Reaction Mechanisms Activation Energy Lecture Short Version

so our lecture had some sound issues so I'm going to do really quick lecture on the notes apologize for potentially not having some of the demos intertwined in here I'll try to splice them in but I'm not sure if that's going to work so today we are talking about reaction mechanisms and activation energy all right so reaction mechanism as you can read on the screen is series of steps in chemical reaction this is the thing reaction may look simple like it may look like it's just reactants making some products but there's always more than meets the eye and an equation doesn't show all the details on EX what is exactly always taking place so have an example here nitrogen dioxide reacts with carbon monoxide and produces nitrogen monoxide and carbon dioxide great here's the thing and it's not your responsibility necessarily to know all the steps but if give you them you're going to figure out how to utilize them this equation actually uses two different steps sorry for being offset and these two equations you'll notice if you add them all up they'll make the overall equation now here are two really important elements that are involved in this the NO2 cancels out once and so does the NO3 by doing that can add up those two reactions to create the overall all reaction well there are some important vocabulary words that go along with this the yellow compounds up above those are called catalyst and catalyst speeds up reaction what is important for you to understand is how to identify the yellow versus the blue is that the Catalyst was introduced it did not enter the equation as byproduct or as product in one of the reactions it was reactant in the beginning and the order of the steps matter if would flip the first step and the Second Step then that changes everything but the yellow ones are catalyst because it is introduced in the reaction notice so it cancels out an intermediate is formed and then consumed so it was created in the reaction but it does not make it in the final reaction the the overall equation so did demo you saw during chemist tween of decomposition of hydrogen peroxide here's the equation hydrogen peroxide breaks down to make H2O and O2 if you did balance that out there should be two in front of the H2O2 and the H2O for the coefficients but if we poured out some hydrogen peroxide it's just colorless liquid and sit and have it sit in beaker it would be an extremely slow an extremely boring reaction you would not see really anything happening as it's slowly turning into water and oxygen but you can add catalyst so what had everybody doing and if you'd like to pause it for second try to figure out the Catalyst and the intermediate in this reaction if you'd like it is important to note that you don't necessarily need to have catalyst or you need to have an intermediate they both don't always exist one may exist and the other one may not but in this case there is one of each the is the Catalyst and the minus is the intermediate so what did in what's called elephant toothpaste is that you add some pottassium iodide so you're adding iodine into the hydrogen peroxide and you add little soap to help capture the oxygen gas and boom you get elephant toothpaste which is big huge foaming reaction all right the other thing we are talking about today is activation energy so have two energy curves up and think about if you're riding bike for second so you're you're sitting on bike where the letter is and you got to Pedal really hard to get up the hill right you got to put lot of energy you got to put lot of energy up in that hill but if you get over that Hill it's smooth sailing you get to easily ride down the bottom of the hill and it's going to happen there isn't much you can do about it it's the same kind of idea with activation energy and chemical reactions certain reactions require more energy than others to get going and if you can achieve that activation energy then that reaction will start and it's almost it's really difficult for it to stop here's an example Ox paper if paper is just sitting out it slowly reacts with oxygen and good example of that and proof of that is that if you look at really old newspapers that they have turned kind of yellowish or an orange right but that is really slow process it's happening over many many many many years in lot of interactions with oxygen but if you take piece of paper and you light it on fire it is not reacting with the fire fire is giving putting in enough energy to allow the paper to react with the oxygen in the atmosphere and boom you've hit the activation energy and you're able to proceed with the reaction so it is important to understand how you measure and where you measure the activation energy so first I'm getting ahead of myself these two reactions are very different the first one is an exothermic reaction because the reactants are higher than the products on an energy scale so you've lost more energy than you started with and the second one the endothermic where the reactants are versus where the products end the products end on higher energy than where they started so that means you have gained energy but how do you measure activation energy it's from where you start to the peak of that that energy curve that's how much energy is required so that is the activation energy how You' measure it for each situation there's one more key element that you could measure off of this so then the difference between the reactants and the products are what we would be calling and I'll talk about the other thing in second is enthalpy then so notice that the enthalpy on the left the EXO that's between the reactants and the products but am losing energy and the reactants the products is very small but I'm gaining energy in that one but sorry got ahead of myself the activation energy you could write this down it's the threshold energy that must be overcome for the reaction to occur all right but you could also kind of combine the two so here is an energy curve and I've already labeled it as an uncatalyzed pathway all right so that that's the pathway that the reaction is going to take from the reactants to the products but what did we just do previously we added catalyst and what does catalyst do Catalyst speeds up the reaction so think about you being on the bike again and if you were on your bike and you started from your reactants and you end to your products if you wanted to get there faster what would you do you got to take shorter Hill or you have to take shorter path so hopefully this makes some sense that this would be catalyzed pathway and that is actually how and why catalyst Works catalyst is lowering the activation energy which causes the reaction to happen more quickly so you should be responsible and understand the difference between those two drawings lastly sometimes we are going to be calculating activation energy this is not an equation you have to have memorized so note it's natural log of two different rate constants equaling the activation energy over our gas constant times two different temperatures that are subtracted from each other to things to discuss one there are two temperatures and two rate constants as we change temperature that's why we have different rate constant if you're at constant temp it's always the same rate constant but if you increase or decrease temperature the molecules are going to be moving faster or slower causing more or less interaction which causes huge effect on rate okay secondly you may see the equation with the K1 and the K2 flipped if that's the case simply you will also notice that the T1 and the t2 will be switched so if it's K1 over K2 on the left then it would be 1/ T2 minus 1 / T1 that would be the only difference be sure to use the proper value have it written there why do know have to use that one because it has jewels in it and above that is activation energy which energy is always dealing with jewels lastly we discussed or ended talking about marble statues and acid rain will try to splice that part in because know did have sound but I'm not quite sure will be able to so hopefully we will you are responsible for determining the rate of reaction and the orders of reaction between hydrochloric acid and calcium carbonate which is major component of marble statues