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IV-18: “Pass the Proton” Acid – Base Theories. The definitions of acids and bases have changed over time. Notes. Arrhenius Definition of Acids and Bases: An acid is any substance that adds hydrogen ion (H+) to solution. A base is any substance that adds hydroxide ion (OH-) to solution.

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the definitions of acids and bases have changed over time

IV-18: “Pass the Proton” Acid – Base Theories

The definitions of acids and bases have changed over time.

Notes

Arrhenius Definition of Acids and Bases:

  • An acid is any substance that adds hydrogen ion (H+) to solution.
  • A base is any substance that adds hydroxide ion (OH-) to solution.

Important: Not all compounds that have OH in them are bases

For example: CH3COOH is acetic acid (vinegar)

Because acids and bases add ions to water, they are both electrolytes and will conduct electricity.

slide2

IV-19: “pHooey!” [H+] and pH

Notes

What does pH measure?

The pH of a solution is determined by:

  • The molarity of the solution AND
  • The identity of the solution (acid, base or neutral)
slide3

IV-19: “pHooey!” [H+] and pH

Notes

Acids tend to dissociate into H+ ions in solution.

Example:

HCl (aq) H+ + Cl –

slide4

IV-19: “pHooey!” [H+] and pH

Notes

Bases tend to dissociate into OH- ions in solution.

Example:

NaOH (aq) Na+ + OH –

slide5

IV-19: “pHooey!” [H+] and pH

Notes

You can think about pH as the “power of hydrogen.”

pH measures the concentration of H+ ions in the solution, [H+]

pH = [H+]

slide6

IV-19: “pHooey!” [H+] and pH

Notes

pOH is the concentration of OH- ions in solution.

pOH = [OH-]

slide7

IV-19: “pHooey!” [H+] and pH

Notes

The pH of water is 7.

Water is considered neutral because the number of H+ is equal to the number of OH-.

[H+] = [OH-]

slide8

IV-19: “pHooey!” [H+] and pH

Notes

pH + pOH = 14

Water is neutral.

Neutral: [H+] = [OH-]

The pH of water is 7, therefore the pOH must be 7 as well.

pH plus pOH adds to 14.

ph poh scale log scale
pH & pOH scale  log scale
  • Each unit of change in pH represents a tenfold difference in H+ concentration
  • EX: A solution with a pH of 3 has an [H+] ten times greater than a solution with a pH of 4

pH = -log [H+]

**Same goes for pOH**

pOH = -log [OH-]

slide11

IV-19: “pHooey!” [H+] and pH

Notes

  • The pH number of a solution tells you the concentration of H+ ions in solution.
  • A pH of 1 corresponds to a H+ concentration of 0.1 moles/L.
  • [pH of 1= 0.1 M H+]
  • Solutions that are more acidic have more H+ ions.
  • Solutions that are more basic have less H+ ions.
slide12

IV-19: “pHooey!” [H+] and pH

Check-In

If you know the pH of a solution, what

else do you know?