In a hydrocarbon solution, the gold compound (CH 3 ) 3 AuPH 3 decomposes into ethane (C 2 H 6 ) and a different gold compound, (CH 3 )AuPH 3 . The following mechanism has been proposed for the decomposition of (CH 3 ) 3 AuPH 3 : Step 1: (CH 3 ) 3 AuPH 3 k 1 ⇌k -1 (CH 3 ) 3 Au + PH 3 (fast) Step 2: (CH 3 ) 3 Au k 2 → C 2 H 6 + (CH 3 )Au (slow) Step 3: (CH 3 )Au + PH 3 k 3 → (CH 3 )AuPH 3 (fast) (c) What is the molecularity of each of the elementary steps?

Hello everyone today. We are being given the following reaction of hydrogen peroxide decomposing quickly into water and oxygen gas in the presence of a platinum catalyst. The decomposition reaction takes place in two steps. So we have step one and step two. And we are then asked to determine the molecular charity of each individual step. So first step one, we have the following molecules, we have a platinum and we have a peroxide molecule. Now keep in mind. Molecular Pretty is the right off to the side here. Molecular charity is equal to the number of molecules that collide to form products. So molecular charity is the number of molecules that collide to form products. And so for step one we have platinum and we have one peroxide that is going to lead us to have two molecules or in other words be by molecular, So by molecular because there's two molecules for step two, we have Platinum and the presence of oxygen plus one peroxide. And so once again we have two molecules colliding, which is therefore going to lead us to have by molecular charity for both steps. I hope this helped and until next time.

Ch.14 - Chemical Kinetics

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Brown 14th Edition

Ch.14 - Chemical Kinetics

Problem 112c

Chapter 14, Problem 112c

In a hydrocarbon solution, the gold compound (CH₃)₃AuPH₃ decomposes into ethane (C₂H₆) and a different gold compound, (CH₃)AuPH₃. The following mechanism has been proposed for the decomposition of (CH₃)₃AuPH₃:
Step 1: (CH₃)₃AuPH₃ k₁⇌k_-1 (CH₃)₃Au + PH₃ (fast)
Step 2: (CH₃)₃Au k₂→ C₂H₆ + (CH₃)Au (slow)
Step 3: (CH₃)Au + PH₃ k₃→ (CH₃)AuPH₃ (fast)
(c) What is the molecularity of each of the elementary steps?

Verified step by step guidance

Identify the molecularity of Step 1: (CH_3)_3AuPH_3 ⇌ (CH_3)_3Au + PH_3. Since one molecule of (CH_3)_3AuPH_3 is involved in the reaction, this step is unimolecular.

Identify the molecularity of Step 2: (CH_3)_3Au → C_2H_6 + (CH_3)Au. This step involves the decomposition of a single molecule of (CH_3)_3Au, making it unimolecular.

Identify the molecularity of Step 3: (CH_3)Au + PH_3 → (CH_3)AuPH_3. This step involves two reactant molecules, (CH_3)Au and PH_3, so it is bimolecular.

Summarize the molecularity: Step 1 is unimolecular, Step 2 is unimolecular, and Step 3 is bimolecular.

Understand that molecularity refers to the number of molecules coming together to react in an elementary step, and it is always a whole number.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecularity

Molecularity refers to the number of reactant molecules involved in an elementary reaction step. It can be classified as unimolecular (one molecule), bimolecular (two molecules), or termolecular (three molecules). Understanding molecularity is crucial for determining the rate law of a reaction and predicting how changes in concentration affect the reaction rate.

Elementary Steps

Elementary steps are individual reactions that occur in a multi-step reaction mechanism. Each step represents a single molecular event, and the overall reaction is the sum of these steps. Analyzing elementary steps helps in understanding the pathway of the reaction and the role of intermediates, which are species formed during the reaction but not present in the final products.

Rate Determining Step

The rate determining step (RDS) is the slowest step in a reaction mechanism that dictates the overall reaction rate. It is crucial to identify the RDS because it has the highest activation energy and thus controls how quickly the reaction proceeds. In the provided mechanism, the slow step is where (CH₃)₃Au decomposes, making it essential for calculating the reaction kinetics.

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Related Practice

Textbook Question

The following mechanism has been proposed for the gasphase reaction of chloroform 1CHCl32 and chlorine:

Step 1: Cl₂(g) k₁⇌ k_-1 2 Cl(g) (fast)

Step 2: Cl(g) + CHCl₃(g) k₂→ HCl(g) + CCl₃(g) (slow)

Step 3: Cl(g0 + CCl3(g) k₃→ CCl₄ (fast)

(e) What is the rate law predicted by this mechanism? (Hint: The overall reaction order is not an integer.)

Open Question

Consider the hypothetical reaction 2 A + B → 2 C + D. The following two-step mechanism is proposed for the reaction: Step 1: A + B → C + X Step 2: A + X → C + D. X is an unstable intermediate. (c) Your result for part (b) might be considered surprising for which of the following reasons: (i) The concentration of a product is in the rate law. (ii) There is a negative reaction order in the rate law. (iii) Both reasons (i) and (ii). (iv) Neither reasons (i) nor (ii).

Textbook Question

In a hydrocarbon solution, the gold compound (CH₃)₃AuPH₃ decomposes into ethane (C₂H₆) and a different gold compound, (CH₃)AuPH₃. The following mechanism has been proposed for the decomposition of (CH₃)₃AuPH₃:

Step 1: (CH₃)₃AuPH₃ k₁⇌k_-1 (CH₃)₃Au + PH₃ (fast)

Step 2: (CH₃)₃Au k₂→ C₂H₆ + (CH₃)Au (slow)

Step 3: (CH₃)Au + PH₃ k₃→ (CH₃)AuPH₃ (fast)

(a) What is the overall reaction?

(b) What are the intermediates in the mechanism?

Textbook Question

Step 1: (CH₃)₃AuPH₃ k₁⇌k_-1 (CH₃)₃Au + PH₃ (fast)

Step 2: (CH₃)₃Au k₂→ C₂H₆ + (CH₃)Au (slow)

Step 3: (CH₃)Au + PH₃ k₃→ (CH₃)AuPH₃ (fast)

(e) What is the rate law predicted by this mechanism?

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Textbook Question

Step 1: (CH₃)₃AuPH₃ k₁⇌k_-1 (CH₃)₃Au + PH₃ (fast)

Step 2: (CH₃)₃Au k₂→ C₂H₆ + (CH₃)Au (slow)

Step 3: (CH₃)Au + PH₃ k₃→ (CH₃)AuPH₃ (fast)

(f) What would be the effect on the reaction rate of adding PH₃ to the solution of (CH₃)₃AuPH₃?

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Textbook Question

Platinum nanoparticles of diameter 2 nm are important catalysts in carbon monoxide oxidation to carbon dioxide. Platinum crystallizes in a face-centered cubic arrangement with an edge length of 3.924 Å. (b) Estimate how many platinum atoms are on the surface of a 2.0-nm Pt sphere, using the surface area of a sphere (4πr²) and assuming that the 'footprint' of one Pt atom can be estimated from its atomic diameter of 2.8 A .

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