When is titration at equilibrium

Acid-Base Equilibria. Search for:. Weak Acid-Strong Base Titrations. Learning Objective Distinguish a weak acid-strong base titration from other types of titrations. Key Points In an acid-base titration, the titration curve reflects the strengths of the corresponding acid and base. If one reagent is a weak acid or base and the other is a strong acid or base, the titration curve is irregular, and the pH shifts less with small additions of titrant near the equivalence point.

Navigation menu Personal tools Log in Request account. Namespaces Page Discussion. Views Read View source View history. Community Community portal Web chat Mailing list. This page was last modified on 26 November , at This page has been accessed 19, times. As you can see from these plots, the titration curve for adding a base is the mirror image of the curve for adding an acid. Suppose that we now add 0. Thus the pH of the solution increases gradually.

At the equivalence point when The pH is initially As the equivalence point is approached, the pH drops rapidly before leveling off at a value of about 0. The titration of either a strong acid with a strong base or a strong base with a strong acid produces an S-shaped curve. The curve is somewhat asymmetrical because the steady increase in the volume of the solution during the titration causes the solution to become more dilute. Due to the leveling effect, the shape of the curve for a titration involving a strong acid and a strong base depends on only the concentrations of the acid and base, not their identities.

The shape of the titration curve involving a strong acid and a strong base depends only on their concentrations, not their identities. Calculate the pH of the solution after A Because 0. To completely neutralize the acid requires the addition of 5.

Because only 4. B The final volume of the solution is As we shall see, the pH also changes much more gradually around the equivalence point in the titration of a weak acid or a weak base.

Thus the pH of a solution of a weak acid is greater than the pH of a solution of a strong acid of the same concentration. Below the equivalence point, the two curves are very different. Note also that the pH of the acetic acid solution at the equivalence point is greater than 7. That is, at the equivalence point, the solution is basic. Above the equivalence point, however, the two curves are identical. In this situation, the initial concentration of acetic acid is 0. However, you should use Equation Inserting the expressions for the final concentrations into the equilibrium equation and using approximations ,.

Thus the pH of a 0. Now consider what happens when we add 5. All problems of this type must be solved in two steps: a stoichiometric calculation followed by an equilibrium calculation.

In the first step, we use the stoichiometry of the neutralization reaction to calculate the amounts of acid and conjugate base present in solution after the neutralization reaction has occurred. The acetic acid solution contained. The stoichiometry of the reaction is summarized in the following ICE table, which shows the numbers of moles of the various species, not their concentrations.

Using the stoichiometry of the reaction, the unknown concentration can be determined. It makes use of the neutralization reaction that occurs between acids and bases and the knowledge of how acids and bases will react if their formulas are known. A strong acid- strong base titration is performed using a phenolphthalein indicator. Phenolphtalein is chosen because it changes color in a pH range between 8. It will appear pink in basic solutions and clear in acidic solutions.

In the case of a strong acid-strong base titration, this pH transition would take place within a fraction of a drop of actual neutralization, since the strength of the base is high. The addition of reactants is done from a burette. The reactant of unknown concentration is deposited into an Erlenmeyer flask and is called the analyte. The other reactant of known concentration remains in a burette to be delivered during the reaction. It is known as the titrant. The indicator—phenolphthalein, in this case—has been added to the analyte in the Erlenmeyer flask.

Titration : Titration of an acid-base system using phenolphthalein as an indicator. Neutralization is the reaction between an acid and a base, producing a salt and neutralized base.

For example, hydrochloric acid and sodium hydroxide form sodium chloride and water:. Neutralization is the basis of titration. A pH indicator shows the equivalence point —the point at which the equivalent number of moles of a base have been added to an acid. It is often wrongly assumed that neutralization should result in a solution with pH 7. What is the unknown concentration of a Titrations are reactions between specifically selected reactants—in this case, a strong base and a weak acid.

A titration curve reflects the strength of the corresponding acid and base, showing the pH change during titration. The titration curve demonstrating the pH change during the titration of the strong base with a weak acid shows that at the beginning, the pH changes very slowly and gradually.

This indicates the formation of a buffer system as the titration approaches the equivalence point. At the equivalence point and beyond, the curve is typical of a titration of, for example, NaOH and HCl. Titration of a weak Acid with a strong base : This figure depicts the pH changes during a titration of a weak acid with a strong base. The initial pH of the solution at the beginning of the titration is approximately that of the weak acid in water.

However, the pH at the equivalence point does not equal 7. This is due to the production of conjugate base during the titration.

The resulting solution is slightly basic. The endpoint and the equivalence point are not exactly the same: the equivalence point is determined by the stoichiometry of the reaction, while the endpoint is just the color change from the indicator.

This conjugate base reacts with water to form a slightly basic solution. Recall that strong acid-weak base titrations can be performed with either serving as the titrant.