Chemical Kinetics Notes

Chemical Kinetics

Definition

• Chemical kinetics, also known as reaction kinetics, is the study of the rates of chemical processes.
• It examines how quickly a reaction proceeds.
• It investigates the influence of experimental conditions on reaction speed.
• It yields information about the reaction's mechanism and transition states.
• It enables the construction of mathematical models to describe reaction characteristics.

Rate of Reaction

• The rate of a reaction is the change in concentration of a product or reactant per unit time.
• $rate = \frac{\Delta[product]}{time}$
• $rate = -\frac{\Delta[reactant]}{time}$

Experimental Methods to Determine Reaction Rate

• Gas collection method
• Colorimetric method
• Precipitation (disappearing cross) method
• Titrimetric method

Factors Affecting Reaction Rate

• Concentration
• Temperature
• Catalysts
• Pressure
• Surface area

Perspectives on Chemical Kinetics

• Chemical kinetics can be analyzed from two primary viewpoints:
  • Collision Theory
  • Transition State Theory

Collision Theory

• Based on kinetic theory.
• Assumes particles must collide with the correct orientation and sufficient kinetic energy for reactants to convert into products.
• For the reaction A + B → products, particles (reactants) with sufficient kinetic energy collide and form products in a one-step process.
• Not all collisions are successful because not all particles possess sufficient kinetic energy (activation energy).
• Activation energy is the minimum energy required for a reaction to occur.

Transition State Theory

• This theory posits that as reactants approach each other, a transitory activated complex (transition state) is formed at a potential energy maximum.
• At the activated complex, original bonds weaken, and new bonds are partially formed.
• Collisions are not necessary for a successful reaction: A + B → [T.S.] → products
• In transition state theory, activation energy is the energy difference between reactants and the potential energy maximum.

Concentration

• As reactant concentration increases, the frequency of molecular collisions increases, raising the probability of successful collisions and, thus, the reaction rate.

Temperature

• Molecules at higher temperatures have greater average kinetic energy.
• While collision frequency increases at higher temperatures, its contribution to the increase in reaction rate is small.
• The proportion of reactant molecules with sufficient energy to react (energy greater than activation energy: E > E_a) is significantly greater, leading to more successful collisions and a faster reaction rate.

Catalyst

• A catalyst is a substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterward.
• Catalysts increase the reaction rate by providing a different reaction mechanism with a lower activation energy, enabling more particles to collide successfully.

Pressure

• Increasing the pressure in a gaseous reaction increases the frequency of collisions between reactant particles.
• This leads to a corresponding increase in the frequency of successful collisions and, thus, an increase in the rate of reaction.

Surface Area

• Reactants in the solid state react faster when subdivided into smaller particles.
• This results in a greater surface area per unit volume, increasing contact with other reactant particles, and thus accelerating the reaction.