Why does activation energy change with temperature

The Arrhenius equations relates the rate of a chemical reaction to the magnitude of the activation energy:. Collision theory provides a qualitative explanation of chemical reactions and the rates at which they occur, appealing to the principle that molecules must collide to react. Collision Theory provides a qualitative explanation of chemical reactions and the rates at which they occur.

A basic principal of collision theory is that, in order to react, molecules must collide. This fundamental rule guides any analysis of an ordinary reaction mechanism.

Consider the elementary bimolecular reaction:. If the two molecules A and B are to react, they must come into contact with sufficient force so that chemical bonds break. We call such an encounter a collision. If both A and B are gases, the frequency of collisions between A and B will be proportional to the concentration of each gas. If we double the concentration of A, the frequency of A-B collisions will double, and doubling the concentration of B will have the same effect.

Therefore, according to collision theory, the rate at which molecules collide will have an impact on the overall reaction rate. When two billiard balls collide, they simply bounce off of one other.

This is also the most likely outcome when two molecules, A and B, come into contact: they bounce off one another, completely unchanged and unaffected. In order for a collision to be successful by resulting in a chemical reaction, A and B must collide with sufficient energy to break chemical bonds. This is because in any chemical reaction, chemical bonds in the reactants are broken, and new bonds in the products are formed.

Therefore, in order to effectively initiate a reaction, the reactants must be moving fast enough with enough kinetic energy so that they collide with sufficient force for bonds to break. This minimum energy with which molecules must be moving in order for a collision to result in a chemical reaction is known as the activation energy.

As we know from the kinetic theory of gases, the kinetic energy of a gas is directly proportional to temperature. As temperature increases, molecules gain energy and move faster and faster. Therefore, the greater the temperature, the higher the probability that molecules will be moving with the necessary activation energy for a reaction to occur upon collision.

Even if two molecules collide with sufficient activation energy, there is no guarantee that the collision will be successful. In fact, the collision theory says that not every collision is successful, even if molecules are moving with enough energy.

The reason for this is because molecules also need to collide with the right orientation, so that the proper atoms line up with one another, and bonds can break and re-form in the necessary fashion. For example, in the gas- phase reaction of dinitrogen oxide with nitric oxide, the oxygen end of N 2 O must hit the nitrogen end of NO; if either molecule is not lined up correctly, no reaction will occur upon their collision, regardless of how much energy they have.

However, because molecules in the liquid and gas phase are in constant, random motion, there is always the probability that two molecules will collide in just the right way for them to react.

Of course, the more critical this orientational requirement is, like it is for larger or more complex molecules, the fewer collisions there will be that will be effective. An effective collision is defined as one in which molecules collide with sufficient energy and proper orientation, so that a reaction occurs.

According to the collision theory, the following criteria must be met in order for a chemical reaction to occur:. Collision theory explanation : Collision theory provides an explanation for how particles interact to cause a reaction and the formation of new products. The rate of a chemical reaction depends on factors that affect whether reactants can collide with sufficient energy for reaction to occur. Explain how concentration, surface area, pressure, temperature, and the addition of catalysts affect reaction rate.

Raising the concentrations of reactants makes the reaction happen at a faster rate. Active 4 years, 4 months ago.

Viewed 24k times. Improve this question. Individual molecules need to have a kinetic energy comparable to the height of the barrier in order to react. When you increase the temperature, it increases the average kinetic energy of the molecules and hence the number of molecules with sufficient energy to react , but does not change the height of the barrier.

So the reactants cross over the barrier more frequently -- hence a faster reaction. Add a comment. Active Oldest Votes. Improve this answer. Email Link. Arrhenius Equation:. Re: Effect of temperature on activation energy Post by Maisha 2L » Mon Mar 07, am Actually, a higher temperature means a faster reaction. The Arrhenius equation can be written in a non-exponential form, which is often more convenient to use and to interpret graphically.

This affords a simple way of determining the activation energy from values of k observed at different temperatures. Depending on the magnitudes of E a and the temperature, this fraction can range from zero, where no molecules have enough energy to react, to unity, where all molecules have enough energy to react. Therefore, A represents the maximum possible rate constant; it is what the rate constant would be if every collision between any pair of molecules resulted in a chemical reaction.

This could only occur if either the activation energy were zero, or if the kinetic energy of all molecules exceeded E a —both of which are highly unlikely scenarios. In a given chemical reaction, the hypothetical space that occurs between the reactants and the products is known as the transition state.

The species that is formed during the transition state is known as the activated complex. TST is used to describe how a chemical reaction occurs, and it is based upon collision theory. If the rate constant for a reaction is known, TST can be used successfully to calculate the standard enthalpy of activation, the standard entropy of activation, and the standard Gibbs energy of activation.

Transition state theory : The activated complex, which a kind of reactant-product hybrid, exists at the peak of the reaction coordinate, in what is known as the transition state. According to transition state theory, between the state in which molecules exist as reactants and the state in which they exist as products, there is an intermediate state known as the transition state. The species that forms during the transition state is a higher-energy species known as the activated complex.

TST postulates three major factors that determine whether or not a reaction will occur. These factors are:. This third postulate acts as a kind of qualifier for something we have already explored in our discussion on collision theory.

According to collision theory, a successful collision is one in which molecules collide with enough energy and with proper orientation, so that reaction will occur. However, according to transition state theory, a successful collision will not necessarily lead to product formation, but only to the formation of the activated complex. Once the activated complex is formed, it can then continue its transformation into products, or it can revert back to reactants. Transition state theory is most useful in the field of biochemistry, where it is often used to model reactions catalyzed by enzymes in the body.

For instance, by knowing the possible transition states that can form in a given reaction, as well as knowing the various activation energies for each transition state, it becomes possible to predict the course of a biochemical reaction, and to determine its reaction rate and rate constant. Privacy Policy. Skip to main content.

Chemical Kinetics. Search for:. Activation Energy and Temperature Dependence Activation Energy Activation energy is the energy required for a reaction to occur, and determines its rate. Learning Objectives Discuss the concept of activation energy. Key Takeaways Key Points Reactions require an input of energy to initiate the reaction; this is called the activation energy E A.

Activation energy is the amount of energy required to reach the transition state. For cellular reactions to occur fast enough over short time scales, their activation energies are lowered by molecules called catalysts.

Enzymes are catalysts. Key Terms activation energy : The minimum energy required for a reaction to occur. The Collision Theory Collision theory provides a qualitative explanation of chemical reactions and the rates at which they occur, appealing to the principle that molecules must collide to react.

Learning Objectives Discuss the role of activation energy, collisions, and molecular orientation in collision theory. Key Takeaways Key Points Molecules must collide in order to react. In order to effectively initiate a reaction, collisions must be sufficiently energetic kinetic energy to break chemical bonds; this energy is known as the activation energy.

As the temperature rises, molecules move faster and collide more vigorously, greatly increasing the likelihood of bond breakage upon collision.

Key Terms activation energy : The minimum energy with which reactants must collide in order for a reaction to occur. Factors that Affect Reaction Rate The rate of a chemical reaction depends on factors that affect whether reactants can collide with sufficient energy for reaction to occur. Learning Objectives Explain how concentration, surface area, pressure, temperature, and the addition of catalysts affect reaction rate.

Key Takeaways Key Points When the concentrations of the reactants are raised, the reaction proceeds more quickly. This is due to an increase in the number of molecules that have the minimum required energy. For gases, increasing pressure has the same effect as increasing concentration. When solids and liquids react, increasing the surface area of the solid will increase the reaction rate. This is due to an increase in the number of particles that have the minimum energy required.