The Equilibrium Constant Of An Ester Hydrolysis Reaction

Topics: Sodium hydroxide, Chemical reaction, Functional group Pages: 11 (1403 words) Published: April 17, 2015


The Equilibrium Constant of an Ester Hydrolysis Reaction

Julia Stanley
CHM 152 LL
Dr. Asmita Kane Budruk

Goal of the lab:

The purpose of this laboratory is to determine the equilibrium constant, Kc, for the acid-catalyzed reaction between an unknown ester and water to produce an unknown alcohol and an unknown carboxylic acid. I was using Unknown Ester #3 with a density of 0.9342 and Molar Mass of 74.08 g/mol; alcohol with density 0.7914 and Molar Mass 32.04 g/mol.

Chemical principle or theory involved:
I used chapter 14, part 14.1 The Concept of Equilibrium and the Equilibrium Constant. Equilibrium is a state of balance in a chemical reaction in which the forward and backward rates are equal. For the reaction below, when A and B were mixed, the reaction proceeds in the forward direction to produce C and D.  However, as time progresses, the concentration of C and D increases causing an increase in the rate of the reverse reaction.  Concurrent with this increased rate of the reverse reaction is a reduction of the forward rate due to the decrease in the concentration of A and B.  At some point, the rate of the forward and reverse reactions will become the same and we will reach a state of dynamic equilibrium: aA  +  bB    cC  +  dD (1)

For a general equilibrium equation we can specify an equilibrium constant, Kc, that relates the concentrations of all product and reactant species, (2)
where [A], [B], [C], and [D] are the molar concentration of all species present at equilibrium.  The exponents, a, b, c, and d represent the stoichiometry coefficients from the balance chemical reaction.  Kc is really the ratio of the rate of the reverse reaction divided by the rate of the forward reaction and so is a dimensionless constant at a given temperature.

Additionally, functional groups are important for this experiment. Functional group: A group of atoms whose bonding is the same from molecule to molecule. A functional group has similar behavior regardless of the molecule that contains it, so molecules with identical functional groups tend to have similar chemical and physical properties. In addition, functional groups with similar structures tend to impart similar chemical and physical properties to the molecules that contain them. (http://www.chem.ucla.edu/harding/IGOC/F/functional_group.html)

Table 1: Some organic functional groups

Some organic functional groups
Structure
Name
Example
R Group
ROH
Alcohol
C2H5OH=ethanol
C2H5-
RCOOH
Carboxylic acid
CH3COOH=acetic acid
CH3-
R1COOR2
Ester
CH3COOC2H5=ethyl acetate

In this experiment I will determine the equilibrium constant for the following hydrolysis of an ester reaction: (4)
(5)
In chapter 16 I read about an acid-base indicator that is used in this experiment to signal the end point of the titration. Phenolphthalein drops are completely colorless in acid, however, in a base solution turn reddish pink. Also, I used Chapter 4 for determination of molarity. In chapter 3 I found all information about stoichiometry. Specifically in part 3.8 I used knowledge of converting from grams to moles, from milliliters to liters.

Procedure:

1. At first, I signed out an unknown ester sample from the stockroom and recorded the unknown number. After I determined the density of deionized water, and of the 3 M HCL solution. Also record the density and molar mass of the ester and alcohol. 2. Prepared the reaction mixtures as given in Table 1. Measured and recorded the volumes to 0.01 ml precision and stored them in my drawer till the mixtures came to chemical equilibrium. 3.After I calculated the mass of sodium hydroxide needed to prepare 0.5 L of a 0.7 M NaOH solution, poured the solution into a polyethylene bottle and added water to bring the total volume of solution to about 500 mL. 4. Next step was to calculate the mass of potassium hydrogen phthalate (KHP)...
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