Reaction Rates: Understanding Chemical Kinetics
Concept of Speed in Chemical Reactions
The reaction rate indicates the rate at which reactants are transformed into products. The measured reaction rate is the decreased concentration of a reagent or the increased concentration of a product. The speed can be expressed as the disappearance of a reactant or the appearance of a product and can be categorized as average speed and instantaneous velocity.
Average Speed
Speed is not constant but varies depending on the time interval.
Instantaneous Speed
The speed at which the reaction has a very small interval of time allows us to calculate the velocity at a given time. The speed can be defined as the derivative of the concentration versus time for the stoichiometric coefficient and turned into a positive amount.
Factors Influencing Reaction Rate
Collision Theory
In order to create a reaction, there has to be contact between the molecules, i.e., there are collisions between molecules of the reactants. It was noted that sometimes there were clashes, but no reaction occurred. For this, it was said that it was necessary to give the shocks in a definite direction and with a given amount of energy. The state in which they have particular energy is called the activated complex, and the energy is the activation energy. The activation energy is the energy that reactants must absorb to form the activated complex. It is also the difference between the activated complex and the energy of the reactants. In reactions, energy is released when activated, and the activated complex forms products. The difference is the net energy.
Other Factors
Speed depends on the physical state in which the reagents are. It also influences the degree of division of the reactants (the more divided, the more rapid). If the reaction is more effective, the contact surface speed increases; this explains why gases react better than liquids. With increasing temperature, the speed of the reaction increases. The culprit of that factor is taking into account the orientation of the molecules. Some reactions are favored by light (photochemical); there are others that give off light. Catalysts are substances involved in the reaction and cause an increase or decrease in the reaction rate. They can be recovered as introduced in the reaction. Catalysts provide an alternate path and cause a reaction that needs less energy. A catalyst does not change the thermodynamic variables; only the activation energy decreases. There are two types of catalysis:
- Homogeneous: Reactants and catalyst are in the same phase.
- Heterogeneous: Catalyst and reactants are in different phases.
Rate Equation
It is defined as the mathematical expression that relates the instantaneous velocity with the concentration of the reagents at the time.
From the rate equation, we can determine the order of reaction and the molecularity. The molecularity is the number of chemical species involved in the reaction as reactants.
Arrhenius Equation
It shows the relationship between the rate constant and the temperature. K = A. E-EA / RT. From this equation, it follows that k is directly proportional to the frequency of collisions, which will be larger the lower the activation energy and increases with temperature.