Section 6.3—Acidity, pH

Section 6.3—Acidity, pH

How does concentration of acid affect the pH of a sports drink?

A Review of Acids & Bases

Acids – Arrhenius Definition

Produce Hydronium ion (H3O+1) in waterHydronium ion is water + a hydrogen

cation

HO

H

water

H+1

HO

H

H +1

By this definition, if an acid is to give a H+1 to water, then all acids will have hydrogen as the cation (first element written).

How do Acids produce Hydronium?

HO

H

H -

water acid

Hydrogen cation with some anion

How do Acids produce Hydronium?

HO

H

H -+1

How do Acids produce Hydronium?

HO

H

H+1 -

Hydronium ion Anion

Bases – Arrhenius Definition

Bases produce the hydroxide ion in water

HO-1

Hydroxide Ion

Characteristics of Acids & Bases

BasesAcids

Produce H3O+1 (hydronium ion) in water

Produce OH-1 (hydroxide ion) in water

Tastes sour Tastes Bitter

React with active metals to form hydrogen gas Feels slippery

Strength versus Concentration

Strong versus Weak Acids

+

++

-

-

-

Strong acidMost of the acid molecules

have donated the H+1 to water

How many hydronium ion – anion pairs can you find?

How many intact acid molecules can you find?

3

1

Strong versus Weak Acids

+

-

Weak acidOnly a few of the acid

molecules have donated the H+1 to water

How many hydronium ion – anion pairs can you find?

How many intact acid molecules can you find?

1

3

Concentrated versus Dilute

solute solvent

Lower concentration

Not as many solute (what’s being dissolved) particles

Higher concentration

More solute (what’s being dissolved) particles

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

Not much acid added and most of what is there stays

together

Strong

Weak

Acids and Bases as Electrolytes

Acids and bases dissociate into ions in water

Free-floating ions in water conduct electricity

Acids & Bases are electrolytes

Strong acids and bases are strong electrolytesWeak acids and bases are weak electrolytes

pH

Is a scale to measure the acidity of a sample

pH Scale

1 14

Highly acidic Very basic (not acidic)

neutral

7

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

pH is a Logarithmic Scale

Logarithm –The number of times a base must be multiplied by itself to reach a given number

yx blog# of multiples

Base

# you’re trying to reach

Calculating pH

pH scale – Logarithmic scale of the acidity of a solution

The pH scale uses base “10”

]log[ 13

OHpH

pHOH 10][ 13

pH has not units

[ ] = concentration in Molarity

The “-” in the pH equation

Concentration of Hydronium ion versus pH

0

0.5

1

0 0.5 1 1.5 2

pH

[H3O

+]

Because pH is the negative log of concentration of hydronium, as concentration increases, the pH goes down.

The lowest pH is the highest concentration of hydronium

What does a “log” scale really mean?

pH

4

3

2

1

10x more acidic

100x more acidic

1000x more acidic

Leve

l of a

cidi

ty in

crea

ses

Every change of 1 in pH shows a change of 10x in concentration of hydronium

An example of calculating pH

Example:Find the pH if the concentration of [H3O+1] is 0.25 M

An example of calculating pH

pH = 0.60

]log[ 13

OHpH

)25.0log( MpH

Example:Find the pH if the concentration of [H3O+1] is 0.25 M

An example of calculating hydronium

Example:Find the [H3O+1] if

the pH is 2.7

An example of calculating hydronium

H3O+1 = 0.0020 M

pHOH 10][ 13

7.213 10][ OH

Example:Find the [H3O+1] if

the pH is 2.7

Auto-ionization of Water

Water will split into ions 2 H2O H3O+1 + OH-1

Water will do this to make sure that at 25°C the following is true:[H3O+1] × [OH-1] = 1 × 10-14

So if you know the hydronium concentration at 25°C (which can be found from pH), then you can also find the hydroxide concentration

An example of calculating hydroxide

Example:Find the [OH-1] if the pH is 10.7

An example of calculating hydroxide

14113 101][][ OHOH

14111 101][]100.2[ OH

Example:Find the [OH-1] if the pH is 10.7

H3O+1 = 2.0 × 10-11 M

pHOH 10][ 13

7.1013 10][ OH

]100.2[101][ 11

141

OHOH-1 = 0.0005 M

Let’s Practice #1

Example:Find the pH if the concentration of

[H3O+1] is 2.5 × 10-5 M

Let’s Practice #1

pH = 4.6

]log[ 13

OHpH

)105.2log( 5MpH

Example:Find the pH if the concentration of

[H3O+1] is 2.5 × 10-5 M

Let’s Practice #2

Example:Find the [OH-1] if

the pH is 3.6

Let’s Practice #2

14113 101][][ OHOH

1414 101][]105.2[ OH

Example:Find the [OH-1] if

the pH is 3.6

H3O+1 = 2.5 × 10-4 M

pHOH 10][ 13

6.313 10][ OH

]105.2[101][ 4

141

OHOH-1 = 4.0 ×10-11 M

Let’s Practice #3

Example:Find the [H3O+1] if

the pH is 11.2

Let’s Practice #3

H3O+1 = 6.3 × 10-12 M

pHOH 10][ 13

2.1113 10][ OH

Example:Find the [H3O+1] if

the pH is 11.2