CBSE Chemistry Project

On Chemical Kinetics

By Badree (XII-B)

New Green Field School,Saket, New Delhi-17

2012-13

The project involves the aim to understand the order of reaction of methyl ethanoate's hydrolysis (catalysed by acid).

PROJECT ON

CHEMICAL KINETICS:

Acid catalysed hydrolysis

of ester.

Contents1. Certificate

2. Acknowledgement

3. Objective

4. Theory I (introduction)

5. Theory II (factors affecting rate of

reaction)

6. Materials Required

7. Experimental Procedure

8. Observation

9. Calculations

10. Result and Conclusion

11. Precautions

12. Bibliography

CERTIFICATE This is to certify that the Project titled 'Chemical Kinetics: Acid catalysed hydrolysis of ester'was completed under my guidance and supervision by Badree, Roll No. ________ a student of XII-B, New Green Field School,Saket within the stipulated time as prescribed by the CBSE.Mr.Harold Perera, Chemistry TeacherNew Green Field School, Saket, New Delhi.

A CKNOWLEDGEMENT

I gratefully acknowledge my sincere thanks to ourrespected chemistry teacher Mr.Harold Perera forhelping me decide on the project of my choice and his remarkable, valuable guidance and supervisionthroughout the project work.

O BJECTIVE

To understand the hydrolysis of methyl ethanoate catalysed by HCL and determine the order of the reaction and rate constant by using the values obtained by titrating it against a standar base, NaOh.

by (i) Graphically,

(ii)Mathematically

THEORY:

INTRODUCTION

CHEMICAL KINETICS

Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. Relatively simple rate laws exist for zero-order reactions (because reaction rates are independent of concentration), first-order reactions, and second-order reactions can be derived for others. In consecutive reactions, the rate-determining step often determines the kinetics. In consecutive first-order reactions, a steady state approximation can simplify the rate law.

FACTORS AFFECTING REACTION RATE NATURE OF THE REACTANTS Depending upon what substances are reacting, the reaction rate varies. Acid/base reactions, the formation of salts, and ion exchange are fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow. Nature and strength of bonds in reactant molecules greatly influence the rate of its transformation into products. PHYSICAL STATE

The physical state (solid, liquid, or gas) is also an important factor of the rate of change. When reactants are in the same phase, as in aqueous solution, thermal motion brings them into contact. But, when they are in different phases, the reaction is limited to the interface between the reactants. For example reaction can occur only at their area of contact; in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking/stirring may be needed to complete the reaction. This means that the more finely divided a solid or liquid reactant the greater its surface area per unit volume and the more contact it makes with the other reactant, thus the faster the reaction.

PRESSURE

Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction

CONCENTRATION

The reactions are due to collisions of reactant species. The frequency with which they collide depends upon their concentrations.The more crowded the molecules are (high conc.), the more likely they are to collide and react. Thus, an increase in the concentrations of the reactants will result in the corresponding increase in the reaction rate,vice versa. For example, combustion that occurs in air (21% oxygen) will occur more rapidly in pure oxygen. FREE ENERGY The free energy change (ΔG) of a reaction determines whether a chemical change will take place. It is possible to make predictions about reaction rate constants for a reaction from free-energy relationships.Although,kinetics describes how fast the reaction is. A reaction can be very exothermic and have a very positive entropy change but will not happen in practice if the reaction is too slow. Also, if a reactant can produce two different products, the thermodynamically most stable one will in (generally) form.

TEMPERATURE

Temperature has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more thermal energy.This makes collisions frequent and the proportion of reactant molecules with sufficient energy to react (energy greater than activation energy: E > Ea) is significantly higher as explained by the Maxwell–Boltzmann distribution of molecular energies.

EQUILIBRIUM

It determines the extent to which reactions occur. In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products no longer change.Component concentrations can oscillate for a long time before finally attaining the equilibrium.

CATALYSTS

A catalyst is a substance that accelerates the rate of a chemical reaction (or lowers activation energy) but remains chemically unchanged afterwards. The catalyst increases rate reaction by providing a different reaction mechanism to occur with a lower activation energy.

M ATERIALS AND EQUIPMENT REQUIRED:

CHEMICALS

Methyl acetate Hcl Naoh Oxalic Acid Phenolphthalein

APPARATUS

Conical flask Thermostat bottles Reagent Burette Pipette Ice

PROCEDURE Step I: Standardization of NaOH using standard Oxalic acid(0.1N)

1. 10ml of given 0.1N standard Oxalic acid is pipetted out into a 100ml conical flask. 2. This solution is titrated against the given unknown concentration of NaOH using phenolphthalein indicator until the end point is colorless to pale pink.3. Tabulate the values and repeat the titration for concurrent readings and determine the unknown concentration of supplied NaOH solution

Step II: Standardization of HCl using NaOH solution. 1. 2 ml of given HCl is pipetted out into a 100ml conical flask.

2. This solution is titrated against the NaOH using phenolphthalein indicator until the end point is colorless to pale pink.

3. Tabulate the values and repeat the titration for concurrent readings and determine the unknown concentration of supplied HCl solution.

Step III: Determination of rate constant (k1) for the acid-catalyzed hydrolysis of methyl acetate.

1. 100 ml of given HCl (whose strenght is determined in step II) solution is taken in a stoppered reagent bottle.

2. 5 ml of methyl acetate solution is added to the HCl solution. Note the time when half the methyl acetate solution is added. The mixture is shaken well. 3.Pipette out 5 ml of the reaction mixture and discharge it into 50 ml of

ice cold water kept in a conical flask.

4.Titrate the reaction mixture against NaOH solution, using phenolphthalein as indicator. This titre value corresponds to Vo . Steps 3 and 4 are repeated at intervals of 5, 10, 15, 20, 30, 45, 60 minutes. Each titre value corresponds to Vt .

5. The remaining solution is taken in a stoppered conical flask and heated to 60oC, and kept at this temperature for 5 minutes.

6.The solution is allowed to cool to room temperature. Repeat Steps 3 and 4. This titre value corresponds to V∞ till concurrent values are obtained.

7. Plot a graph of log (V∞ -Vt ) versus time (t) and determine the slope.

8. Report the theoretical and graphical value of rate constant (k1).

OBSERVATIONS

Table corresponding to step I

Standardization of NaOH using standard Oxalic acid(0.1N)S.No. Vol.(oxalic acid)

in mlBurette (initial)in ml

Burette(final)in ml

Volume of NaOhin ml

1.2.3.

Table corresponding to step II

Standardization of HCl using NaOH solutionS.No. Vol.(HCl)

in mlBurette (initial)in ml

Burette(final)in ml

Volume of NaOhin ml

1.2.3.

Table corresponding to step III

Determination of Rate Constant S.No. time

(min.)Volume of sol. in ml

Burette(Initial)in ml

Burette(Final)in ml

Volume of NaOhin ml

V∞-Vt

1.2.3.4.

CALCULATIONS

TABLE I

(HCOO)2 N1 V1 = N2 V2 (NaOH)

Concentration of NaOH, N2 = _______________

TABLEII

(NaOH) N2V2 = N3 V3 (HCl)

Concentration of HCl, N3 = ______________

TABLE III

CALCULATING THE RATE CONSTANT (I) MATHEMATICALLY:

In the presence of an acid, this reaction should be of second order, since two molecules are reacting. But, it is found to be first order. This may be explained in the following way :The rate of the reaction is given by

dx / dt = k’[CH3COOCH3] [ H2O ] ,

where k’ is the rate constant.Since water is present in large excess, its active mass (molar concentration) virtually remains constant during the course of the

reaction. Therefore, its active mass gets included in the constant, and the above equation reduces to :dx / dt = k1 [CH3COOCH3]

Thus, the rate of the reaction is determined by one concentration term only (that is, by a single power of the concentration term only).Hence, the reaction is first order. Such reactions are also called pseudo first order reactions.

K1=2.303/t(logV∞-Vo/V∞-Vt)S.No. K1

1.2.3.4.

(II)GRAPHICALLY

In order to determine the rate law for a reaction from a set of data consisting of concentration (or the values of some function of concentration) versus time, make three graphs.

[A] versus t (linear for a zero order reaction)

ln [A] versus t (linear for a 1st order reaction)

1 / [A] versus t (linear for a 2nd order reaction)

ZERO ORDER

FIRST ORDER

SECOND ORDER

Therefore it is easy to infer the order of a reaction graphicallyHence the plots for concentration of ester are:

RESULT &CONCLUSION

Hence the acid catalysed hydrolysis is a (pseudo) first order reaction, verified graphically and mathematically whose rate constant is :

PRECAUTIONS 1.Ethyl acetate is highly flammable. Harmful if swallowed in quantity. Vapours may cause drowsiness.

2.Hydrochloric acid is corrosive. In the event of skin contact, wash well with water.

3.The dilute sodium hydroxide solution caustic,and one should not handle them with hands

4.When diluting the hydrochloric acid, remember to add the concentrated acid to water to avoid splattering. Take care in handling the container as the dilution will generate heat. 5.Dissolving of sodium hydroxide generates heat. Take care in handling the dilution container.

6. All solutions in this experiment should be disposed of in the proper waste containers as provided in the laboratory.

BIBLIOGRAPHY

1.www. chem-guide.blogspot.com

2.www.wikipedia.org

3.NCERT PART I