Determination of Equilibrium Constant in Liquid Phase

Ratification Page

Complete report of experiment in physical chemistry with title “Determination of Equilibrium Constant in Liquid Phase”, was made by:

Name : Dhidi Erwanto
Registration Number : 081304167
Group : VI
Class : International Class Program of Chemistry

After checked by assistant and coordinator assistant, so this report have accepted.

Makassar, 6th April 2010
Coordinator assistant Assistant


Mahyuddin Ezzar Fitriyani

Known by:
Responsibility Lecturer


Drs. H. Muhammad Yudi, M.Si
 
The Title of Experiment
The title of this experiment is “determination of equilibrium constant in liquid phase”.
The Objectives
The objective is determine the equilibrium constant KC eshterification of acetic acid.
Background
Equilibrium constants are determined in order to quantify chemical equilibria. When an equilibrium constant is expressed as a concentration quotient, it is implied that the activity quotient is constant. In order for this assumption to be valid equilibrium constants should be determined in a medium of relatively high ionic strength. Where this is not possible, consideration should be given to possible activity variation. (Atkins. 2001. 201).

aA + bB ↔ cC + dD

The equilibrium expression above is a function of the concentrations [A], [B] etc. of the chemical species in equilibrium. The equilibrium constant value can be determined if any one of these concentrations can be measured. The general procedure is that the concentration in question is measured for a series of solutions with known analytical concentrations of the reactants. Typically, a titration is performed with one or more reactants in the titration vessel and one or more reactants in the burette. Knowing the analytical concentrations of reactants initially in the reaction vessel and in the burette, all analytical concentrations can be derived as a function of the volume (or mass) of titrant added (Atkins. 2001. 205)
The Equilibrium Constant, Kc, relates to a chemical reaction at equilibrium. It can be calculated if the equilibrium concentration of each reactant and product in a reaction at equilibrium is known. The equilibrium expression below, formed from the general chemical equilibrium, is universally true. The chemical components happen to be gases.
aA(g) + bB(g) ↔ cC(g) + dD(g)
The equilibrium constant is defined as:
Kc=(〖[C]〗^c 〖[D]〗^d)/(〖[A]〗^a 〖[B]〗^b )
The notation [A] signifies the molar concentration of species A. An alternative expression for the equilibrium constant involves partial pressures.
Kp= (Pc^c PD^d)/(PA^a PB^b )
Note that the expression for the equilibrium constant includes only solutes and gases; pure solids and liquids do not appear in the expression. For example, the equilibrium expression for the reaction
CaH2 (s) + 2 H2O (g) →Ca(OH)2 (s) + 2 H2 (g)


Is :
Kc= 〖[H_2 ]〗^2/〖〖[H〗_2 O]〗^2
Observe that the gas-phase species H2O and H2 appear in the expression but the solids CaH2 and Ca(OH)2 do not appear. The equilibrium constant is most readily determined by allowing a reaction to reach equilibrium, measuring the concentrations of the various solution-phase or gas-phase reactants and products, and substituting these values into the Law of Mass Action (David, N, Blauch. 2001. 102).
Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid, which gives vinegar its sour taste and pungent smell. It is a weak acid, in that it is only a partially dissociated acid in an aqueous solution. Pure, water-free acetic acid (glacial acetic acid) is a colorless liquid that absorbs water from the environment (hygroscopic), and freezes at 16.5 °C (62 °F) to a colourless crystalline solid. The pure acid, and concentrated solutions, are dangerously corrosive (Jones, R.E. Templeton, D.H.1958. 309).
The major esters of acetic acid are commonly used solvents for inks, paints and coatings. The esters include ethyl acetate, n-butyl acetate, isobutyl acetate, and propyl acetate. They are typically produced by catalyzed reaction from acetic acid and the corresponding alcohol:

H3C-COOH + HO-R → H3C-CO-O-R + H2O, (R = a general alkyl group)
(Author. 1. 2010)

The condensation product of two molecules of acetic acid is acetic anhydride. The worldwide production of acetic anhydride is a major application, and uses approximately 25% to 30% of the global production of acetic acid. Acetic anhydride may be produced directly by methanol carbonylation bypassing the acid, and Cativa production plants can be adapted for anhydride production.

O O O O
+ + H2O

OH HO O

Acetic acid Acetic acid Acetic acid anhydride

Acetic anhydride is a strong acetylation agent. As such, its major application is for cellulose acetate, a synthetic textile also used for photographic film. Acetic anhydride is also a reagent for the production of aspirin, heroin, and other compounds (Author. 2. 2010).

Apparatus and Chemicals
Apparatus:
Separatory funnel 100 ml I pieces
Erlenmeyer flask 250 ml 2 pieces
Erlenmeyer flask 500 ml 1 pieces
Picnometer 50 ml 1 pieces
Volumetric glass 250 ml 1 pieces
Spoon
Analytical balance
Stative and clem
Beaker glass 250 ml 1 pieces
Stirrer
Dropping pipette
Hot plate
Spraying bottle
Chemicals:
C2H5OH 95% 200 ml
CuSO4.%H2O sufficient
Glacial acetic acid (CH3COOH.2H2O)
NaCO3 sufficient
H2SO4 © sufficient
Aquadest
Tissues





Instructions
200 ml CuSO4 anhydride was mixture with ethanol till the solution is not appear the blue color and also same treatment with 250 ml acetic acid dilute with CuSO4. Than determine the density each other. The two solution entrance into a big Erlenmeyer flask than added a little sulphuric acid. Heating the solution till got the balancing condition.
50 ml of a mixture was took and entrance in the separatory funnel. Adding as continuously into saturated Na2CO3. Addition of saturated Na2CO3 stopped after not formed the layers again. Separating the two phase by using separatory funnel. Then balanced the mass of acetic acid. Rapidly and reply the fourth and fifth steps as three times and determine the average value.
Result
CH3COOH 250 ml + Crystals of CuSO4 (shaked) ⃗ precipitate (filtered) ⃗ colorless.
C2H5OH 200 ml + crystals of CuSO4 (shaked) ⃗ precipitate (filtered) ⃗ colorless.
The measure the density of CH3COOH and C2H5OH:
Picnometer empty weight = 36, 39 grams
After fuelled the picnometer by the two solution:
Density of CH3COOH =(104,51 g)/(100 ml)=1,0451 g/ml
Density of C2H5OH =(81,31 g)/(100 ml)=0,8131 g/ml
Mixturing the two solution into Erlenmeyer flask + H2SO4 colorless
Colorless 50 ml of a mixture + Na2CO3 colorless.
Separating the solution by separatory funnel + sample solution 2 layers.
Take the white crystals.
The weight remains:
Filter paper = 0, 46 g
After take crystals = 1, 03 g
Total weight of crystals = 1, 03 g – 0, 46 g = 0, 57 g.
Data analysis
Known: Density of acetic acid = 1, 0451 g/ml
Density of ethanol = 0, 8131 g/ml
Volume of ethanol = 200 ml
Volume of acetic acid = 250 ml
Mm C2H5OH = 46, 9 g/mole
Mm CH3COOH = 60, 0 g/mole
Mm CH3COOC2H5 = 88 g/mole
Mass of CH3COOH = ρ. V
= (1. 0451 g/mole) (250 ml)
= 261, 275 g
Mole of CH3COOH = mass/mm
= (261.275 g)/(60 g/mole)
= 4. 354 mole
Mass of C2H5OH = ρ. V
= (0. 813 g/mole) (200 ml)
= 162. 62 g
Mole of CH3COOH = mass/mm
= (162.62 g)/(46 g/mole)
= 3. 523 mole
Volume of CH3COOC2H5 = 50 ml of ethyl acetic take from 450 ml.
= (450 ml)/(50 ml)
= 9
Practicum mass of CH3COOC2H5 in 450 ml = practicum mass x g
= 0. 57 g x 9
= 5, 13 grams.
Mole of CH3COOC2H5 = mass/mm
= (5,13 g)/(88 g/mole)
= 5.8 x 10-2 mole
The chemical reaction:
CH3COOH + C2H5OH ↔ CH3COOC2H5 + H2O
Initially 3.5 mole 4.3 mole - -
Reaction 5.8 x 10-2 mole 5.8 x 10-2 mole 5.8 x 10-2 mole 5.8 x 10-2 mole
Equil 3.442 mole 4.242 mole 5.8 x 10-2 mole 5.8 x 10-2 mole

Kc= (〖[CH〗_3 COOC_2 H_5][H_2 O])/([〖CH〗_3 COOH] 〖[C〗_2 H_5 OH])

= ([5.8 x〖10〗^(-2) ][5.8 x〖10〗^(-2))/( [3.442] 4.242])
=〖3.364 x 10〗^(-2)/14.6
= 2.3 x 10-4
Discussion
On this experiment, the addition of anhydride CuSO¬¬4 to ethanol functioned to know and identifying if there is not water again was attached in the ethanol. It’s viewing by the ethanol when added CuSO4, will not produce blue color again. In this cases, must be free of water because, according to the theory, if the volume in the solver or solution is bigger, influence the equilibrium (Adi, Krisbiyantoro. 2006:60). Thus, it this ethanol still bounding the water, absolutely the concentration is higher also. Same case with the addition of CuSO4 in glacial acetic acid. Because, both of them want to know how big the density. So, if there is H2O still banded in the solution, will be influence the density value.
After through out the water from the two solution, we measuring the density
both of them. Before that, the picnometer must be really dry and free from H2O and another disturb. Because, it’s will be influence the occur of density each solution after measure the density of each compound, CH3COOH is 1.04151 g/ml, and the density of acetic acid based on the theory is 1.0491 g/ml. And the density of ethanol is 0.8131 g/ml. It’s also similar with the theory, is 0.8134 g/ml (Anonim.2010).
The addition of sulphuric acid is as the acceptor acid condition, because according from the theory, CH3COOHC2H5 where will produced from the ethanol and acetic acid is have volality characteristic low condition. So that to kept the CH3COOC2H5 from it’s volality added by H2So4. Input the solution in separatory funnel and added by Na2CO3 drop by drop. It’s treated as the equilibrium director. And if the addition not carefully, the crystals that was formed before, will be soluble again. The equilibrium can be see when the crystal white was formed. The weight of crystals is 0.59 gram. And the value of equilibrium constant is 2.3 x 10-4. The chemical reaction is :
CH3COOH + C2H5OH ↔ CH3COOC2H5 + H2O
(white crystals)






 
Conclusion
After done the experiment, can be concluded:
The density of CH3COOH is 0.4151 g/ml and density of C2H5OH = 0,8131 g/ml.
Ester of acetic and can likewise be formed via esterification, and the weight of it’s crystals is 5.13 gram
The ester was formed by white crystals
Suggestion
Especially for the next students, and practicum, be carefully when added the saturated solution into the separatory funnel to formed white crystals and big amount.
 
BIBLIOGRAPHY

Author. 1. 2010. Equilibrium Constant. Online. http://www.wikipedia.com. Access on 3th April 2010.

Author. 2. 2010. Acetic Acid. Online. http://www.chem-is-try.com. Access on 3th April 2010.

Atkins. 2001. Physical Chemistry, Third Edition. Oxford University.
David, N, Blauch. 2001. Determination Of Stability Constant. Mc.Crew. Till press
Jones, R.E. Templeton, D.H.1958. The Crystals Structure Of Acetic Acid. Acta: Crystallog.