[Music] and this video we're going to be looking at topic 11 a further kinetics and this is going to focus on the first half to the first 30 subtopics of this unit this is part of the ie2 chemistry course from in Excel and is the first and unit four so we're gonna be looking at key terms within those topics or looking at things such as rate equations rate constants and rate determining steps how we can calculate the half-life of a reaction using data from a suitable graph being able to select and justify a suitable experimental technique how we can use particular experiments to investigate rates of reactions and particular the initial rate method and a continuous monitoring method how we can deduce the order of a reaction with respect to a substance and every equation using graph data or from the initial rate method and we're going to be specifically looking at the acid catalyzed IO IO donation of propanone this comes up and core practicals as well and how we can deduce the rate determining step from a rate equation so let's look at what we already know about rates of reaction you have discussed that back and topic name as well as an IG CSE so this part should not be new to you we can measure the rate of a reaction and by following how much product is being formed over a specific time frame so a rate of reaction is always linking concentration of a product to time we can also look at the concentration of a reactant and we can look at how the reactant decreases but most of the time we'll focus and the change in concentration of a product so the equation that we use is this one at the bottom here now you have used the variation of this end GCSE where you're looking at possibly the change in mass or the change in volume it's the same thing but we're just going to always be expressing the change as a concentration and last topic because we're wanting to make it universal across all of our equations and all of our reactions so rate has a unit of moles haier decimeter cubed hair second as a measure of the change in concentration and concentration is always measured and moles per decimeter cubed and per unit of time and we use a standard unit of time of seconds so our unit as most hair decimeter cubed per second now the rate of reaction can be followed using various different measurements or monitoring techniques you need to know sex possible ways to do that and we need to be able to identify it what is the most efficient or effective way of measuring the information that we need for it depending on what reaction is that you're carrying it so the sex ways are looking at the volume of a gas looking at the change and mass the change in color intensity change and concentration using a titration a change in pH or a change in electrical conductivity and let's look at some of these and a little bit more detail you'll have seen some of these before but one or two may be new to you so the volume of gas evolved you will have seen back and IGCSE when you had the marble chips and the hydrochloric acid experiment in order to make carbon dioxide gas and you collected the carbon dioxide gas and you measured how much was made and a specific time frame and there are two ways that we can measure we can either collect over water which is our first picture down at the bottom here or we can use a gas range and that is our second picture and the method of collection will depend on the level of precision that's required if you're using a very small volume of gas that you're expecting then should reusing a gas that and because these tend to people with no more than a hundred centimeters cubed but it could even go all the way down to ten centimeters cubed whereas for a measuring cylinder you could be upwards of two hundred and fifty centimeters cubed we also need to be careful that the gas doesn't dissolve and the water is if it does them we should not use collection over water because it's good to throw our results off so we would always use a gas Siddhant or something like that now you may also have seen the change in mass and this is again a link to when I gas it's fun so you'll have seen this during deformation specifically of carbon dioxide so again with our marble chest and our hydrochloric acid and this works best when the gas that's being released has a high density because we see a clear difference and our mass and instead of looking at how much gas is being produced we're looking at what mass is being lost due to that so we would see the mass decreasing and we like to see how much the mass decreases over time so you would maybe take the mass measurements every 30 seconds or every minute and order for you to get your data and then stopping obviously once your reaction is finished now one that you may not have seen before is called cullen amatory and this uses something called a colored ammeter to allow for looking at the intensity and change of color now this is usually things that we cannot detect with the human eye so we're looking at intensity changes so what we sometimes see has no change in a color eye color ammeter well built rather than seeing during the color itself is be able to quantify it and it gives a specific number or specific absorbance so we look at something called absorbance and that is all to do with the darker the color the list and the more absorbance you're going to get now you're not going to have to go into a lot of detail it that's just simply a method that you need to know then of course we've met titrations before we've seen this bat an eye GCSE and we also seen it and talked 8d and we're going to revisit titrations quite a few times right that's course so what we do is we look at taking small samples or at the property as an Alcott being removed from a reaction mixture at regular intervals so we would have a separate mixture that is reacting and we would tick the sample it a specific time maybe every minute or every five minutes and we would then quench the reaction and that means just to stop the reaction so that we can measure exactly how much of a product is in that sample at that particular time and usually by quenching we do that by adding and another chemical or putting it in an ice bath and we're going to come back to this when we start to talk about the continuous method lift it on in this video the alikom after that has been quenched is then going to have a specific amount of product in it and we will then titrate it in order to determine that concentration and we carry out the titration exactly the same way as we have through all of our other examples of this so exactly what you did and topic 8d you carry out here there's no deference to the titration the only part that's different is what you do to get your sample and then example of this would be when we titrate iodine against sodium thiosulfate which again we'll discuss later on and topic 16 when we're looking at many dots titrations now we can also look at pH change and this is going to be again used in conjunction with titrations and this is specific for acid-base reactions but it works very much the same way we can use a titration to determine what the ph of a substance sets because we look to see how much of the acid or how much of the base is being used in order to neutralize the electrical conductivity is one that doesn't come up that often but it still can be used and it's where we look at the change and the number of charged particles that are being formed so looking at the number of ions that are available and does that number decrease our entities over a specific type we can also use any other physical property that is going to give us a significant change so what we may see as the volume of liquid changing we should see something known as chirality which we will discuss later on in topic 15 or you can even look at the refractive index of a substance and you'll have talked about that and IGCSE physics and that is all to do with the diffraction of light so how late is going to change direction as it and speed and direction as it passes through a particular liquid now as I said these ones are not as common but you do need to be aware that they are possible methods the ones that are going to come up most often are going to be your titration colorimetry change in volume or change and mass so as we want to looking at rate equations and rate constants so and most chemical reactions the rate of a reaction is going to be directly proportional to the concentration of a reactant for example if the concentration doubles then the rate is going to double if the coin concentration chapel's then the rate will travel ever half the rate will have we call this being directly proportional so whatever we do to the concentration is going to happen to the rate and we use a rate equation as a mathematical relationship twin the rate of reaction and the concentration of the reactant so we're going to America like these two things together and an equation and a rate equation may look something like this at the bottom here and we call we're a type of reaction where we get this direct proportionality a first-order rate equation and we won't talk about that later when we talk about or additive reactions so don't worry too much about that just now what we want to see as how we express a rate equation and what we do is we have the red is equal to K and the concentration of a reactant so when you see something written and these square brackets that is a shorthand way of writing concentration so when we're looking at square brackets of a what we're actually saying is the concentration of a and K is known as the proportionality constant this is something that you may have come across and maths and basically what it means is that if we were to take the ratio of our concentration of a and our R it we would get a constant value and that constant value is going to be K and K is what is known as our rate constant and typically unequip and a question and topic 11 this as the key thing that they are going to be looking at the rate constant they're going to be looking at how we calculate the rate constant and actually be able to value on this for every single reaction because all reactions are going to have different rate constants now what we need to understand is that not every single chemical reaction is going to have a rate that is directly proportional to the concentration the majority of them well but sometimes they don't sometimes we can get a reaction where our concentration is doubled and what we see is our rate actually increasing by a factor of four or the rate as quadrupled and when that happens we call this a second order rate equation so first order is where desire dinettes proportion second order would be where we see the rate quadrupling when we double our concentration and we get every equation that looks like this which reads as the rate is equal to the rate constant multiplied by the concentration of a squared now we can also get something that is known as a zero order reacquaint that means that the rate is not affected in any way by the concentration of a specific reactant now what you may find an equation or in our reaction is that you have two or three different reactants and maybe only one or two of them will affect their the rate one of the concentrations may have no effect on the rate and that's what we call a zero order and when we get this we have the rate being equal to the rate constant times the concentration of a to the power zero that you should know that anything to the power zero is going to be one you should go back from maths therefore for a zero order reaction your rate is going to be equal to the rate constant now another key question that comes up is going to ask you to determine the units of a rate constant and it is going to depend on the type of reaction that you have so if you have a zero order reaction first order reaction a second or even a third and we're going to come back to the orders of reactions as we go through this topic it's going to be something that is very commonly being used and what you need to be able to do is you need to be able to determine the units of a rate constant depending on the origin of it and I'm gonna go through one example here and then I will show you the other examples and this is the key table to memorize if you memorize what the units are it's then easier for you to watch out so we're gonna look at a second-order reaction so if we have a second-order reaction R R it is going to be equal to the rate constant multiply it by the concentration of a squared now what we can do is we know the units of race and we know the units of the concentration and we really enjoyed equation to figure out the rate constant so our unit of rate is going to be moles per decimeter cubed per centimeter and that's gonna be equal to K which we don't know yet and then the concentration units is going to be moles per decimeter cubed but we're squaring it so actually we have that same thing twice now what we can do is we can rearrange it so that key is now equal to moles per decimeter cubed per second all divided by moles per decimeter cubed times moles per decimeter cubed and what we can now see is we can start to cancel certain things out so we can now cancel it this top one and one of our moles per decimeter cubed so k is now equal two seconds to the minus one divided by moles per decimeter cubed and what we can do is we can rate that so that they're all on the same lane and we have seconds to the minus one moles to the minus one decimeters cubed and we typically rate it so that the seconds is at the end so we're going to have moles to the minus one pair decimeter cubed per second to the minus one and you can see that that's what we get here no typically it's not going to matter what order you do the units on whether you put the decimeters cubed or the moles first as long as you have the cadet subscripts so the collector superscripts for each of them now you can see here that we've got the reaction or the calculation for each of the different types of units for a zero order first order a second order and a third order reaction and it just uses the rate equations here and we just substitute and all of the units that we know and is a case of cancelling out what we do have and then conveying what is left and we get the units that are an death table here what I would suggest is that you practice how to calculate and how to the units for each of these types of reaction so take sampling the rate equation and then can you get to the units of rate of the rate constant it does take a little bit of time but the more you practice that the more used to it you're going to get now we've been talking about the order of a reaction and what this is basically going to be or what we mean by less is how does changing the concentration of a specific reagent or a specific reactant affect the overall concentration so it's going to have an effect of what does each reactant do but also the overall order and that's by looking at the individual reactants so when we're talking about the order as the effect changing the concentration of a reactant has on the rate so if it's first order doubling the reaction concentration is going to double the rate okay and then we can figure out the overall order by simply adding each of them so if we have a rate equation that looks like this we get our rate constant multiplied by the concentration of a by the concentration of B and by the concentration of C squared now what this tells us is that the reaction is first order with respect to a and 2b and second order with respect to C and what we actually mean by that as F I take a or B if I double the concentration of any of those two things I'm going to double the rate however if I take C and I double its concentration I'm going to quadruple the rate that's what these superscripts and are tailing us here so f I then take my first order for a and B and second order for C I add these together and the overall reaction is what we call fourth order so I just suddenly add one plus one plus two now we can use these orders of reaction that we've worked at for each individual reactant to determine a possible mechanism for a reaction and we linked this to something called the rate determining step now when we write a stoichiometric equation that is just a fancy word for a balanced chemical equation what we're actually writing there as a summary of all of the reactants and the products that doesn't necessarily mean that they are going to go through a single step and we've seen this when we were looking at organic chemistry mechanisms such as electrophilic addition or nucleophilic attack reactions now the majority of our reactions do not occur via a single step because f they dead because of the number of moles present they would be very very slow remember an order for a reaction to occur they have to collide with the connect energy and geometry and if you have something like 14 molecules of reactant they are not going to occur with the kool-aid worth we collect energy and the connect geometry and one go so what we can do is we can then look at the mechanism there for the reaction and we can only determine this using experimental methods so any mechanisms that you have already met have been determined using experimental methods you know the slowest step and any reaction mechanism is the one that is going to determine the overall rate of reaction and we call this the rate determining step or sometimes shortened to RDS and that's how we're going to refer to it throughout this video now only the reactants that are involved and the IDS are going to be expressed and the rate equation and that is because they are the only reactants whose concentrate Asians affect the rate so if you have a reactant that is zero order which means that has no effect on the rate it is not going to appear and the RDS if it does not affect the rate only the concentrations of the reactants that are going to have an effect on the rate will appear and not rate determining step so if we look at an example our equation could be a plus B plus C it gives us D plus E and we determine that the rate equation as the rate is equal to K and then multiplied by the concentration of a and the concentration of B what that tells us is that C is present in such large excesses that it cannot affect the rate okay so C would be zero order for the rate has no effect on how fast or slow the reaction is going to happen so because it has no effect then it does not appear and the rate determining step it is not the step in the mechanism that affects how fast or how slow something is going to happen so what that must mean is that a and B must react to form some sort of intermediate and that is your rate determining step and that intermediate then reacts with C and we can summarize it as we see here so our slow step is the rate determining one so it reacts with B to make an intermediate of 's it and then reacts with C and it makes the products of D and E and that is a fairly fast reaction so it doesn't have the effect on our rate it's not the thing that determines how fast or how slow the reaction is going to happen so if we apply the same logic to a real-life example that may help you visualize it a bit better so imagine that we have three who are going to be arranging some sheets of paper and to a set of knots there are three different steps that we can do the first step is to collect the knots that are arranged in ten different piles and be able to take one sheet from each pile the first student is going to do this the second student then takes the set of papers and shuffles them so that they are tidy and ready for stapling and the third student is then going to steeple the set of notes together now that's overall gripped the overall process will take a specific amount of time but what we can do is we can actually break it down into the different steps to see which one is going to have the biggest effect on the writ and what we can determine is that step one the actual collection of the sheets is going to determine how fast that task can be done and it is because this is the slowest possible step the tithing of the sheets and the stapling is very quick but the actual collection is very slow so how fast that step is done will determine how long it takes for the entire tasks to be completed and no matter how quickly the tithing or the stapling is done they will have to wait for the first student to collect all of the sheets so if we break it down as if it's a mechanism the overall reaction would be going from 10 and read your sheets to a stapled set of notes so having 10 sheets to a stapled set of notes that would be equivalent to your stoichiometric equation when we break it down into the different steps that we see here this would be like our mechanism and student 1 collecting the sheets is going to be the slow step so that is going to be the step that determines how fast the overall task can be done so we call that the rate determining step now when we're trying to determine orders of reactions there are two different methods that we can use to determine the reaction with respect to a specific reactor and they are called the content method and the initial rate method both methods involve experiments because we have to have experimental data to be able to do this and then analyzing the data but they are slightly different whilst they have the overall same effect how we physically do them is going to be very different and you need to understand both methods and these do link and to the core practicals specifically at core practical 9 a and then B one uses the continuous and one uses the initial rate method so if we start with the continuous method this is where we have one reaction mixture and it continuously reacts and what we do is we take samples and withdraw them at specific time intervals so we have a reaction setup and once it's set up we do not change it or we simply do as we take samples from that reaction mixture we stop the reaction by quenching that we talked about earlier on and then we determine the concentration which is usually using a titration from that information we can then determine and create a concentration time graph so we know for each sample that we've taken so for a minute 1 minute 2 and minute 3 we can determine the concentration and from that information we can then determine the half-life now you will have met half-life and IGCSE physics and we're going to take the same concept of half-life but we're just in this case not linking it to radioactivity so we are looking for when the concentration halves okay and looking at what is the time difference for each time the concentration half so that's what we mean by half-life here after our half-life is constant then the reaction is first-order so after the time taken for the concentration to fall by 1/2 as exactly the same each time then it's going to be a first-order reaction at for half-life doubles then a reaction and second-order so we would get get off like this so we have our concentration of reactant and it doesn't matter what your unit we use we're not interested in that what we are interested in is how it actually changes so if we start with a concentration of 120 then after 100 seconds that falls to sexy after 200 seconds it falls to 30 and after 300 seconds it falls to 15 what we can see is the half-life here as 100 seconds so every 100 seconds my concentration is going to fall so this is going to be a first-order reaction because my half-life is a constant value so I've taken my data and I've been able to make a constant constant value over half-life telling me the order so this is how we physically work out what the order does and then we determine our it equation now after you get a graph that is a straight line with a negative gradient meaning that it goes downwards and we don't have that nice curve then it means the rate is not changing with the concentration and that tells us that as zero order so if we have a curve it could be first or second order and we'll have to check the half-life and if we get a straight line then as a zero order reaction and we don't need to check anything to do with the half-life so that's using the continuous method in order to determine my order of reaction now to determine the rate of a reaction using a concentration time graph or we can even use a volume time graph would be the same what we have to do because it's a curve we have to draw a tangent to the curve and then we determine the gradients of that particular line so if we wanted to figure it the rate of reaction for this point here what we do is we draw a tangent to the to the line and our tangent should touch the x and the y axis and we then look at these particular numbers so we are looking at test number here and this number here and we then use those values in order to determine our gradient I remember gradient would be y2 minus y1 over x2 minus x1 and you'll have looked at that and maps and also back and topic name okay if we have a gas being evolved so if we have a positive gradient circulating going upwards again if I wanted to determine the rate at time T I draw my tangent to the line here and then I just picked two points that are going to be on the line for this one because obviously I can't touch both axes here because of my tangent is going to be going away from the y-axis so here I would just pick any two points and then I can determine the gradient and this is what we call an instantaneous reaction rate and it means the reaction rate at a specific time okay rather than the overall rate of the reaction we're looking at the rate of reaction at a specific time now if we look at the initial rate method this is slightly different where it involves carrying out the same reaction several times but we actually vary the concentration of one of our reactants so as we have a reaction where we have a plus B gives you C we're going to carry out the reaction between a and B four times but you can see that we're going to change some of the values here so in the first case we have got 0.1 moles per decimeter cubed away and 0.1 moles per decimeter cubed of B and I get this particular rate I have n change one of the concentrations so in this case I'm going to double be and I get a change in my rate I'm then going to double e and I record what happens here and then I'm going to double a and double B and again I record my initial rate so I've carried out my reaction I've figured out what my rate is and then I look at the mathematical relationships between the data so from this information what we can see is the FI double either of my concentrations then my rate is going to double okay so if I go from our natural rate of 0.02 when I double my concentration of B I am doubling my rate of reaction I can look at the same so here I'm looking at number one and number two I can look at the same for a number one and number three where I double the concentration of a and I don't change B and again I'm getting my rate doubling if I wanted to compare what happens when I double both of them why should see a compounded effect there so doubling both of the reactions should have a quadruple effect on the rate and we can see that it does so what we can determine from that information is that the reaction is first order with respect to both of our reactants so I can write my rate equation as R it is equal to K times the concentration of a time's the concentration of B so I have certainly carried out the same experiment four times but changed one variable and then look to see how changing that variable has affected my rate and from that I can then determine the orders now I can also do this graphically and I can plot a graph of the concentration of it versus one over T this one over T is going to be proportional to my rate and based on the shape of that particular glass then we can determine the origin of reaction so actually have the concentration of the inverse is 1 over T and we get a straight line horizontally this means that changing the concentration has no effect on the rate and that tells us that this must be a zero order reaction if I get linear straight line that means that they are directly proportional to each other so my rate and my concentration is directly proportional so I have first order and if I get a curve like this then I can determine that is going to be a second order but in order for us just to check that what we do is we plot instead of the concentration of V on its own we do the concentration of a squared and we should see a straight line because again as we've seen previously if you have the concentration of a squared being directly proportional to the writ that tells us that we have a second order reaction now well this is quite difficult to guess what you may find is that some of the questions are actually a bit easier than you think it is all about looking at the data and can you then take information from that and be able to determine the rate of a reaction so we'll look at a path paper question to tell us how to do that this is from the June 2019 paper so we're gonna take this particular question and splutter unto to the first section we're gonna do in this video and then the second part of the question we're going to do and the second video of topic 11 because it talks about the mechanisms so in this reaction we've got nitrogen oxide reacting with hydrogen to produce nitrogen and water and partly is simply asking us to rate the equation for this so we're going to have n whole + h2 reacting to give into and each tool and of course I have to balance it and I balance it by putting twos in front of three of my substances and the equation now moving on to Part B you can see that we carried the same experiment three times so we must be using the initial rate method here and we're going to use the information from the table to deduce the origin of reaction with respect to the nitrogen oxide and to the hydrogen and we have to justify our answers by looking at our information from the table so if we start off by looking at the nitrogen oxide we can see that I have three different concentrations here so I'm gonna have to do some comparisons and I want to end this case look at number one and number three because my hydrogen stays the same and both of these and what I can see as I go from number one to number three I'm multiplying my concentration by four if I then compare that to my rate going from five point five times ten to the minus three to eight point eight times ten to the minus two I am multiplying by 16 so if I multiply my concentration by four and I see I multiplied by 16 for my natural rate that must mean that it is increasing by a factor of four so that tells me that the Nano is going to be second order and we would have a rate equation as looking at the concentration squared for the nitrogen oxides then we compare the hydrogen now with the hydrogen we can't compare number one and number three because the concentration is the same so we have to be looking at number two and number one so I've doubled my hydrogen here but I do need to be careful in the fact that yes I may have doubled my hydrogen but I have also doubled my nitrogen oxide so I have to take into account the fact that is also going to have an effect on the rate so as we just had the nitrogen oxide being doubled what I should see as my rate increasing by a factor of four but when I compare number one and number two I actually see my rate increasing by a factor of eight so that tells me that the hydrogen must be doubling it as well because if the nitrogen oxide is all red multiplying by four to get up to it I have to double it so my hydrogen is going to be first order now if me right now the Huayra justify these for the N or we compare experiment one and three the concentration has multiplied by four and the R it is multiplied by 16 now of course you would write this out in sentences I'm just writing out in shorthand for the sake of time and that's video but what we're seeing is that the concentration is multiplying by four and the rate is multiplied by 16 and the hydrogen concentration is constant so that's our justification for it being second-order for the hydrogen we compare experiments 1 and 2 and changing the concentration of the N or by 2 should multiply the rate by 4 but that is actually by it so the change and the hydrogen must be doubling the rate we're taking into account the fact that both of the concentrations are having an effect on the rate now we want to write the rate equation for this reaction so we're always going to write it in the same way that RIT is equal to K and then we're going to have our concentrations here and for the concentration of n all we need to square that because it's second order and for the concentration of hydrogen we can write a 1 but we don't have to we could just leave the one out and that would give you the correct answer now please be aware if you do get the orders of reaction wrong and part one you do get transferred at our max here so you know lose the marquee as long as you use the connect orders that you calculated and part one then we want to calculate the value of the rate constant and then give its units so in order to calculate the value of a rate constant we simply take values from the table so what we're gonna do is we're just gonna look at the very first experiment and we have concentrations of zero point zero two for the nitrogen oxide and the hydrogen and then we have a rate of five point five times ten to the minus three so K is equal to the rate divided by the concentration of N or squared times the concentration of hydrogen so if I substitute my numbers in I get five point five times ten to the minus three as divided by zero point zero zero two squared times zero point zero zero two and I simply substitute all of that into my calculator and I get an answer of six point eight seven five times ten to the four and that's the value of my rate constant and because it is a second or a third order reaction overall we think back to the table that we looked at previously so my unit I'm gonna rewrite down so down here so my units are going to be DM to the power six moles to the minus two seconds to the minus one so you'll get one mark for your value and one mark for your units and there is your mark scheme for that and that's everything for part one of the kinetics topic hopefully everything is made sense but if there's anything you're not shoot off please feel free to leave a comment below and we hope to see you back on the channel soon to check out part 2 on the kinetics topic [Music] you