Section 2.5—Characteristics of Acids and Bases

Objectives: •investigate the characteristics of acids & bases•distinguish between strong & weak acids & bases•distinguish between concentration & strength•explain the pH scale and use of indicators

Characteristics of Acids & Bases

BasesAcids

•Produce H3O+1 (hydronium ion) in water

•Produce OH-1 (hydroxide ion) in water

•Tastes sour•May sting or burn on contact

•Tastes Bitter

•React with active metals to form hydrogen gas

•Feels slippery

•Turn blue litmus paper pink •Turns pink litmus paper blue

Acid Strength

Review of how acids produce ions

H

OH

H -

water acid

Hydrogen cation with some anion

Review of how acids produce ions

H

OH

H -+1

Review of how acids produce ions

H

OH

H+1 -

Hydronium ion Anion

Strong versus Weak Acids

+

++

-

-

-

Strong acidMost of the acid molecules have

donated their H+1 to water

How many hydronium ion – anion pairs can you find?

How many intact (whole) acid molecules can you find?

3

1

Strong versus Weak Acids

+

-

Weak acidOnly a few of the acid

molecules have donated their H+1 to water

How many hydronium ion – anion pairs can you find?

How many intact acid molecules can you find?

1

3

Strong versus Weak Acids

A strong acid is an acid that dissociates (comes apart) and ionizes (forms ions)almost completely in water.

A weak acid is an acid that dissociates and ionizes only slightly in water.

Concentrated versus Dilute Acids

A Note About Solutions

We saw in previous slides how an acid compound combines with water to form a hydronium ion and an anion. When this occurs, the acid is really an acid solution.

Solutions are homogeneous mixtures. There is a even mixing of the 2 parts throughout the mixture.

The 2 parts are called the solute – the substance that is dissolved – and the solvent – the substance that does the dissolving.

Concentrated versus Dilute Solutions

solute solvent

DiluteNot as many solute (what’s being dissolved) particles

ConcentratedMore solute (what’s being

dissolved) particles

Concentrated versus Dilute Acids

A concentrated acid will have a lot of the acid compound mixed into the water.

A dilute acid will not have a lot of the acid compound mixed into the water.

Concentration vs. Strength

Combinations of Concentration & Strength

DiluteConcentrated

A lot of acid added & most

dissociates

Not much acid added, but most of

what’s there dissociates

A lot of acid added, but most stays together

(does not dissociate)

Not much acid added and most of what is there stays

together

Strong

Weak

All of these same ideas about strength and concentration apply to bases as well

Many people think all acids are dangerous and all bases are more safe but . . .Vinegar is an acid we eat…some acids are safe!Sodium hydroxide is a very caustic base…not all

bases are less harmful than acids!The stronger an acid or base is (and the more

concentrated it is), the more dangerous it is for you!!

Other notes

pH

Is a scale to measure acidity (concentration of hydronium ions)

pH Scale

1 14

Highly acidic Very basic (not acidic)

neutral

7

Chapter 6 will give more detail about how pH is calculated!

pH

Neutral Substances These are substances that are not considered to be an acid or a base. They have a pH around 7.

Acidic Substances These substances have a pH LESS THAN 7. In fact, the lower the pH, the greater the concentration of hydronium ions. The lower the pH, the more ACIDIC the substance is considered.

pH

Basic SubstancesThese substances have a pH GREATER THAN 7. In fact, the higher the pH, the lower the concentration of hydronium ions. The higher the pH, the more BASIC the substance is considered.

Indicators change color based on pHLiquid indicators – various indicators change colors at

different pH’sUniversal indicator – a combination of liquid indicators to

produce a “rainbow” changing colors at several pH’s

Paper IndicatorsPaper with a liquid indicator on it (Litmus paper or pH

paper)

pH meters or pH probesElectronically determine pH and give a read-out

Ways to measure pH

Common pH indicators – pg. 56

This picture © 1998 David Dice

pH of common substancesSUBSTANCE PH

0.1M HCl 1

Stomach contents

2

Vinegar 2.9

Soda pop 3

Grapes 4

Beer 4.5

Pumpkin pulp 5

Bread 5.5

Intestinal contents

6.5

Milk 6.5

Urine 6.6

Bile 6.9

Saliva 7

Blood 7.4

Eggs 7.8

0.1M NH3 (aq) 11.1

0.1M NaOH 13•Which substance is most acidic? least?•Which substance is most basic? least?•Is bread acidic or basic? eggs?•Is any substance is neutral?