| 1. | Phosgene, COCl2, one of the poison gases used during World War I, is formed from chlorine and carbon monoxide. The mechanism is thought to proceed by:
| step 1: | Cl + CO → COCl |
| step 2: | COCl + Cl2 → COCl2 + Cl |
a. Write the overall reaction equation.
b. Identify any reaction intermediates.
c. Identify any catalysts. |
| a. The overall reaction:
| step 1: | Cl + CO → COCl |
| step 2: | COCl + Cl2 → COCl2 + Cl |
| overall: | CO + Cl2 → COCl2 |
| b. | The reaction intermediate is COCl - it is produced during the first step but immediately used up in the second step. |
| c. | The catalyst is Cl - a catalyst can be identified by the fact that it is added as a reactant but emerges (as a product) unchanged; thus it will always first appear as a reactant but will later (not necessarily the next step) appear as a product in the reaction. Since it appears on both sides of the equation it will be cancelled out of the net or overall equation, just as are reaction intermediates. |
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| 2. | We have typically been simplifying our potential energy curves somewhat; for multistep reactions, potential energy curves are more accurately shown with multiple peaks. Each peak represents the activated complex for an individual step.
Consider the PE curve for a two-step reaction:
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| | Answers |
| a) | What is ΔH for the overall reaction? | - 20 kJ |
| b) | What is ΔH for the first step of the reaction mechanism? | + 20 kJ |
| c) | What is ΔH for the second step of the reaction mechanism? | -40 kJ |
| d) | What is ΔH for the overall reverse reaction? | +20 kJ |
| e) | What is Ea for the first step? | +80 kJ |
| f) | What is Ea for the second step? | +40 kJ |
| g) | Which is the rate-determining step - step 1 or step 2? How do you know? | step 1 because it has the largest activation energy, Ea |
| h) | What is Ea for the reverse of step 1? | +60 kJ |
| i) | Is the overall reaction endothermic or exothermic? | exothermic |
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