so start off on kinetics what is kinetics the study of first off rates how fast that's it so later on in the semester you guys got a little dose of it earlier last semester but a little later in the semester we have a whole chapter on what's called thermodynamics so in thermodynamics can tell you everything about a chemical reaction whether it should happen or not we call it spontaneity so whether it's endothermic or exothermic gives off heat or absorbs heat whether it results in an increase in entropy or a decrease in entropy randomness so it turns out all the equilibrium constants are actually going to come from thermodynamics as well so everything about a reaction comes from thermodynamics except one thing and that one thing is how fast and that's what kinetics tells us so if we look at this lovely stack of papers here should this stack of papers react with the o2 molecules in the air and burst into flames actually it should so thermodynamics says it's going to happen but kinetic says not so fast literally not so fast it turns out that the activation energy for this combusting is huge it's a huge activation energy and at room temperature virtually zero molecules have enough energy to overcome that activation energy barrier now notice if I wanted this to burn I might stick a flame under it stick under the paper or something like that but what if I just stuck it in the oven cranked up the oven with the paper burn in the oven even without a flame eventually yeah high enough temperature eventually some of the ules at higher temperatures would get enough energy to overcome that activation energy barrier and the paper would indeed burn at higher temperatures so thermodynamics tells me yeah paper's going to burn but kinetics says not so fast all right so kinetics first thing what to talk about AR raay Expressions so array expression is simply that it's a way of expressing the rate of a reaction so and the one I'm look at here is this one so let's say I had this chemical reaction going how many different ways would there be to measure the rate of this reaction or at least easy ways that'd be three I could measure how fast he's disappearing I could measure how fast he's appearing or he's appearing there's three ways if I have a way to measure any of these three concentrations I can monitor how fast this reaction is going so in this case one way to do that would be to monitor the change in the concentration of nitrogen over time so and that would be a valid rate expression a way of measuring the rate of this reaction just follow the concentration of nitrogen and see how it changes over time I could do the same thing with hydrogen and do the same thing but I have a problem do are nitrogen and hydrogen produced at the same rate no which one which ones produce faster hydrogen how many times faster three times faster for every one nitrogen molecule I get three hydrogen molecules it's produce three times faster and so if I call this the rate well this can't be also the rate because they're two different numbers this is a three times bigger than this quantity right here and so for a valid rate expression we divide by the coefficient and that way now yeah these are now equal and both are valid rate Expressions valid ways of measuring the rate of this reaction so I can do the same thing with a reactant so but is this lovely quantity equal to the two I've already put up here no so what do I got to do based on what we just said about coefficients got to divide by two or same thing as multiplying by2 same diff but is the concentration of ammonia being a reactant going up or going down it's going down when something's going down it's decreasing what kind of change is that it's a negative change this is actually a negative number these are both positives and that's fine we always Define rates as positive numbers but this unfortunately a negative number so if I want to call it a rate for reactants we got to put a big fat negative sign out front that way a negative times a negative comes out to give me an overall positive rate everybody cool with that so these are valid rate Expressions you may get a question on the exam that just says for this reaction which of the following is a valid rate expression and any one of these three could be the correct answer or they might put all three and then put Choice d as all the above right cool the other kind of question you might see with this though is just comparing actual numerical rates of different reactants and products in the same reaction for example if I told you that the rate of change of hydrogen over time was equal to 12 m per minute 12 mol per minute then the question is how quickly is nitrogen being produced well here's the deal if you're actually dealing with numbers I would highly recommend you don't actually use the rate Expressions don't go there just look at the St ometry so who's being produced faster nitrogen or hydrogen hydrogen how many times faster three times faster and so if the hydrogen is being produced at 12 m per minute how fast is the nitrogen being produced yeah three times left less four molar per minute cool and what if I asked for the rate of change of amonia what would his rate of change over time be oh which is what is 2/3 of 12 not quite six not quite six that's a half eight but it's not just eight it's awesome it's negative 8 now here's the deal and this one's just a little bit tricky What If instead of this last question here I asked for the rate of I can spell it consumption of ammonia NH3 so this time I've asked for the rate of consumption because I've used the word consumption what do I know I know it's being used up and its concentration going down and if I already know it's going down I already know it's negative you don't include a negative as part of your answer this would just simply be eight molar per minute eight molar per minute you sat in the front row it was your first mistake today what's your name sh shav shahab shahab so shahab and if I'm butchering your name I'm really sorry but so what you guys don't know is that shahab is a big time Gambler Big Time Gambler and so if I knew he went to the casino last night so and I asked him what was your net change in your worth as of your visit last night to the casino so and if he said negative $5,000 I like oh rough night because that would tell me he lost $5,000 right however if I knew he was a terrible Gambler and so I immediately instead of saying what's your change in your net worth I said how much did you lose last night because I knew he lost you know he always loses so it's how the casino stay in business by the way so how much did you lose and if he says negative $5,000 I'm G scratch my head and be like does that mean you gained $5,000 but notice if I already said how much did you lose you're never going to use that negative sign then you just say I lost 5,000 same thing here if I'm already talking about a rate of consumption then we already know it's going down and so we just give the absolute value eight mol per minute but if I just simply ask for the rate of change well then I don't know if it's going up or down you got to tell me and so for reactants they're going to be negative everybody see the difference in the way the question might be worded if it's rative change for reactants they're negative for products they're positive so and that's so the the only time you really got to worry about because notice products are always going to be positive numbers but for the reactant if I just ask for rate of change that's when it's negative but if I say rate of consumption that's just the absolute value cool