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Ch.14 - Chemical Kinetics
Chapter 14, Problem 30b,c

Consider the following reaction:
2 NO(g) + 2 H2(g) → N2(g) + 2 H2O(g)
(b) If the rate constant for this reaction at 1000 K is 6.0 × 104 M-2 s-1, what is the reaction rate when [NO] = 0.035 M and [H2] = 0.015 M?
(c) What is the reaction rate at 1000 K when the concentration of NO is increased to 0.10 M, while the concentration of H2 is 0.010 M?

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1
Identify the rate law for the given reaction. Since the reaction is 2 NO(g) + 2 H2(g) → N2(g) + 2 H2O(g), the rate law can be expressed as: rate = k[NO]^2[H2]^2, where k is the rate constant.
For part (b), substitute the given values into the rate law. Use k = 6.0 × 10^4 M^-2 s^-1, [NO] = 0.035 M, and [H2] = 0.015 M. The expression becomes: rate = (6.0 × 10^4 M^-2 s^-1) × (0.035 M)^2 × (0.015 M)^2.
Calculate the rate for part (b) by evaluating the expression from the previous step. This involves squaring the concentrations of NO and H2, multiplying them together, and then multiplying by the rate constant.
For part (c), substitute the new concentration values into the rate law. Use k = 6.0 × 10^4 M^-2 s^-1, [NO] = 0.10 M, and [H2] = 0.010 M. The expression becomes: rate = (6.0 × 10^4 M^-2 s^-1) × (0.10 M)^2 × (0.010 M)^2.
Calculate the rate for part (c) by evaluating the expression from the previous step. This involves squaring the new concentrations of NO and H2, multiplying them together, and then multiplying by the rate constant.

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Key Concepts

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

Rate Law

The rate law expresses the relationship between the rate of a chemical reaction and the concentration of its reactants. For a reaction of the form aA + bB → products, the rate can be described by the equation Rate = k[A]^m[B]^n, where k is the rate constant, and m and n are the orders of the reaction with respect to reactants A and B, respectively. Understanding the rate law is essential for calculating reaction rates based on varying concentrations.
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Order of Reaction

The order of a reaction refers to the power to which the concentration of a reactant is raised in the rate law. It indicates how the rate of reaction is affected by the concentration of that reactant. For example, if the reaction is first order with respect to NO, doubling the concentration of NO will double the reaction rate. Identifying the order of each reactant is crucial for accurately determining the overall reaction rate.
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Units of Rate Constant

The units of the rate constant (k) depend on the overall order of the reaction. For a second-order reaction, like the one given (2 NO + 2 H2 → N2 + 2 H2O), the units of k are M^-2 s^-1. This means that the rate of reaction is proportional to the product of the concentrations of the reactants raised to their respective orders. Understanding these units is vital for correctly applying the rate law to calculate reaction rates.
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