Strong Acid-Base Titrations

Chapter 17

Neutralization Reactions ReviewGenerally, when solutions of an acid and a base are combined, the products are a salt and water.

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

Neutralization ReactionsWhen a strong acid reacts with a strong base, the net ionic

equation is…

HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)

H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) Na+ (aq) + Cl- (aq) + H2O (l)

Net Ionic Equation: H+ (aq) + OH- (aq) H2O (l)

The K for this reaction is 1014

Neutralization reactions

• When a strong base is added to a solution of weak acid, the following neutralization reaction occurs:

Conj. Acid + OH- conj. Base + H2O

• When a strong acid is added to a solution of a weak base, a neutralization reaction occurs:

Conj base + H3O+ conj acid + H2O

Titration

A known concentration of base (or acid) is slowly added to a solution of acid (or base).

Titration

A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.

• The initial pH (initial acid) is determined by the initial concentration of the strong acid.

• Major substances present: H+ and Cl-

Titration of a Strong Acid with a Strong Base

• From the start of the titration to near the equivalence point, the pH goes up slowly.

• As strong base is added, the pH increases slowly because the base is neutralized by the acid.

• Major substances present: H+ , Cl-, Na+

Titration of a Strong Acid with a Strong Base

Titration of a Strong Acid with a Strong Base

• The pH of the solution before the equivalence point increases quickly because it is determined by the concentration of the aced that has not yet been neutralized.

Titration of a Strong Acid with a Strong Base

• At the equivalence point, moles acid = moles base based on stoichiometry of reaction, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

• The pH at equivalence point for a strong acid and strong base is always 7 exactly.

Titration of a Strong Acid with a Strong Base

• The pH after the equivalence point is determined by the concentration of the excess NaOH.

• As more base is added, the increase in pH again levels off

Practice ProblemCalculate the pH when 49.0 mL of 0.100 M

NaOH solution have been added to 50.0 mL of 0.100 M HCl solution.

Step 1: figure out how many moles of H+ are present in solution

0.0500 L soln x 0.100 mol H+/1 L soln = 5.00 x 10-3 mol H+

Step 2: figure out how many moles of OH- are added

0.0490 L soln x 0.100 mol OH- /1 L soln = 4.90 x 10-3 mol OH-

Calculate the pH when 49.0 mL of 0.100 M NaOH solution have been added to 50.0 mL of 0.100 M HCl solution.

Step 3: Because the equivalence point has not been reached, there are more moles of H+ present. ICE Table

H+ OH- H2O(l)

Initial 5.00 x 10-3

mol0 -

Change 4.90 x 10-3 mol

-

Equilibrium 0.10 x 10-3 mol

0 -

H+ OH- H2O(l)

Initial (before addition)

5.00 x 10-3

mol0 -

Change (addition)

4.90 x 10-3 mol

-

Equilibrium (after addition)

0.10 x 10-3 mol

0 -

Step 4: Determine the total volume of the mixed solution

0.050 L + 0.049 L = 0.0990 L

Step 5: Determine concentration of H+ and then pH

[H+] = 0.10 x 10-3 mol/0.0990 L = 1.0 x 10-3 M

pH = -log(1.0x10-3)