Acids and Bases

• Acids and bases have distinct properties:

–Acids give foods a tart or sour taste.

–Aqueous solutions of acids are strong or weak electrolytes.

–Acids cause certain dyes, called indicators, to change color.

–Many metals, such as zinc and magnesium, react with aqueous solutions of acids to produce hydrogen gas.

• Acids and bases have distinct properties.

–Soap is a familiar material that has the properties of a base.

• The bitter taste is a general property of bases.

• The slippery feel of soap is another property of bases.

–Bases will cause an indicator to change color.

–Bases also form aqueous solutions that are strong or weak electrolytes.

Acid/Base Theories

• 1.) Arrhenious Acids and Bases

• 2.) Brønsted-Lowry Acids and Bases

• 3.) Lewis Acids and Bases

1.) Arrhenious Acids and Bases

• Proposed in 1887

• Swedish Scientist named Svante Arrhenious

• Acids are compounds that produce hydrogen ions (H+) in water.

• Bases are compounds that produce hydroxide ions (OH–) in water

Acids –> Form H+ in water

Some Common Acids

Name Formula

Hydrochloric acid HCl

Nitric acid HNO3

Sulfuric acid H2SO4

Phosphoric acid H3PO4

Ethanoic acid CH3COOH

Carbonic acid H2CO3

–monoprotic acid –

• Ex. nitric acid (HNO3) has one ionizable hydrogen.

–diprotic acids –

• Ex. sulfuric acid (H2SO4), has two ionizable hydrogens

–triprotic acids –

• Ex. phosphoric acid (H3PO4) has three ionizable hydrogens

• When a compound loses it’s H+ ion to water, where do they go?

• A hydronium ion (H3O+) is the ion that forms when a water molecule gains a H+ ion.

Bases –> Form OH- in water

Some Common Bases

Name Formula Solubility in Water

Sodium hydroxide NaOH High

Potassium hydroxide KOH High

Calcium hydroxide Ca(OH)2 Very low

Magnesium hydroxide Mg(OH)2 Very low

• Strong bases completely dissociate in water.

2.) Brønsted-Lowry Acids and Bases

• Proposed in 1923

• Danish chemist Johannes Brønsted and English chemist Thomas Lowry

• Were working independently, but proposed the same definition of acids and bases.

• Brønsted-Lowry theory-

• an acid is a hydrogen-ion (H+) donor

• a base is a hydrogen-ion (H+) acceptor.

– This includes all the acids and bases that Arrhenius defined AND…

– Includes some compounds that Arrhenius did not classify as bases.

Arrhenious theory would NOT consider ammonia to be a base.

Brønsted-Lowry theory would.

• Ammonia is a Brønsted-Lowry base because it accepts hydrogen ions.

• Water is a Brønsted-Lowry acid because it donates hydrogen ions.

A conjugate acid - the ion or molecule formed when a base gains a hydrogen ion.

Conjugate Acids and Bases

A conjugate base is the ion or molecule that remains after an acid loses a hydrogen ion.

Conjugate Acids and Bases

Conjugate acids are always paired with a base, and conjugate bases are always paired with an acid.

• A conjugate acid-base pair consists of two ions or molecules related by the loss or gain of one hydrogen ion.

Conjugate Acids and Bases

Find the conjugate acid/base pairs

A substance that can act as either an acid or a base is said to be amphoteric.

• Water is amphoteric.

Amphoteric Substances

3.) Lewis Acids and Bases

• Proposed by Gilbert Lewis

• A Lewis acid accepts a pair of electrons

• A Lewis base donates a pair of electrons

• This definition is more general than those offered by Arrhenius or by Brønsted and Lowry.

• Identify the Lewis acid and Lewis base.

Acid/Base Theory Summary

Acid-Base Definitions

Type Acid Base

Arrhenius H+ producer OH– producer

Brønsted-Lowry

H+ donor H+ acceptor

Lewiselectron-pair acceptor

electron-pair donor

Hydrogen Ions and Acidity

• The pH scale is a system to express the concentration of hydrogen ions [H+] is the pH scale.

• Ranges from 0 – 14

• Closer to zero is acidic

• Closer to fourteen is basic

Which is Bigger?

• 1 x 10-4 or 1 x 10-8

• 1 x 10-9 or 1 x 10-7

• 1 x 10-14 or 1 x 10-3

• How does a solution with a pH of 7 relate to the H+ concentration?

• A pH of 7 = 1 x 10-7 M of H+ ions

• a pH of 3 = 1 x 10-3 H+ ions

• a pH of 11 = 1 x 10-11 H+ ions

• How are [H+] and [OH−] related in an aqueous solution?

• [ ] = concentration of

• In aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide-ion concentration equals 1.0 × 10−14

• [H+] x [OH-] = 1.0 × 10−14

This is called:

ion-product constant for water (Kw).

Kw = [H+] × [OH−] = 1.0 × 10−14

If the [H+] in a solution is 1.0 × 10−5 M, is the solution acidic, basic, or neutral? What is the [OH−] of this solution?

Acidic or Basic?

• [H+] is 1.0 × 10−5M, which is greater than 1.0 × 10−7M.

• Thus, the solution is acidic.

What is the [OH-] ?

[OH−] =

Kw

[H+]

Kw = [H+] × [OH−]

= 1.0 × 10−9M

[OH−] =1.0 × 10−14

1.0 × 10−5

The pH of a solution is the negative logarithm of the hydrogen-ion concentration.

The pH Concept

pH = −log[H+]

• In pure water or a neutral solution, the [H+] = 1 × 10−7M, and the pH is 7.

• pH = −log(1 × 10−7)

• = −(log 1 + log 10−7)

• = −(0.0 + (−7.0)) = 7.0

pH = −log[H+]

• If the [H+] of a solution is greater than 1× 10−7M, the pH is less than 7.0.

• If the [H+] is less than 1× 10−7M, the pH is greater than 7.0.

What is the pH of a solution with a hydrogen ion concentration of 4.2 × 10−10M?

• pH = −log(4.2 × 10−10)

• = −(−9.37675)

• = 9.37675

• = 9.38

pH = −log[H+]

The pH of an unknown solution is 6.35. What is the hydrogen-ion concentration of the solution?

• pH = −log[H+]

• −log[H+] = pH

• −log[H+] = 6.35

• log[H+] = −6.35

• [H+] = antilog(−6.35)

On most calculators, use the 2nd or INV key followed by log to get the

antilog.

[H+] = 4.5 × 10−7

What is the pH of a solution if [OH−] = 4.0 × 10−11M?

Kw = [OH−] × [H+]

[H+] =Kw

[OH−]

[H+] =1.0 × 10−14

4.0 × 10−11= 0.25 × 10−3M

= 2.5 × 10−4M

pH = −log[H+]

= −log(2.5 × 10−4)

• Use a calculator to find the log.

= −(−3.60205)

= 3.60