The data below were determined for the reaction shown below. s2o82– + 3i – (aq) → 2so42– + i3– expt. # [s2o82–] (m) [i –] (m) initial rate 1 0.038 0.060 1.4 × 10 – 5 m/s 2 0.076 0.060 2.8 × 10 – 5 m/s 3 0.076 0.030 1.4 × 10 – 5 m/s the rate law for this reaction must be:

The rate law for this reaction must be = k[S₂O₈⁻][I⁻].

Explanation:

Exp.               [S₂O₈⁻],M                [I⁻],M                    initial rate

1                       0.038                   0.060                 1.4 x 10⁻⁵ M/s

2                      0.076                   0.060                 2.8 x 10⁻⁵ M/s

3                      0.076                   0.030                  1.4 x 10⁻⁵ M/s

where, k is the rate constant of the reaction,

a is the order of the reaction with respect to [S₂O₈⁻].

b is the order of the reaction with respect to [I⁻].

From Exp 1 and 2,

(initial rate)₁ = k[S₂O₈⁻]₁ᵃ[I⁻]₁ᵇ         (1)

(initial rate)₂ = k[S₂O₈⁻]₂ᵃ[I⁻]₂ᵇ       (2)

By dividing (1) over (2)

∴ (initial rate)₁ / (initial rate)₂ = [k[S₂O₈⁻]₁ᵃ[I⁻]₁ᵇ] / [k[S₂O₈⁻]₂ᵃ[I⁻]₂ᵇ]

∴ (1.4 x 10⁻⁵ M/s) / (2.8 x 10⁻⁵ M/s) = [k[0.038]ᵃ[0.06]ᵇ] / [k[0.076]ᵃ[0.06]ᵇ]

∴ (0.5) = [0.038]ᵃ / [0.076]ᵃ = [0.5]ᵃ

Taking log for the both sides,

log (0.5) = a log (0.5)

∴ a = 1.

∴ the reaction is first order reaction with respect to [S₂O₈⁻].

From Exp. 2 and 3,

(initial rate)₂ = k[S₂O₈⁻]₂ᵃ[I⁻]₂ᵇ         (3)

(initial rate)₃ = k[S₂O₈⁻]₃ᵃ[I⁻]₃ᵇ         (4)

By dividing (3) over (4)

∴ (initial rate)₂ / (initial rate)₃ = [k[S₂O₈⁻]₂ᵃ[I⁻]₂ᵇ] / [k[S₂O₈⁻]₃ᵃ[I⁻]₃ᵇ]

∴ (2.8 x 10⁻⁵ M/s) / (1.4 x 10⁻⁵ M/s) = [k[0.076]ᵃ[0.06]ᵇ] / [k[0.076]ᵃ[0.03]ᵇ]

∴ (2.0) = [0.06]ᵇ / [0.03]ᵇ = [2.0]ᵇ

Taking log for the both sides,

log (2.0) = a log (2.0)

∴ b = 1.

∴ the reaction is first order reaction with respect to [I⁻].

∴ the rate law for this reaction must be = k[S₂O₈⁻][I⁻].

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.