Reaction Rate and Rate Constant of the Hydrolysis of Ethyl Acetate With

Reaction rate and rate constant of the hydrolysis of ethyl acetate with sodium hydroxide Ikhazuangbe, Prosper Monday Ohien and Oni, Aisosa Babalola Madonna university, Faculty of Engineering Chemical Engineering Department, P.m.b 302 Enugu, Nigeria ABSTRACT Hydrolysis is a chemical decomposition involving breaking of a bond and the addition of elements of water. In this hydrolysis of ester (ethyl acetate) with an alkali (sodium hydroxide), HCl was used as catalyst to accelerate it. 1ml and 2ml of ethyl acetate was injected separately into the 500ml reactor vessels which contains 0.05N of NaOH and thoroughly mixed. At regular time interval, 0, 10, 20, 30, 40, and 90 minutes, 25ml of each of the samples were withdrawn into a 250ml conical flask containing 0.05N HCl, and titrated against 0.05N NaOH solution using phenolphthalein as indicator. From the titre values, the hydrolysis involving the 1ml ethyl acetate was faster than that of the 2ml ethyl acetate, indicating that the higher the concentration the faster the rate of reaction. The rate constant after evaluation from the graphs was approximately 0.003min-1cm-3 for the 1ml and 2ml ethyl acetate, signifying that while the rate of reaction is concentration dpendent, the rate constant is not dependent on concentration Key words: Alkali, concentration, ester, hydrolysis and reactor INTRODUCTION product is formed. The rate at which the reactant ‘a’ is disappearing is proportional to its concentration at Chemical reaction takes place when a detectable any instance, number of molecules of one or more species have lost their identity and assumed a new form by a I.e. rate (r) (a - x) change in the kind or number of atoms in the Rate (r) = k (a - x) compound and by a change in structure or configuration of these atoms. In this classical Where k = rate constant approach to chemical change, it is assumed that the total mass is neither created nor destroyed when a The concept of rate of reaction is very important in chemical reaction occurs. evaluating chemical reacting systems. It is the core factor in the development of performance models to Chemical kinetics is the part of physical chemistry stimulate reactor functional parameters. that studies reaction rates. The reaction rate for a reactant or product in a particular reaction is Factors which determine rate of reaction intuitively defined as how fast a reaction takes place. a. Availability of reactants and its surface area. For example, oxidation of iron under the atmosphere The greater the surface area of a solid, the is a slow reaction which can take years, but the greater the rate of reaction. combustion of butane in a fire is a reaction that takes b. Concentration: increase in concentration place in seconds. increase the rate of reaction Rate of reaction: The rate of reaction is defined as c. Pressure: increase in pressure results in an the change in the number of molecules of reacting increase in the rate of reaction, if the species per unit volume per unit time. It is also reactants and products are gaseous defined to be proportional to the concentration of d. Catalyst: The presences of a catalyst reacting species raised to a certain power called the generally increase the rate of reaction. There order of reaction. It is usually taken as the rate at are however, negative catalysts that lower which the reactant disappear or the rate at which the the rate of reactions.

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e. Temperature: increase in temperature measure of the rate of reaction, so the greater the increase rate of reaction value of the rate constant, the faster the reaction. Each reaction has a definite value of the rate Rate constant: Rate constant, K quantifies the constant at a particular temperature and the value of speed of a chemical reaction. For a chemical reaction the rate constant for the same reaction changes with where substance A and B are reacting to produce C, temperature and the values do not depend upon the the reaction rate has the form: concentration of reaction but depend upon order of Reaction: A + B C reaction. r = K[A]m[B]n Hydrolysis of an ester such as acetate in presence of a mineral acid: Hydrolysis is a Where K is the rate constant that depends on chemical decomposition involving breaking of a bond temperature and the addition of elements of water. The use of an A is the concentration of substance A in moles per acid catalyst accelerates the hydrolysis. The reaction volume of solution assuming the reaction is taking rate is expressed in terms of chemical composition of place throughout the volume of the solution. the reacting species. Rate constant is the rate of reaction when the i. The hydrolysis of an ester such as ethyl acetate in concentration of each reaction is taken as unity. That the presence of a mineral acid gives acetic acid and is why it is also known as specific reaction rate. It is a ethyl alcohol. CH3COOC2H5 + H2O CH3COOH + C2H5OH H+ Ethyl acetate acetic acid ethyl alcohol Therefore, the kinetics of the reaction can be studied And verifying the constancy of the value of rate by taking a known quantity of ethyl acetate and constant, K or in this case, (a - x) ( - Vo) and a mixing it with a relatively large quantity of HCl. An ( Vo) aliquot of the reaction mixture is withdrawn at different intervals of time and titrated against a K = log standard alkali. Obviously, as the reaction proceeds, 2 the value of alkali required to neutralize the acid (HCl Where, V = initial titre value present as catalyst + CH3COOH produced by o hydrolysis of the ester) progressively increases. = final titre value at the end of the experiment and The fact that this is a first order reaction is Vt = titre value at the various time intervals chosen established by substituting the results in the first order rate expression; As the hydrolysis proceeds, there will be proportional increase in the concentration of acetic acid formed. K = log ii. The hydrolysis of ethyl acetate illustrates a 1 bimolecular reaction that gives sodium acetate and ethanol as the product from which second order rate constant can be calculated:

CH3COOC2H5 + NaOH CH3COONa + C2H5OH Ethyl acetate sodium hydroxide sodium acetate ethanol As the reaction proceeds, each hydroxide ion present in it direct or indirect titration with a standard removed in the formation of ethanol removes one solution of an acid. molecule of ethyl acetate, being the number of moles - The fact that it is a second order reaction can be of either OH or ethyl acetate so removed, the established by the consistency of the values of K concentration of the reactants decreases. If we start determined by substituting the titration results at with equal concentrations of the reactants, the different time intervals in the second order rate concentration of NaOH can be conveniently followed equation as follows: at different time intervals by withdrawing an aliquot from the reaction mixture and determining the NaOH

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K = . slope, the value of the rate constant, K can be evaluated. 3 Experimental procedure: 25ml of 0.05N HCl was Where Vo is the volume of the acid equivalent to the measured with a pipette into each of eight (6) alkali present at the beginning of the experiment and numbered 250ml conical flasks. 400ml of 0.05N Vt is the volume of the acid equivalent to the alkali NaOH solution was placed in the reactor (500ml present at any selected time interval. Obviously, Vt is conical flask immersed in a thermostatic water bath proportional to (a - x) and Vo is proportional to ‘a’ in set at 28oC). At time zero, 1ml of ethyl acetate was the original rate equation for the second order also measured with a syringe into the reaction vessel reaction of TYPE I, viz: and the mixture thoroughly shaken for ten seconds. K = . 25ml of the sample was withdrawn ( and at time 10, 20, 30, 40 and 90minutes after the reaction has gone 4 to completion) and discharged as rapidly as possible If we start with different concentrations of the into the respective 250ml conical flask containing the reactants, the second order rate equation TYPE II 25ml 0.05N HCl already and titrated against 0.05N of should be used for establishing the reaction to be of NaOH solution using two drops phenolphthalein as second order. indicator. The experiment was repeated with the volume of ethyl acetate increased to 2ml at the same K = . log temperature.

5 All reagents used in this experiment are of analar If ‘b’ moles of ester and ‘a’ moles of NaOH are taken grade. initially, then Table 2: Temperature = 28oC and volume of ethyl acetate = 1ml ‘a’ Vo (the volume of acid equivalent to the amount of NaOH present initially) Time Titre Vt - (Vo - Log

(min) (ml) ) (a - x) Vt (the volume of acid equivalent to the amount of NaOH present at time, t)

(a - b) (the volume of acid equivalent to the 0 5.60 -16.70 -16.70 1.00 0.00 amount of NaOH present at the end of the reaction) 10 17.80 -4.50 -16.70 3.71 0.57 X = a – (a - x) = Vo - Vt b= a – (a - b) = Vo - 20 20.90 -1.40 -16.70 11.93 1.08 Therefore, (b - x) = a – (a - b) – [a – (a - x)] 30 21.60 -0.70 -16.70 23.86 1.38 = a – (a - b) – a + (a - x) = (a - x) – (a - b) = Vt - 40 21.90 -0.40 -16.70 41.75 1.62 By substitution, equation 5 becomes

K = . log 90 22.30 0.00 -16.70 - -

K = [ ] o Table 1: Temperature = 28 C and volume of ethyl

Log = + log acetate = 2ml

Time Titre V - (V - Log t o 7 (min) (ml) ) Since this equation is of the form: y = mx + c, this represents an equation for a straight line. 0 9.70 -15.20 -15.20 1.00 0.00 Therefore, a plot of log vs. t gives a straight 10 22.80 -2.10 -15.20 7.24 0.86 line with a slope equal to . From the value of the

20 24.00 -0.90 -15.20 16.89 1.23

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30 24.40 -0.50 -15.20 30.40 1.48 gives a slope which is equal to , from which K is

40 24.70 -0.20 -15.20 76.00 1.88 evaluated: Slope = 0.034min-1 = 90 24.90 0.00 -15.20 - -

Therefore,

( ∞ ) 0.034 = Log ( ) 1.8푡 ∞ K = 1.6 K = 0.0031 1.4 -1 -3 1.2 K 0.003min cm 1 Also, for the 2ml ethyl acetate: 0.8 From equation 7, Log = + log . Using

0.6 table 1, a plot of Log against time in figure 1, 0.4 gives a slope which is equal to , from which K is 0.2 evaluated: 0 Slope = 0.027min-1 = 0 10 20 30 40 50 Tiime (min) Therefore, Fig.2: graph for 1ml of ethyl acetate at 28oC 0.027 =

K =

( ∞2) Log = 0.0028 ( 푡 1.8 ∞ ) -1 -3 1.6 K 0.003min cm 1.4 Conclusion: In conclusion, it can be seen from this experiment that rate of reaction is concentration 1.2 dependent while rate constant is not. Also, that the 1 rate constant for the hydrolysis of ethyl acetate with o 0.8 sodium hydroxide using HCl as a catalyst at 28 C is approximately 0.003min-1cm-3. 0.6 0.4 REFERENCE 0.2 E.U. Akusoba and G.O. Ewelukwa, (2006); Calculations in chemistry for senior secondary schools, Africana first 0 publishers limited, Owerri, Nigeria. Pg 123 – 124 0 10 20Time (min)30 40 50 H.S. Fogler (2008); Elements of chemical reaction o engineering, Fourth edition, Pearson Education Fig.1: graph for 2ml of ethyl acetate at 28 C international, Massachusetts, USA. Pg. 4-7 For the 1ml ethyl acetate: Octave Levenspiel, (2008); chemical reaction engineering, 3rd edition, Choudhary Press, Delhi, India. Pg 27 -28 From equation 7, Log = + log , using S.S. Dara (2009); A textbook of engineering chemistry, S. table 2, a plot of Log against time in figure 2, Chand and company Limited, Ram Nagar, New Delhi.

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