Acids and basesCommon strong acids are:HCl; H,SO4; HNO3Common weak acids are:ethanoic, CH,COOH; carbonic, H,CO3Common strong bases are:NaOH; KOHAmmonia is a weak base6
6 Acids and bases Common strong acids are: HCl; H2SO4 ; HNO3 Common weak acids are: ethanoic, CH3COOH; carbonic, H2CO3 Common strong bases are: NaOH; KOH Ammonia is a weak base
Acids and basesWordsWords and Expressionsdonor; acceptor; donate; acceptconjugate: conjugate acid; conjugate baseAmpholyte7
7 Acids and bases Words donor; acceptor; donate; accept conjugate: conjugate acid; conjugate base Ampholyte Words and Expressions
Strength of acidsStrength ofacidsThe strength of an acid (how well it protonates) is measured bythe equilibrium constant for the reaction:HA+H,O →H.O++A[H3O+] × [A-][H,O] is nearly constant at 55.5 mol[HA] × [H2O]dm-3 in most bench solutions, so thisexpression can be simplifiedKa= K× 55.5_[H30+] ×[A-][HA]K, is called the acidity or dissociation constant for the acid.If K, is greater than 102 the acid is strong, while ifit is lessthan 10-2 the acid is weak.8
8 Strength of acids Strength of acids The strength of an acid (how well it protonates) is measured by the equilibrium constant for the reaction: HA + H2O → H3O+ + A- [H2O] is nearly constant at 55.5 mol dm-3 in most bench solutions, so this expression can be simplified Ka is called the acidity or dissociation constant for the acid. If Ka is greater than 102 the acid is strong, while if it is less than 10-2 the acid is weak
Strength of acidsIn the same way, it is possibleFor an acid HA:to write a Kh expression for aHA+H2O=H3O++A-base and a Kw expression for[H3O+] [A-]water. Compare the three:Ka=[HA]For the conjugate base A-:For waterA-+H2O=HA++OH-H2O+H2O=H3O++OH[HA] [OH-]Kb=Kw= [H3O+] [OH-][A-]Now look what happens[HAT[OH-][H3O+][A-]KaXKp=when we multiply the[AL[HATfirst two together and= [H3O+] [OH-] = Kwcancel out terms on theKw is called the dissociation constant fortop and bottom lineswater and has the value91 X 10-14 mol2 dm-6 at 250C
9 Strength of acids For an acid HA: For the conjugate base A- : In the same way, it is possible to write a Kb expression for a base and a Kw expression for water. Compare the three: For water Now look what happens when we multiply the first two together and cancel out terms on the top and bottom lines: Kw is called the dissociation constant for water and has the value 1×10-14 mol2 dm-6 at 25oC
Strength of acidsComplementary strengths of acids and their conjugate bases.K, X Kp = Kw, which is a constant. So it follows that if K, is large.K, must be small, and if K, is small Kp must be large. This meansthat if an acid is strong, its conjugate base must be weak, and viceversa.So strong acids have weak conjugate bases10
10 Strength of acids Complementary strengths of acids and their conjugate bases. Ka×Kb = Kw, which is a constant. So it follows that if Ka is large, Kb must be small, and if Ka is small Kb must be large. This means that if an acid is strong, its conjugate base must be weak, and vice versa. So strong acids have weak conjugate bases