- When you are ready to start the experiment, pour the potassium peroxodisulphate(VI) solution into the mixture in the beaker. Immediately start timing, and carefully stir to ensure that everything is properly mixed. Record the time taken for the blue colour to appear.
- Repeat the procedure with mixtures 2 – 5, and record the time taken for the blue colour to appear in each case.
DCP:
Amount of substances needed for carrying each measurement were recorded in Table 1.
Later on the experiment was carried and the time results were recorded in Table 2.
The average value of time was calculated:
taverage=
Rounded to 1 decimal place
Also the uncertainty of the measurement was calculated according to the formula:
Uncertainty=
The results were recorded in Table 3.
Rounded to 1 decimal place
To determine the order of reaction using the iodine clock method, some of the calculation were necessary.
At first, I calculated the [I-] concentration in each of 5 solutions using the equation:
C[I-]= where VKI is the volume of KI solution
[KI] is the KI concentration
Vtotal is the total volume of solution
On the example of 5th mixture:
C[I-]=
Having calculated the concentration of [I-] ions, it is important to determine whether any reagent is in excess. To do that, I needed to calculate the concentration of iodide ions and the concentration of peroxodisulphate(VI) ions in mixture 5th.
C[I-]=
C[S2O3 2-]=
It can be seen that the [I-] ions are in excess. The calculated S2O3 2- concentration shows and determines how many moles of iodine will be produced during the reaction.
To calculate the amount of iodine moles that will be produced I need the calculations above.
I will use the formula:
n= and the equation S2O82-(aq) + 2I-(aq) → 2SO42-(aq) + I2(aq)
n[iodine]= ==0.00008 moles
So 8*10-5 moles of I2 will be produced.
Next, having the values for KI and K2S2O8 I needed to calculate the number of moles of thiosulphate.
n===0.00002 moles are added in each mixture
This value will be needed for calculation of iodine moles that will be used by S203 2- . According to the equation:
2S2O32- (aq) + I2 (aq) → S4O62- (aq) + 2I- (aq)
We can see that:
2 moles S2O32- — 1 mole of I2
0.00002 moles — x
X=0.00001 moles = 1*10-5 moles of I2
1*10-5 moles of I2 are used by S2O32- in this reaction.
Since I have all necessary values, I am now able to calculate the initial rate of this reaction.
Rate of reaction=
Graph1: The relationship between the iodide ions concentration and rate of the reaction.
The shape of the curve on the graph indicates that it is the reaction of first order.
Conclusion and evaluation:
The aim of the experiment was to find the order of the reaction using the iodine clock method. Using raw and calculated data I managed to determine its order and compare it with the literature. Rate of reaction and time in which it occured were necessary for the experiment.
The graph showing the relationship between the iodide ions concentration and the rate of reaction was the key concept for establishing accurate order of reaction. The range of [I-] concentration was not very wide, but more than satisfying. The rate of reaction in the given range increases constantly but not directly proportional. The shape of the curve on the graph allow me to recognize the first order of reaction. Sadru Damji in "Chemistry 2nd Edition" present more accurate graph for the first order of reaction:
rate
[A]concentration
As can be observed, the graph I created and that presented by Sadru Damji follows similar trend. My graph is not directly proportional, but its shape indicates it's a first order reaction.
Due to some errors occured in experiment, the disparency might be observed:
- Small volumes of reactans made it quite difficult to notice the first moment that blue colour occured
- The temperature was not measured; as it is a major factor that influences the rate of reaction the temperature had a strong influence
- The mixtures were not stirred during measurements as it wasn't in the method; there was just said to stir it to mix the reactants
- Once made starch solution was not completely soluble, the sediment was present and there is a posibility that during measuring the volume of solution some of the sediment got into pipette and was used in the reaction
To improve the experiment and the results obtained I could have:
- Take bigger volumes of reactants in order to have better access to the beaker and to better see the blue colour appear
- Change the proportions and calculate orders with respect to different reactants
- Use newly made starch solution and use the liquid part of it only
- Some changes to the method could be made, for example monitoring the temperature and keeping it stable or stirring the mixtures during measurements
References:
-
Geoffrey Neuss "Chemistry for the IB DIPLOMA"
-
Sadru Damji "Chemistry, 2nd edition"