#Reaction Mechanisms: Unveiling the Steps of Chemical Change

Hey there, future AP Chem superstar! 👋 Ready to dive into the nitty-gritty of how reactions actually happen? Forget those simple, one-step reactions; we're going complex! This guide will break down reaction mechanisms, catalysts, intermediates, and rate laws, making sure you're totally prepped for exam day. Let's get started!

#Introduction to Reaction Mechanisms

#What is a Mechanism?

Reaction mechanisms are like the behind-the-scenes story of a chemical reaction. They break down the overall reaction into a series of elementary steps, showing exactly how reactants transform into products. Think of it as zooming in to see the molecular dance!💃

#Elementary Steps

• Elementary steps are the individual, simple reactions that make up a complex reaction. They show what actually happens at the molecular level.
• Each step has its own rate constant and activation energy. 💡
• When you add up all the elementary steps and cancel out the spectators, you get the overall balanced chemical equation (the net equation).

#Example: Decomposition of Hydrogen Peroxide (H₂O₂)

Let's look at the decomposition of hydrogen peroxide (H₂O₂):

• Step 1: H₂O₂ + I⁻ → H₂O + IO⁻

• Step 2: H₂O₂ + IO⁻ → H₂O + O₂ + I⁻

• Notice how the iodite ion (IO⁻) is formed in the first step and consumed in the second. This makes it an intermediate. Also, the iodide ion (I⁻) is present in both the reactants and products, making it a catalyst.

#Catalysts and Intermediates

#Catalysts

• Definition: A catalyst speeds up a reaction without being consumed in the process. It's like a helpful assistant that gets the job done faster, but doesn't change itself. 🏃
• Catalysts affect the mechanism of a reaction, not the overall products.
• They lower the activation energy of a reaction, making it easier for the reaction to occur.

#Intermediates

• Definition: Intermediates are species formed during a reaction and then consumed in subsequent steps. They're not reactants or products, but they're crucial to the reaction mechanism.
• They exist in significant concentrations only while the reaction is happening.

#Mechanisms and Rate Laws

#Rate-Determining Steps

• Each elementary step has its own rate constant and activation energy.
• The rate-determining step (also called the slow step) is the slowest step in a reaction mechanism. It controls the overall rate of the reaction. 🐌
• The overall rate of a reaction is equal to the rate of the rate-determining step. 💡
• Typically, the rate-determining step has the highest activation energy.

#Example of Rate-Determining Step

Let's consider this two-step mechanism:

• Step 1 (Slow): H₂ + ICl → HI + HCl
• Step 2 (Fast): HI + ICl → I₂ + HCl

1. Overall Reaction: H₂ + 2ICl → I₂ + 2HCl
2. Rate-Determining Step: Step 1 (slow step)
3. Rate Law: R = k[H₂][ICl]

#When the Slow Step Has Intermediates

• If the rate-determining step includes an intermediate, you'll need to use equilibrium concepts to substitute it out. This usually involves using the Keq of a fast equilibrium step. (More on this in the next guide!)

#Example Mechanism

Let's tackle a real example from the 2019 AP Chemistry exam:

• Step 1 (Slow): NO₂ + NO₂ → NO + NO₃

• Step 2 (Fast): NO₃ + CO → NO₂ + CO₂

• Rate-Determining Step: Step 1

• Rate Law: R = k[NO₂]²

#Final Exam Focus

• Key Concepts:
  • Understanding the difference between elementary steps and overall reactions.
  • Identifying catalysts and intermediates.
  • Determining the rate-determining step.
  • Writing rate laws based on the rate-determining step.
• Common Question Types:
  • Identifying catalysts and intermediates in a given mechanism.
  • Determining the rate law from a given mechanism.
  • Explaining how a catalyst affects the rate of a reaction.
• Last-Minute Tips:
  • Pay close attention to the wording of the questions. Are they asking about an elementary step or the overall reaction?
  • Practice writing rate laws from mechanisms.
  • Don't forget that rate laws can only be determined experimentally or from elementary steps.
  • Stay calm and confident! You've got this! 💪

#

Practice Question

Practice Questions

#Multiple Choice Questions

1. The rate law for the reaction 2A + B → C is found to be rate = k[A][B]². Which of the following mechanisms is consistent with this rate law? (A) A + B → X (slow); X + B → C (fast) (B) 2A + B → C (single step) (C) A + B → X (fast); X + A → C (slow) (D) A + B → X (slow); X + A → C (fast)

2. Which of the following statements about a catalyst is correct? (A) A catalyst is consumed in the reaction. (B) A catalyst increases the activation energy of a reaction. (C) A catalyst is a reactant in the overall reaction. (D) A catalyst provides an alternative reaction pathway with a lower activation energy.

3. Consider the following reaction mechanism: Step 1: A + B ⇌ C (fast equilibrium) Step 2: C + D → E (slow) What is the rate law for the overall reaction? (A) rate = k[A][B] (B) rate = k[C][D] (C) rate = k[A][B][D] (D) rate = k[A][B]/[C]

#Free Response Question

The following mechanism is proposed for the reaction between nitrogen dioxide and carbon monoxide:

• Step 1: NO₂ + NO₂ → NO + NO₃ (slow)
• Step 2: NO₃ + CO → NO₂ + CO₂ (fast)

(a) Write the overall balanced equation for the reaction. (b) Identify any intermediates in the mechanism. (c) Identify any catalysts in the mechanism. (d) What is the rate law for the overall reaction? (e) Sketch a potential energy diagram for the reaction, labeling the reactants, products, transition state(s), and activation energy for the rate-determining step.

#Scoring Breakdown for FRQ

(a) (1 point) NO₂ + CO → NO + CO₂

(b) (1 point) NO₃

(c) (1 point) None

(d) (1 point) Rate = k[NO₂]²

(e) (3 points) * Correctly labeled axes (Potential Energy vs. Reaction Coordinate) * Reactants (NO₂ + CO) and Products (NO + CO₂) at correct relative energy levels * Two transition states and a clear activation energy for the first step