The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, F. Sherwood Rowlan

Question

The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, F. Sherwood Rowland, and Mario Molina for their work concerning the formation and decomposition of ozone in the stratosphere. Rowland and Molina hypothesized that chlorofluorocarbons (CFCs) in the stratosphere break down upon exposure to UV radiation, producing chlorine atoms. Chlorine was previously identified as a catalyst in the breakdown of ozone into oxygen gas. Using the enthalpy of reaction for two reactions with ozone, determine the enthalpy of reaction for the reaction of chlorine with ozone.

Explanation / Answer

Answer – We are given the reactions with standard enthalpies of reactions and we need to calculated the standard enthalpy of third reaction. We need to use Hess’s law for this one –

ClO(g) + O3(g) -----> Cl(g) + 2O2(g), Horxn = -122.8 kJ/mol …..1

2 O3(g) -----> 3O2(g)    Horxn = -285.3 kJ/mol …….2

Now we need to calculate the standard enthalpy of following reaction –

O3(g) + Cl(g) -----> ClO(g) + O2(g)

So we need to reverse the reaction number 1 and then added to the reaction number 2

2 O3(g) -----> 3O2(g)    Horxn = -285.3 kJ/mol

Cl(g) + 2O2(g) ----->   ClO(g) + O3(g) Horxn = 122.8 kJ/mol

+_____________________________________________________

   O3(g) + Cl(g) -----> ClO(g) + O2(g)    Horxn = -162.5 kJ/mol

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