen that some ionic compounds are not

soluble in water. This is only partially true.

Factors that Affect SolubilityCommon-Ion Effect

ExampleCalculate the solubility of CaF2 at 25oC in 0.01M

Ca(NO3)2. (Ksp = 3.9 x 10-11).

Solubility and pHThe pH of a solution will affect the solubility of any

substance whose anion is basic.

If a compound contains an anion that is the conjugate base of a weak acid its solubility will increase as the solution becomes more acidic.

Formation of Complex IonsMetal ions can act as Lewis bases in the presence

of a Lewis acid.

Precipitation and Separation of IonsThe equilibrium of BaSO4(s) in water can be

achieved in either direction:

Selective Precipitation of ions Ions can be precipitated out of a solution based on

the solubilities of their salts.