Chemical Kinetics: Reaction Rates and Equilibrium

Chemical Kinetics

Chemical kinetics is the study of the speed with which chemical reactions occur.

Rate of Reaction

For a homogeneous reaction between reactive gases in solution, the average speed is defined as the ratio between the variation of the concentration (expressed in mol/liter) of one of the reactants or products and the time interval in which that variation occurs.

Instantaneous speed is the speed that the reaction has at a given moment.

Collision Theory

The energy required for a collision to be effective and break the bonds necessary to produce the reaction is called activation energy.

Rate Equation

The rate equation is defined as the equation, determined experimentally, used to predict the dependence of the velocity of a chemical reaction on reagent concentrations.

Factors Influencing Reaction Rate

• Nature of reagents: Gaseous substances react faster than substances in solution, and these, in turn, react faster than solids.
• An increase in the total pressure on the mixture increases the reaction rate.
• For solid reactants, the reaction rate increases with the degree of division.

Temperature of Reaction

The relationship between reaction rate and temperature was established by Arrhenius in 1889 through the expression that relates the rate constant with temperature.

Catalysts

A catalyst is a substance that speeds up the reaction without being consumed in the process. Catalysts can work by forming an intermediate compound or by adsorbing reagents.

Law of Mass Action

The law of mass action states that the equilibrium constant is the product of the concentrations of the products raised to their stoichiometric coefficients, divided by the product of the concentrations of the reactants raised to their stoichiometric coefficients.

Value of the Equilibrium Constant (Kc)

The value of the equilibrium constant, Kc, is specific to each chemical reaction, independent of the initial concentrations of reactants and products. It only depends on the temperature.

Degree of Dissociation

The degree of dissociation indicates the amount of a reagent that has reacted. It is equal to the ratio between the amount (expressed in moles) of reagent that has reacted and the initial amount of reactants.

Reaction Quotient

The reaction quotient is calculated in the same way as the equilibrium constant, but it uses the initial concentrations of reagents and products.

Le Chatelier's Principle

When a system at equilibrium undergoes a change in concentration of the reacting species, pressure, or temperature, the system responds by reaching a new equilibrium that partially offsets the effect of the modification.

Examples of changes include:

• Adding or removing a species in a gaseous state.
• Adding an inert gas while keeping the volume constant.
• Changing the system volume.