Students study the reaction S2O8^2- + 2 I- -> 2 SO4^2- + I2 using a clock method (starch indicator + thiosulfate). They measure time t for the blue color to appear in three trials at 25 C with constant ionic strength. The fixed amount of thiosulfate used means the average rate over time t corresponds to consumption of 1.00e-4 mol/L of S2O8^2-. Trial 1: [S2O8^2-]=0.040 M, [I-]=0.040 M, t=84 s Trial 2: [S2O8^2-]=0.080 M, [I-]=0.040 M, t=42 s Trial 3: [S2O8^2-]=0.040 M, [I-]=0.080 M, t=42 s
To determine the activation energy, the student should next:
- A
change ionic strength while holding T constant
- B
repeat Trial 1 with different initial concentrations only
- Ccheck_circle
repeat Trial 1 at several different temperatures and plot ln(k) vs 1/T
- D
measure absorbance of I2 at 25 C
Explanation
Arrhenius: ln(k) = -Ea/RT + ln(A). A plot of ln(k) versus 1/T from runs at different T gives slope = -Ea/R.