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. 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
What is the rate law?
- Acheck_circle
rate = k[S2O8^2-][I-]
- B
rate = k[S2O8^2-][I-]^2
- C
rate = k[S2O8^2-]^2[I-]
- D
rate = k[S2O8^2-]
Explanation
Doubling [S2O8^2-] halves t (doubles rate) -> first order. Doubling [I-] also halves t -> first order. Overall rate = k[S2O8^2-][I-].