The purpose of this lab is to determine the rate constant and order of reaction for an acid-base indicator, bromophenol blue. Bromophenol blue (BP) displays a color change from yellow to purple in the pH range of 3 to 4.5. In strong alkaline solutions, the dye becomes unstable and decomposes due to a reaction with hydroxide ions, as follows: . UV-vis spectroscopy will be used to record the absorbances at the wavelength of maximum absorbance.
The full differential rate law can be derived from the above reaction: , where k is the rate constant and x and y are the orders of reaction for bromophenol blue and the hydroxide ions respectively.1 However, a large excess of hydroxide will be used, allowing the concentration to remain constant throughout the experiment and allowing one to simplify the equation with respect to BP. The rate law for the disappearance of bromophenol blue is derived: .
[...] The R2 value is not very close to denoting that the order of reaction with respect to bromophenol blue is not zero order. Graph The first order plot provides a fairly accurate linear fit. The R2 value is almost identical to denoting that the order of reaction with respect to bromophenol blue is first order. Graph The second order plot does not provide an accurate linear fit. The R2 value is not very close to denoting that the order of reaction with respect to bromophenol blue is not second order. [...]
[...] This could also have led to an inaccurate starting point for the absorbance readings. Yet, as stated previously, these errors were minimal and had little effect on the lab itself. This shows that the lab was performed with high accuracy and good technique. Conclusion: The purpose of this lab, to determine the order of the reaction of bromophenol blue with hydroxide ions as well as the rate constant, was successfully achieved in this experiment. The individual orders of reaction for bromophenol blue and [...]
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