THE STRENGTH OF ACIDS AND BASESSection 6.5 pg. 254-259

What do you remember?

ACIDS BASES

Fill in as many empirical properties for acids and bases as you can ...

Do all acids and bases have the same properties and to the same degree?

Two different acids of the same concentration were tested with a pH meter.

HCl(aq) 1.0 mol/L = pH = 1CH3COOH 1.0 mol/L = pH = 4 ??? How does this happpen ???

Strong and Weak Acids The concept of strong and weak acids was

developed to describe and explain the differences in properties of acids.

An acid can be described as a weak acid if its characteristic properties (under the same conditions) are less than those of a common strong acid

There are relatively few strong acids: ( 6 to be exact )

The Difference: Using the Modified Arrhenius Theory

Strong Acids:

These empirical properties suggest many ions are present (lots of H3O+ ions present); which is consistent with the idea that strong acids react completely _________ with water to form hydronium ions

The Difference: Using the Modified Arrhenius Theory

Weak Acids:

These empirical properties suggest fewer hydronium ions are present

Based on this evidence, a weak acid reacts incompletely _________ with water to form relatively few hydronium ions

-

WEAK ACIDS Because of this difference, weak acids are

much safer to handle – you can even eat and drink many of them.

Lactic acid in many dairy

products and

builds up in

muscles

Citrus acid in fruits

Stearic Acid in Animal Fat

Carbonic acid in drinks

Tannic Acid in Tea

Practice – Strong and Weak Acids

Pg. 255 #1-5

Strong and Weak Bases Strong bases – have a high electrical

conductivity, fast reaction rate and a very high pH (>>7)

Weak bases – have a low electrical conductivity (if molecular bases), slower reaction rate and a pH closer to, but greater than, 7

How do we explain the difference?

Strong and Weak Bases

Strong Bases –

Weak Bases – an ionic or molecular substance that reacts partially ________ with water to produce relatively few hydroxide ions

Practice

Explain the weak base properties of baking soda.

Explain why sodium acetate has a pH of 8 when dissolved in water.

Practice

Pg. 257 #6, 7, 9

Summary

Strong Acids

Weak Acids

Strong Bases

Weak Bases

Empirical properties (need same concentration & temperature)

Very low pH Med to low pH

Very high pH

Med to high pH

High conductivity

Low conductivity

High conductivity

Low conductivity

*

Fast reaction

rate

Slow reaction

rate

Fast reaction

rate

Slow reaction

rate

Modified Arrhenius Theory

Completely react with water to

form H3O+(aq)

ions

Partially react with water to

form H3O+(aq)

ions

Completely react with water to

form OH-(aq)

ions

Partially react with water to

form OH-(aq)

ions

* Applies only to weak bases that are molecular

Investigation 6.3 The evidence from the demonstration clearly shows that

acids with the same initial concentration can have different degrees of acidic properties.

This difference was shown in the different conductivity measurements and the different rate of reactions.

What were the two important controlled variables?concentration and temperature

Why? A very dilute strong acid could have a higher pH than a more concentrated weak acid – which is incorrect and the conductivity and rate of reaction measurements would be misleading.