Kinetics: Reaction Mechanism
Question
The reaction of CO with Cl gives phosgene (COCl2), a nerve gas used in World War I. One proposed mechanism is:
[latex]\begin{aligned} &\textbf{Step 1: }\mathrm{CO}\,\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}\rightarrow\mathrm{COCl}\,\mathrm{(g)}\\ &\textbf{Step 2: }\mathrm{COCl}\,\mathrm{(g)}+\mathrm{Cl}_2\mathrm{(g)}\rightarrow\mathrm{COCl}_2\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)} \end{aligned}[/latex]
- Write the overall reaction equation.
- Identify any reaction intermediates.
- Identify any catalysts.
Show/Hide Answer
- [latex]\mathrm{Cl}_2\mathrm{(g)}+\mathrm{CO}\,\mathrm{(g)}\Rightarrow\mathrm{COCl}_2\mathrm{(g)}[/latex]
- COCl (g)
- Cl (g)
Refer to Section 4.7: Reaction Mechanism (1).
Strategy Map
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Check out the strategy map.
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| Strategy Map Steps |
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1. Add up each elementary reaction step to get the overall reaction.
Show/Hide HintTwo of the same species on the same side of the arrow add together; 2 of the same species on opposite sides cancel each other out. |
2. Identify which elements are used up in the elementary steps and not present in the overall reaction.
Show/Hide HintThese are the intermediates formed in one step and used up in another step. |
3. Identify the species present at the start and end of the reaction in the same form.
Show/Hide HintThese are the catalysts used in the reaction and then regenerated. |
Solution
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Check out this solution.
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a. Write the overall reaction equation.
Add the 2 step reactions together:
[latex]\begin{array}{rcl} \mathrm{CO}\,\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}&\rightarrow&\mathrm{COCl}\,\mathrm{(g)}\\ \mathrm{COCl}\,\mathrm{(g)}+\mathrm{Cl}_2\mathrm{(g)}&\rightarrow&\mathrm{COCl}_2\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}\\ \hline \mathrm{CL}_2\mathrm{(g)}+\mathrm{CO}\,\mathrm{(g)}&\Rightarrow&\mathrm{COCl}_2\mathrm{(g)}\\ \end{array}[/latex]
Answer: [latex]\mathrm{CL}_2\mathrm{(g)}+\mathrm{CO}\,\mathrm{(g)}\Rightarrow\mathrm{COCl}_2\mathrm{(g)}[/latex]
b. Identify any reaction intermediates.
COCl (g) is formed in step 1 and used up in step 2, so it is not present in the overall reaction, meaning it is an intermediate.
Answer: COCl (g)
c. Identify any catalysts.
Cl (g) is present at the start of the reaction and at the end of the reaction in the same form, meaning it is a catalyst.
Answer: Cl (g)
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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| Guided Solution Ideas |
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This question is a theory type problem where you are asked to analyze the given elementary steps of a reaction, identify the overall reaction that they add up to, and identify any intermediates and catalysts.
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Start by adding up elementary reactions:
Show/Hide Think About This!In reaction 1, COCl is formed. In reaction 2, COCl is used up. In reaction 1, Cl (g) is a reactant. In reaction 2, Cl (g) is a product. So, these species will cancel out and not be present in the overall reaction. |
Recall the terms intermediate and catalyst:
Show/Hide Don’t Forget!
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| Complete Solution |
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| a. Write the overall reaction equation.
Add the 2 step reactions together: [latex]\begin{array}{rcl} \mathrm{CO}\,\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}&\rightarrow&\mathrm{COCl}\,\mathrm{(g)}\\ \mathrm{COCl}\,\mathrm{(g)}+\mathrm{CL}_2\mathrm{(g)}&\rightarrow&\mathrm{COCl}_2\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}\\ \hline \mathrm{CO}\,\mathrm{(g)}+\mathrm{Cl}\,\mathrm{(g)}+\mathrm{COCl}\,\mathrm{(g)}+\mathrm{CL}_2\mathrm{(g)}&\rightarrow&\mathrm{COCl}\,\mathrm{(g)}+\mathrm{COCl}_2\mathrm{(g)}+\mathrm{CL}\,\mathrm{(g)} \end{array}[/latex] Cancel any species that are exactly the same on the left and right side of the reaction: [latex]\require{cancel} \begin{gathered} \mathrm{CO}\,\mathrm{(g)}+\cancel{\textcolor{blue}{\mathrm{Cl}\,\mathrm{(g)}}}+\cancel{\textcolor{blue}{\mathrm{COCl}\,\mathrm{(g)}}}+\mathrm{CL}_2\mathrm{(g)}\rightarrow\cancel{\textcolor{blue}{\mathrm{COCl}\,\mathrm{(g)}}}+\mathrm{COCl}_2\mathrm{(g)}+\cancel{\textcolor{blue}{\mathrm{CL}\,\mathrm{(g)}}} \end{gathered}[/latex] This gives the overall reaction equation: [latex]\mathrm{CL}_2\mathrm{(g)}+\mathrm{CO}\,\mathrm{(g)}\Rightarrow\mathrm{COCl}_2\mathrm{(g)}[/latex] |
| b. Identify any reaction intermediates.
COCl (g) COCl (g) is formed in step 1 and used up in step 2, so it is not present in the overall reaction, meaning it is an intermediate. |
| c. Identify any catalysts.
Cl (g) Cl (g) is present at the start of the reaction and at the end of the reaction in the same form, meaning it is a catalyst. |
Check Your Work
Summary of what we would expect based on the related chemistry theory
Your overall reaction should have unique chemical species on the right and left sides and should be balanced. Any identified intermediate or catalyst will not show up in the overall reaction.
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Refer to Section 4.7: Reaction Mechanism (1).
Does your answer make chemical sense?
Show/Hide Answer
The overall reaction is balanced and has unique species on the left and right sides. Intermediates are formed and then used up, which is true of COCl (g). Catalysts are present at the start and end of a reaction in the same form, which is true of Cl (g).
Show/Hide Don’t Forget!
Both Cl and COCl are present in the elementary steps but not in the overall reaction.
PASS Attribution
- LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (2).
- Question 4.79 from LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (3) is used under a CC BY 4.0 license.
- Question 4.79 is adapted from question 12.E.6.11: Q12.6.11 from LibreTexts Chemistry 1e (OpenSTAX) (4), which is under a CC BY 4.0 license.
- Question 12.E.1.6: Q12.1.6 is question 6 from OpenStax Chemistry 2e (5), which is under a CC BY 4.0 license. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
References
1. OpenStax. 4.7: Reaction Mechanisms. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/04%3A_Kinetics/4.07%3A_Reaction_Mechanisms.
2. OpenStax. TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2024. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520).
3. OpenStax. 4.E: Kinetics (Exercises). In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/04%3A_Kinetics/4.E%3A_Kinetics_(Exercises).
4. OpenStax. 12.E: Kinetics (Exercises). In Chemistry 1e (OpenSTAX); LibreTexts, 2023. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/12%3A_Kinetics/12.E%3A_Kinetics_(Exercises).
5. Flowers, P.; Theopold, K.; Langley, R.; Robinson, W. R. (2019). Ch. 12 Exercises. In Chemistry 2e; OpenStax, 2019. https://openstax.org/books/chemistry-2e/pages/12-exercises.
