Acids and BasesChapter 19

DHS Chemistry

Definition

Definition - Acids

•An acid is a molecule or ion that acts as a proton (hydrogen ion) donor.

HCl + H20 H30+ + Cl-

acid

Definition - Base

•A base is a molecule or ion that acts as a proton (hydrogen ion) acceptor.

HCl + NH3 NH4+ + Cl-

base

Examples - Acids

•An acid is basically a compound that produces hydronium (H30+) ions when dissolved in water. Typically acids start with hydrogen in their formulas.

Ex. HCl, H2S04, HNO3

Examples - Bases•A base is basically a compound that produces hydroxide ( OH- ) ions when dissolved in water. Most bases (not all) have OH in their chemical formulas.

Ex. NaOH, KOH, Mg(OH)2 , NH3

Properties - Acids•sour taste

– Like orange juice or vinegar

•contain hydrogen and most react with active metals to produce hydrogen gas

Properties - Acids•turn blue litmus paper red (pink)

•react with bases to produce salt and water (neutralization)

•electrolytes (may be strong or weak)

© Prentice Hall

Properties - Acids

•bitter taste•feel slippery•turn red litmus paper blue•react with an acid to produce salt and water (neutralization)

•electrolytes (may be strong or weak)

Properties - Bases

Litmus Paper

Aci ase Re lueBD

Common Ones

A. Common AcidsAcid Formul

aMajor Uses

sulfuric acidH2SO4

Car batteries, production of metals

phosphoric acid

H3PO4 Found in soft drinks

Nitric acid HNO3Production of

explosives

hydrochloric acid

HCl Cleaning of metals

Common Bases

Base Formula

Major Uses

ammonia NH3 Refrigerant, household cleaners

sodium hydroxide

NaOH Drain cleaner

potassium hydroxide

KOH To increase pH of acidic soils

Strength of Acids and Bases

•One way to measure the strength of an acid or a base is to measure how much of the original molecule remains after it has been added to water. If little remains, the acid or base is strong. If a lot of the original molecule remains, the acid or base is weak.

Acids•acids will form ions (electrolytes) when dissolved in water in a process called ionization.

Ex:HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

Bases

•Bases also will form ions (electrolytes) when dissolved in water.

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

Strength

•Strong acids: Completely ionize.

•Weak acids: Partially ionize.

REMEMBER STRONG AND WEAK

ELECTROLYTES !

6 H-A

H+

A-

H+

H+

H+

H+

H+

A-

A-

A-

A-A-

STRONG ACI D100 % I ONI ZED I N WATER

6 H-A

A-H+

H-A

H-A

H+

A-

H-A

H-A

WEAK ACI DLI TTLE I ONI ZATI ON I N WATER

A-

H+

water

acid

proton

anion

H-A

6 H-A

H+

A-

H+

H+

H+

H+

H+

A-

A-

A-

A-A-

6 H-A

H+

A-

H+

H+

H+

H+

H+

A-

A-

A-

A-A-

STRONG ACI D100 % I ONI ZED I N WATER

6 H-A

A-H+

H-A

H-A

H+

A-

H-A

H-A

WEAK ACI DLI TTLE I ONI ZATI ON I N WATER

A-

H+

water

acid

proton

anion

H-A

A-

H+

water

acid

proton

anion

H-A

• Do not confuse the terms strong and weak with the terms concentrated and dilute. Strength refers to what % of the original molecules convert to ions in water. Concentrated or dilute refer to how many total moles there are in water.

Strong Acids

•HCl, HBr, HI, HNO3, H2SO4

These acids ionize almost completely in water.

*memorize those that are highlighted

Weak Acids

•HF, HCN, HC2H3O2, H2CO3

These acids weakly ionize in water.

*memorize those that are highlighted

•The strength of a base depends on its solubility in water.

Strong bases

•NaOH, KOH, Ca(OH)2, LiOH, CsOH, RbOH, Sr(OH)2

These bases ionize almost completely in water.

*memorize those that are highlighted

Weak bases

•CH3N2, NH3, NaCN, Mg(OH)2

These bases weakly ionize in water.

*memorize those that are highlighted

A. Neutralization Reactions

When an acid and a base react, they are neutralized and a salt and water are the produced:

Acid + Base Salt + Water

GenericNeutralization Reaction(not in your notes)

HA + BOH B A + H2O

ex. HCl(aq) + NaOH(aq) NaCl (aq) +

H2O(l)

acid base salt water

Cation

Anion

Add this to your notes!

•All neutralization reactions are double-replacement reactions

The salt is a combination of the cation from the base and the anion from the acid. Water is produced if a hydroxide base is used. (This is different from table salt!)

A. Neutralization Reactions

Ex. The salt is highlighted in each case.

HCl + NaOH → NaCl + H2O HC2H3O2 + KOH KC2H3O2 + H2O

H2SO4 + 2 NH4OH (NH4) 2SO4 + H2O

HC2H3O2 + NH4OH NH4C2H3O2 + H2O

Strong acid plus strong base

Weak acid plus strong base

Strong acid plus weak base

Weak acid plus weak base

Practice1. Complete and balance these

reactions involving acids and bases.a) nitric acid + potassium hydroxide

b) calcium hydroxide + carbonic acid

(H2CO3)

c) acetic acid + sodium hydroxideHC2H3O2(aq) + NaOH(s) NaC2H3O2(aq) + H2O(g)

HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)

Ca(OH)2 (aq) + H2CO3 (aq) CaCO3(aq) + 2H2O(l)

B. pH Scale

Hydronium vs Hydroxide

•Hydronium H3O+

– A hydrogen ion in water

– H+ + H2O H3O+

– H+ and H3O+ used interchangeably

– For acids

•Hydroxide OH-

– For bases

1. Background•Any aqueous solution contains both hydronium ions and hydroxide ions. This stems from the fact that water will ionize to a very small amount:

Note: for pure water, the number of hydronium ions is

equal to the number of hydroxide ions.

H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

•when an acid is added to water, the number of hydronium ions increases

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

(aq)

•when a base is added to water, the number of hydroxide ions increases

H2O(l)

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

(aq)

•it is the ratio of hydronium ions to hydroxide ions that determines whether a solution will be an acid, a base, or neutral

•acid: contains more hydronium ions than hydroxide ions

•base: contains more hydroxide ions than hydronium ions

•neutral: the # of hydronium ions is equal to the # of hydroxide ions

2. pH Scale•a measure of the number of hydronium or hydroxide ions is the pH scale

•it is based on the concentration of hydrogen and hydroxide ions in solution

•pH is defined as the negative logarithm of the hydrogen ion concentration

•[ ] = concentration (molarity)

•notice pH is based on a base 10 scale

pH = -log([H+]) = -log([H3O+])

pH Scale

•the typical pH scale runs from 0 to 14 .

•a pH of 7 is considered neutral which means that the concentration of hydrogen ions and the concentration of hydroxide ions are equal

•as you go down on the pH scale (< 7), solutions are considered acidic

•solutions with pH’s greater than 7 are considered basic

Summary of the pH scale

pH Category Concentration of ions

< 3 strong acid

many H3O+

3-7 Weak acid H3O+ > OH-

7 neutral H3O+ = OH-

7-11 Weak base H3O+ < OH-

> 11 strong base

many OH-

Practice1. Determine whether the following are

a strong acid, a weak acid, a strong base, a weak base, neutral solution

a. pH = 2.5b. lots of hydroxide ions, hardly any hydronium ions

c. little more hydroxide ions than hydronium ions

Strong Base

Weak Base

Weak Acid

•There is also something called pOH, which is a measure of the concentration of hydroxide ions.

•pH and pOH are related by the following:

14 = pH + pOH

•Again, for strong, single hydroxide bases, [OH-] = molarity of the base.

Ex: What is the pOH of KOH if the pH is 14? pH + pOH = 14

14 + pOH = 14pOH = 0

IV. Titrations

A. Titrations

•The concentration of an acid (or base) in solution can be determined by performing a neutralization reaction.

acid + base salt + water

•An indicator is used to show when neutralization has occurred.

•An indicator is a substance that forms different colors in different pH solutions. Phenolphthalein is a common indicator used in acid-base titrations. It will change from colorless in acidic environments to pink in basic environments.

50

40

30

20

10

0

Titrant- standarized base in burette

Titrand-unknown acid containing indicator and a means f or stirring