Chemistry - Reaction Rates

What are Reaction Rates?

A reaction rate is simply a measure of how fast a chemical reaction occurs. It describes how quickly the reactants are used up or how quickly the products are formed over a specific period of time. You can think of it like the speed of a car, but instead of measuring miles per hour, chemists measure change in concentration per second or minute.

Determining Rate Laws from Experimental Data

To find the rate law, we look at how the initial rate of a reaction changes when we change the concentration of one reactant at a time. 

Rate Law Format: Rate = k[A]^x[B]^y

k = Rate constant

x = Order of reactant A

y = Order of reactant B

Practice Problem 1: Determining Reaction Orders

Consider the following reaction: 2 NO(g) + Cl2(g) -> 2 NOCl(g)

The following data was collected at a constant temperature:

Trial 1: [NO] = 0.10 M, [Cl2] = 0.10 M, Initial Rate = 0.18 M/s

Trial 2: [NO] = 0.10 M, [Cl2] = 0.20 M, Initial Rate = 0.36 M/s

Trial 3: [NO] = 0.20 M, [Cl2] = 0.10 M, Initial Rate = 0.72 M/s

Questions:

1. What is the reaction order for Cl2 (the value of y)?

2. What is the reaction order for NO (the value of x)?

3. What is the overall rate law for the reaction?

4. Calculate the value of the rate constant (k).

Practice Problem 2: The Zero Order Case

Consider the reaction: A + B -> Products

Trial 1: [A] = 1.0 M, [B] = 1.0 M, Initial Rate = 0.05 M/s

Trial 2: [A] = 2.0 M, [B] = 1.0 M, Initial Rate = 0.05 M/s

Trial 3: [A] = 1.0 M, [B] = 2.0 M, Initial Rate = 0.10 M/s

Questions:

1. What is the reaction order for A (the value of x)?

2. What is the reaction order for B (the value of y)?

3. Write the rate law for this reaction.

Answer Key

Problem 1 Answers:

1. Order for Cl2 (y): Compare Trials 1 and 2. [NO] stays the same, but [Cl2] doubles. The rate also doubles (0.18 to 0.36). This means y = 1.

2. Order for NO (x): Compare Trials 1 and 3. [Cl2] stays the same, but [NO] doubles. The rate quadruples (0.18 to 0.72). Since 2 squared is 4, x = 2.

3. Rate Law: Rate = k[NO]^2[Cl2]^1

4. Rate Constant (k): Use Trial 1 data. 0.18 = k[0.10]^2[0.10]. Solving for k gives 180 M^-2s^-1.

Problem 2 Answers:

1. Order for A (x): Compare Trials 1 and 2. [A] doubles, but the rate stays exactly the same. This means x = 0.

2. Order for B (y): Compare Trials 1 and 3. [B] doubles and the rate doubles. This means y = 1.

3. Rate Law: Rate = k[B] (Reactant A is not included because its order is 0).