E reac. reac. A2 + B2 ----------------------H r H 2AB E prod. prod. -------------------------------------
reaction path Graph’s Graph’s details: The graph represented above is a reaction profile (progress) for an uncatnalyzed, combination reaction: A2 + B2 2AB < 0 r H < The curve refers to an exothermic process, because the potential energy of reactants (A 2 and B2) is greater than the potential energy of the obtained product (AB), hence certain amount of heat energy is given out to the surroundings in attempt of the reactants to reduce the energy i.e. r H = Epprod. Epreact. < 0 (the reaction enthalpy is negative for this synthesis reaction but positive for the reversed process, the process of decomposition of AB) Graph’s trend: The reaction profile depicts the variation in potential energy of the two molecules A 2 and B2 that: - approach (as they came closer the Ep increases so the curve goes up); - collide (if they have proper orientation and enough energy the collision will be effective); - form a temporary unstable arrangement of all the atoms of the reactants called activated complex – the peak of the curve schematically described by the scheme. The bonds which connect the atoms A-A and atoms B-B will break up and the electrons in them will redistribute in order to connect the new atoms A-B in the product, then follows the separation of the two new born molecules A-B which result in decreasing in potential energy – the curve rapidly falling down; - if the input of energy is not enough to activate the molecules of the reactants, instead product may break apart to re-form the reactants. But only small fraction of the reacting molecules posses the minimum amount of energy (activation energy, E A), needed for the reacting particles to react. So, the process of breaking the bonds in A 2 and B2 molecules thus requires energy called activation energy. Here from the temperature should be increased in order to increase the fraction of active molecules i.e. molecules that have energy equal to or greater than E A to overcome the energy barrier to form product.
A+B
peak 2
E A2
catalyzed reaction
E A2
E reac.---------------------------------------------A+K
AK + B
AB
E prod. prod. AB + K
reaction path Graph’s details: The graphs represented above show two reaction profiles, the curve refers to an uncatnalyzed reaction while the red colored curve is a catalytic, combination combination reaction: A + B AB < 0 r H < However both curves refer to an exothermic process, because the potential energy of reactants (A and B) is greater than the potential energy of the obtained product (AB), hence certain amount of heat energy is given out to the surroundings in attempt of the reactants to reduce the energy energy i.e. reaction enthalpy is Epreact. < 0 (the reaction r H = Epprod. negative for this reaction but positive for the reversed process; decomposition of AB) Graph’s trend: We will pay attention to the reaction in which a catalyst is used. The reaction profile depicts the variation in potential energy of the two molecules A and B during the course of this catalytic reaction. The reaction takes place in two stages; I) A + K AK the substance (reactant A) which combines with the catalyst K is called substrate; First, catalyst reacts with one of the reactants (in this case A) to form intermediate compound AK (peak 1) and in the second phase, the unstable intermediate reacts with the second reactant, B, forming an unstable arrangement of atoms involved in this elementary act, called activated complex (peak 2). AB compound is obtained and the catalyst, K, is released for further interactions; II) AK + B AB + K Finally it is important to point out that catalyst modifies the reaction path. The activation energies for the non-catalyzed (E A1) and catalyzed ( E A2) differ; E A1 is higher than E A2 because there are two steps with respective activation energies. The two intermediate steps have lower activation energy thus the catalytic reaction proceeds at higher rate.
