Physical Chemistry: Kinetics

Rate of Reaction

The rate of reaction refers to the speed at which reactants are converted into products.
The units for rate of reaction are usually concentration per unit time, i.e. mol dm^-3 s^-1.
A fast reaction is one where the reactants are quickly converted into products.
A slow reaction takes a long time to convert reactants into products.

Factors Affecting the Rate of Reaction

Several factors can influence the rate of reaction, including temperature, concentration, pressure, the presence of a catalyst, and surface area.
Temperature: The higher the temperature, the faster the particles move, and hence the greater the likelihood of collisions leading to a reaction.
Concentration and Pressure: An increase in the concentration of reactants or pressure in gaseous reactions increases the frequency of collisions and hence increases the rate of reaction.
Catalysts: They reduce the activation energy needed for a reaction to occur, thereby increasing the rate of reaction.
Surface Area: By increasing the surface area of a reactant, more particles are exposed, allowing for more chance of collisions and hence faster reaction rates.

Collision Theory

The collision theory states that for a reaction to occur, particles must collide with enough energy (known as the activation energy), and with the correct spatial orientation.
Only a certain proportion of all collisions lead to a successful reaction. This proportion can be increased by raising the temperature or adding a suitable catalyst.

Activation Energy

The minimum amount of energy required for a reaction to occur is the activation energy, often represented by the symbol Ea.
The activation energy can be thought of as the energy barrier that must be overcome for reactants to form products.
A reaction with higher activation energy will occur more slowly than a similar reaction with a lower activation energy.

Rate Equations

The rate equation shows how the rate of reaction depends on the concentration of each reactant.
The general form of the rate equation is rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are concentrations of reactants, and m and n are the orders of reaction.
The order of reaction represents the dependence of rate on the concentration of that particular reactant.
A reactant’s order (i.e. zero, first or second order) determines whether the rate stays the same, increases linearly, or increases exponentially with increasing concentration of that reactant.

Reaction Mechanisms

Reaction mechanisms outline the series of individual steps that occur during a reaction.
Each step of the reaction mechanism is called an elementary process, which describes one molecular event such as breaking or forming a bond.
The slowest step of these events is known as the rate-determining step and it controls the rate of the whole reaction.

These key topics provide a basic understanding of kinetics, which is fundamental in understanding how and why reactions occur with different speeds under varying conditions.