Self-ionization of Water and pH Objectives: 1. Identify the ions present in pure water and give their concentrations. 2. Define pH and explain how the pH scale is used. 3. Identify what a buffer is and how the strength of it is related to the buffer capacity.

Arrhenius Definition An acid is a substance that releases (dissociates) Hydrogen ions (H+) in water monoprotic - releases 1 H+ ion (HCl) diprotic - releases 2 H+ ion (H2SO4) triprotic - releases 3 H+ ion (H3PO4) An base is a substance that releases (dissociates) Hydroxide ions (OH-) in water ex: KOH, NaOH, Ca(OH)2, Mg(OH)2

Bronsted-Lowry Definition An acid is a proton (H+) donor A base is a proton acceptor conjugate acid-base pairs are substances related by the loss or gain of a H+ ion strong acids have a weak conjugate base weak acids have a strong conjugate base Conjugate base/acid pair Conjugate acid/base pair

Lewis Acid Definition Lewis Acid and Bases An acid is an electron pair acceptor A base is an electron pair donor H+ + OH- g H2O In this reaction the H+ ion accepts the electron pair donated by the OH- ion to form H2O

Self ionization Water has many unusual properties. One of these properties is water's ability to act as either an acid or a base. (H2ODH3O++ OH-) Amphoteric is the name given to a substance that can act as either an acid or a base. Water becomes hydronium and hydroxide ions through self ionization The concentrations of each for pure water are equal at a pH of 7

Ion-Product The ion-product (Kw) can be used to find the concentration of acid and base in any solution based on the following assumptions: The formula for Kw for ALL solutions at 25oC is Kw = [H3O+][OH-] = 1x10-14 pH of pure water is 7, so the concentration of H3O+ is 1 x10-7 The concentration of base therefore is [OH-] = 1x10-14/1x 10-7 = 1 x10-7 EX: If a solution has 1 x10-2 H3O+ions, what is the concentration of the base ions? [OH-] = 1x10-14/1x 10-2 = 1 x10-12

pH Scale The pH scale was developed to help people interpret the amount of hydronium ions compared to the amount of base in a solution. The scale was developed by Soren Sorenson, a Danish biochemist, and is based on the -log of the hydronium ion (H3O+) concentration in solution.{-log[H3O+]}. The pH scale contains values between 0 and14. Each whole number change in the pH of a substance represents a 10 fold increase in the number of hydronium ions. Scale is from 0 - 14 0-7 acidic (solutions with a higher hydronium ion (H3O+)concentration than 1 x10-7 are acidic ; 0 most acidic) 7 neutral 7-14 basic (solutions with a higher hydroxide ion (OH-)concentration than 1 x10-7 are basic ; 14 most basic) can be measured using a chemical indicator a weak acid or base that changes color as the pH changes (gain or lose H+ ions)

pH Scale pH scale can be used to determine the amount of hydronium and hydroxide concentrations in solution with the equation pH + pOH = 14 and the ion product (Kw). Consider the following examples. example: A solution with 1 x 10-2 H3O+ ions, what is the pH and pOH of the solution? to get the pH simply change the negative exponent to a positive pH pH = 2 pOH = 14 -2 =12 A solution with 1 x 10-2OH- ions Use the ion product constant 1st to find the concentration of hydronium Kw=1 x 10-14/1 x 10-2OH-= 1 x 10-12 H3O+ ions pH = 12 pOH = 2