Relationship between activation energy and rate of reaction

The Activation Energy of Chemical Reactions

relationship between activation energy and rate of reaction

Second order reaction: For a second order reaction (of the form: rate=k[A]2) the and thus we can calculate the activation energy from the above relation. First. However, if a catalyst is added to the reaction, the activation energy is lowered Enzymes affect the rate of the reaction in both the forward and reverse represents the difference in energy between the ground state and the. Relations between Activation Energy, Rate of Reaction, Bond Dissociation Glasstone, Laidler, and Eyring, The Theory of Rate Processes (McGraw‐Hill Book.

The enzymes lose their activity upon heating or changing the pH or adding certain chemical reagents. This is due to deformation of the configuration of the active site.

Surface Area of Reactant: The rate of a reaction increases with increase in the surface area of solid reactant if any used.


The surface of a solid can be increased by grinding it to a fine powder. This is also true with the solid catalysts, which are usually employed in finely powdered form while carrying out a chemical reaction.

  • The Arrhenius Law: Activation Energies
  • Arhenius Equation

The Intensity of Light: The rate of some photochemical reactions, which occur in presence of light, increases with increase in the intensity of suitable light used. With the increase in the intensity, the number of photons in light also increases.

Hence number of reactant molecules get energy by absorbing more number of photons and undergo a chemical change. The solvent may affect the rate in many ways as explained below: The solvents are used to dissolve the reactants and while doing so they help in providing more interactive surface between reactant molecules which may be otherwise in different phases or strongly bonded in the solid phase.

Usually, solvents help in breaking the cohesive forces between ions or molecules in the solid state. The polar molecules tend to dissolve more in polar solvents with more dielectric constants and react faster in them. Whereas nonpolar molecules prefer nonpolar solvents. In case of diffusion controlled reactions, the viscosity of the solvent plays a major role.

The rate decreases with increase in the viscosity of the solvent. Effect of Change of Temperature on the Rate of Reaction: The two distribution graphs are shown below for a lower temperature T1 and a higher temperature T2. The area under each curve represents the total number of molecules whose energies fall within the particular range. The shaded regions indicate the number of molecules which are sufficiently energetic to meet the requirements dictated by the two values of Ea that are shown.

It is clear from these graphs that the fraction of molecules whose kinetic energy exceeds the activation energy increases quite rapidly as the temperature is raised. This the reason that virtually all chemical reactions and all elementary reactions proceed more rapidly at higher temperatures.

Activation energy - Wikipedia

Arrhenius came up with an equation that demonstrated that rate constants of different kinds of chemical reactions varied with temperature. This equation indicates a rate constant that has a proportional relationship with temperature. For example, as the rate constant increases, the temperature of the chemical reaction generally also increases.

relationship between activation energy and rate of reaction

The result is given below: A is called frequency factor or pre-exponential factor and proportional to the frequency of collisions between reacting molecules. A is independent of the absolute temperature T. Equations 12 and 3 are different forms of Arrhenius equation.

relationship between activation energy and rate of reaction

Arrhenius Equation and Temperature Variation: The relation between rate constant k and the absolute temperature T of the reaction is given by For two different temperatures say T1 and T2 we have Determination of Activation Energy: We have By knowing Values of K1 and K2 at temperatures T1 and T2 using experiments, the value of activation energy can be calculated. Only a small fraction of the collisions between reactant molecules convert the reactants into the products of the reaction.

In order for the reaction to occur, the nitrogen atom in NO must collide with the chlorine atom in ClNO2.

relationship between activation energy and rate of reaction

Another factor that influences whether reaction will occur is the energy the molecules carry when they collide. Not all of the molecules have the same kinetic energy, as shown in the figure below.

relationship between activation energy and rate of reaction

This is important because the kinetic energy molecules carry when they collide is the principal source of the energy that must be invested in a reaction to get it started. The vertical axis in this diagram represents the free energy of a pair of molecules as a chlorine atom is transferred from one to the other. The horizontal axis represents the the sequence of infinitesimally small changes that must occur to convert the reactants into the products of this reaction.

To understand why reactions have an activation energy, consider what has to happen in order for ClNO2 to react with NO. First, and foremost, these two molecules have to collide, thereby organizing the system. Not only do they have to be brought together, they have to be held in exactly the right orientation relative to each other to ensure that reaction can occur. Both of these factors raise the free energy of the system by lowering the entropy.

As the temperature of the system increases, the number of molecules that carry enough energy to react when they collide also increases. The rate of reaction therefore increases with temperature.

As a rule, the rate of a reaction doubles for every 10oC increase in the temperature of the system. Purists might note that the symbol used to represent the difference between the free energies of the products and the reactants in the above figure is Go, not Go.

Activation energy

A small capital "G" is used to remind us that this diagram plots the free energy of a pair of molecules as they react, not the free energy of a system that contains many pairs of molecules undergoing collision.

If we averaged the results of this calculation over the entire array of molecules in the system, we would get the change in the free energy of the system, Go. Purists might also note that the symbol used to represent the activation energy is written with a capital "E". This is unfortunate, because it leads students to believe the activation energy is the change in the internal energy of the system, which is not quite true. Ea measures the change in the potential energy of a pair of molecules that is required to begin the process of converting a pair of reactant molecules into a pair of product molecules.

Activation Energy

Catalysts and the Rates of Chemical Reactions Aqueous solutions of hydrogen peroxide are stable until we add a small quantity of the I- ion, a piece of platinum metal, a few drops of blood, or a freshly cut slice of turnip, at which point the hydrogen peroxide rapidly decomposes.

Four criteria must be satisfied in order for something to be classified as catalyst. Catalysts increase the rate of reaction.

Catalysts are not consumed by the reaction.

relationship between activation energy and rate of reaction