Chemical Reaction Kinetics: Rates, Mechanisms, and Factors
Reaction Rate: Definition
Reaction Rate: The measure of how quickly reactants are consumed or how quickly products are formed in a chemical reaction.
Rate Law Equation
The Rate Law Equation describes the relationship between the rate at which a chemical process occurs and the concentrations of the reactants.
The rate constant (k) depends on the specific reaction and the temperature at which it takes place.
Reaction Order Explained
Reaction Order: The sum of the exponents to which reactant concentrations are raised in the rate equation.
Reaction Mechanisms
When reaction orders do not match the stoichiometric coefficients, it indicates that the reaction proceeds via a series of elementary steps. A reaction mechanism refers to the sequence of all elementary steps that, when combined, yield the overall reaction.
Not all elementary steps occur at the same speed. The slowest step in the mechanism is called the rate-determining step (or rate-limiting step), as it dictates the overall reaction rate. The rate equation for the overall reaction is derived from this dominant step.
We define molecularity as the number of molecules, atoms, or ions involved in each elementary reaction. In this specific case, the molecularity does coincide with the stoichiometric coefficients of that elementary step.
If a reaction occurs in a single step, then its molecularity and reaction order are identical, and both correspond to the stoichiometric coefficients of the reactants.
Theories of Chemical Reactions
Collision Theory
This theory assumes that for a chemical reaction to occur, reactant particles must collide. However, for these collisions to be effective and lead to a reaction, two conditions must be met:
- The energy of the reactants must be sufficient to break existing bonds in the reactive particles. Every chemical reaction needs a certain amount of energy to proceed, which is known as activation energy.
- For the collision to be effective, it’s not enough for the energy to be sufficient; the reactant particles must also collide with the correct orientation.
Transition State Theory (Activated Complex Theory)
This theory incorporates all the requirements of Collision Theory but adds that the reaction proceeds through an intermediate state called the activated complex. This activated complex is a quasi-molecule where bonds in the reactants are partially broken, and new bonds for the products are partially formed. The activated complex possesses a high amount of energy, making it extremely unstable, and it rapidly degenerates into products, releasing excess energy.
Factors Influencing Reaction Rates
Any factor that increases the number of effective collisions between reactant molecules will increase the reaction rate.
Nature and Physical State of Reactants
In the gaseous state, reactant particles are more mobile than in the liquid state, and even more so than in the solid state. Therefore, gases generally react faster than liquids, and liquids react faster than solids. For solids, the more finely divided they are, the greater their surface area available for collisions, thus increasing the reaction rate. The inherent reactivity of substances also varies, depending on their electronic configuration and the types of bonds present.
Concentration of Reactants
A higher concentration of reactants means more particles per unit volume, leading to a greater frequency of collisions and, consequently, a higher reaction rate.
Temperature
Increasing the temperature increases the kinetic energy of particles, which in turn increases both the frequency and the energy of collisions, leading to a higher reaction rate. Svante Arrhenius proposed an equation that relates the rate constant (k) with temperature.
Where R is the ideal gas constant (8.31 J/mol·K). The rate constant (k) is highly dependent on the temperature at which the reaction occurs.
Catalysts
A catalyst is a substance that participates in a chemical reaction without being consumed, altering the reaction rate by providing an alternative reaction pathway with a lower activation energy, thus speeding up the process. Catalyzed reactions are crucial in industry, as they accelerate product formation.
Some specialized catalysts are known as biocatalysts (enzymes), which exhibit high selectivity and stereospecificity. Conversely, inhibitors are substances sometimes referred to as negative catalysts, as they slow down or prevent certain reactions.
Reaction Enthalpy
Reaction Enthalpy (ΔH): The energy difference between the products and the reactants.
Intermediate State
Intermediate State: A transient state that occurs between elementary steps in a multi-step reaction.