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AP.Chem:

TRA‑4 (EU)

, TRA‑4.B (LO)

, TRA‑4.B.1 (EK)

, TRA‑4.B.2 (EK)

, TRA‑4.C (LO)

, TRA‑4.C.1 (EK)

, TRA‑4.C.2 (EK)

, TRA‑4.C.3 (EK)

to think about collision theory let's consider the following reaction here we have atom a reacting with a diatomic molecule BC to form a new diatomic molecule a B and C according to collision Theory molecules must collide to react so for this example atom a has to collide with molecule B C in order for the reaction to occur next the collisions must have the correct orientation in space to be an effective collision for example let's say for this reaction right we have our molecule B C a molecule B C approaches a in this orientation and since we're forming a bond between a and B let's say this is the proper orientation right so this is the way our collision has to occur in order for the reaction to occur if the diatomic molecule BC approaches in the opposite direction so let's say we have our atom a here and then we have and then we have C and we have C B right so the atom C approaches the atom a here this is not the proper orientation for the reaction to occur so this would be no so there has to be a collision but the collision has to be in the proper orientation and finally collisions must have enough energy so if the collision doesn't have enough energy the molecules or in this case the atom and the molecule will just bounce off of each other if you do have enough energy the colliding molecules will vibrate strongly enough to break bonds so let's go ahead and draw this in we're starting with a certain energy for our reactants so right here we're going to draw in the energy for our reactants all right so we have our atom a and we have our molecule BC at this point and let's say our total energy is 20 kilojoules per mole all right when the atom and the molecule collide right they need enough energy they need enough energy to break this bond between B and C right so we're trying to break this bond in here and so we can find that energy on our diagram here so we're starting with 20 kilojoules per mole and we need to get up to here to 60 all right so this is how much energy is how much energy we need for the reaction to occur and we call this the activation energy which is symbolized by e sub a here so this is the this is the activation the activation energy and the activation energy is important because this is the minimum amount of energy that's required to initiate a chemical reaction and for this reaction we can see we need to get 260 kilojoules per mole right so this point right here this point right here is that sixty kilojoules per mole we're starting out with twenty right so 60 minus twenty would of course be forty so the activation energy for this reaction so EA according to our diagram is positive 40 kilojoules per mole so the energy of the collision must be greater than or equal to the activation energy and at the top at the top right here we're going to get a transitional structure so let me go ahead and draw in a possible transitional structure for this reaction so we have a bond forming between a and B at the same time we have a bond breaking between B and C and we call this transitional structure right here we call this the the transition state right so our structure right here is called the transition state all right you might also see this called the activated complex right so the transition state or the activated complex all right you can see I've drawn in partial bonds here right so the bond between B and C is breaking the same time we have the bond forming between a and B and so let's think about in an analogy here let's say we have let's see we have a hill so here's my hill right here and if we have a ball all right let's say we have a ball right here at this end of the hill well it takes energy to push the ball up the hill and let's say we have enough energy to get the ball to right here right well in that case that's not enough for the all to roll down the other side of the hill here the ball is going to roll back to the starting position right so that's that's like thinking about having not enough energy and the molecule is just bouncing off of each other all right but if you have enough energy all right so if you're starting out with the ball right here and you have enough energy to bring the ball to the top of the hill so just barely to the top here right the ball can now roll down and so the ball is going to end up at the bottom of the hill right here right and that's that's thinking about formation of your products so for our example right our products would be our new diatomic molecule a B so let me draw that in here so here we have a B and we also have we also have plus C at this point so this would be this would be our products and this represents the energy of our products so let's go here and find that on our diagram so we have our products here so what energy is that so we go over to here all right and we find the energy and let's say that's at 10 right let's say this is 10 right here so the energy of our products is equal to 10 kilojoules per mole and the energy of our products for this example is lower than the energy of our reactants we start out with 20 kilojoules per mole and we ended up with 10 kilojoules per mole right so to find that difference in energy right to find that change in energy that would be the energy of the products minus the energy of the reactants and for this example the energy the products is 10 kilojoules per mole so we have 10 minus the energy of the reactants we started out with 20 kilojoules per mole so 10 minus 20 gives us gives us a change in energy equal to negative 10 kilojoules per mole so on our diagram we can see this change in energy right here all right so it's the change in energy this right here is a negative change in energy so I'm writing Delta e is equal to U is negative so this is an exothermic reaction so heat is given off here remember it takes energy to break bonds and energy is given off when bonds form and for this reaction right we're giving off we're giving off heat so the represents an energy diagram for an exothermic reaction now let's think about another reaction so down here we have reaction progress so we're starting out with the energy of our reactants right here so this is the energy of our reactants at 20 kilojoules per mole let's say right and we know that to get to this point right here this represents the the energy of the activated complex so right there would be our activated complex that's at 80 kilojoules per mole all right so this difference in energy this is our activation energy so this is EA right that represents our activation energy which for this reaction would be 80 minus 20 which would be equal to 60 kilojoules per mole so we get 2 here we get to the transition state or the activated complex right and then this would represent the energy of our products so this is our energy of our products here which for this reaction you can see the energy of our products is greater than the energy of our reactants and so if we go over to here let's say it's set 40 right so let's say this level is at 40 kilojoules per mole all right so what's the change in energy here so the change in energy for this reaction once again is the energy of the products minus the energy of the reactants so that would be 40 kilojoules per mole so 40 kilojoules role - we started out with 20 so 40 minus 20 gives us a change in energy equal to positive 20 kilojoules per mole so positive 20 kilojoules per mole so that would be that would be this difference right here so our change in energy our change in energy for this reaction is positive and so this represents an endothermic reaction so the previous example was an exothermic reaction where heat is given off and this is an endothermic reaction this represents an endothermic reaction where heat is absorbed

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