CFCs have been largely replaced by hydrogenated chlorofluorocarbons (HCFCs) beca

Question

CFCs have been largely replaced by hydrogenated chlorofluorocarbons (HCFCs) because they are degraded more rapidly by OH radical, the scrubber of the troposphere.

(a) Use the following information to determine the reaction rate and lifetime of HCFC-22 (CHF2Cl).

                 [CHF2Cl] = 1.0 ppbv (part per billion by volume)

A.) Reaction Time _________h^-1

B.) Lifetime _________ yr

C.) In comparison, the lifetime of CFC-14 (CF3Cl) is 680 years. Determine the removal rate for CFC-14.

      _________ yr^-1

Explanation / Answer

Molar mass of CHF2Cl is 86.5 g/mol

1ppbv = 1x10-9 g/cm3        

CHF2Cl concentration is

(1x10-9 g/cm3 / 86.5 g/mol ) x 6.02 x1023 molecules/mol = 6.92x1012 molecules/cm3

k = 1.44 x10-11 (molecules/cm3)-1·h-1

Looking at the measurements units of k, the reaction is second order.

- dc/dt = k [CHF2Cl][HO·]

Assume [HO·] is constant.

-d[CHF2Cl] = 1.44x 10-11 (molecules/cm3)-1·h-1 x 5. x105 molecules/cm3=

           = 7.20x10-6 h-1 [CHF2Cl]      (this a pseudo first order reaction).

A. (Apparent) Reaction rate k = 7.2x10-6 h-1

B. Half life = 0.693/k = 0.693/7.20x10-6 = 96250 h = 11 years

    Mean lifetime = 1/k = 15.9 years

C.

680 y = 680 x 365 d/y x 24 h/day = 5.96x106 h

Removal rate is k = 1/5.96x106 h = 1.68x10-7 h-1

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