Experiment 17 Determining a Rate Law and Rate Constant Chemistry 1220 136 September 24th 2013 Raffeal Bennett Purpose The purpose of this experiment was to understand the different rate laws and be able to determine a rate law and rate constant using absorbance measures Procedure Refer to pages 41 43 of General Chemistry 1220 Laboratory Manual by The Ohio State University Department of Chemistry 2013 2014 Calculations Please see attached calculations Discussion By evaluating the absorption of light of a substance the rate constant and a reaction order can be determined As Cr H20 63 is combined with H2EDTA2 one of the products of the reaction is CrEDTA can be determined and used to calculate how fast the reaction is occurring First to do so the absorbance of CrEDTA after the complete reaction occurs needs to be established Then using this and the known concentration of each solution a Beer s law graph can be made and evaluated The slope of this graph yields a value equal to the molar absorptivity times the path length of light through the solution Experimentally this value was determined to be 183 01 Then by dividing m by the absorption at each time the concentration was determined The first was a zero order graph that was chromium ion concentration vs time the second was the first order graph which is the natural log of the vs time and finally the second order graph which was one over the natural log of the concentration of the chromium ions By comparing these graphs the order of the reaction can be discovered The graph with the lines that are most parallel is the correct reaction order In this experiment the reaction was a first order reaction which is illustrated on the graphs The slopes that are being compared are actually the rate constant for the reaction To obtain a more accurate rate constant the average of all the slopes was taken creating a final rate constant that best represents the data The rate constant in this experiment was 00925m 1 plus or minus 0007 There were a number of factors that cause potential error First was the error in the measuring instruments The machine that read the absorbance had an uncertainty of plus or minus 001 Moreover the buret used to measure each chemical into the test tubes had an uncertainty of plus or minus 01 mL The cause of the majority of the error came from errors in technique First readings were taken every 5 minutes However whenever the solutions were not being read the temperature affected the reaction rate which in part had a large part to do with the readings taken and their accurateness Another potential error was the cuvette Two sides of the cuvette were not fully transparent If this side was facing the absorbance reader it would cause disruption of the data Conclusion In this experiment the rate constant of a reaction and the order of the reaction were successfully calculated This was done by relating the absorption of light to the completeness of the reaction Additionally zero first and second ordered graphs of the reaction were effectively created and used to determine the correct order of the reactions Lastly the experiment provided an example and practical use of Beer s law and how it relates to rate of reaction and the reaction constant Without creating a proper Beer s law graph the rate constant could not have been computed