In chemistry an activated complex is defined by the International Union of Pure and Applied Chemistry (IUPAC) as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential energy surface".[1] In other words, it refers to a collection of intermediate structures in a chemical reaction that persist while bonds are breaking and new bonds are forming. It therefore represents not one defined state, but rather a range of transient configurations that a collection of atoms passes through in between clearly defined products and reactants.
It is the subject of transition state theory - also known as activated complex theory - which studies the kinetics of reactions that pass through a defined intermediate state with standard Gibbs energy of activation ΔG°‡.[2] The state represented by the double dagger symbol is known as the transition state and represents the exact configuration that has an equal probability of forming either the reactants or products of the given reaction.[3] The activated complex is often confused with the transition state and is used interchangeably in many textbooks. However, it differs from the transition state in that the transition state represents only the highest potential energy configuration of the atoms during the reaction while the activated complex refers to a range of configurations near the transition state that the atoms pass through in the transformation from products to reactants. This can be visualized in terms of a reaction coordinate, where the transition state is the molecular configuration at the peak of the diagram while the activated complex can refer to any point near the maximum. Activated complex has partial reactant and partial product character.[4]
An activated complex is an intermediate state that is formed during the conversion of reactants into products. An activated complex is the structure that results in the maximum energy point along the reaction path. The activation energy of a chemical reaction is the difference between the energy of the activated complex and the energy of the reactants. Consider a chemical reaction between reactants A and B to form products C and D. The reactants must collide with each other and interact in order to form the products. Several factors improve the chances that A and B will encounter each other, including increased temperature, increased concentration of reactants, or adding a catalyst. In a reaction with an activated complex, A and B form the complex A-B. The complex only forms if sufficient energy (the activation energy) is present. The energy of the activated complex is higher than that of either the reactants or products, which makes the activated complex unstable and temporary. If there isn't enough energy for the activated complex to form the products, it eventually breaks apart into the reactants. If enough energy is available, the products form. Some textbooks use the terms transition state and activated complex interchangeably, but they mean different things. The transition state refers only to the highest potential energy of the atoms participating in a chemical reaction. The activated complex covers a range of atom configurations that atoms form on their way from reactant to products. In other words, the transition state is the one molecular configuration that occurs at the peak of the energy diagram of the reaction. The activated complex may be present at any point near the transition state.
A reaction is second order with respect to a reactant. How is the rate of rection affected if the concentration of the reactant is
Let the reaction A → B is a 2nd order reaction w.r.t A and conc. of A is ‘a’ mol/L, then rate of reaction can be written as: dxdt=k[A]2 =ka2 (i) When conc. [A] is doubled i.e., [A’] = 2a mol/L Then new rate of reaction d'(x)dt=k[2a]2 = 4ka2 = 4dxdt Thus rate of reaction will become four times where concentration is doubled.
d'(x)dt = ka22 = 14ka2 = 14dxdt The rate of reaction will become one-fourth of the initial rate of reaction.  Text Solution Solution : When the reactant molecules absorb energy their bonds are loosened and there is formation of new loose bonds between them. This intermediate thus, formed is known as activated complex or transition state complex, which is highly unstable and immediately dissocoates into products and some energy is released during this dissociation. <br> e.g. <br> <img src="https://doubtnut-static.s.llnwi.net/static/physics_images/ARH_CHSE_ODI_13Y_SP_CHE_XII_C04_E03_014_S01.png" width="80%"> Velcro is a synthetic material that allows fabric (among other things) to stick together. Another, more unusual, use for Velcro is the sport of "Velcro-jumping". The participant wears clothing made of Velcro and jumps at a Velcro-covered wall. Sometimes the collision with the wall will result in the person sticking to the wall. Other times, the person simply bounces off the wall and does not connect. Reactant particles sometimes collide with one another and remain unchanged by the collision. Other times, the collision leads to the formation of products. The state of the particles that is in between the reactants and products is called the activated complex. An activated complex is an unstable arrangement of atoms that exists momentarily at the peak of the activation energy barrier. Because of its high energy, the activated complex exists for an extremely short period of time (about \(10^{-13} \: \text{s}\)). There is equal likelihood that the activated complex either reforms the original reactants or goes on to form products. The figure below shows the formation of a possible activated complex between colliding hydrogen and oxygen molecules. Because of their unstable nature and brief existence, very little is known about the exact structures of many activated complexes. Summary
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What is the role of the activated complex in a chemical reaction
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