- The Strength of Acids and Alkalis -

Our Goals for todayTo state the uses of pH scaleTo relate pH value with acidic or alkaline properties of a substancesTo relate the concentration of hydrogen and hydroxide ions with pH valuesTo relate strong or weak acids/alkalis with the degree of ionisationTo conceptualise qualitatively strong and weak acids/alkalis.

What is the pH scale?The pH scale measures how acidic or basic a solution is.

The pH scaleThe pH scale is the concentration of hydrogen ions in a given substance.

Identifying Acids and BasesAcids have a pH from 0-7Lower pH value indicates a stronger acid.The lower the pH value, the higher concentration of H ions

Cont.Bases have a pH from 7-14Higher pH value indicates a stronger base.The higher the pH value, the higher the concentration of OH ions.

Did we Miss something??What happens when the pH of a substance is 7?

Ans: A pH level of 7 indicates a Neutral Substance i.e: Water!

Apparatus to measure pH valuepH meterpH paper Universal Indicator

pH METERS

Universal Indicators ColorsUniversal Indicator is a mixture of different indicators which covers the full range of pH values

How to measure the pH value?take measurements and record the results

[ H + ] = 10For bases

[ H + ] = 3For acids

WHEN THE CONCENTRATION OF HYDROGEN IONS [H+] IN SOLUTION INCREASE SO THE pH VALUES ARE LOWER WHEN THE CONCENTRATION OF HYDROXIDE IONS [OH+] IN SOLUTION INCREASE SO THE pH VALUES ARE INCREASES

Concentration of acids and bases with pH values

Strong and Weak AcidsDepends on degree of ionisation :Complete dissociation/ ionization in water produces high concentration of H ionsPartial dissociation / ionisation in water produces low concentration of H ionsHA (aq) H+ (aq) (high) + A-(aq) HA (aq) H+ (aq) () + A-(aq)

Diagram of strong acid dissociation Before dissociationAfter dissociationHAHA (aq) H+ (aq) + A-(aq) H2OBackHAAAHH H+ H+ H+ H+A-A-A-A-Example ?

Diagram of weak acid dissociationBefore dissociationHA (aq) H+ (aq) + A-(aq) H2OBackAfter dissociationAHHHHAAAA-A-A-A- H+ H+ H+ H+Example ?

Strong acids definition?Weak acids definition?

Diagram of strong alkali dissociationBefore dissociationBOH (aq) B+ (aq) + OH-(aq) H2OAfter dissociationBackBBBB B+OHOHOHOH OH- B+ B+ B+ OH- OH- OH-

Diagram of weak alkali dissociationBackBefore dissociationBOH (aq) B+ (aq) + OH-(aq) H2OAfter dissociationBBBB B+OHOHOHOH OH- B+ B+ B+ OH- OH- OH-

Definition of strong alkali?Definition of weak alkali?

-must consist of:1)Degree of ionisation2)Concentration of OH ions3)pH value