Carrying on with Acid-Base (Part 2)

The Autoionization of Water Water is never just water  . A glass of water is constantly undergoing what is referred to as ‘autoionization’ . It can be illustrated as: H2O(l) + H2O   H3O+(aq) + OH-(aq) It has been shown, experimentally, that water molecules will react with one another to form hydronium and hydroxide ions.

This ionization process is very small, with about 2 water molecules in a billion undergoing ionization ( 2.0E-11 %). This ‘water equilibrium constant’ can be expressed as: Kw = [H+(aq)][OH-(aq)] ➗ [H2O(l)] = [H+(aq)][OH-(aq)] It has been shown, through very precise measurements, that the concentrations of both the hydrogen and hydroxide ions in pure water are: 1.0E-07 mol/L so Kw is equal to: Kw = [H+][OH-] = [1.0E-07mol/L][1.0E-07mol/L] = 1.0E-14 Please note this value for Kw is for STP (Standard Temperature and Pressure) only.

Type of Aqueous Solution In neutral aqueous solutions of pure water the concentration of [H+] ions and [OH-] ions is equal. In acidic solutions, the concentration of [H+] ions is greater than [OH-] ions. In basic solutions, the concentration of [OH-] ions is greater than the [H+] ion concentration. Type of Aqueous Solution Concentration Neutral [H+] = [OH-] Acidic [H+] > [OH-] Basic [H+] < [OH-]

Strong acids are acids that dissociate completely in water to form [H+] ions. If the label on the bottle of HCl reads ‘contains 6M HCl’ we can assume there is virtually no HCl present in the bottle. There are 6 moles (6M) of [H+] ions and 6 moles of [Cl-] ions in the bottle. The most common strong acids are: HCL, H2SO4, and HNO3 In the case of both nitric acid and sulfuric acid, the hydrogen atom is attached to the oxygen atom. Monoprotic Acids – a monoprotic acid has 1 hydrogen ion to contribute. Diprotic Acids – a diprotic acid, like sulfuric acid, has 2 hydrogen ions to contribute. Triprotic Acids – a triprotic acid, H3PO4, has 3 hydrogen ions to contribute.

We can use these ideas of complete dissociation of strong acids and of Kw to calculate the hydrogen ion or the hydroxide ion concentrations of strong acids. Ex. A 0.15M solution of HCl is found to have a hydrogen ion [H+] concentration of 0.15mol/L. What is the hydroxide ion [OH-] concentration? Solution: Kw = [H+][OH-] 1.0E-14 = [0.15][x] [OH-] = 1.0E-14 ➗ [0.15] = 6.67E-14

Example 2: Calculate the hydroxide ion concentration in a 0 Example 2: Calculate the hydroxide ion concentration in a 0.25M solution of HBr acid. Solution: Kw = [H+][OH-] 1.0E-14 = [0.25][x] [OH-] = 1.0E-14 ➗ [0.25] = 4.0E-14 Example 3: Calculate the hydrogen ion concentration in a 0.39M solution of NaOH. 1.0E-14 = [x][0.39] [H+] = 1.0E-14 ➗ [0.39] = 2.56E-14

Strong Bases – that last example brings us to the idea of strong bases Strong Bases – that last example brings us to the idea of strong bases. A strong base, like a strong acid, dissociates in water to produce hydroxide [OH-] ions. All hydroxides of the Group 1 metals form strong hydroxides. Group 2 metals also form strong hydroxides BUT they produce 2 moles of hydroxide ions for every mole of these metals dissolved in water. Ex: Ba(OH)2 dissolves readily in water to form 1 mole of [Ba2+] ions and 2 moles of [OH-] ions. You use the same math to determine hydrogen ion concentrations in solutions of strong bases.

Example 1: What is the hydrogen ion concentration in a 0 Example 1: What is the hydrogen ion concentration in a 0.11M solution of NaOH? Kw = [H+][OH-] 1.0E-14 = [x][0.11] [H+] = 1.0E-14 ➗ [0.11] = 9.09E-14 Example 2: You dissolve 2.6g of NaOH in 1L of water. What are the [OH-] and [H+] concentrations? Calculate the number of moles of NaOH. NaOH has a mole mass of 40g/mol. 2.6g➗40g/mol = 6.5E-2 mol of NaOH ([OH-] is 6.5E-2) [H+] = 1E-14 ➗ [6.5E-2] = 1.54E-13 mol

Assignment No.2 due Monday See the handout