Lab Report 4 (Physical Chemistry)

EXPERIMENT 4: ACTIVATION ENERGY OF AN IONIC REACTION

Objectives 1. To investigate the rate of reaction of Peroxydisulphate ions (S2O82-) with large excess of iodide at temperature between 30oC and 70oC. 2. To determine the activation energy of the reaction. 3. To show how faster the reaction is proceeding.

Abstract Activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration. peroxydisulphate ions start to oxidize to the iodide and to liberate iodide as soon as the solution is mixed. The thiosulphate immediately reacts with the generated iodide, so no color is seen for as long as there is thiosulphate in solution. When all the thiosulphate has reached any further iodide generated gives a color change. Because the same volume of the reagents is used at each temperature, the same fraction of the origin peroxydisulphate has reached each time when the color appears. Thus, the graph In1/τ against 1/T will get linear graph and the activation energy for reaction can be determined. Peroxydisulphate ions are powerful oxidizing agents and react with iodide to form iodine via the overall reaction. The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants rose to some powers. The rate of reaction is dependant on the concentration of the peroxydisulphate and iodide ions. The rate of reaction will appear to depend only on the concentration of peroxydisulphate ions. Because the relationship of reaction rate to activation energy and temperature is exponential, a small change in temperature or activation energy causes a large change in the rate of the reaction. Many reactions have rate laws which depend only on the concentrations of the reactants :

rate = k[A]x[B]y

Introduction In order to react, the colliding molecule must have total potential energy equal to or greater than the activation energy (Ea), which is the minimum amount of energy required to initiate a chemical reaction. The species temporary form by the reactant molecule as the result of the collision before they form the product is called the activation energy. A rate law describes the behavior of a reaction. The rate of a reaction is a function of temperature (through the rate constant) and concentration.

Peroxydisulphate ions are powerful oxidizing agents and react with iodide to form iodine via the overall reaction:

S2O8- + 2I-

2 SO4- + I2

The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers The rate of reaction is dependant on the concentration of the peroxydisulphate and iodide ions. The rate of reaction will appear to depend only on the concentration of peroxydisulphate ions. Reaction order is always defined in terms of reactant (not product) concentrations. The rate is:

-d [S2O8-] = k’[S2O8-]

Ea = action energy of reaction (J/mol) R = molar gas constant (8.314 J/K•mol) T = the absolute temperature A = frequency factor .

k = A exp

In 1/τ

m=

1/T T

In this experiment peroxydisulphate ions start to oxidize to the iodide and to liberate iodide as soon as the solution is mixed. The thiosulphate immediately reacts with the generated iodide, so no colour is seen for as long as there is thiosulphate in solution. When all the thiosulphate has reached any further iodide generated gives a color change. Because the same volume of the reagents is used at each temperature, the same fraction of the origin peroxydisulphate has reached each time when the color appears. Thus, from graph above, activation energy for reaction can be determined.

Materials 1. 2. 3. 4.

0.2 M Potassium Iodide (KI) 0.05 M Sodium Thiosulphate (Na2S2O3) 0.1 M Potassium Peroxydisulphate (K2S2O8) Iodine Indicator

Equipment 1. Beaker 2. Test Tube (10 test tubes) 3. Thermostat

Procedure 1. In this experiment, five thermostatically controlled water-baths are being used. The temperatures used are 30, 40, 50, 60 and 70ºC. By using a mercury thermometer immersed in the water, a more accurate measurement of water temperature was obtained. 2. Five test tubes were obtained and they were labeled with the differences temperature. 5 cm 3 of 0.05M sodium thiosulphate (Na2S2O3) and 10 cm3 of 0.2 M potassium iodide (KI) were placed in five test tubes. All the test tubes were placed in the 5ºC ice bath. 3. Other five test tubes (new test tubes) were placed in with 10 cm 3 of 0.1 M Potassium Peroxydisulphate (K2S2O8) and 5 drops of iodine indicator were added into them. All the test tubes were placed in the plastic rack in the 30, 40, 50, 60 and 70ºC of water-bath. 4. By using a thermometer to monitor the temperature, the contents of the tubes were allowed to reach the temperature of the thermostat. 5. The temperature of the water bath was recorded. The contents of one Erlenmeyer flask were poured quickly into the test tube, and then the stopwatch was started. The reactant was stirred and the time taken for the colorless solution to turn dark yellow was recorded. 6. The results were recorded and graph ln 1/τ against 1/T was plotted.

Results and Discussion Results:

Temperature,

303.15

313.15

323.15

333.15

343.15

Time, t (s)

302

195

131

84

59

1/T (K-1)

0.0033

0.0032

0.0031

0.0030

0.0029

1/τ

0.0033

0.0051

0.0076

0.0119

0.0169

ln 1/τ

-5.7138

-5.2785

-4.8796

-4.4312

-4.0804

T(K)

TABLE 1

GRAPH ln 1/τ AGAINST 1/T 0 -1 -2 ln 1/τ (s¯¹)

-3 -4 -5 -6 1/T (K¯¹)

FIGURE 1 Calculations: From Figure 1,

Slope , m

=

∆Y ∆X

=

−4.0804−(−3.7) 0.0029−0

= -131.17 From the value of the gradient, m we can calculate the activation energy, R = 8.314 J/mol.K m

=

−E a

−

Ea R

= mR

= (-131.17) (8.314) = - 1090.547 J/mol 

Ea

= 1090.547 J/mol

Ea

.

Discussion: Based on the graph in FIGURE 1, we can conclude that ln 1/τ is directly proportional to the 1/T. When the value of 1/T increases, the ln 1/τ also increases. This is because according to the Arrhenius equation, the rate of reaction depends on the temperature. Arrhenius Equation:

k = A exp

ln k = ln A -

[ ] −E RT

E RT

Where E is the activation energy of the reaction, R is the molar constant (8.314 J/mol.K) and A is known as the A-factor, which is related to the frequency of the collision between the reacting molecules. We have already shown that k is proportional to 1/τ. From this theory, high temperature will made the time of solution taken to turn color from colorless to dark yellow is short because higher and greater number of collision that consequently the rate of reaction is faster. These means that the reaction of solution is depends on temperature. The rate of the reaction can be determined by the reciprocal of time because a constant amount of iodine is produced in each reaction and a graph of the ln 1/τ against 1/T can be plotted. This graph can be used to determine the order of the reaction. Based on our graph, the gradient that we get from the graph in FIGURE 1 is m= -131.17. To calculate activation energy (Ea) of the reaction, we can use the formula of gradient = Ea/R. Value of R is given as 8.314 J/mol.K. Finally, we obtained Ea = 1090.547 J/mol. Then, the objectives of the experiment were achieved.

Conclusion & Recommendations Conclusion: The factor affecting rate of a chemical reaction is temperature and time. Time decreases as temperature increases. That’s why the rate of reaction increases with the rise of temperature. The other factor is temperature. It can be explain by the fact that at higher temperature, a greater fraction of colliding molecules posse the necessary energy of activation. Thus, the graph In 1/τ against 1/T will get linear graph and the activation energy for reaction can be determined that’s is 1090.547 J/mol. By doing this experiment, the rate reaction of peroxydisulphate with large excess of iodide at a number of temperatures between 30 oC and 70oC can be investigated. For the precautions, never transfer the thermometer from one tube to the other because to avoid contamination of the solution. Overall of this experiment can be concluding that all objective of this experiment is success.

Recommendations: 

Use a different amount of peroxydisulphate ion S2O8for same temperature. We can investigate the rate of reaction from the graph amount of peroxydisulphate ion against the temperature.



Use iodide in small concentration than peroxydisulphate, so the concentration of iodide will easily change during the experiment.



Use catalyst to make the reactant become more faster to turns into dark yellow.



Use a different scale of thermometer, so we can avoid transferring the thermometer to wrong tube.

For safety, 

Always wear protective gloves and glasses during the experiment.



Waste solutions were placed in large beaker provided.



Solutions must not easily pour into the sink because they were strongly oxidizing solutions.



Tubes must always be in the water bath as long as the solutions do not turn into dark yelllow in color.



Use thermometer only for the same solutions.

Questions 1. How good is the approximation that the iodide concentration remains constant? Use evidence both from the experimental method and from your results to justify your argument. In this experiment, we want to investigate the rate of reaction of peroxydisulphate ions (S2O82-) when it reacts with iodide ion to form iodine. Actually, the rate of reaction for this reaction is depends on the concentrations of peroxydisulphate and iodide ions. If iodide is present in much higher concentration than peroxydisulphate, then the concentration of iodide will hardly change during the experiment. This will affect the rate of reaction. Therefore, the rate of reaction in this experiment will depend on the concentration of iodide. This is totally opposite with our objectives. Hence, to achieve our objectives we must constant the concentration of iodide ion so that the rate of reaction for this experiment will depend only on the temperature and time. Because of that, we must use different temperature of waterbath to prove this experiment. As the same volumes of reagents are used at each temperature, the same fraction of the original peroxydisulphate has reacted each time when the color appears. Then, we had plotted the graph In 1/τ against 1/T and we get linear graph. From the graph, we can prove that when temperature is higher, the time taken for the iodine, τ, to change color is shorter. Thus, the rate of reaction for this reaction will become faster. This situation will help us a lot and much better for our experiment.

References  

http://en.wikipedia.org/wiki/Activation_energy Internet sources: Experiment 5: Fast, Faster, Fastest: Exploring Chemical CatalysisLarsen



4:20 Spring 2000. Internet sources: Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann



Arbor, Michigan, 1990. Lecture note Chemistry Module Semester 2 Kolej Mara Kuala Nerang, 2010 topic of Reaction Constant.