When performing part 2 of this lab a group of students forgot to place the test
ID: 1068329 • Letter: W
Question
When performing part 2 of this lab a group of students forgot to place the test tubes Into the cold-water bath for 10 min before starting the reaction. Only after adding persulfate they placed the test tubes Into the cold-water bath. How will this mistake affect their data (elapsed time) and their results (rate constant)? It is possible to determine k' with a single measurement? Explain how. Why did we bother to perform multiple measurements to determine k if we can do It with a single measurement? Be specific.Explanation / Answer
Ans
2.
The purpose of putting both the test tubes (one containing I- solution and the other containing S2O82-solution) in the cold bath for 10 minutes is to equilibrate the temperatures of both the solutions to a low temperature. At low temperatures, the reaction occurring between the two reactants (when they are mixed) will be significantly slow and the time taken to bring about a colour change will be significantly higher. However, if the test tubes are not allowed to stand in the cold-bath before mixing, then the solutions in the test tube will be at room temperature. Upon mixing, the solutions will react much faster (the time taken will be almost same as the time taken for the colour change at room temperature) and hence the time noted will be much lower than it should have been. Since the rate constant is defined as the rate of change of concentration with time and the time noted is erroneously short, the calculated rate constant will be much higher.
3a) It is indeed possible to determine k’ from the instantaneous rate of a reaction. The change in concentration of a reactant at a particular instant of time can be used to calculate k’.
b) The instantaneous rate of a reaction varies with time and hence k’ will also vary. The value of k’ obtained by plotting logarithm of rate vs time gives a time-averaged rate constant which remains constant over the course of the reaction. Hence, this method is a more accurate method for calculating k’.
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