Saturday 12 November 2011

Determination of the activation energy for the reaction of bromide and bromate ions in acid solution.

Objectives

1. To understand the chemistry of activation energy.

2. To determine the activation energy for the reaction of bromide and bromate ions in acid solution.

3. To understand the chemistry of rate of reaction.

Introduction

Activation energy is defined as the minimum energy that must be overcome in order for a chemical reaction to occur. It is usually denoted as Ea, and given in unit of kJ/mol. For a chemical reaction to proceed at a reasonably rate, there should exist an appreciable number of molecules with energy equal to or greater than the activation energy. The concept of activation energy also can be related to the collision theory. According to the collision theory, in order for a reaction to occur, the reactant particles must collide. However, not all collision brings about reaction. Only a certain fraction of the total collisions cause chemical change, which are called successful collision. The successful collisions have sufficient energy to overcome the activation energy barrier at the moment of impact to break the existing bonds and form new bonds, resulting in the production of products.

The activation energy of a reaction can be measured by using Arrhenius equation,

k = A

where k is the rate constant of a reaction with absolute temperature, T, is the energy of activation of the reaction and R is the gas constant. The pre-exponential term, A is the property of particular reaction related to the collision frequency of the reactive species and thus is temperature dependent. However, according to the equation, the dependence of k on temperature is dominated by the strong exponential term, so the dependence of A on temperature is usually ignored as a first approximation. By taking logarithms of both sides,

lo k = - /2.303RT + lo A

= - /2.303RT + constant

So, when a reaction has a rate constant that obeys Arrhenius equation, a plot of lo k versus 1/T gives a straight line. The gradient of the straight line is - /2.303R while the interception of the straight line on the y-axis of the graph can be used to determine the values of lo A.

Now, the rate of reaction is higher when the time taken for a fixed amount of reaction to complete is shorter. This makes the time taken, t to complete a fixed amount of reaction is inversely proportional to the rate constant, k.

t α 1/k

or t = constant/k

By taking logarithms of both sides,

lo t = - lo k + constant

= /2.303RT + constant

A plot of lo t versus 1/T gives a straight line as well and the slope of the graph is /2.303R. Thus, if t is measured at several temperatures then the energy of activation can be found.

In this experiment, the above method is applied to the reaction of bromide and bromate ions in an acid solution which occurs slowly at room temperature.

KBr + 5 KBr + 3 3 + 3 B + 3

or Br + 5 B + 6 3 B + 3

The time required for a fixed amount of the reaction to be completed, t is found by adding a fixed amount of phenol and some methyl red indicator to the reaction mixture. The bromine produced in the first reaction reacts very rapidly with the phenol to form tribromophenol.

+ 3 B OH + 3 HBr

When all the phenol has reacted, the bromine continuously produced in the first reaction will then react with the methyl red indicator and bleaches its colour.

Methyl red + colourless compound

Apparatus : 1 dm-3 beaker, 3 100cm3 beakers, 2 boiling tubes, 1 5cm3 pipette, 1 10-cm3 pipette, thermometer (0 - 110°C), stopwatch

Material : 0.01 mole dm-3 aqueous phenol solution, bromide/bromate solution (0.0833 mole dm-3 potassium bromide and 0.0167 mole dm-3 potassium bromate, equivalent to 0.05 mole dm-3 bromine), 0.3 mole dm-3 sulphuric acid, methyl red indicator.

Procedures

1. 10 cm3 of phenol solution and 10 cm3 of bromide/bromate solution were pipette into one boiling tube.

2. Four drops of methyl red indicator were added to the mixture.

3. 5 cm3 of sulphuric acid was pipette into another boiling tube.

4. The two boiling tubes were immersed in the water bath of (75 ± 1) °C.

5. The contents of the two tubes were mixed by pouring rapidly from one tube to the other twice and the stopwatch was started at the same time.

6. The boiling tube containing the reaction mixture was kept immersed in the water.

7. The time required for the red colour of the methyl red indicator to disappear was determined.

8. The whole experiment was repeated at 65, 55, 45, 35, 25 and 15 °C.

9. Ice was used to achieve the lowest temperature.

Results & Calculations

(Assume R = 8.314 J K-1 mol-1)

From Graph 1,

Slope of the graph = Ea / 2.303R

Ea / 2.303R = (2.94 – 2.20) / (3.35×10-3 – 3.10×10-3)

Ea /2.303R = 0.74 / (2.50×10-3)

Ea = 2960*2.303R

Ea = 2960*2.303*8.314

Ea = 56676 J/ mol

Energy of activation, Ea = 56676 J /mol

Discussion

The reaction between bromide and bromate ions in acid solution is a slow reaction at room temperature. This may be due to the high activation energy of the reaction, which required 56.676 kJ of energy in order for a reaction to proceed favorably. The high activation energy is one of the factors that reduce the effectiveness of the collisions in bringing about reaction because according to collision theory, it is not sufficient that molecules simply collide can brings about reaction to occur. Colliding molecules without energy equal to or higher than activation energy will not cause any reaction to happen. There are also other factors that can reduce the effectiveness of collisions of molecules such as the order of colliding molecules which are not included in the objectives of this experiment.

According to the data obtained in Table 1, we can observed that the higher the temperature of the reactant species, the shorter the time taken for the disappearance of red colour of methyl red indicator. The shorter time taken indicates that the reaction is faster and higher rate of reaction. Although the activation energy for the reaction to occur remains the same at all the temperature and unaffected, but the rate of reaction increased as the temperature increased. This means that the rate of reaction is dependence on the temperature of the reactant species and this is indeed the case. This has also been proven in Arrhenius equation where rate constant, lo k is proportionally to 1/T with the slope of the graph, - /R and a constant of lo A.

lo k = - /2.303RT + lo A

The higher temperature caused the value of the - /2.303RT closer to the value of 0. The constant lo A will then minus off the value of /2.303RT and results in a larger value. The larger value will caused the value of the rate constant, k to become larger as well. Larger value of k will then results in faster reaction. Hence, it can be said that the higher the temperature of reactants, the reaction will proceed faster with the higher rate of reaction.

Besides, the rate of reaction roughly doubles for every 10 °C rise in temperature. This is because increase in temperature increase the kinetic energy possessed by the reacting molecules, hence increase the speed of the reacting molecules. With the higher speed of moving, the reacting molecules will now collide more frequently and increase the amount of successful collision. As the kinetic energy of the molecules increase, the molecules can therefore easily overcome the activation energy barrier during the collision and bring about reaction to occur. Hence, the higher the temperature, the higher the kinetic energy possessed by the molecules, the faster the molecules move, the higher the frequency of collision, the easier the molecules to overcome the activation energy barrier at the moment of impact, the higher the amount of successful collision, the faster the reaction and therefore the higher the rate of reaction.

Fixed amount of phenol and methyl red indicator were added to the mixture contents every time the experiment repeated. This is because phenol can provides an intermediate state before the bromine molecules produced in the reaction between bromate and bromide ions in acid solution able to bleach the methyl red. In other words, phenol is used to observe the time taken for the bromine molecules to react completely with phenol and then bleach the colour of the methyl red indicator. The methyl red indicator was added to provide a distinct colour to ease the observation. This is because when the sulphuric acid was poured to the bromate/bromide ions solution, the methyl red indicator turns to pink colour. The bleaching effect from the bromine molecules caused the methyl red indicator to turn colourless after all the phenol is used up and this provides a clear difference of colour in order to stop the stopwatch. The amount of phenol added must be fixed in every repeated experiment in order to compare the time taken for the disappearance of colour of methyl red.

Phenol is chosen to be used to provide an intermediate state because phenol can react with bromine molecules very fast to produce tribromophenol and hydrogen bromide before the bromine molecules react with the methyl red.

image

This is very important because if other substances are chosen to replace phenol, the substance chosen might not react with the bromine so fast and caused the bromine to react straight away with the methyl red, bleaches its colour without going through the intermediate phase. The –OH group in the phenol has the effect of making the benzene ring much more reactive than it would be with a 2, 4-directing effect. This means that the incoming groups will normally go into the 2-position or the 4-position, but hardly go into the 3-position because of the 3-isomer is produced too slowly. The bromine molecules undergo substitution reaction in this reaction by substituting three hydrogen atoms from the benzene ring with three bromine atoms to maintain the aromaticity of the ring in phenol.

The reaction between bromate and bromide ions in acid solution is a redox reaction.

Br + 5 B + 6 3 B + 3

The potassium and sulphate ions act as spectator ions in this experiment and they are not involved in any of the redox reaction or changing of their state. The bromide ions undergo oxidation by donating one of its electrons to the bromine atom in the bromate ions. The bromine atom in the bromate ions then undergoes reduction by receiving electron from the bromide ions. The hydrogen ions and oxygen atom in bromate ions does not involve in increase or decrease in oxidation number but they were involved in changing the state from the ions in aqueous solution to the water molecule in liquid state.

On the other hand, there are some precautions steps that we need to take into considerations during the experiment. Care must be taken when using phenol and sulphuric acid as they can cause burn to the skin. Gloves and protective cloth must be worn at all times in the laboratory. The boiling tubes containing the mixture should be kept immersed in the water to ensure that the mixture remains its temperature while the reaction proceeding.