You have been provided with three solutions. Solution A contains iron(III) ions, Solution B contains — Kinetics Chemistry Question
Problem Context
You have been provided with three solutions. Solution A contains iron(III) ions, Solution B contains iodide ions, and Solution C contains thiosulfate ions (and some starch indicator).
Devise and carry out an experimental procedure to determine the kinetic order of the reaction that occurs upon mixing them with respect to [Fe 3+] from Solution A and with respect to [I‒] from Solution B.
NOTE: In carrying out your approved experimental plan, mix Solutions B and C before adding Solution A, and record the time required for the resulting mixture to turn a distinct, permanent color.
Give a brief description of your experimental plan.
Model Answer
[1 point] mix & record time
[1 point] describing how to vary concentration
[1 point] stating how volume increments will be added (e.g., pipet, grad cylinder)
[1 point] planned to do replicates (either mentioned here or actually performed, as evidenced in the answer to question #2)
Example response: I will mix equal volumes of the three solutions as provided and measure the reaction time. I will dilute first solution A and then solution B, keeping the total volume of the mixture constant, and measure and record those reaction times. I will repeat each trial at least once (twice if I have time to do so).
Record your data/observations.
Model Answer
[3 points] clear data table (volumes & times)
[2 points] need 3 trials
[1 point] observed color change
[2 points] observed times consistent with 1st order in [Fe3+]
[2 points] observed times consistent with 2nd order in [I-]
Example response: Observations –upon mixing, there is a brief flash of purple-blue color then the solution is colorless for about 10 seconds before turning a dark blue-black color.
Data:
Volume of A (mL) | Volume of B (mL) | Volume of C (mL) | Volume H2O (mL) | Time (s) Trial 1 | Time (s) (Trial 2)
10 | 10 | 10 | 0 | 7 | 8
5 | 10 | 10 | 5 | 15 | 13
10 | 5 | 10 | 5 | 29 | 32
Show all calculations.
Model Answer
[3 points] comparing ratios of concentrations to ratios of rates (conc./time) for [Fe3+]
[3 points] comparing ratios of concentrations to ratios of rates (conc./time) for [I-]
[1 point] averaging data (or justifying not doing so)
Example response: The average time for reaction when equal volumes of the 3 solutions are mixed is (7 s +8 s)/2 or 7.5 s. The average reaction time when Solution A is diluted to one-half of its initial concentration, approximately doubles (14 s versus 7.5 s). This indicates that the reaction order with respect to [Fe3+] is one (first order).
The average reaction time when Solution B is diluted to one-half of its initial concentration, is approximately 4 times longer (31.5 s vs 7.5 s), indicating that this reaction is second order with respect to [I-].
The kinetic order of this reaction with respect to [Fe 3+] is _____________.
Model Answer
The kinetic order of this reaction with respect to [Fe3+] is first (or 1st).
The kinetic order of this reaction with respect to [I ‒ ] is _______________.
Model Answer
The kinetic order of this reaction with respect to [I-] is second (or 2nd).
Write the rate expression for this reaction, referring to the kinetic order with respect to [S2O3 2‒] as “x”.
Model Answer
answer should be based on their data (1 point for form of expression and 1 point for exponents).
rate = k[Fe3+][I-]^2[S2O3 2-]^x