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 ) + O 3 ( g ) ⟶ Cl ( g ) + 2 O 2 ( g ) Δ H ∘ rxn = − 122.8 kJ mol (2) 2 O 3 ( g ) ⟶ 3 O 2 ( g ) Δ H ∘ rxn = − 285.3 kJ mol (3) O 3 ( g ) + Cl ( g ) ⟶ ClO ( g ) + O 2 ( g ) Δ H ∘ rxn = ?

Answer 1

Answer:

$\Delta H_{rxn3}=-162.5 kJ/mol$

Explanation:

The reaction we need to calculate:

$O_3 (g) + Cl (g) \longrightarrow ClO (g) + O_2 (g)$

1) $O_3 (g) + ClO (g) \longrightarrow Cl (g) +2 O_2 (g)$

$\Delta H_{rxn}=-122.8 kJ/mol$

We need the ClO in the products side, so we use the inverse of this reaction:

$Cl (g) +2 O_2 (g) \longrightarrow O_3 (g) + ClO (g)$

$\Delta H_{rxn1}=122.8 kJ/mol$

2) $2 O_3 (g) \longrightarrow 3 O_2 (g)$

$\Delta H_{rxn2}=-285.3 kJ/mol$

Now we need to combine this two:

$Cl (g) +2 O_2 (g) + 2 O_3 (g)\longrightarrow O_3 (g) + ClO (g) + 3 O_2 (g)$

$Cl (g) + O_3 (g)\longrightarrow ClO (g) + O_2 (g)$

The enthalpy of reaction:

$\Delta H_{rxn3}=\Delta H_{rxn1}+ \Delta H_{rxn2}=122.8 kJ/mol-285.3 kJ/mol$

$\Delta H_{rxn3}=-162.5 kJ/mol$