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A topic from the subject of Kinetics in Chemistry.

  • 1 year ago
  • 9 min read

Rate Laws and Order of Reaction

The rate law for a chemical reaction expresses the relationship between the reaction rate and the concentrations of the reactants. It is determined experimentally, and it generally does not match the stoichiometry of the balanced chemical equation.

A typical rate law takes the form:

Rate = k[A]m[B]n

where:

  • Rate is the speed of the reaction (often expressed as the change in concentration per unit time).
  • k is the rate constant, a proportionality constant specific to the reaction and temperature.
  • [A] and [B] represent the concentrations of reactants A and B.
  • m and n are the orders of reaction with respect to reactants A and B, respectively. These are experimentally determined exponents and are not necessarily equal to the stoichiometric coefficients in the balanced equation.

Order of Reaction

The overall order of reaction is the sum of the individual orders (m + n in the example above). It indicates how sensitive the reaction rate is to changes in reactant concentrations.

Examples:

  • Zero-order reaction: The rate is independent of the concentration of reactants (m = n = 0). Rate = k
  • First-order reaction: The rate is directly proportional to the concentration of one reactant (m = 1 or n = 1, but not both). Rate = k[A] or Rate = k[B]
  • Second-order reaction: The rate is proportional to the square of the concentration of one reactant (m = 2 or n = 2) or the product of the concentrations of two reactants (m = 1 and n = 1). Rate = k[A]2 or Rate = k[A][B]
  • Higher-order reactions: Reactions can have orders greater than two, though these are less common.

Determining the rate law: The rate law is determined experimentally, often using the method of initial rates. This involves measuring the initial rate of reaction at different initial concentrations of reactants and analyzing how the rate changes in response to concentration changes.

Rate Laws and Order of Reaction
Introduction

The rate law is an equation that expresses the relationship between the rate of a chemical reaction and the concentrations of the reactants. The order of reaction describes how the rate is affected by changes in reactant concentrations. It's determined experimentally, not from the stoichiometry of the balanced equation.

Key Points
  • Rate law: r = k[A]a[B]b, where r is the rate, k is the rate constant, [A] and [B] are the concentrations of reactants A and B, and a and b are the orders of reaction with respect to A and B, respectively. The overall order of the reaction is a + b.
  • Order of reaction: The order of reaction with respect to a reactant indicates how the rate changes when the concentration of that reactant changes. A first-order reaction, for example, will double in rate if the concentration of that reactant is doubled.
  • Determining order of reaction: The order of reaction is determined experimentally, often using the method of initial rates. This involves measuring the initial rate of the reaction at different initial concentrations of reactants.
  • Zero-order reaction: The rate is independent of the concentration of reactants. Rate = k
  • First-order reaction: The rate is directly proportional to the concentration of one reactant. Rate = k[A]
  • Second-order reaction: The rate is proportional to the square of the concentration of one reactant, or the product of the concentrations of two reactants. Examples: Rate = k[A]2 or Rate = k[A][B]
  • Third-order reaction: The rate is proportional to the cube of the concentration of one reactant, or other combinations of reactant concentrations that sum to three. Examples: Rate = k[A]3 or Rate = k[A]2[B]

Main Concepts
  1. Rate of reaction: The rate of a reaction is the change in concentration of a reactant or product per unit time. It's usually expressed in units of mol L-1 s-1.
  2. Rate constant (k): The rate constant is a proportionality constant that relates the rate of a reaction to the concentrations of reactants. It's temperature-dependent and is unique for each reaction.
  3. Reaction mechanism: The reaction mechanism is a series of elementary steps that describe the actual process by which reactants are transformed into products. The rate law is determined by the slowest step (rate-determining step) in the mechanism.
  4. Activation energy (Ea): The activation energy is the minimum energy required for the reactants to overcome the energy barrier and form the activated complex (transition state), leading to product formation. It is related to the rate constant through the Arrhenius equation.
Experiment: Rate Laws and Order of Reaction

Objective: To determine the rate law and order of a chemical reaction.

Materials:

  • Two beakers
  • Stopwatch
  • Pipettes
  • Graduated cylinders (for accurate volume measurement)
  • Reagents (e.g., sodium thiosulfate, iodine, sulfuric acid, distilled water)
  • Thermometer (to monitor temperature)

Procedure:

  1. Prepare several solutions of sodium thiosulfate (Na2S2O3) with varying concentrations using distilled water. Record the exact concentrations.
  2. Prepare a solution of iodine (I2) and sulfuric acid (H2SO4).
  3. In a beaker, mix a known volume of the iodine/sulfuric acid solution with a known volume of one of the sodium thiosulfate solutions. Record the volumes and concentrations used.
  4. Start the stopwatch immediately.
  5. Observe the reaction. The reaction between iodine and sodium thiosulfate will produce a colorless solution. Note the time it takes for the solution to become colorless (the reaction endpoint). Record this time.
  6. Repeat steps 3-5 using the other sodium thiosulfate solutions with different concentrations, keeping the volume and concentration of the iodine/acid solution constant.
  7. Maintain a constant temperature throughout the experiment. Record the temperature.

Key Procedures:

  • Use a stopwatch to accurately measure reaction time.
  • Keep the temperature constant throughout the experiment (use a water bath if necessary).
  • Vary the initial concentrations of sodium thiosulfate systematically while keeping the iodine concentration constant (or vice versa in a separate set of trials to determine the order with respect to each reactant).
  • Use graduated cylinders for accurate volume measurements.
  • Properly dispose of chemical waste according to safety guidelines.

Data Analysis:

The rate of the reaction is inversely proportional to the time taken for the solution to become colorless. Calculate the rate for each trial. Then, use the method of initial rates to determine the order of the reaction with respect to each reactant and the overall order. This involves comparing the rates of reaction at different initial concentrations and determining the exponents in the rate law (Rate = k[Na2S2O3]m[I2]n, where m and n are the orders with respect to thiosulfate and iodine, respectively, and k is the rate constant).

Significance:

This experiment allows students to observe and analyze the effects of initial reactant concentrations on reaction rate. By determining the rate law and order of the reaction, students gain insight into the reaction mechanism. This experiment demonstrates the fundamental principles of chemical kinetics and how reaction rates are influenced by concentration.

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