>>chem.: Factors Affecting the Rate of a Chemical Reaction

What is Chemical Reaction?

Chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input ofenergy, or non-spontaneous, often coming about only after the input of some type of energy,heat, light or electricity.

There are many things that we should understand and explore about chemical reaction, but this tutorial will only focus on the factors that affect the rate of a certain chemical reaction.

By the way,what do we really mean about FACTOR?

FACTOR/S: It is a Constituent or element that brings about certain effects or results, or indicates a specific multiple, number, or quantity.

FACTOR no.1

TEMPERATURE

As always,temperature is the measurement of the average kinetic energy of the molecules in an object or system and can be measured with a thermometer or a calorimeter. It is a means of determining the internal energy contained within the system.

But,how can temperature affect the rate of a chemical reaction?

Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of thumb for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10°C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop.

ALWAYS REMEMBER:

All reaction rates increases with an increase in temperature.This happens because the kinetic energy of the reactants increase with a rise in temperature.
The faster the reactant particles move,the greater the chance they will collide and react with each other.
As general rule,9 or 10 degree Celsius rise in temperature increases the reaction rate twice.

FACTOR no.2

CONCENTRATION OF REACTANTS

Concentration effect is important because chemical reactions are usually carried out in solutions.

The concentration of reactants plays an important role in chemical kinetics. It is usually true that by increasing the amount of reactants, the rate of reaction is increased. As we know that an increase in concentration of reactants increases the number of molecules. According to collision theory, the greater the number of molecules the higher is the collision ratio, consequently faster is the rate of reaction.

EXAMPLE: The tragedy in 1967,the ground test of Apollo I.

FACTOR no.3

NATURE OF REACTANTS

Different reactants have different energies of activation. Reaction between polar or ionic molecules is very fast. Oxidation-Reduction reactions are slower than ionic reactions because they involve transfer of electrons and bond rearrangement. The physical state of reacting substances are important in determining their reactivities. The reaction in which ionic solutions are involved also take place at high speed.

POLAR to POLAR-reacts easily
NON-POLAR to NON-POLAR-reacts easily
POLAR to NON-POLAR-do not react easily

FACTOR no.4

PRESENCE OF CATALYST

Presence of Catalysts and Competitors

Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. Catalysts work by increasing the frequency of collisions between reactants, altering the orientation of reactants so that more collisions are effective, reducing intramolecular bonding within reactant molecules, or donating electron density to the reactants. The presence of a catalyst helps a reaction to proceed more quickly to equilibrium. Aside from catalysts, other chemical species can affect a reaction. The quantity of hydrogen ions (the pH of aqueous solutions) can alter a reaction rate. Other chemical species may compete for a reactant or alter orientation, bonding, electron density, etc., thereby decreasing the rate of a reaction.

POSITIVE CATALYST:
A positive catalyst increases the rate of reaction by lowering the energy of activation. Thus in the presence of a positive catalyst, the greater fraction of the total molecule will posses lower energy of activation and collided successfully in a short period of time, there by increasing the rate of reaction

NEGATIVE CATALYST OR INHIBITOR:
A negative catalyst retards the rate of reaction. Negative catalyst do not lower the energy of activation rather they are combined with reactant molecule thus decreasing the number of colliding reactant molecules. This decreases the effective collisions, hence rate of reaction.

What is the role of the positive catalyst?

A positive catalyst functions by providing an alternate path to the reaction or by the formation of a transition (intermediate) compound having low energy of activation. The activation energy of this path is lower. As a result rate of reaction is increased.

What is the Role of the Negative catalyst?

A negative catalyst does not lower the energy of activation rather it combines with the reactant molecules, thus decreasing the number of colliding reactant molecules. This decreases the effective collisions , hence rate of reaction.

FACTOR no.5

SURFACE AREA

In heterogeneous reactions, the rate of reaction depends upon the surface area of solid reactant. Greater the surface area, higher is the rate of reaction. For example finely divided calcium carbonate (marble) reacts more quickly with hydrochloric acid than calcium carbonate chips. It is due to the fact that powered calcium carbonate offers larger surface area to the reacting acid. In other words, by increasing the surface area of reactant, rate of reaction increases due to greater contact between individual particles and also due to the fact that the surface molecules reacts more quickly.

ALWAYS REMEMBER: