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. 1) ClO (g) + O3 (gO -----> Cl (g) + 2O2 (g) ?H rxn = -122.8 kJ 2) 2O3 (g) ------> 3O2 (g) ?Hrxn= -285.3 kJ 3) O3(g) + Cl (g) ---------> ClO(g) + O2 (g) ?Hrxn = ? Find the missing ?Hrxn for reaction

Answer 1

Answer:

The enthalpy of reaction when chlorine reacts with ozone is -162.5 kJ/mol.

Explanation:

$ClO (g) + O_3 (g)\rightarrow Cl (g) + 2O_2 (g) ,\Delta H_{rxn,1}= -122.8 kJ/mol$..[1]

$2O_3 (g) \rightarrow 3O_2 (g) \Delta H_{rxn,2}= -285.3 kJ/mol$..[2]

$O_3(g) + Cl (g)\rightarrow ClO(g) + O_2 (g) \Delta H_{rxn,3} = ?$...[3]

Applying Hess's law:

Hess’s law of constant heat summation states that the amount of heat absorbed or evolved in a given chemical equation remains the same whether the process occurs in one step or several steps.

[2] - [1]  = [3]

$\Delta H_{rxn,3} =\Delta H_{rxn,2} -\Delta H_{rxn,1}$

$=-285.3 kJ/mol-(-122.8 kJ/mol)=-162.5 kJ/mol$

The enthalpy of reaction when chlorine reacts with ozone is -162.5 kJ/mol.