The Equivalence Point: Acid/Base Titrations

The equivalence point is a fundamental concept in acid-base titrations. It represents the moment of perfect balance when the moles of a known solution (the titrant/standard) equal those of the unknown solution (the analyte). This critical point holds immense significance as it unveils the pH of the solution upon reaction completion.

The equivalence point offers a clear and definitive measure of when the reaction reaches completion. It not only helps in determining the concentration of the unknown analyte but also aids in understanding the behavior and interactions of different chemical species within the solution.

Moreover, the pH value at the equivalence point serves as a critical indicator of the solution’s acidic or basic (strong or weak) natures.

Titrations

Titrations are an analytical technique used to determine the concentration of an unknown reactant by measuring its reaction with a solution of known concentration.

This process involves the careful mixing of two different solutions: the titrant and the analyte. The titrant, usually placed in a burette, is of known concentration, while the analyte, placed in a conical flask, is of unknown concentration.

The heart of titration lies in understanding the equilibrium point when the moles of the titrant precisely neutralize the moles of the analyte. This balance is often signaled by an indicator that changes color when the reaction is complete. Acid-base titrations, in particular, rely on the measurement of pH throughout the titration process to determine the equivalence point.

Acid-Base titrations

Acid-base titrations are monitored by the change in pH as the reaction progresses. For a reaction between an acid and a base, the titration technique is useful for measuring the pH at various points throughout the reaction and especially at the equivalence point.

At the equivalence point in an acid-base titration:

moles of base = moles of acid

Usually, the progress of the acid-base titration reaction is represented in a titration curve, which is a graph of the pH as a function of the amount of titrant (acid or base) added.

There can be titrations between strong and weak acids and bases (4 pairs) with different equivalence points on the graph.

1. Strong Acid-Strong Base Titrations

At the equivalence point (halfway up the steep curve)

moles of NaOH added = moles of HCl in the analyte

At this point, H₃O⁺ ions are completely neutralized by OH^–. The solution only has salt (NaCl) and water and therefore the pH is neutral i.e. pH = 7.

2. Titration of a weak acid with a strong base

At equivalence point

moles of NaOH added = moles of CH₃COOH in the analyte

At this point, H₃O⁺ ions are completely neutralized by OH^–.

The solution contains CH₃COONa salt and H₂O. This will dissociate to CH₃COO- and Na⁺, but CH₃COO^– is a strong base so it will react with H₂O to give the weak conjugate acid CH₃COOH and HO^– which explains the high pH at (9) at the equivalence point.

3. Titration of a strong acid with a weak base

Titration curve of strong acid and weak base

At equivalence point

moles of NH₃ added = moles of HCl in the analyte

At this point, H₃O⁺ ions are completely neutralized by OH^–.

The solution contains NHN₄Cl salt and H₂O. This will dissociate to NH₄⁺ and Cl^–, but NH₄⁺ is a strong acid so it will react with H₂O to give the weak conjugate base NH₃ and HCl which explains the high pH at (5) at equivalence point.

Significance of Equivalence point

The significance of the equivalence point in acid-base titrations is pivotal in various scientific and industrial applications. Here are some key points highlighting its importance:

Concentration Determination
Endpoint Identification
pH Measurement
Chemical Behavior
Quantitative Analysis
Accuracy and Reliability
Calibration
Environmental Monitoring
Educational Tool
Quality Control

Role of Chemical Indicators in Equivalence points

Indicators play a vital role in acid-base titrations and other types of titrations as they help determine the equivalence point or endpoint of the titration. Their uses are as:

Endpoint Detection
Transition Ranges
Color Changes

Concepts Berg

Why is the pH in a titration of a weak acid with a strong base at an equivalence point not equal to 7?

For a weak acid/strong base titration, the pH at the equivalence point is > 7. This is because the solution is composed of water and the strong conjugate base of the weak acid, this will dissociate with water to give HO^– (basic) which is responsible for the high pH.

Why is the equivalence point at a pH of 7 for a strong acid?

The pH at the equivalence point is 7.0 because the solution only contains water and salt that is neutral. Since neither H⁺ nor OH^– molecules remain in the solution, we can conclude that at the equivalence point of a strong acid – strong base reaction, the pH is always equal to 7.0

What is the endpoint of a titration?

An endpoint is a point in a titration that signifies the completion of the titration by a change in the color or intensity of the solution, it should not be confused with the equivalence point which is the point at which the added titrant is chemically equivalent to the sample analyte.

Why is methyl orange a suitable indicator for titrating weak bases and strong acids?

Methyl orange is a good chemical indicator in strong acid weak base titrations as it changes its color in this range. Methyl Orange shows a red color in an acidic medium and a yellow color in a basic medium, along with a range of color changes in favor of strong acids (low pH).

What is the half-equivalence point in the titration?

The half-equivalence point of an acid-base titration is the point at which the concentration of an added base is equal to half of the original concentration of the acid.

Because of the incomplete dissociation of the acid, the reaction is in equilibrium, with an acid dissociation constant, Ka, which is specific to that acid. point is the same. Therefore, at the half-equivalence point, the pH is equal to the pKa.

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