Unit 18

Acids and Bases

Properties of acids and Bases

Acids 1. Tastes sour 2. Burns 3. Reacts w/metal to make

H2

4. Electrolyte 5. Formula usually starts

with “H” 6. Indicators

1. Litmus: blue to red 2. Phenolphthalein: pink to

clear

Bases 1. Tastes bitter

2. Slippery

3. Does not react with metals

4. Electrolyte

5. Formula usually ends with “OH”

6. Indicators 1. Litmus: red to blue

2. Phenolphthalein: clear to pink

Acidic or Basic Ionization of Water

H2O(l) ⇌ H+(aq) + OH-

(aq)

• Normally, [H+] = [OH-]

• Acidic solution : [H+] is greater than [OH-]

• Basic solution : [OH-] is greater than [H+]

H+ OH-

Arrhenius Model (1887)

• Acid - compound that ionizes to make H+ (proton). – monoprotic acid - 1 ionizable hydrogen (a

proton), HCl.

– diprotic acid - 2 protons, H2SO4.

– triprotic acid - 3 protons, H3PO4.

• Base - compound that ionizes to make OH-.

Arrhenius definition is easy to use, but is too limited.

Some things act like acids and bases without H+ or OH-.

Bronsted- Lowery Model (1923) • Acid - proton or hydrogen-ion

donor.

HX + H2O ⇌ H3O+ + X-

• Base - proton or hydrogen-ion acceptor.

Acid Base

Bronsted- Lowery Model (1923)

Acid Base NH3 + H2O ⇌ NH4

+ + OH-

In the forward reaction, a hydrogen-ion or proton, leaves water and combines with NH3 to make NH4

+. H2O is the proton donor, acid, and NH3 is the proton acceptor, base.

Bronsted- Lowery Model (1923)

Acid Base Conjugate Acid

Conjugate Base

NH3 + H2O ⇌ NH4+ + OH-

In the reverse reaction, NH4+

donates (acid) the proton to OH-

(base). Because they are formed from NH3 and H2O, they are called conjugates (differ by a proton).

Bronsted- Lowery Model (1923) • Conjugate acid-base pair – two

substances that differ from each other by a proton.

NH3 + H2O ⇌ NH4+ + OH-

Acid Base Conjugate Acid

Conjugate Base

Bronsted- Lowery Model (1923) • Amphoteric - a substance that can

act as either an acid or a base.

HCO3- + H2O ⇌ H2CO3 + OH-

HCl + H2O ⇌ H3O+ + Cl-

Strong Verses Weak Acids and Bases • Strong Acid/Base

– Acids and bases that ionize completely

– This means that they are converted into 100% products and are represented in the reaction as a single arrow pointing to the products.

Strong Verses Weak Acids and Bases

Strong Acid

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

Strong Base

NaOH(s) Na+(aq) + OH-

(aq)

Some metallic hydroxides have low solubilities(like Ca(OH)2), but they still considered strong becauses the particles that dissolve, ionize 100%!

Strong Verses Weak Acids and Bases

• Weak Acids and Bases – Weak acids and bases only partially ionize and

are therefore equilibrium reactions.

Weak Acid

HC2H3O2(aq) + H2O(l) ⇌ H3O+(aq) + C2H3O2

-(aq)

Weak Base

CH3NH2(aq) + H2O(l) ⇌ CH3NH3+(aq) + OH-

(aq)

Strong Verses Weak

Acids and Bases

• Because they are equilibrium reactions,

we can determine the Keq

HCl(aq) + H2O(l) ⇌ H3O+

(aq) + Cl-(aq)

Ka

should be small for a weak acid

The larger the Ka the stronger the acid

[HCl]

]][ClO[HK 3

eq

aK

Strong Verses Weak Acids and Bases

• The same applies for weak bases – Keq = Kb

– Kb is small for a weak base – The larger the Kb the stronger the base

Concentrated/dilute vs.

Strong/weak

Concentrated Weak Acid Dilute Strong Acid

Concentrated Strong Acid Dilute Weak Acid

HA

What is pH? • Remember:

H2O(l) ⇌ H+(aq) + OH-

(aq)

• Normally, [H+] = [OH-]

• Acidic solution : [H+] > [OH-]

• Basic solution : [OH-] > [H+]

What is pH?

H2O(l) ⇌ H+(aq) + OH-

(aq)

]][OH[HKeq

]][OH[HKw

For water, the Keq is known as Kw

What is pH?

At 298 K for pure water [H+] and [OH-] both equal 110-7 M.

]][OH[HKw

]101][101[K -7-7

w

-14

w 101K

[H+][OH-] = 110-14

What is pH?

What is the [OH-] if the [H+]=2.310-4?

[H+][OH-] = 110-14

2.310-4 [OH-] = 110-14

[OH-] = 4.310-11 M

4

14

103.2

101][OH

What is pH?

Because [H+] and [OH-] are such small numbers, scientists invented pH to

simplify these odd numbers!

pH = 7 neutral pH < 7 acidic pH > 7 basic

What is pH?

• Where does pH come from? – p (in pH) means the negative log of the

H+ concentration

pH = -log[H+]

pOH = -log[OH-]

pH Problem #1 What is the pH if [H+]=2.310-4?

pH = 3.64

What is pH?

• How do you find the original concentration from pH and pOH? – Just do the opposite!

• To go to pH we used the -log[H+]

– [H+] = antilog(-pH)

– [OH-] = antilog(-pOH) • Antilog is “10x” on your calculator!

pH Problem #2 What is the [H+] if the pH is 4.20?

[H+] = 6.310-5 M

What is pH?

• How are pH and pOH related?

pH + pOH = 14

pOH 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Finding pH from solution

concentrations Strong acids

• Monoprotic acids fully ionize

HCl H+ + Cl- – [H+] = solution concentration

– 0.002 M HCl [H+] = 0.002 M

Strong bases

• All strong bases fully ionize

NaOH Na+ + OH-

Ca(OH)2 Ca2+ + 2OH- – [OH-] = (solution concentration)(# hydroxide ions)

– 0.002 M NaOH [OH-] = 0.002 M

– 0.002 M Ca(OH)2 0.004 M

pH Problem #3

• What is the pH of 2.410-5 M Mg(OH)2?

Mg(OH)2 ⇌ Mg2+ + 2OH-

[OH-] = 2(2.410-5 M )

= 4.810-5 M

pOH = -log(4.810-5 M)

= 4.32

pH + pOH = 14

pH = 9.68

Finding pH from solution concentrations

• Weak Acids and Bases – We’ll just leave this for AP Chem!

Types of Acid-Base Reactions

• Hydrochloric acid, HCl, is a common household and laboratory acid.

• Sodium hydroxide, NaOH, is a common strong base found in drain cleaning

• The reaction of an acid and a base to produce a salt and water is called a neutralization reaction.

Salt – ionic compound made from the cation from a base and the anion from an acid.

• A solution of hydrochloric acid, HCl, is added to exactly the amount of a solution of basic sodium hydroxide, NaOH, that will react with it.

Neutralization Reactions

Click box to view movie clip.

Neutralization Reactions

• Litmus papers show that the resulting salt solution is neither acidic nor basic.

• A typical type of acid-base reaction is one in which both the acid and base are strong.

Strong Acid + Strong Base

• Instead of an overall equation, an ionic equation, in which substances that primarily exist as ions in solution are shown as ions, can be written.

Strong Acid + Strong Base

• When ions common to both sides of the equation are removed from the equation, the result is called the net ionic equation for the reaction of HCl with NaOH.

Spectator Ions and the Net Ionic Reaction

Acid-Base Titrations

• The general process of determining the molarity of an acid or a base through the use of an acid-base reaction is called an acid-base titration.

• The known reactant molarity is used to find the unknown molarity of the other solution.

• Solutions of known molarity that are used in this fashion are called standard solutions.

• In a titration, the molarity of one of the reactants, acid or base, is known, but the other is unknown.

Acid-Base Titrations

• You know that NaOH and HCl react completely.

• You know the concentration of the NaOH solution, so it is your standard solution.

Acid-Base Titrations

• You can use the reaction, the volumes of acid and base used, plus the molarity of the base to determine the molarity of the unlabeled HCl.

Acid-Base Titrations

Titration

• At neutralization (called the equivalence

point),

moles H+ = moles OH-

– How do you find moles?

• Stoichiometry

Finding Concentrations Using

Titrations

1. Start by writing the balanced equation for the

acid-base reaction.

2. Use stoichiometry to convert the volume (in

liters) of the known substance to the number of

moles of the unknown needed to neutralize it.

• This will be a 2 step problem.

• Step 1 – Molarity as a conversion factor (L to moles)

• Step 2 – Mole ratio

3. To find the concentration, divide the number of

moles with the unknowns volume in liters.

Titration Problem #1

What is the molarity of a HCl solution

if it takes 68 mL of 1.2 M NaOH to

neutralize 25 mL of HCl?

HCl + NaOH

mol NaOH

L NaOH

1.2

1

.0816 0.068 L NaOH

= mol HCl

mol NaOH

mol HCl 1

1

0.0816 mol HCl

0.025 L HCl = 3.3 M HCl

NaCl + HOH

Titration Problem #2

What is the concentration of a NaOH

solution if it takes 12.4 mL of 0.24 M

H2SO4 to neutralize 9.5 mL of NaOH?

H2SO4 + NaOH

mol H2SO4

L H2SO4

0.24

1

0.005952 0.0124 L H2SO4

= mol NaOH

mol H2SO4

mol NaOH 2

1

0.005952 mol NaOH

0.0095 L NaOH = 0.63 M NaOH

Na2SO4 + HOH 2 2

Finding pH from solution concentrations

• Weak Acids and Bases – Weak Acid/Bases are an equilibrium

dissolving reaction so we can make the following assumption:

HA(aq) ⇌ H+ + A-

[0.01 M] [x] [x]

[HA]

]][A[Ha

Kx]-[0.01

[x][x]

Since x must be very small, we’ll also assume 0.01 – x ≈ 0.01!

[0.01]

[x][x]

Finding pH from solution concentrations

• This means the [H+] of any weak acid (and the [OH-] of any weak base) is:

[HA]K][H a

pH Problem #4

• What is the pH of the a weak acid

Finding the Ka from the pH of a Weak Acid

• For a weak acid: HA(aq) qe H+ + A-

[x] [x]

[HA]

]][A[Ha

K

[x] = [H+] = [A-] = antilog(-pH)

[HA] = [original concentration] – [x]

Ka Problem What is the Ka if the pH of a 0.100 M

solution of formic acid (HCOOH) is 2.38?

[H+] = antilog(-2.38)