The peroxodisulfate ion is one of the strongest oxidants that are known, although the oxidation reac — Physical Chemistry — Kinetics Chemistry Question
Chemical Kinetics of the Peroxodisulfate Ion
The peroxodisulfate ion is one of the strongest oxidants that are known, although the oxidation reaction is relatively slow. Peroxodisulfate ions are able to oxidize all halides, except fluoride, to halogens.
The initial rate (r0) of the iodine–formation according to
S2O8 2– + 2 I – 2 SO4 2– + I2
was determined as a function of the initial concentrations (c0) of the reactants at 25°C:
Draw the line–bond structure of the peroxodisulfate ion and determine the oxidation states of all atoms.
Write down the rate equation for the reaction shown above.
Model Answer
r = k c(S2O8 2–) c(I – )
Write down the total order and the partial orders of the reaction shown above.
Model Answer
reaction order: 2
partial reaction order of S2O8 2–: 1
partial reaction order of I – : 1
Prove that the rate constant of the reaction is 0.011 dm 3 mol –1 s –1 .
Model Answer
1.1 × 10–8 mol dm–3 s–1 = k × c(S2O8 2–) × c(I–)
1.1 × 10–8 mol dm–3 s–1 = k × 0.1 × 10–3 × 1 × 10–2 mol2 dm–6
k = 0.011 dm3 mol–1 s–1
The activation energy of the reaction mentioned above is 42 kJ mol –1 .
What temperature (in °C) has to be chosen to decuple the rate constant?
Model Answer
Using the Arrhenius equation we may write
k1 / k2 = e^(–Ea/RT1) / e^(–Ea/RT2)
because k1/k2 = 1/10, it follows that
ln(1/10) = –Ea/R (1/T1 – 1/T2) ⇒ 1/T2 = 1/T1 + R/Ea ln(1/10)
=> T2 = 345 K ≈ 72 °C
Iodine reacts with thiosulfate ions (S2O3 2–) forming iodide ions rapidly.
Write down the reaction scheme of this reaction.
Model Answer
2 S2O3 2– + I2 2 I – + S4O6 2–
Write down the rate equation for the reaction
S2O8 2– + 2 I – → 2 SO4 2– + I2
assuming that there is an excess of thiosulfate ions relative to the peroxodisulfate ions and the iodide ions in the solution.
Model Answer
It has to be noticed that the concentration of the iodide ions does not vary any longer, because iodine formed reacts quickly with thiosulfate ions (which are available in excess according to the precondition) forming iodide ions again. Therefore the reaction is of pseudo first–order and the rate equation is given by
r = k’ c(S2O8 2–)
(It is important to note that the rate constant k’ is different from k of the parts 6.2 – 6.5 of this problem, because it includes the pseudo–constant concentration of the iodide ions).