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Physical Chemistry
March 13, 2025 | UR Gate

Kinetic Study of the Hydrolysis of Ethyl Acetate Catalyzed by Hydrochloric Acid


Objective of the Experiment:

  1. Determine the specific reaction rate constant.
  2. Determine the half-life (t1/2​) of the reaction.

Theoretical Principle:

The study of chemical kinetics focuses on the quantitative analysis of reaction rates and their changes over time. It examines the factors influencing these rates, such as catalysts, exposure to radiation, temperature, and molar concentration or pressure in gaseous phases. For example, the formation of water from hydrogen and oxygen gases can take years unless triggered by an electric spark, which causes immediate water formation. Similarly, the hydrolysis of organic esters is faster in the presence of an acid or base than in neutral solutions. In such cases, acids, bases, and electrical sparks are considered catalysts.

In the presence of a large amount of water, this reaction follows first-order kinetics concerning the concentration of ethyl acetate. The hydrolysis process occurs very slowly and is not measurable under normal conditions. Therefore, the reaction is catalyzed by acid (hydrogen ions), following pseudo-first-order kinetics. Despite involving two reactant molecules, the excess water ensures that only ethyl acetate concentration appears to change. Additionally, the excess water prevents any observable reverse reaction.

The increase in acid concentration serves as a measure of the ethyl acetate concentration, while the hydrochloric acid concentration remains constant throughout the experiment. If  V0, Vt , and V∞ ​ represent the volume of the titrant consumed for neutralizing hydrochloric acid and acetic acid at times 0, t, and ∞, respectively, then the concentration of ethyl acetate at time t (a − x) is proportional to V∞ ​ −Vt ​ , and the initial concentration is proportional to V∞ ​ −V0 ​.
The rate constant can be determined using the following equation:




Experimental Procedure:

(A) Determination of Vt :

  • Take 5 ml of ethyl acetate in an Erlenmeyer flask and add 100 ml of 0.1 M hydrochloric acid. Start timing the reaction.
  • After 5 minutes, withdraw 10 ml of the reaction mixture, transfer it to a clean Erlenmeyer flask, and add 20 ml of cold water. Then, add two drops of phenolphthalein indicator and titrate against a base.
  • Repeat the titration for additional 10 ml portions of the reaction mixture at intervals of 5, 10, 15, 20, 25, and 30 minutes.

(B) Determination of  V∞ :

  • Withdraw 10 ml of the mixture immediately after mixing and place it in a clean, dry Erlenmeyer flask.
  • Heat the mixture at 60°C for 60 minutes, then allow it to cool for 5 minutes.
  • Add 20 ml of ice-cold distilled water and titrate against 0.1 M base using two drops of an indicator.


Results and Calculations:

Note:
This is an illustrative example performed in the laboratory.


Graphical Representation:





Discussion:

Q1: What is the mechanism of the acid-catalyzed hydrolysis of ethyl acetate?




Q2: Why is this reaction considered pseudo-first order?
Although two reactants are involved (ethyl acetate and water), the reaction follows first-order kinetics concerning ethyl acetate and zero-order kinetics concerning water. This is because water is present in large excess, making its concentration effectively constant.


Q3: Why is ice-cold water added before titration?
To prevent any reverse reaction from occurring.


Q4: Briefly discuss the major factors affecting reaction rates.
  1. Nature of Reactants: Different chemical reactions require varying amounts of time to reach completion, depending on the properties of the reactants and products.
  2. Concentration: Higher concentrations of reactants lead to faster reactions, as more molecules are available for collisions, increasing the likelihood of forming products.
  3. Surface Area in Heterogeneous Reactions: In solid-liquid or solid-gas reactions, the reaction occurs at the solid’s surface. The reaction rate depends on the available surface area, which is inversely related to the size of solid particles.
  4. Temperature: Higher temperatures increase the kinetic energy of reactants, enhancing their collision frequency and energy, which accelerates the reaction. In general, a 10°C rise in temperature approximately doubles the reaction rate.
  5. Stirring: Mixing reactants enhances their interactions, leading to a faster reaction rate.
  6. Catalysts: A catalyst is a substance that increases the reaction rate without being consumed in the reaction. Theoretically, a catalyst can be recovered and reused after the reaction.


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