Acids and BasesThe concept of acidic and basic

solutions is perhaps one of the most important topics in chemistry. Acids

and bases affect the properties of foods, biochemical reactions,

pharmaceuticals, and industrial materials.

Acids and Bases• Properties of Acids• Sour or tart taste.• Corrosive (deteriorate).• Electrolytes• Electrolytes are able to conduct an

electrical current because of the presence of ions in aqueous solutions.

Acids and Bases• Properties of Acids (cont.)• Will react with most metals to form

hydrogen gas.• Some acids are ‘stronger’ than others• All acids contain a hydrogen that they

can give away.

Acids and Bases• Properties of Bases• Bitter tasting• Slippery• Caustic – They will degrade biological

tissue. Chemical burns from strong bases are nasty.

• Bases form the hydroxide ion (OH-1) in water.

Acids and Bases• Arrhenius Acids • Svante Arrhenius (1900) defined an

acid.• Acids are hydrogen containing

compounds that yield a hydrogen ion (H+) in water.

• An Arrhenius acid donate an H+ ion.

Acids and Bases• Arrhenius Acids • HCl(s) H+

(aq) + Cl-(aq)

• H2SO4(s)

• H2O(l)

Acids and Bases• Arrhenius Acids • Acids that have one hydrogen ion to

donate are called monoprotic.• Acids that have 2 hydrogen ions to

donate are called diprotic.• Acids that have 3 hydrogen ions to

donate are called ____protic.

Acids and Bases• Arrhenius Bases • Compounds that produce the

hydroxide ion(s) (OH-1) in water are called Arrhenius Bases.

• NaOH(s)

Acids and Bases• Arrhenius Acids and Bases • Problem with the Arrhenius

definition; Some bases can form OH- ions in solution but not have an OH- ion in the chemical formula.

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

Acids and Bases• Bronsted-Lowry Acids and Bases• A better definition of an acid and a base• Bronsted-Lowry Acid – A molecule that

donates an H+ to another molecule.• Bronsted-Lowry Base – A molecule that

accepts an H+ from the B-L acid.• Identify the Bronsted-Lowry acid and base;

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

Acids and Bases• Bronsted-Lowry Acids and Bases• Conjugate Acid – becomes the H+ donor in

the reverse reaction.• Conjugate Base – becomes the H+

acceptor in the reverse reaction.• Identify the BL acid, base, conjugate acid

and conjugate base;HClO2(aq) + H2O(l) H3O+1

(aq) + ClO2-(aq)

Acids and Bases• The pH Scale – Measures the

concentration of H+ ion in an aqueous solution.

Acids and Bases• The pH Scale • Remember that a water molecules

ionizes;H2O(l) H+

(aq) + OH-(aq)

• In pure water, the concentration of H+ and OH- each is 1.0 x 10-7M

Acids and Bases• The pH Scale

H2O(l) H+(aq) + OH-

(aq)

• Therefore, the product of [H+] and [OH-] must be equal to 1.0 x 10-14M2.

• An aqueous solution will always have the concentration of H+ and OH- equal to 1.0x10-14M2.

Acids and Bases• The pH Scale

H2O(l) H+(aq) + OH-

(aq)

Keq = [H+] x [OH-]

Kw = [1.0x10-7] x [1.0x10-7] = 1.0x10-14

Kw is called the autoionization constant for water.

Acids and Bases• The pH Scale

Keq = [H+] x [OH-]

[1.0x10-7] x [1.0x10-7] = 1.0x10-14

• If the addition of an acid makes the [H+] increase, then the [OH-] will decrease.

• If the addition of a base makes the [OH-] increase, the [H+] will decrease.

Acids and Bases• The pH Scale • Calculate the [H+] if 0.05 moles of HCl

is added to 1.0 L of water.

Acids and Bases• The pH Scale • pH is a measure of the concentration

of H+.

pH = -log[H+]

Acids and Bases• The pH Scale • What is the pH of an aqueous solution

where [H+] = 1.0 x 10-7?

• What is the pH of an aqueous solution where [H+] = 1.0 x 10-2?

Acids and Bases• The pH Scale • What is the pH of an aqueous solution

where [H+] = 2.7 x 10-1?

• What is the pH of an aqueous solution where [H+] = 8.0 x 10-12?

Acids and Bases• The pH Scale • Since Kw = 1 x 10-14 = [H+] x [OH-]

14 = pH + pOH

Acids and Bases• The pH Scale • Calculate the pOH of an aqueous

solution that has an [H+] = 1.0 x 109.

Acids and Bases• The pH Scale • Calculate the [H+] of an acid with a pH

of 4.

• Calculate the [H+] of a base with a pH of 10.8.

Acids and Bases• The pH Scale • Calculate the [OH-] of an acid with a

pH of 2.

• Calculate the [OH-] of a base with a pH of 12.9.

Acids and Bases• Strong Versus Weak Acids• What makes some acids ‘strong’ and

some ‘weak’?

Acids and Bases• Strong Versus Weak Acids• We can quantify the relative strength

of an acid by using it’s equilibrium expression (Ka).

• Ka = [products]x = [H+] x [conj. base] [reactants]y [acid]

Acids and Bases• Strong Versus Weak Acids• Write the Ka expression for HCl.

• Ka =

Acids and Bases• Strong Versus Weak Acids• Write the Ka expression for H3PO4.

• Ka =

Acids and Bases• Strong Versus Weak Acids• As the [H+] increases, Ka increases.

Therefore the greater the value of Ka, the more [H+] present, the stronger the acid.

Acids and Bases• Weak Acids

The pH of a 1.0 M solution of formic acid, HCOOH, is 2.38. Calculate the Ka for formic acid.

Acids and Bases• Weak AcidsCalculate the Ka of a 0.220 M solution of H3AsO4

that has a pH of 1.50.

Acids and Bases• Weak AcidsCalculate the Ka of a 0.0400 M solution of HClO2

that has a pH of 1.80.

Acids and Bases• Weak Acids

Calculate the pH of a 8.6 x 10-3 M solution of H3PO4. Ka of H3PO4 is 1.4 x 10-3.

Acids and Bases• Salt Hydrolysis• Sometimes an ion from a salt can

make an aqueous solution acidic or basic.

• What happens when sodium bicarbonate dissolves in water?

Acids and Bases• Salt Hydrolysis• NaHCO3(aq) Na+

(aq) + HCO3-(aq)

• HCO3-(aq) + H2O(l) H2CO3(aq) + OH-(aq)

Now there will more OH- than H+ in the solution making it basic.

Acids and Bases• Salt Hydrolysis• Will a solution of ammonium chloride,

NH4Cl, be acidic or basic?

Acids and Bases• Buffers• A buffer is a solution that resists

changes in pH when either an acid or a base is added.

• Buffers consist of either a weak acid with one of its salts, or a weak base with one of its salts.

Acids and Bases• Buffers• For example, if a solution is make by

dissolving carbonic acid (weak acid) and sodium bicarbonate (salt of the acid) we get the following;

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3

-1(aq)

Acids and Bases• Buffers

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3

-1(aq)

• If we add a base to this buffered solution, the H3O+ will scoop it up.

H3O+(aq) + OH-

(aq) 2H2O(l)

Acids and Bases• Buffers

H2CO3(aq) + H2O(l) H3O+(aq) + HCO3

-1(aq)

• If we add an acid to this buffered solution, the HCO3

-1 will scoop it up.

H+(aq) + HCO3

-1 (aq)

H2CO3(aq)

Acids and Bases• Buffers• Write the chemical reaction for the

phosphoric acid – dihydrogen phosphate buffer reaction.

Acids and Bases• Neutralization Reactionso Occurs when an equal number of H+ ions

and OH- ions react.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

o The products of a neutralization reaction are always a salt and water.

Acids and Bases• Neutralization Reactions

KOH(aq) + HNO3(aq)

Ca(OH)2(aq) + HClO3(aq)

H2SO4(aq) + LiOH(aq)

Acids and Bases• Titrations – Reacting an acid with a

base to a specific pH.

Acids and Bases• Titrations o The purpose of a titration is to determine

the concentration of either the acid or the base being titrated.

o Usually, the buret contains a known concentration of base while the acid is of an unknown concentration.

Acids and Bases• Performing a Titration

1. Add a desired amount of the acid into the flask.

2. Load the buret with the base of a known concentration.

Acids and Bases• Performing a Titration

3. Record the initial volume of the base in the buret. A buret can give a volume reading to 0.01 mL.

4. Add a few drops of the indicator.

Acids and Bases• Performing a Titration

5. Slowly dispense the base into the acid. Your goal is to change the color of the acid from clear to pink with the addtion of one drop of the base.

Acids and Bases• Titrations o After the titration is completed, we use the

equation MAVA = MBVB to determine the concentration of the acid.

MA = Concentration of Acid

MB = Concentration of Base

VA = Volume of Acid

VB = Volume of Base

Acids and Bases• Titrations o Calculate the concentration of 25.0 mL of an

HCl solution that was titrated with 0.25 M NaOH. The initial volume in the buret read 3.50 mL and the final volume read 18.20 mL after the titration was completed.