Question

Predict whether the equivalence point of each of the following titrations is below, above, or at \(\mathrm{pH} 7 :(\mathbf{a}) \mathrm{NaHCO}_{3}\) titrated with \(\mathrm{NaOH},(\mathbf{b}) \mathrm{NH}_{3}\) titrated with \(\mathrm{HCl},(\mathbf{c}) \mathrm{KOH}\) titrated with HBr.

Short answer

The pH at the equivalence point for the given titrations are as follows: (a) NaHCO3 titrated with NaOH: above 7, as it involves a weak base and a strong base. (b) NH3 titrated with HCl: below 7, as it involves a weak base and a strong acid. (c) KOH titrated with HBr: equal to 7, as it involves a strong base and a strong acid.

Step-by-step solution

Analyze the given titrations.

For each titration, we first classify the acid and base as strong or weak. Then, we predict the pH at the equivalence point based on this classification.

(a) NaHCO3 titrated with NaOH

For the first titration, we have NaHCO3 (sodium bicarbonate) as the analyte and NaOH (sodium hydroxide) as the titrant. NaHCO3 is a weak base, as it does not completely dissociate in water. On the other hand, NaOH is a strong base, meaning it completely dissociates in water.

Predict the equivalence point of (a)

Since the titration involves a weak base (NaHCO3) and a strong base (NaOH), the pH at the equivalence point will be above 7, as the strong base will be in excess.

(b) NH3 titrated with HCl

For the second titration, we have NH3 (ammonia) as the analyte and HCl (hydrochloric acid) as the titrant. NH3 is a weak base, as it does not completely dissociate in water. On the other hand, HCl is a strong acid, meaning it completely dissociates in water.

Predict the equivalence point of (b)

Since the titration involves a weak base (NH3) and a strong acid (HCl), the pH at the equivalence point will be below 7, as the strong acid will be in excess.

(c) KOH titrated with HBr

For the third titration, we have KOH (potassium hydroxide) as the analyte and HBr (hydrobromic acid) as the titrant. KOH is a strong base, as it completely dissociates in water. On the other hand, HBr is a strong acid, meaning it completely dissociates in water as well.

Predict the equivalence point of (c)

Since the titration involves a strong base (KOH) and a strong acid (HBr), the pH at the equivalence point will be equal to 7, as both the acid and the base will neutralize each other completely. In conclusion, the pH at the equivalence point for: (a) NaHCO3 titrated with NaOH is above 7. (b) NH3 titrated with HCl is below 7. (c) KOH titrated with HBr is equal to 7.

Key concepts

Equivalence Point

In acid-base titrations, the equivalence point is the moment where the amount of added titrant is chemically equivalent to the amount of substance present in the analyte. At this juncture, the acid and base react completely, resulting in a solution that is neither acidic nor basic when titrating a strong acid with a strong base.

However, the situation varies when dealing with weak acids or bases. For instance, when a weak base is titrated with a strong acid, the equivalence point pH will be below 7 due to the presence of the conjugate acid of the weak base, which slightly ionizes and renders the solution acidic. Conversely, when a weak acid is titrated with a strong base, the result is a conjugate base that makes the solution mildly basic, with a pH above 7.

Understanding the concept of the equivalence point is crucial for determining the concentration of an unknown solution and for the analytical chemistry that underpins various industries and research fields.

pH Level

The pH level is a numerical representation of the acidity or basicity of a solution. It is measured on a scale that typically ranges from 0 to 14. A pH of 7 is considered neutral, which is the pH of pure water at 25°C. Values less than 7 indicate acidity, while values greater than 7 indicate basicity.

The pH of a solution at the equivalence point in an acid-base titration is particularly informative. For a titration between a strong acid and a strong base, the pH at the equivalence point will be neutral, hovering around 7. If a strong acid is titrated with a weak base, or vice versa, the pH at the equivalence point will shift away from 7, reflecting the strength of the excess ions in solution.

For students, grasping the changes in pH levels during a titration curve can assist in visualizing the titration process and predicting the outcomes for different acid-base combinations.

Strong and Weak Acids and Bases

Acids and bases are classified as strong or weak depending on their ability to dissociate in water. Strong acids and bases dissociate completely, releasing all of their hydrogen ions (H+) or hydroxide ions (OH-) into the solution. Common examples of strong acids include hydrochloric acid (HCl) and strong bases like sodium hydroxide (NaOH).

Weak acids and bases only partially dissociate in water, establishing an equilibrium between the undissociated and dissociated forms. Acetic acid (CH3COOH) is a typical weak acid, and ammonia (NH3) is a weak base. The strength of an acid or a base significantly affects the pH of the solution during a titration and thus determines the pH at the equivalence point.

It is fundamental for students to distinguish between strong and weak acids and bases to predict the outcome of a titration, understand buffer systems, and comprehend equilibrium concepts in chemistry.