COLLISION THEORY: Energy and Orientation Barrier to Reactions We know that the basic requirement for a reaction to occur is that the reacting species must undergo collision with one another. This is basis of the collision theory for the reactions. The number of collisions that takes place per second per unit volume of the reaction mixture is called collision frequency. The value of collision frequency is generally very high. Under ordinary conditions of temperature and pressure in a gaseous system, the collision frequency of binary collisions is of the order 1025 to 1028 This theory explains that how chemical reactions take place and why rates of reaction changes. For a reaction to occur, the reactant particles must collide. Only a certain fraction of the total collisions results in the formation of products. Such collisions are called effective or successful collisions. The successful or effective collisions have sufficient energy (activation energy) during the time of collision to break the bonds present in the reactant molecules and to form new bonds, resulting in the products of the reaction. There are some factors which bring about more collisions like increasing the concentration of the reactants and raising the temperature and bring about more effective collisions which increases the rate of reaction. Presence of catalyst alo increases the rate of reaction because catalyst provides large surface area for the reactants to undergoes collision and less energy is required for the chemical change to take place. Hence more collisions have sufficient energy for reaction to occur. The reaction rate therefore increases.There are two important barriers to a reaction namely(1) Energy barrier (2) Orientation barrier which has to be overcome for the formation of the products.
Energy barrier : For the reacting species to make effetive collisions they should have sufficient energy to break the chemical bonds in the reacting molecules. The minimum amount of energy which the colliding moleccules must possess is known as threshold energy. This means that only those collisions of reactants which possess energies greater than threshold energy will give products.
Orientation barrier : The colliding molecules should also have proper orientation so that the old bonds may break and new bonds are formed. For example, consider the reaction
During this reaction, the products are formed only when the colliding molecules have proper orientationat the time of collisions .These are called effective collisions. On the other hand when molecules are not properly orientedat the of collision,they result in ifeffective collisionsand donot form products. Take another example ,the reaction between bromoethane with OH- ions to form methanol. The OH- ions must attack the positively charged
carbon to form an intermediate whch changes to product after elimination of Br- ion. If OH- does not get proper site for attack, the reaction will not occur.
It is clear that proper orienttion of reactants leads to the bond formation while improper orientation simply makes them to bounce back and no product is formed. The main points of collision theory are smmed up below: • There must be collision between the reacting species for the formation of products. • Only certain number of collisions are effective. • The reacting molecules should have sufficient energy as well as orientation. • The fraction of effective collisions , under ordinary conditions may vary from zero to one. Thus, the rate of reaction is proportional to the a) the number of collisions per unit volume per second(collision frequency,Z) between the reactng species b) the fraction of effective collisions ( properly oriented and possessing sufficient energy) f;
Dependence of reaction rates on temperature Teperature effects the rate of reaction effectively.In general increase in temperature increases the rate of reactionand vice versa. This is possible for both exothermic as well as endothermic reactions For example, the rate contant for the decomposition of N2O5 is 7.87 x 10 –7s-1 at 273k but it becomes 3.56 x 10-5s-1 at 298K it means for a rise of 25ºC in the temperature, the rate constant is increased by about 45 times. Similarly if we consider the time taken for the decomposition of N2O5 to half of its original concentration it is 10 days at 0oC,5 hours at 250C. In general the aproximate rule for the effect of temperature on the reaction rates is that th rate of reacytion or rate constantbecomes almost double for every 100C rise in temperature. This is called temperature coefficient.
Reason for the increases in rate of reaction with rise in temperature As we know collision theory of chemical reactions tells that rate of a reaction depends upon the collision frequency(Z) and fraction of the effective collisions (f) .These factors are discussed below. Increase in the collision Frequency: As the temperature of the reaction is increased ,the average kinetic energy of the molecules also increases which leads to the increases in the number of the collision per unit time (Z). The average kinetic energy of themolecules is directly proportional to the absolute temperature. When the increase in the tempearture is of 10oC the increase in the kinetic energy is only 3%. It means the increase in rate of reaction with temprature is only due to the increase in collision frequency by a factor of 3% which is very small in comparison to the experimentally observed factor of two(2). Therefore the increase of rate in reaction is not only due to the increase in collision frequency. Effective collisions: According to collision theory, only a small fraction of collisions is effective in bringing about the chemical reaction . For effective collisions the colliding molecuels must have minimum energy called threshold energy. The reacting molecuels having energy less than threshold energy will not undergo effective collisions and are not able to form products. Thus the energy of collisions determines the possibility of a reaction. There are some collisions which are highly energetic while others are not. This can be understood by taking an example . As we know that all
the molecules in a substance don’t possess the same kinetic energy because of the collision between the moving molecuels. During the collision ,the energies of the moving molecules is transferred from one molecule to another molecule. Thus there is a distribution of kinetic energies among the reacting molecules. If we plot a graph between energy of molecules and fraction of molecules at a particular temprature then a curve is obtained.This is called Maxwell’s distribution of energies. The fraction of molecules having very low or very high energies is very small. The peak of the graph shows that most of the molecules have intermediate kinetic energies. E corresponds to the minimum or threshold energy required for the effective collisions. The molecules having energy equal to or greater than threshold energy will result in the formation of product and this fraction of molecules which is capable of effective collisions is very small. It may be noted that Lower the value of activation energy larger will be the fraction of colliding molecules for effective collisions.This will increase the rate of reaction. Effect of increase in temprature on the number of effective collisions : This can be explained by drawing a graph between fraction of molecules and energy distribution of molecule at two different tempratures T1 and T2. T2 = T1+ 100 From the graph we can get that at high temperature the curve is shifted towards the right which indicates that at high temperature the molecules have higher energies. And it is also clear that the curve at high temperature is flatter than at lower temperature. This means that the number of molecules with higher energy have increased .
The number of molecules having energies equal to or greater than threshold energy is proportional to the area abcd at temperature T1 and area abef at temperature T2. The area abef is roughly twice as large as abcd. It may be interpreted that the fraction of molecules possessing activation energy has increased approximately twice with every 100 rise in temperature. From the above discussion and graphs we may conclude that increase in the rate of reaction with the rise in temperature is mainly due to the increase in number of effective collisions. Effect of catalyst on reaction rates As we know that raising the temperature can increase the rate of reaction, this raise is within certain limits because in certain cases at high temperature the reactant becomes unstable and gets decomposed. So there must be some other methods to increase the rates of reactions. It has been observed that many reactions are made to proceed at an increased rate by the presence of some other substance. For example mixture of H2 and O2 does not react at room temperature but becomes vigorous in the presence of finally divided platinum. So the substance, which accelerates the rate of reactions without undergoing any change in itself, is called catalyst. The phenomenon of increasing the rate of reaction by the use of catalyst is called catalysis. It is observed that catalyst is not consumed during the progress of the reaction. Actually in a catalyzed reaction the catalyst is used in one step and regenerated in subsequent step. In this way it is used again and again without undergoing any permanent change. Catalyst provides a new path for the reaction in which reactants are converted to products quickly. Catalyst forms a new activated complex of lower potential energy. This means that for the catalyzed reactions the activation energy is lower than that for the uncatalyzed reaction.
As a result the fraction of total number of collisions possessing lower activation energy is increased and hence the rate of reaction also increases. This can be explained with the help of a figure.
Addition of a catalyst C results into the formation of new activated complex of lower activation energy.
Regarding the function of catalyst the following points should be kept in mind: 1) A catalyst may undergo intermediate physical change
2) Although catalyst speeds up reaction but does not shift the position of equilibrium. It is because the presence of catalyst reduces the height of barrier by providing alternative path for the reaction and lowers the activation energy. The lowering in activation energy for forward and backward reaction is the same. As a result the increase in the rate of forward reaction and backward reaction is also same. Hence the position of equilibrium remains unaltered. 3) Catalysts are highly specific in nature. Catalyst, which can be used for one reaction, may not have any effect on the other reaction. 4) The catalyst does not change ∆E or ∆H of reaction. From the above figure it is clear that the addition of a catalyst does not change energy of reactants or products. Effect of radiation on reaction rates Photochemical reactions: There are certain reactions where the rate of chemical reaction is affected by certain radiation. The photons of these radiations having frequencies possess sufficient energy to overcome the activation barriers. Such types of reactions, which are initiated by absorption of radiation, are called photochemical reactions. There are various examples like preparation of HCl, photosynthesis, polymerization, photography, sterilization of water, photo etching etc. Generally ultraviolet and visible radiations are used for carrying out such types of reactions because their photons possess energies approximately of the order of 420 kJ/mole, which is comparable to most of the bond energies. Infra red radiations can’t be used because energy of their photons is of the order of 60kJ, which is very very less for breaking the bond.
Observations noticed during photochemical reactions: 1) Reactants should absorb the light. 2) The energy of light depends upon the frequency of light used. If light of different colors is used, the reaction may not be initiated by all the colors. This is because every color of the light has its own frequency. For example frequency of red light is less than that of blue light. So the reactions, which are initiated by blue light, can’t be initiated by red light due to its less energy. 3) Rate of reaction depends on the intensity of light 4) In some cases the molecule that absorbs light may transfer it’s energy to another molecule, which undergoes a reaction. This is called photosensitizations Mechanism of photochemical reaction: Consider an example of the reaction between H2 and Cl2. The reaction does not occur at room temperature but reacts violently when the mixture of both is exposed to sunlight.
The sunlight starts the reaction by breaking the chlorine molecule into atoms, which are also called free radicals. The reaction proceeds as follows. 1) First step involves dissociation of Cl2 molecules into atoms. This step is initiated by the absorption of photon of energy, which breaks the bond between atoms. So this is called initiation step.
2) The chlorine atom formed in the first step attacks the hydrogen molecule and forms HCl and hydrogen atom.
3) This highly reactive hydrogen atom formed in step 2 further combines with Cl2 molecule to give HCl and chlorine atom H + Cl2 → HCl + Cl (Chain propagation step)
4) These steps are repeated again and again till whole of H2 and Cl2 have reacted to form HCl. 5) There are some reactions, which remove the active species needed for the propagation of the reaction. This called chain-terminating step.
Differences between photochemical reaction and thermo chemical reaction:
Photosensitization: There are some reactions which don’t get initiated directly when exposed to the radiations. For the initiations of such type of reactions a small amount of a foreign material, which can absorb light, is added to the reaction mixture. The added substance does not undergo any chemical change but absorbs energy and transfers the excess energy to one of the reactants. Such a substance which when added to the reaction
mixture helps to start a photochemical reaction without itself undergoing any chemical change is called photosensitiser. The reaction carried by the presence of photo sensitizer is called a photosensitization reaction. For example, the dissociation of X2 can occur in the presence of a photo sensitizer as:
Some examples of photosensitizing reactions are: 1) Hydrogen molecule does not undergo dissociation when exposed to light. But it gets dissociated to atomic hydrogen in the presence of mercury vapors when exposed to light radiation of some wavelength. The reaction is supposed to proceed as:
In this reaction mercury acts as photo sensitizer. 2) Similarly carbon dioxide and water vapors present in air are unable to absorb radiations emitted by the sun. However chlorophyll absorbs the visible light energy from the sunlight. After absorbing the radiation it transfer its energy to carbon dioxide and water molecules that then react chemically and series of the reactions starts and finally form the carbohydrates and oxygen.
Fast Reaction: The rate of chemical reactions varies to a very large extent. Some reactions are very slow but there are some reactions, which are very fast and occur in the order of10-12s (1 Pico second) or even less then this. Such type of reaction are initiated by using a pulse of laser light which lasts for a very short time as short as a few pico seconds. When the reaction is initiated it can be studied by various methods like light absorption, light emission, electrical or magnetic properties. Some examples are: i)
ii)
Recently many new techniques have been developed to study the fast reactions. These are flow methods, relaxation methods and many spectrophotomeric techniques. For example, photosynthesis reaction was studied by flash photolysis.
