Chemical Reaction Dynamics: Kinetics and Equilibrium Factors

Chemical Kinetics and Reaction Rate

The branch of chemistry dealing with the speed of reactions is known as chemical kinetics. It investigates the stages or phases of the reaction mechanism and studies the factors that can affect this speed.

Defining Reaction Rate

The speed of a reaction (or reaction rate) is defined as the amount of reactant that disappears per unit time in this reaction.

Reaction Systems

• Homogeneous System: The reaction occurs throughout the volume of the mixture.
• Heterogeneous System: The reaction occurs only at the interfaces. The reaction rate depends on the surface area common to both phases.

Factors Influencing Reaction Speed

Several factors significantly influence the rate at which a chemical reaction proceeds:

• Concentration of Reactants

  Increasing the concentration of reactants increases the number of collisions between molecules. This results in a greater probability of effective collisions and, consequently, increases the reaction rate.

• Reaction Temperature

  Increased temperature causes an increase in the kinetic energy of the molecules and the percentage of activated molecules. This increases the number of effective collisions and, therefore, increases the reaction rate.

• Degree of Division (Surface Area)

  A higher degree of division of solid particles increases the surface contact area. This leads to a higher probability of effective collisions and increases the reaction speed.

• Catalysts

  Catalysts are substances that change the activation energy, thereby altering the reaction speed (either speeding it up or slowing it down). They do not experience permanent alteration during the reaction.

Catalysis: Positive and Negative Agents

• Positive Catalysts

  These substances decrease the activation energy value, achieving a greater number of effective collisions, which increases the reaction rate. They provide a reaction mechanism different from the usual one, with lower activation energy.

• Negative Catalysts (Inhibitors)

  These substances increase the activation energy value, reducing the effective number of collisions and decreasing the reaction rate. They prevent the reaction mechanism from using its usual pathway.

Chemical Equilibrium: Le Chatelier's Principle

The position of chemical equilibrium can be shifted by changes in external conditions, as described by Le Chatelier's principle:

• Concentration

  The value of the equilibrium constant remains unchanged. If the concentration of a substance in equilibrium is increased, the system shifts in the direction in which this substance is consumed. If the concentration decreases, the system shifts in the direction which produces these substances.

• Temperature

  Temperature changes the equilibrium constant. If the temperature increases, the system shifts to consume heat (the sense of the endothermic reaction). If the temperature decreases, the system shifts to release heat (the sense of the exothermic reaction).

• Pressure

  Pressure changes do not affect the value of the equilibrium constant (assuming volume varies). Increasing the total pressure of a gaseous system in equilibrium causes the system to shift toward the side with fewer moles of gas. If the pressure decreases, the system shifts toward the side with a greater number of moles of gas.