Titration Wrap-Up

Strong vs. Weak Acids and BasesTitration Curves

Naming Acids and Bases

Strong acidsAcids can be strong or weak.

Strong acids dissociate fully into solution, so that all their H+ ions are released into the mixture.

Hydrochloric acid is a typical strong acid, so the dissociation reaction is complete:

HCl H+ + Cl–

HA H+ + A–

The dissociation of a weak acid in water is a reversible reaction:

Weak acids

Weak acids do not dissociate fully, some of their H+ ions stay attached to the acid molecule.

Ethanoic acid is a typical weak acid, with its ions in dynamic equilibrium with the un-dissociated acid. The reaction is moving both directions at the same rate.

HA ⇌ H+ + A–

CH3COOH ⇌ H+ + CH3COO–

Properties of strong and weak acids

Strong or weak?

pH of strong and weak acids

have lower pH values

are better conductors of electricity

react more quickly.

pH is a measure of the number of H+ ions in solution, with a lower pH meaning more H+ ions.

Because strong acids dissociate fully in solution, they contain more H+ ions per molecule of acid, producing a lower pH.

Compared to weak acids of the same concentration, strong acids:

high H+ low H+

Strong Acids

• HI• HBr• HCl• HNO3

• H2SO4

• HClO4

• HClO3

Properties of strong and weak basesBases can be classified as strong and weak in the same way as acids. A strong base, such as sodium hydroxide, fully dissociates in solution.

A weak base, such as ammonia, does not fully dissociate, and some of the OH– ions are not released into solution.

NH3 + H2O ⇌ NH4+ + OH–

NaOH Na+ + OH–

Comparable levels of ion dissociation mean that strong and weak bases have similar properties to strong and weak acids.

Properties of strong and weak alkalis

Strong Bases

• NaOH• KOH• LiOH• RbOH• CsOH• Ca(OH)2

• Ba(OH)2

• Sr(OH)2

Strong and weak acids and bases

Titration Curves

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Titration & Titration Curves• Titration: the adding of one solution of an known

concentration into another solution– standard solution: a solution with a known concentration

• Titration curve: a graph showing pH vs volume of acid or base added – The pH shows a sudden change near the equivalence point– The Equivalence point (a.k.a. stoichiometric point) is the

point at which the moles of OH- are equal to the moles of H3O+

• End point- Point at which titration is complete; indicator color change

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Strong acid-strong base Titration Curve

• At equivalence point, Veq:

Moles of H3O+ = Moles of OH-

• There is a sharp rise in the pH as one approaches the equivalence point

• With a strong acid and a strong base, the equivalence point is at pH =7

15_327

01.0

Vol NaOH added (mL)

50.0

7.0

13.0

pH

100.0

Equivalencepoint

pH

mL base added

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Weak acid-strong base Titration Curve

• The increase in pH is more gradual as one approaches the equivalence point

• With a weak acid and a strong base, the equivalence point is higher than pH = 7

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Indicators