THE PH SCALE

Lesson 15

The Strength of Acids and Bases The strength of acids and bases are not

all equal. Some acids and bases are safe enough to eat while others can eat through clothing and metal.

The Strength of Acids and Bases Some acids and bases are classified as

being strong while others are called weak.

-Strong acids and bases are extremely reactive and corrosive.

-Weak acids and bases are mostly unreactive and corrosive.

The Strength of Acids and Bases The strength of an acid is determined by

its ability to produce hydrogen ions (H+) when it is mixed with water. Common acids are recognized because they start with hydrogen. When a strong acid is mixed with water the H+ ion dissociates, leaving it separated from the rest of the molecule.

The Strength of Acids and Bases Weak acids have fewer molecules

dissociate or break up and thus the number of hydrogen ions is less. The more hydrogen ions there are in the water the lower the pH value and the stronger the acid.

The pH Scale

The pH scale is used to represent how acid or basic a solution is.

The scale ranges from 0-14 with very acidic being 0, neutral being 7, and very basic being 14.

The pH Scale

Every point on the scale represents a 10 base exponent difference. Ex lemons (pH = 2.0) are 100 times

more acidic than tomatoes (pH = 4.0)

The pH Scale

7.0 is neutral (neither acidic nor basic (alkaline)). Acids range from 0-6.9 Bases range from 7.1 -14

The pH Scale

Questions

Describe what would happen to the pH of a base was added to neutralize an acidic solution with a pH of 3.

Where would you expect to find the approximate pH for each of the following? A very concentrated base A very concentrated acid A dilute basic solution Tap water

How much more acidic is a solution with a pH of 4.5 than a solution with a pH of 5.5 6.5

Acid - Base Indicators: The most common method to get an idea

about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a “colour dye".

Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators , which do respond to a change in the hydrogen ion concentration.

Acid - Base Indicators:

Most of the indicators are themselves weak acids. The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2. 

 Color  Blue Litmus  Red Litmus

 Acid  turns red  stays same

 Base  stays same turns blue

Acid - Base Indicators:

Other commercial pH papers are able to give colors for every main pH unit. Universal Indicator, which is a solution of a mixture of indicators, is able to also provide a full range of colors for the pH scale.

Acid – Base indicators A variety of indicators change color at

various pH levels. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample.

An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The weak acid form (HIn) will have one color and the weak acid negative ion (In-) will have a different color.

The weak acid equilibrium is: HIn H+ + In- 

For phenolphthalein: pH 8.2 = colorless; pH 10 = red

For bromophenol blue: pH 3 = yellow; pH 4.6 = blue

Lab