CHM 101/102 Laboratory Manual Kinetics General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington

CHM 101/102 Laboratory Manual Kinetics Purpose Purpose  To determine the rate law and the value of the reaction rate constant for a chemical reaction using the method of initial rates. Safety Considerations Safety Considerations  Wash your hands thoroughly after handling chemical reagents and before leaving the lab.  All chemical wastes should be placed in the labeled jar in the hood.

CHM 101/102 Laboratory Manual Kinetics Background Background  The chemical reaction being studied in this experiment is: 6 I - + BrO H + 3 I 2 + Br H 2 O  The general rate law for this reaction can be written as: Rate = = k [I - ] x [BrO 3 - ] y [H + ] z tttt  [BrO 3 - ]  By measuring the rate of the reaction as we vary the concentrations of I -, BrO 3 -, and H +, we can determine the values of x, y, and z. bromate ion

CHM 101/102 Laboratory Manual Kinetics Background Background  Since BrO 3 - is colorless, we need some way to determine when a certain amount of BrO 3 - has reacted.  When all of the S 2 O 3 2- is used up, any I 2 that remains will react with the starch to produce a deep-blue colored complex.  To do this, we will add S 2 O 3 2- and starch to the reaction mixture. S 2 O 3 2- reacts very rapidly with I 2 as shown below: I S 2 O I - + S 4 O 6 2- tetrathionate ionthiosulfate ion

CHM 101/102 Laboratory Manual Kinetics Background Background  The concentration of S 2 O 3 2- in each trial is 2.0x10 -4 M. Since there are 6 moles of S 2 O 3 2- reacted for every 1 mole of BrO 3 - reacted, then the concentration of BrO 3 - reacted in each trial is 3.3x10 -5 M.  By measuring the time it takes for the blue color to appear in each trial, we can determine the rate of the reaction for each trial. Rate = tttt  [BrO 3 - ] = tttt 3.3 x M

CHM 101/102 Laboratory Manual Kinetics Procedure Procedure  Each trial is performed in the same manner. Use a 10 mL graduated cylinder to measure the quantities shown in Table 1 for Trial 1 into flasks labeled “A” and “B”.

CHM 101/102 Laboratory Manual Kinetics Procedure Procedure  Add 4 drops of starch solution to the flask labeled “B”. B  Quickly pour the contents of flask “B” into flask “A” and start the stopwatch. Record the time it takes for the solution to turn blue. Record the temperature of the blue solution. B A