Question

(i) The rate constant of the reaction H2O2(aq) + (aq) + H+(aq) + H2O(l) + HIO(aq) is sen- sitive to the ionic strength of the aqueous solution in which the reaction occurs. At 25°C, kr = 12.2dmmol-2min -1 at an ionic strength of 0.0525. Use the Debye-Hckel limiting law to estimate the rate constant at zero ionic strength. (ii) At 25°C, kr = 1.55dm@mol-2min-1 at an ionic strength of 0.0241 for a reaction in which the rate-determining step involves the encounter of two singly charged cations in an aqueous solution. Use the Debye-Hckel limiting law to estimate the rate constant at zero ionic strength.

Answer 1

(i) ln(k_r/k₀) = 1.02*ZA.ZB * (I)^1/2

Where ZA and ZB are the charges of the ions A and B present in the solution.

ZA = -1, ZB = +1

'I' is the ionic strength of the solution = 0.0525

i.e. ln(12.2/k₀) = 1.02*(-1)*(+1)*(0.0525)^1/2 = -0.234

i.e. 12.2/k₀ = e^-0.234 = 0.7916

Therefore, the rate constant at zero-ionic strength (k₀) = 15.4 dm⁶.mol^-2.min^-1

(ii) ln(1.55/k₀) = 1.02*(+1)*(+1)*(0.0241)^1/2 = 0.158

i.e. 1.55/k₀ = e^0.158 = 1.1716

Therefore, the rate constant at zero-ionic strength (k₀) = 1.32 dm⁶.mol^-2.min^-1