Determination of Kc for an Esterification Reaction (Lab Report X)
INTRODUCTION
A system is in equilibrium when the concentrations of the reactants and products are both
constant. Kc is called the equilibrium constant, for a given temperature, therefore it is
temperature-dependent. For any reaction, we are able to calculate the equilibrium constant
by inputting the concentrations of reactants and products into the equilibrium constant
expression:
[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]
Kc = [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠]
If Kc is less than one, we say that at equilibrium, the concentration of reactants is greater than
that of products, and the forward reaction has hardly proceeded. If the equilibrium constant
is greater than one, then the concentration of products is greater than that of reactants, and
the reaction is said to have almost gone to completion. If it is equal to one, at equilibrium,
there are equal amounts of both reactants and products present in the equilibrium mixture.
The magnitude of Kc can therefore tell us about how the concentrations of products and
reactants will look like for a certain reaction at a certain temperature at equilibrium.
In this lab experiment we aim to calculate Kc for an acid-catalysed esterification reaction
between water and ethyl ethanoate. We will be using eight mixtures with different volumes
of hydrochloric acid (catalyst), water, ethyl ethanoate (ester), ethanoic acid (carboxylic acid)
and ethanol (alcohol), and determining their equilibrium concentrations through
stoichiometry in order to calculate Kc for each mixture. We must leave the mixtures in the
dark for one week beforehand, so that equilibrium is already established by the time we do
the titration. The reaction is as follows:
Ethanol + ethanoic acid ⇌ water + ethyl ethanoate
This is an example of an esterification reaction, which is a reaction in which an alcohol and an
acid form an ester as a reaction product. Esters are organic compounds where the -COOH
group of a carboxylic acid is replaced by a hydrocarbon group of some kind. In ethyl
ethanoate, the carboxyl group is replaced by an ethyl group, which is where its name comes
from. Esters are typically characterized as having a pleasant, fruity odor.
, Image 1.0 – Esterification reaction of ethyl ethanoate
(http://wiki.chemprime.chemeddl.org/articles/e/s/t/Esters_in_Foods_ff8b.html)
The advantage of studying this particular reaction is that it is kinetically slow, so it is possible
to measure the concentration of a reactant or product by titration without significantly
disturbing the equilibrium in the short time period it takes to carry out the titration. To
determine Kc, we will first titrate the acid in the mixtures with sodium hydroxide to determine
the amount of ethanoic acid present in each mixture. Then, we will calculate the number of
moles of each reagent present in the original mixture (before the reaction had taken place).
The last step is to find Kc, and we do this by producing an ICE table, which shows us the initial
and final (equilibrium) concentrations of each reagent, as well as the change that had taken
place in order to come to the final concentrations. We will plug the final concentrations into
the equilibrium constant expression to calculate the equilibrium constant. We use the final
concentrations because these are the concentrations at equilibrium.
Since this reaction is reversible, it is possible to use either the forward and backward reactions
to calculate Kc (we do not have to use just the forward one). In these calculations the
backward reaction is used, unless otherwise stated.
METHODOLOGY
The independent variable for this experiment is the equilibrium concentrations of the
reactants and products, which will be determined through calculation.
The dependent variable is the value of the equilibrium constant K c for each mixture, also
determined through calculation using the independent variable.
Here are the controlled variables:
Variable Why it was controlled How it was controlled
1. Temperature 1. The equilibrium 1. Placing the
2. Volume of HCl constant is equilibrium mixtures
3. Concentration of temperature- in the same place for
NaOH used to titrate dependent. The one week while
the acid in each temperature must be equilibrium is taking
mixture kept constant while place. The reaction
4. Amount of the equilibrium is vessels were also
phenolphthalein being established. kept closed.
indicator in titration 2. The HCl is used as a 2. Using the same
catalyst for the volume of HCl in each
esterification mixture.
reaction. 3. Using samples from
3. To determine the the same one-molar
acid concentration in solution of NaOH.
INTRODUCTION
A system is in equilibrium when the concentrations of the reactants and products are both
constant. Kc is called the equilibrium constant, for a given temperature, therefore it is
temperature-dependent. For any reaction, we are able to calculate the equilibrium constant
by inputting the concentrations of reactants and products into the equilibrium constant
expression:
[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]
Kc = [𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠]
If Kc is less than one, we say that at equilibrium, the concentration of reactants is greater than
that of products, and the forward reaction has hardly proceeded. If the equilibrium constant
is greater than one, then the concentration of products is greater than that of reactants, and
the reaction is said to have almost gone to completion. If it is equal to one, at equilibrium,
there are equal amounts of both reactants and products present in the equilibrium mixture.
The magnitude of Kc can therefore tell us about how the concentrations of products and
reactants will look like for a certain reaction at a certain temperature at equilibrium.
In this lab experiment we aim to calculate Kc for an acid-catalysed esterification reaction
between water and ethyl ethanoate. We will be using eight mixtures with different volumes
of hydrochloric acid (catalyst), water, ethyl ethanoate (ester), ethanoic acid (carboxylic acid)
and ethanol (alcohol), and determining their equilibrium concentrations through
stoichiometry in order to calculate Kc for each mixture. We must leave the mixtures in the
dark for one week beforehand, so that equilibrium is already established by the time we do
the titration. The reaction is as follows:
Ethanol + ethanoic acid ⇌ water + ethyl ethanoate
This is an example of an esterification reaction, which is a reaction in which an alcohol and an
acid form an ester as a reaction product. Esters are organic compounds where the -COOH
group of a carboxylic acid is replaced by a hydrocarbon group of some kind. In ethyl
ethanoate, the carboxyl group is replaced by an ethyl group, which is where its name comes
from. Esters are typically characterized as having a pleasant, fruity odor.
, Image 1.0 – Esterification reaction of ethyl ethanoate
(http://wiki.chemprime.chemeddl.org/articles/e/s/t/Esters_in_Foods_ff8b.html)
The advantage of studying this particular reaction is that it is kinetically slow, so it is possible
to measure the concentration of a reactant or product by titration without significantly
disturbing the equilibrium in the short time period it takes to carry out the titration. To
determine Kc, we will first titrate the acid in the mixtures with sodium hydroxide to determine
the amount of ethanoic acid present in each mixture. Then, we will calculate the number of
moles of each reagent present in the original mixture (before the reaction had taken place).
The last step is to find Kc, and we do this by producing an ICE table, which shows us the initial
and final (equilibrium) concentrations of each reagent, as well as the change that had taken
place in order to come to the final concentrations. We will plug the final concentrations into
the equilibrium constant expression to calculate the equilibrium constant. We use the final
concentrations because these are the concentrations at equilibrium.
Since this reaction is reversible, it is possible to use either the forward and backward reactions
to calculate Kc (we do not have to use just the forward one). In these calculations the
backward reaction is used, unless otherwise stated.
METHODOLOGY
The independent variable for this experiment is the equilibrium concentrations of the
reactants and products, which will be determined through calculation.
The dependent variable is the value of the equilibrium constant K c for each mixture, also
determined through calculation using the independent variable.
Here are the controlled variables:
Variable Why it was controlled How it was controlled
1. Temperature 1. The equilibrium 1. Placing the
2. Volume of HCl constant is equilibrium mixtures
3. Concentration of temperature- in the same place for
NaOH used to titrate dependent. The one week while
the acid in each temperature must be equilibrium is taking
mixture kept constant while place. The reaction
4. Amount of the equilibrium is vessels were also
phenolphthalein being established. kept closed.
indicator in titration 2. The HCl is used as a 2. Using the same
catalyst for the volume of HCl in each
esterification mixture.
reaction. 3. Using samples from
3. To determine the the same one-molar
acid concentration in solution of NaOH.
