Chapter 16

Acids and Bases

16.1 Defining Acids and Bases

• Since the 17th century, chemists have known about acids and bases… however, it took a while before they were understood and could be defined.

Properties of Acids and Bases• An acid looks the same as a base in water. However,

there are many other ways to tell them apart.• Taste :

– Acid- has a sour or tart taste– Base- has a bitter taste

• Touch:– Acid- if dilute, feels like water. If not, has a sting– Base- feels smooth or slippery

• Reactions with metals:– Acid- reacts VIGEROUSLY with metals– Base- does not react

• Electrical conductivity:– Both conduct electricity

• Indicators: a substance that changes color in an acid but a different color in a base.– Acid- has many indicators but most common is

LITMUS PAPER… turns Blue to Red– Base- has many indicators but most common is

LITMUS PAPER… turns Red to Blue

• Neutralization: a reaction between an acid and a base… the acid neutralizes the base and vice versa. What is formed is an ionic compound or a SALT.

The Arrhenius Definition• Swedish Chemist, Svante

Arrhenius, defined acids and bases in 1884.

• Acid- substance that dissociates in water to produce hydrogen ions (H+)

• Base- substance that dissociates in water to produce hydroxide ions (OH-)

Common Arrhenius Acids & Bases

Acid Formula Base Formula

Hydrochloric acid HCl Sodium hydroxide NaOH

Nitric acid HNO3 Potassium hydroxide

KOH

Acetic acid HC2H3O2 Magnesium hydroxide

Mg(OH)2

Sulfuric acid H2SO4 Calcium hydroxide Ca(OH)2

Carbonic acid H2CO3 Barium hydroxide Ba(OH)2

Phosphoric acid H3PO4

• Arrhenius acids and bases, in a acid-base neutralization reaction, will always produce water and a salt.

Hydrochloric acid + sodium hydroxide → water + sodium chloride (table

salt)

– HCl + NaOH → H2O + NaCl (double replacement reaction)

• What would be the product of HNO3 and KOH?

The Brønsted-Lowry Definition

In 1923, two chemists independently developed a new definition of acids and bases. Danish Chemist , Johannes Brønsted and English Chemist, Thomas Martin Lowry said:•An acid is any substance that can donate H+ ions•A base is any substance that can accept H+ ions

• Brønsted-Lowry expands the definition of Arrhenius acids/bases.

1.Defines acids/bases independently of how they behave in water.

2.Focuses only on the H+ ions and ignores the OH- ions.

The Hydronium Ion

• Because H+ will combine with water… a Hydronium ion is actually created.

• Water acts as an acid and a base… substances that do so are called amphoteric.

Conjugate Acid-Base Pairs• Conjugate means… joined together or coupled.

• An acid and a base which differ by a proton are said to form a conjugate acid base pair or the pairs of substances which can be formed from one another by the gain or loss of protons are known as conjugate acid base pairs.

Determining the Strengths of Acids & Bases

1M Acetic acid• Is a weak acid. We can eat it

and use it to clean.

1M Hydrochloric acid• Is a strong acid. Will eat

through your clothes and burn your skin. CAUTION!

Strong & Weak Acids

• Strong acids disassociate 100%. Ex. HCl

– HCl + H2O → H3O+ + Cl - 100%

– Excellent conductors

• Weak Acids only disassociate a small amount. Ex. HC2H3O2

– HC2H3O2 + H2O ↔ H3O+ + C2H3O2 - 0.4%

– Poor conductors

Strong & Weak Bases

• Strong bases have the strongest affinity for H+ meaning they react 100% with water.

Ex. CaO– O2- + H2O → 2OH-

• Weak bases only partially react with water. Ex. CO3

2-

– CO32- + H2O ↔ HCO3

- + OH-

Strength of Conjugate Acid-Base Pairs

The stronger the acid the weaker the conjugate base and vice versa.

Acid-Base Properties of Salts• When salts disassociate in water, they break

down into cations and anions.– NaCl + H2O → Na+ + Cl -

• Many of these ions are weak Brønsted-Lowry acids or bases.

• Salt hydrolysis reactions- rxn of ions from salts to form H30+ or OH-

• We can predict if the salt soln. will be acidic (formed H30+ ) or basic (formed OH-)

• Salts of:– STRONG acids + STRONG bases = neutral soln.– STRONG acids + weak bases = acidic soln.

• formed H30+

– Weak acids + STRONG bases = basic soln.• formed OH-

– Weak acids + weak bases = neutral soln.

16.2 The Self-ionization of Water and pH

• Pure water does not have H2O molecules only. It also contains H3O+ ions and OH- ions.• Water is amphoteric, can act as an acid or a base with other

chemicals.

• Self-ionization-

Mathematical Definition• “In pure water at 25°C, both H3O+ and OH- ions are

found at concentrations of 1.0 X 10-7 M.”

Kw= [H3O+] [OH-]

• Kw= 1.0 X 10-14

– [H3O+] = 1.0 X 10-7

– [OH-] = 1.0 X 10-7

• [H3O+] > 1.0 X 101.0 X 10-7-7> [OH-] Acidic Soln

• both = 1.0 X 101.0 X 10-7-7 Neutral soln• [OH-] > 1.0 X 101.0 X 10-7-7> [H3O+] Basic Soln

Practice Problems• If the concentration of H3O+ in blood is 4.0x10-8 M,

the blood is acidic, basic or neutral?– What is the concentration of OH- ions?

• What is the concentration of OH- ions in chocolate milk if [H3O+ ] = 4.5x10-7 M? – Is it acidic, basic, or neutral?

• What is the concentration of H3O+ ions in black coffee if [OH-] = 1.3x10-9 M.– Is it acidic, basic, or neutral?

The pH Scale• In 1909, Søren Sørensen

developed the pH scale.– 0-6.9: acid– 7.1-14: base

• Its based on logarithms:• pH= -log [H3O+]

• Using a calculator, try:• -log(8.7 x 10-4)• What is the pH of a soln with

[H3O+] = 7.3x10-5 M?

[H3O+] = 6.23x10-4 M?

[OH-] = 5.0x10-2 M? (14=pH + pOH)

Measuring pH

• 2 Common methods1. Use an indicator, (litmus paper) – can use a

combination of indicators to get a more precise pH reading

2. Use an pH meter.

Some Acid-Base Indicators

Acid-Base Titration• Acid-base titration- a

carefully controlled neutralization reaction.

• To test an unknown solution you must use a standard solution to compare against.

• Also need an indicator to determine the pH range.

• Begin the titration by adding the standard solution to the unknown… eventually enough of the standard solution is added to neutralized the acid/base of the unknown. This is the equivalence point.

• End point- the point at which the indicator changes.

Calculating Concentration

• You can calculate the concentration of the acid or base using the data from the titration with this formula.total moles of H+ @ end point = total moles of OH- @ end point

Choosing the Proper Indicator

• Picking the indicator depends on the strength of the acid and base.

• 3 combinations:– Strong acid/Strong base

• neutral

– Weak acid/Strong base• basic

– Weak base/Strong acid• acidic

16.3 Buffers

• There are times when you want to control the pH of a solution.– Ex: your blood has to stay with in a pH of 7.35-7.45 or

you can become very sick and could die!

• We can use a buffer to control the pH– BUFFER- a mixture that is able to release or absorb

H+ ions, which keeps the pH level constant.

• Most buffer solns are made from a weak acid and their conjugate base (or their salt).

• All buffers have a limited capacity to neutralize acids and bases.– BUFFER CAPACITY – amount of acid or base a

buffer can neutralize.– http://www.youtube.com/watch?v=g_ZK2ABUjvA