Aqueous Solutions and Aqueous Solutions and the Concept of pHthe Concept of pH

Chapter 16

Hydronium and Hydroxide IonsHydronium and Hydroxide Ions

provided by acids and bases in aqueous solutions

also provided by the solvent, waterwater is considered the “ripper” that

causes both acids and bases to dissociate in water solution

water actually causes molecules to separate into their ions

Equilibrium ExpressionEquilibrium Expression

Kw = [H3O+][OH-] = 1.0 x 10-14 Memorize this!!

Square brackets indicate the concentration of the ion in moles per liter. (Molarity)

Kw 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, Kw, increases as temperature increases.

Neutral, Acidic, and Basic Neutral, Acidic, and Basic SolutionsSolutions

-because [H3O+] and [OH-] are the same in pure water, it is neutral

[H3O+] = [OH-] = 1 x 10-7

-any solution where they are equal is also neutral

-if [H3O+] is greater it is acidic; in basic solutions the [OH-] is greater

Acid-Base IndicatorsAcid-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 + H2O H3O+ + In- (weak acid) HIn and In- are different colors-range over which an indicator changes

colors also variesTransition interval – the pH range over

which an indicator changes color

Color Ranges of Indicators

Table page 495

pH MeterpH 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 [H3O+] changes

TitrationTitration-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 TerminologyTitration 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 colorLitmus: broad range pH 5.5 – 8.0Bromthymol 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 Titration Curve for Strong Acid/Strong BaseAcid/Strong Base

Titration Curve for Strong Base Titration Curve for Strong Base and Weak Acidand Weak Acid

Titration CompletedTitration 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