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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.

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The definitions of acids and bases have changed over time.

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  1. 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.

  2. 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)

  3. IV-19: “pHooey!” [H+] and pH Notes Acids tend to dissociate into H+ ions in solution. Example: HCl (aq) H+ + Cl –

  4. IV-19: “pHooey!” [H+] and pH Notes Bases tend to dissociate into OH- ions in solution. Example: NaOH (aq) Na+ + OH –

  5. 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+]

  6. IV-19: “pHooey!” [H+] and pH Notes pOH is the concentration of OH- ions in solution. pOH = [OH-]

  7. 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-]

  8. 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.

  9. 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-]

  10. 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.

  11. IV-19: “pHooey!” [H+] and pH Check-In If you know the pH of a solution, what else do you know?

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