Acids, Bases & pH

What are Acids?

• Acids taste SOUR

–Lemons, vinegar

• Compounds that have Hydrogen (H+) as their cation.

• Examples:

–HCl – Hydrochloric Acid

–H2SO4 – Sulfuric Acid

Acids Form Hydronium Ions• When acids dissolve in water, their

ions separate. The Hydrogen ion (H+) combines with H2O to form a Hydronium ion (H3O+)

• Acids are PROTON DONORS (they give their hydrogen ion away)

HCl + H2O +H3O + Cl-

More Info About Acids

• Anything with high concentration of Hydronium ion is considered to be an acid.

• Acids turn blue litmus paper RED

What are Bases?

• Bases are bitter and slippery.

• Bases either have Hydroxide (OH-) as their anion OR they will form Hydroxide in solution

• Examples:

–NaOH – Sodium Hydroxide

–NH3 - Ammonia

Bases Form Hydroxide Ions• If the Hydroxide Ion is not already present

in the Base, the base will make the Hydroxide ion by taking a Hydrogen ion from water.

• Bases are PROTON ACCEPTORS (they take a hydrogen ion)

NH3 + HOH (Water)

NH4+ OH-+

More Info About Bases

• Anything with a high concentration of hydroxide ion is considered to be a base.

• Bases turn Red litmus paper BLUE

Can you tell the difference between an acid and a base?

• Let’s do a Lab to find out!

What is pH?• The pH of a solution indicates its

concentration of Hydronium Ions (H3O+) or its concentration of Hydroxide Ions (OH-)

• pH is short for the “power of Hydrogen”

• Something with more (OH-) will be a base

• Something with more (H3O+) will be an acid

• The pH scale ranges from 0-14

The Scale

1 2 3 4 5 6 7 8 9 10 11 12 13 14• In a neutral solution, the pH is 7

– The amount of H3O+ = OH-

• Acids have a pH of less than 7

– The amount of H3O+ > OH-

• Bases have a pH of greater than 7

– The amount of OH- > H3O+

Lots of OH-Lots of H3O+ Neutral

Calculating pH

• Concentration is written as Molarity, or the number of moles per liter of solution

• Example: the Hydronium concentration of pure water is 0.0000001 mol/L or 10-7 M

• This means that in one liter of water, we have 0.0000001 mol of Hydronium ions

Calculating pH• pH is the negative of the power of 10

used to describe the concentration of Hydronium ions

• Example:

hydronium concentration of

pure water = 10-7

-(-7) = pH of 7

Small Differences in pH Mean LARGE Differences in Acidity

• Each number on the pH scale is a difference of a factor of 10

• pH of 2 is 10 times stronger than a pH of 3• pH of 1 is 100 times stronger than a pH of 3• Example problem: the pH of apple juice is

3. the pH of coffee is 5. What is the acidity difference?

• There are 2 spaces between 3 and 5. • 102 = 100 - Apple juice is 100 times more

acidic than coffee

Review Questions• Which ions do acids form in solution?

– H3O+

• Which ions do bases form in solution?– OH-

• Classify the following as acidic, basic, or neutral.– Soap (pH = 9)

• basic– Sour liquid (pH = 5)

• acidic– Solution with 4x as many hydronium as hydroxide ions

• acidic– Pure water

• neutral• Arrange the following substances in order of increasing

acidity: vinegar (pH = 2.8), gastric juices from your stomach (pH = 2.0), soft drink (pH = 3.4)– Soft drink, vinegar, gastric juice

• Determine the pH of a 0.0001 M solution of HCl0.0001 = 10-4 -(-4) = pH of 4

• What is the pH of 0.01 M solution of HClO4, perchloric acid?

0.01 = 10-2 -(-2) = pH of 2

• The concentration of hydronium ions in a certain acid is 10000 times the concentration of hydronium ions in a second acid solution. If the second solution has a pH of 6, what is the pH of the first solution?

10000 = 104 4 steps down pH scale 6-4 = pH of 2

• Hand soap has a pH of 10. Drain cleaner has a pH of 14. How much more acidic is the hand soap than the drain cleaner?

4 steps = 104 = 10,000 times more acidic

Can you find the pH of the solutions you just tested?

• Let’s do a lab to find out!