Aqueous Solutions and the Concept of pH Chapter 16

Hydronium and Hydroxide Ions provided by acids and bases in aqueous solutions also provided by the solvent, water water is considered the “ripper” that causes both acids and bases to ionize/dissociate in water solution water actually causes molecules to form ions

Self-Ionization of Water -2 water molecules produce a hydronium ion and a hydroxide ion by transfer of a p+ Equation: Since this equation is reversible and does not go to completion in the forward direction but rather reverses itself after only a very few ions are actually formed, we could write an equilibrium expression for this equation.

Equilibrium Expression K w = [H 3 O + ][OH - ] = 1.0 x Memorize this!! Square brackets indicate the concentration of the ion in moles per liter. (Molarity) K w is the equilibrium constant for water. Its very small value indicates that few ions form before the equation reverses itself. Also known as the ionization constant of water, K w, increases as temperature increases.

Neutral, Acidic, and Basic Solutions -because [H 3 O + ] and [OH - ] are the same in pure water, it is neutral [H 3 O + ] = [OH - ] = 1 x any solution where they are equal is also neutral -if [H 3 O + ] is greater it is acidic; in basic solutions the [OH - ] is greater

The pH Scale - French for “hydrogen power”; exponent power (see notes) -pH indicates [H 3 O + ]; pOH indicates [OH - ]

Approximate pH Ranges of Common Materials

Acid-Base Indicators -compounds whose colors are sensitive to pH -change colors because they are either weak acids or weak bases -come in many colors HIn + H 2 O  H 3 O + + In - (weak acid) HIn and In - are different colors -range over which an indicator changes colors also varies Transition interval – the pH range over which an indicator changes color

Color Ranges of Indicators Table page 495

pH Meter -determines the pH of a solution by measuring the voltage between the 2 electrodes that are placed in the solution -used to find exact pH -voltage changes as [H 3 O + ] changes

Titration -the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration -sensitive means for determining chemically equivalent volumes of acid and base

Titration Terminology Equivalence Point – -the point at which the 2 solutions used in a titration are present in chemically equivalent amount -indicators and pH meters can be used to determine equivalence point -pH meter will show large voltage change End Point – -the point in a titration at which an indicator changes color Litmus: broad range pH 5.5 – 8.0 Bromthymol blue: limited transition pH 6.0 – 7.6 -indicators that undergo transition at about pH 7 are used to determine equivalence point of strong-acid/strong-base titrations

Titration Curve for Strong Acid/Strong Base

Titration Curve for Strong Base and Weak Acid

Titration Completed Standard solution – -the solution that contains the precisely known concentration of a solute -often called the “known” solution Primary standard – -a highly purified solid compound used to check the concentration of the known solution in a titration -to be certain of concentration of known solution, titration is done with the primary