Kinetics of a Chemical Reaction: A Clock Reaction

Chemical kinetics involves the study of reaction rates, the factors affecting these rates, and the mechanisms by which reactions proceed. Using a clock reaction method, this experiment examined the kinetics of the reaction between iodide ions (I⁻) and peroxy-disulfate ions (S₂O₈²⁻). The primary reaction studied is 2I^- + S₂O₈²⁻ →I₂ + 2SO^2-_4. This relatively slow reaction rate depends on the reactant concentrations, temperature, and presence of catalysts or inhibitors. Rate= k gives the rate law for this reaction [I^-]^m [S₂O₈²⁻]. A secondary reaction involving thiosulfate ions (S₂O₃²⁻) serves as a clock to measure the reaction rate: S₂O₃²⁻−+I₂→S₄O₆²⁻+2I⁻ Starch is added to the reaction mixture, turning blue when S₂O₃²⁻ is consumed, allowing the determination of the reaction rate through color change timing.

Objectives

The primary objectives of this experiment were to:

1. Determine the reaction order of the iodide and peroxy-disulfate ions.
2. Investigate the effect of temperature on the reaction rate.
3. Examine the influence of potential catalysts or inhibitors on the reaction kinetics.

Methods

Materials/ Apparatus Used and:

• • Solutions of 0.20% starch, 1.2x10⁻² M Na₂S₂O₃, 0.2 M KI, 0.2 M KNO₃, 0.2 M (NH₄) ₂S₂O₈, and 0.2 M (NH₄) ₂SO₄
  • The apparatus used was Two 50 mL Erlenmeyer flasks labeled "A" and "B." A thermometer, stopwatch, and water baths for temperature control

Procedure

We conducted the experiments by following these steps:

1. Preparation of Solutions:

We prepared solutions in two flasks, labeled "A" and "B," according to the specified concentrations.

1. Room Temperature Experiments:

We mixed the solutions from flasks "A" and "B" together and started the timer immediately upon mixing.

1. Temperature-Controlled Experiments:

We placed flasks "A" and "B" in water baths set to the desired temperatures and allowed them to equilibrate for 15 minutes before mixing.

After equilibration, we mixed the solutions and started the timer immediately.

1. Recording Observations:

We recorded the time for the color change to occur in each experiment.

1. Repetition for Accuracy:

To ensure accuracy, we repeated each experiment twice and documented all observations meticulously.

1. Temperature Variations:

We conducted experiments at approximately 10°C, 30°C, and 40°C …. using water baths. Results were recorded in table1.

1. Catalyst/Inhibitor Addition:

A drop of Ag⁺ or Cu²⁺ solution to some runs and compared with control runs. Results were recorded in Table 2

The reaction rate was calculated using Equation 1.

ate=1/2 Δ[S₂O₃²⁻]/ Δt Equation 1

Results and Discussion

The initial and final concentrations of S₂O₃²⁻, reaction times, temperatures, and calculated rates are summarized in Table 1.

Table 1

Chemical reaction before adding catalyst

Table 2

Chemical reaction after adding catalyst

1. Determine the Order of the Reaction:

To find the order of the reaction concerning iodide ions (I⁻), we used the reaction rate data from experiments 1 and 2, where the temperature is constant at 283 K:

Experiment 1: Rate = 8.33 × 10⁻⁴M/s, K= 1.67 × 10²s^-1

Experiment 2: Rate = 1.11 × 10⁻³ M/s, k=2.22 × 10² s^-1

Assuming the concentration of S₂O₈²⁻ is constant, the rate law is:

Rate= k [I^-]^m

Calculating the ratio of the rates:

Rate₂/Rate₁= k [I^-]^m/ k [I^-]^m

1.11 × 10⁻³/8.33 × 10⁻⁴= (0.421/0.421) ^m

1.33=(I^-)/(I^-)

Since the iodide ion concentration remains the same, the reaction order m is;

M=log (1.33)/ log (1) =1

Therefore, the reaction is first-order concerning iodide ions (I⁻).

1. 2. Plot a graph of reaction rate(M/s) versus temperature (K), then determine the Effect of Temperature on the Reaction Rate

Figure 1 A Graph showing the effect of temperature on the reaction rate

The reaction rate increases with temperature (Punith et al., 2021). This indicates that the reaction rate positively correlates with temperature, suggesting an activation energy effect where higher temperatures increase the reaction rate.

1. 3. Using a graph, show and state the effect of catalysts/ inhibitors on the rate of chemical reaction.

Figure 2 Effect of catalyst in a reaction rate

Adding Ag⁺ and Cu²⁺ catalysts increase the reaction rate compared to the control, with Cu²⁺ being more effective (Chen et al., 2022). At 283 K and 293 K, the reaction rates with Cu²⁺ are significantly higher than those with Ag⁺. Both catalysts reduce the reaction time, indicating their role in speeding up the reaction.

The outcomes of this experiment reveal how factors such as temperature and the presence of catalysts affect the reaction rate of iodide ions (I⁻) and peroxy-disulfate ions (S₂O₈²⁻). The determined reaction order of one concerning iodide ions (I⁻) is consistent with the rate law as the reaction rate is directly proportional to the concentration of I⁻. The experiments carried out at different temperatures show that the rate of the reaction increases with the increase in temperature, as postulated in the Arrhenius equation, where higher temperatures are associated with more kinetic energy of the reactant molecules. Thus, they overcome the activation energy barrier and react more readily. The presence of catalysts, Ag⁺ and Cu²⁺ ions were found to enhance the reaction rate even more than when the reaction occurred in the absence of the two; the Ag⁺ ion was found to be more effective than the Cu²⁺ ion. This observation also emphasizes the fact that catalysts help decrease the activation energy and increase the rate of the reaction while they are not used up.

Conclusion

In this experiment, the clock reaction method was employed effectively in the study of the kinetics of the reaction between iodide and peroxy-disulfate ions. It was also established that the reaction was first-order with respect to iodide ions, which indicated a direct relationship with the concentration of these ions. Fluctuations in temperature showed that the reaction rate is proportional to the temperature and, hence, the activation energy effect. The Ag⁺ and Cu²⁺ catalysts improved the reaction rate; however, the Ag⁺ catalyst showed a higher increase in the reaction rate than Cu²⁺. These results clearly indicate the effect of the concentration of reactants, temperature, and the use of catalysts on reaction rates. They are, therefore, helpful in understanding the processes that govern chemical reactions.