Chemical Reaction Fundamentals: Kinetics, Thermodynamics, and Rate Laws

Fundamental Principles of Chemical Reactions

1. Chemical Affinity and Electronic Structure

The chemical affinity of a substance depends on its electronic constitution.

2. Gas-Liquid Reactions and Governing Phenomena

When a chemical reaction involves a reactive gas and another fluid, the governing phenomena include:

• The reaction occurs in a homogeneous liquid phase.
• There is a diffusion component of the gas through the interface, with subsequent dissolution and reaction within the liquid phase (known as absorption).

3. Components of a Chemical Equation

A chemical equation is the simple expression of a reaction. It has two members joined by an equals sign (=).

According to Lavoisier's principles, the equation contains:

• The first member contains molecules or atoms of the reactants.
• The second member contains the resulting bodies (products) from the reaction.

Both reactants and products are preceded by numerical coefficients.

Thermodynamic Principles

4. Hess's Law of Constant Heat Summation

The heat developed in a chemical reaction is the same whether the reaction takes place in one step or through several intermediate steps.

5. Berthelot's Principle (Maximum Work)

When a reaction occurs, it can form two or more products (bodies). The reaction tends toward the formation of the body in which the most heat is evolved (exothermic reaction), unless an external action forces the reaction energy to move in a particular direction.

Chemical Kinetics: Energy and Rate Laws

9. Key Kinetic Energy Concepts

Activation Energy

Activation energy is the energy required to change the reaction rate.

Reaction Energy

Reaction energy is the change in energy between reactants and products.

10. Reaction Rate Constants (k) and Units

The rate constant ($k$) for a simple order reaction is numerically equal to the speed of reaction (rate coefficient). Numerically, it is equal to the rate when the concentrations of all reactants are unity.

Units of the Rate Constant

• First Order ($v = kc$): Units are sec ^-1.
• Second Order: Units are mol ^-1 liter sec ^-1. (Derived from the rate being proportional to concentration squared).
• Reaction Order $n$: The general units are liters ^n-1 mol ^1-n sec ^-1.

11. Methods for Kinetic Data Analysis

Method of Integration

This method assumes a kinetic equation is applicable. For example, if the reaction is assumed to be first order, we start with the differential rate law: -dc/dt = kc, where c is the concentration of the reagent. This equation is transformed by integration into another equation that expresses c in terms of time (t). The integrated equation is then compared with the experimental variation of c with respect to t. If there is good agreement, the value of the rate constant can be determined using a simple graphic procedure. If no agreement is found, another rate equation must be sought, and the process repeated to obtain a satisfactory result.

Differential Method

The differential rate equation is used in its differential form, without integration. The values of dc/dt (the instantaneous rate) are obtained by plotting concentration (c) versus time (t), determining the slope at various points, and comparing these slopes with the rate equation. The difficulty is that the slopes cannot always be obtained accurately. Despite this, this method is often considered the safest, especially when the kinetic behavior is complex or incomplete.