I can also support this with points off the graph:
As seen above, when the concentration doubles so does the rate of reaction. Henceforth the rate of reaction is directly proportional to the concentration.
Therefore: [H2O2]1
The overall rate equation is therefore:
Rate = k [I-]1 [H2O2]1
The overall order of reaction is order 2, because the sum of the two orders equals 2. This is known as an overall second order reaction.
Determining the Rate Constant: Varying the Concentration of Potassium Iodide
Now that I have proved the rate equation experimentally, I want to find a value for the constant, ‘k’. This is called the rate constant and its value remains the same when concentrations are changed. Using my results I should be able to calculate a value for ‘k’. I will use the following equation:
In this experiment I varied the concentration of Potassium Iodide while the concentration of Hydrogen peroxide was kept constant at 1 mol dm-3.
The mean value for k, after ignoring the highlighted anomaly is:
Mean value of K= 0.789584
Determining the Rate Constant: Varying the Concentration of Hydrogen Peroxide
In this experiment I varied the concentration of Hydrogen Peroxide while the concentration of Potassium Iodide was kept constant at 0.2 mol dm-3.
The mean value for k, after ignoring the highlighted anomaly is:
Mean value of K= 0.457721
Determining the Rate Constant: Varying the Temperature of Reaction
Now that I have proved the rate equation experimentally, I want to find a value for the constant, ‘k’. This is called the rate constant, its value changes with temperature. Using my results at a certain temperature I should be able to calculate a value for ‘k’.
In this experiment I varied the temperatures, while the concentration of all reactants was kept the same throughout. Hence the only variable was temperature.
From the calculations made it is clear that the value of k does not change with a change in concentration, however with a change in temperature it does. This is positive as my research had told me that the value of k only changes with temperature and my results back this up.
The Reaction Mechanism:
Many reaction mechanisms involve numerous different steps. The rate equation I have just calculated gives me the information needed to determine the slowest step in the reaction mechanism, often referred to as the rate determining step. Below is the rate equation I calculated.
Rate = k [I-]1 [H2O2]1 [H+]0
This states that the reaction is first order with respect to hydrogen peroxide, first order with respect with respect to iodide ions and zero order with respect to hydrogen ions. Therefore the rate equation can also be written as.
Rate = k [I-]1 [H2O2]1
As H+ ions do not occur in the rate equation, it is therefore possible to state that H+ ions do not take part in the rate determining step.
After research I have been able to identify the reaction mechanism for the reaction between Hydrogen Peroxide and Potassium Iodide.
Step 1: H2O2 + I- H2O + IO- (slow)
Step 2: H+ + IO- HIO (fast)
Step 3: H+ + I- + HIO I2 + H2O (fast)
Explanation:
Step 1 is the rate determining step because it is the slowest step, this means that step 1 controls the rate of the whole reaction. This explains why the rate of reaction only depends upon the concentration of hydrogen peroxide and the concentration of iodide ions. Step 1 involves 1 molecule of hydrogen peroxide colliding with 1 iodide ion, this is why the reaction is first order with respect to hydrogen peroxide and Iodide ions. Since the hydrogen ions do not become involved in the rate determining step, the reaction is zero order with respect to hydrogen ions.
Calculating the Activation Enthalpy:
This is the equation for calculating the activation enthalpy for a given reaction.
First I needed to plot the graph of log 1/t against the log of 1/T. After I calculated the gradient of the graph (this graph can be found in the results section).The step by step calculations are below.
This means that the reactants in my reaction need to have at least this amount of energy in order for the reaction to occur. The energy must be sufficient enough to break the bonds in the reactants, and form new bonds in the products. Only those particles with enough kinetic energy on collision will overcome the activation enthalpy for the reaction. Therefore at least 29.19 kJ are required to break the bonds in reactants and form new bonds within the products.
Activation Enthalpy Further:
As I earlier mentioned in my research, a 10K increase in temperature, doubles the rate of reaction. Therefore I can analyse the number of particles with activation enthalpy at temperatures with a 10K difference.
To do this I will use the following equation.
At 20oC (293.15K) the number of particles with activation enthalpy is:
At 30oC (303.15K) the number of particles with activation enthalpy is:
As visible there is not a double in the number of particles with activation enthalpy, for a double there would be a 100% percentage increase. However these unpredicted results could be due to an error in equipment which I will later discuss. To test if this is due to an error I shall work out the number of particles with activation enthalpy at two different temperatures with a 10K difference.
At 40oC (313.15K) the number of particles with activation enthalpy is:
At 50oC (323.15K) the number of particles with activation enthalpy is:
Once again the number of particles with activation enthalpy does not double with a 10K increase in temperature. My results seem to suggest a 70% increase rather than a 100%. These results can be explained due to percentage error of equipment I used and also human error. I shall examine this in my evaluation. If the results of my experiment had worked as the Maxwell-Boltzman Curves suggest then my results would have been as below: