Acids and Bases• Chapter 20

Properties of….

Acids:

• Sour taste

• Change the color of indicators

• Electrolyte

• React with base to form water and salt

• React with certain metals to produce H2 gas.

Properties of….

Bases:

• Bitter taste, slippery feel

• Change the color of indicators

• Electrolyte

• React with acid to form water and salt

Names and Formulas of Acids

• Acids have the general formula HX, where X is a monatomic or polyatomic anion.

• When the compound HCl(g) dissolves in water, to form HCl(aq), it is named as an acid, hydrochloric acid.

• How are acids named?

Three rules for naming acids:

Name the following acids:

• HNO3 (hydrogen nitrate) is called…

• Nitric acid

• H2SO3 (hydrogen sulfite) is called…

• Sulfurous acid

• HF (hydrogen fluoride) is called…

• Hydrofluoric acid

Naming bases

• A base produces hydroxide ions when dissolved in water.

• NaOH is called sodium hydroxide.

• What would you call Ca(OH)2?

The Self-Ionization of Water

• When water molecules collide, a hydrogen ion is occasionally transferred from one molecule to another in the following reaction, the self-ionization of water:

H2O + H2O H3O+ + OH-

H3O+ is named the hydronium ion.

The Self-Ionization of Water

• This reaction can also be written as…

H2O H+ + OH-

(Basically, the two equations are the same, but the first is more realistic, because H+ always becomes H3O+ in water.)

The Ion-Product Constant for Water (Kw)

The product of [H3O+] and [OH] is always

constant at 25oC. When the concentrations are multiplied, the

product is 1.0x10-14.

This is called the…

Ion-product constant for water, Kw.

[H3O+] x [OH] = 1.0x10-14 = Kw at 25oC

Concentrations of [H3O+] and [OH]

• In pure water,

H2O + H2O H3O+ + OH-

the [H3O+] will always be

equal to the [OH-].1.0 x 10-7

In Water

• In pure water, since [H3O+] = [OH-],

• water is neither acidic or basic.

• Water is neutral.

• [H3O+] = [OH-] = 1.0x10-7M

In Acids and Bases…

• In acidic solutions, the [H3O+] is greater than 1.0 x 10-7M

[H3O+] > 1.0 x 10-7M

• In basic solutions, the [H3O+] is less than 1.0 x 10-7M

[H3O+] < 1.0 x 10-7M

Ion Concentrations

Determine the [H3O+]…

• What is the [H3O+] in a solution if the [OH-] = 6.30 x 10-5M? Is the solution acidic, alkaliine (basic) or neutral?

• Kw = 1.0x10-14 = [H3O+] x [OH-]• 1.0x10-14 / [OH-] = [H3O+] • 1.0x10-14 / 6.30 x 10-5M = [H3O+] • Answer: [H3O+] = 1.59 x 10-10M,• acidic because [H3O+] < 1 x 10-7M

Determine the [OH-]…

• What is the [OH-] in a solution if the [H3O+] = 3.50 x 10-11M?

• Kw = 1.0x10-14 = [H3O+] x [OH-]

• 1.0x10-14 / [H3O+] = [OH-]

• 1.0x10-14 / 3.50 x 10-11M = [OH-]

• Answer: [OH-] = 2.86 x 10-4M

1. To understand pH and pOH

2. To learn to find pH and pOH for various solutions

3. To use a calculator to find pH

4. To learn methods for measuring pH of a solution

5. To learn to calculate the pH of strong acids   

Objectives

The pH Scale

• pH is the negative log of the hydrogen ion concentration.

• pH = –log [H+]

• The “p scale” is used to express small numbers (“p” means –log).

Determine the pH…

• What is the pH of a solution that is 1.0 x 10-3M [H+] ?

• pH = –log [H+] = –log(1.0 x 10-3M) • Answer: pH = 3.00 • (really, no calculator required for this one.

When the coefficient is 1.0, you can figure this without a calculator.)

Determine the pH…• What is the pH of a solution that is

5.6 x 10-8M [H+] ?

• pH = –log [H+] = –log(5.6 x 10-8M)

• Answer: pH = 7.25 • (there is no unit, because this is a log)

Significant figures for logarithms

Determine the [H+]…• What is the [H+] if the pH of a solution is

9.00?

• pH = –log [H+] • 9.00 = –log [H+] • – 9.00 = log [H+] (Move the negative sign)

• 10–9 .00 = [H+] (Take the inverse log of both sides)

• Answer: [H+] = 1.0 x 10-9M

• (Again, no calculator is needed if the pH is a whole number.)

Using a Calculator to Determine the [H+] from pH

Determine the [H+]…

• What is the [H+] if the pH of a solution is 2.75?

• pH = –log [H+]

• 2.75 = –log [H+] • – 2.75 = log [H+] (Move the negative sign)

• 10–2.75 = [H+] (Take the inverse log of both sides)

• Answer: [H+] = 1.8 x 10-3M

The pH Scale • Because the pH scale

is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

The pH Scale

• Acidic solutions have a pH that is less than 7.

• Neutral solutions have a pH of 7.

• Alkaline (basic) solutions have a pH that is greater than 7.

The pH ScaleIn acidic solutions,

[H3O+] > 1.0 x 10-7M,so the pH < 7

In neutral solutions, [H3O+] = 1.0 x 10-7M,

so the pH = 7In basic solutions,

[H3O+] < 1.0 x 10-7M,So the pH > 7

(The negative exponent makes this confusing.)

The pOH Scale

• pOH is just as valid as pH, but it is not as commonly used, except in Chemistry.

pOH = log [OH]

Determining pH from pOH

You can take the “p” (or –log) of each part of

the Kw expression, and you get…

Kw = 1.0x10-14 = [H3O+] x [OH-]

pKw = 14.00 = pH + pOH

so, pH + pOH = 14.00

You can easily convert pOH to pH using pKw.

Determine the pH…

• What is the pH of a solution if the pOH is 11.60?

• pH + pOH = 14.00

• 14.00 – 11.60 = pH

• Answer: pH = 2.40

Determine the pH…

• What is the pH if the [OH-] = 5.74 x 10-4M?

• pOH = -log [OH-] = -log (5.74 x 10-4M)• pOH = 3.241• pH = 14 - 3.241 • Answer: pH = 10.759

• (Remember, you need 3 sig figs, which are places to the right of the decimal for logs.)

Acid Base Indicators

• Indicators – weak acids or bases that exhibit different colors in acidic and basic solutions

– In an acid solution the indicator will be in the HIn form. – In a basic solution the indicator will be in the In form.

Acid Base Indicators

• One common indicator is

litmus paper.

• Red litmus paper turns blue in base.

• Blue litmus paper turns red in acid.

Acid Base Indicators

• Another common indicator is

bromothymol blue.

• Bromothymol blue is blue in base and yellow in acid.

Acid Base Indicators

• Another common indicator is phenolphthalein.

• Phenolphthalein is bright pink in base, and clear in acidclear in acid.

1. To learn about two models of acids and bases

2. To understand the relationship of conjugate acid-base pairs

3. To understand the concept of acid strength

4. To understand the relationship between acid strength and the strength of the conjugate base

5. To learn about the ionization of water

Objectives

The Arrhenius Model• Svante Arrhenius defined acids and bases:

• Arrhenius Acid – produces hydrogen ions in aqueous solution – Example: HCl H+ + Cl-

• Arrhenius Base – produces hydroxide ions in aqueous solution – Example: KOH K+ + OH-

• Monoprotic acids have one ionizable hydrogen (proton). (Example: HCl)

• Diprotic acids have 2 ionizable hydrogens. (Example: H2SO4)

• Triprotic acids have 3 ionizable hydrogens. (Example: H3PO4)

• (Diprotic and triprotic acids are called polyprotic acids.)

• Is HC2H3O2 (acetic acid, found in vinegar) a monoprotic, diprotic, or triprotic acid?

• The only hydrogen that is ionizable is the one attached to the oxygen (so it is monoprotic). This H is ionizable because it is in a very polar bond, so this hydrogen is slightly positive and attracted to water.

• The Arrhenius model does not explain why ammonia (NH3) is a base.

• A new model was created by Brønsted and Lowry. This model explains why NH3 and other molecules are bases, even though they contain no hydroxide.

The Brønsted-Lowry Model

• Brønsted Acid – Hydrogen ion donor

• Brønsted Base – Hydrogen ion acceptor

Brønsted Equations

• In the reaction,

• NH3 + H2O NH4++ OH-

• The NH3 is accepting H+ to form NH4+, so

the NH3 is a base.

• The H2O is donating H+ to form OH-, so the water is an acid.

• NH3 + H2O NH4++ OH-

• Base Acid

• Now, in the reverse reaction, the NH4+ acts

as an acid and the OH- acts as a base. We call these the conjugate acid and conjugate base.

Conjugate Acid-Base Pairs

• NH3 + H2O NH4++ OH-

• Base Acid Conjugate Conjugate acid base

• The base always forms the conjugate acid and the acid always forms the conjugate base.

• NH3 and NH4+ are a conjugate acid-base

pair. They differ from each other by one H+ ion.

Identify the base, acid, the conjugate acid and the conjugate base:

HNO3 + H2O NO3- + H30+

– Acid Base CB CA

–Here, water is acting as a base.

Identify the conjugate acid-base pairs:

• HCl + H2O H30+ + Cl-

Water: Is it an acid or a base?

• Water is amphiprotic. It can behave as either an acid or a base, by donating or accepting H+.

• (Some books use the term amphoteric to describe the same property.)

B. Acid Strength

• Strong acid – completely ionized or completely dissociated

B. Acid Strength

• Weak acid – most of the acid molecules remain intact

B. Acid Strength

• A strong acid has a relatively weak conjugate base.

B. Acid Strength

• Common strong acids are – Sulfuric acid, H2SO4

– Hydrochloric acid, HCl

– Nitric acid, HNO3

– Perchloric acid, HClO4

1. To learn about acid-base titrations

Objectives

Acid-Base Titrations • Titration – (volumetric analysis) delivering a

measured volume of a solution of known concentration into the solution being analyzed

• Equivalence point – when just enough base has been added to react with all of the acid solution being analyzed (where moles of acid = moles of base)

Acid-Base Titrations

• Buret – device used for accurate measurement of the delivery of a liquid

Acid-Base Titrations

• Titration curve (pH curve) – plot of the data (pH vs volume) for a titration