Rate and Extent of Reaction Exam Practice Question Part 1 May/June 2025

Added: 2026-05-12

[00:00:00]In this video, we're going to take a look at a rates of reaction past paper question, this one over here. They also give me a table and a graph and ask me all of these questions. This is from the May-June 2025 paper, so let's go. So, rates of reaction is often asked in the format of an experiment, and that's what they've got over here. So, it says the reaction between magnesium carbonate, which is a solid, and excess dilute hydrochloric acid, aqueous, is used to investigate the effects of concentration on the rate of a reaction. So, the variable that we measure in these experiments is the rate of a reaction.

[00:00:38]And then, the independent variable is the variable that we change. I, independent, is the variable that I change in the reaction. And I actually think that's one of the questions coming up, so we'll get there. When you're given something like this, it's very important to understand the reaction, understand what's independent, dependent, and understand the states that each of these things are in, and what is excess and what is limiting. So, HCl they told me is in excess, and that means that magnesium carbonate is limiting. Right.

[00:01:08]So, my first question, define the term rate of reaction. So, we're going to have to give the definition. You need to learn this off by heart. According to your exam guidelines, reaction rate can be defined as the change in concentration of reactants or products per unit time. It's very important to say per unit time. My next question, write down one controlled variable for this investigation. So, I already mentioned that the dependent variable is rate of the reaction, that's what we're measuring. My independent variable is concentration. It is the variable that we are changing, and therefore, one thing that has to be controlled, there's a few answers you could have given here.

[00:01:45]You could have said temperature, or the initial amount or mass of magnesium carbonate, or the surface area or state of division. So, remember, controlled variable is a variable that must be kept the same, because if it's not kept the same, then it's not a fair investigation. So, you need to think about other things that also influence the rate of a reaction. So, temperature is definitely a big one. It's probably the one that I would have gone with or I probably would have gone with state of division or surface area of magnesium.

[00:02:14]State of division, otherwise known as surface area. And because magnesium is a solid, state of division makes sense to be said here. They then give me a table.

[00:02:25]Three experiments using different concentrations of HCL are carried out and it says the concentration during the experiment does not change. Okay, so in experiment one they use 0.1 and that doesn't change throughout the duration of experiment one and so on. As you can see, experiment three has the highest concentration. Experiment one has the lowest concentration and they give me three different graphs that look like this and they say the incomplete graph incomplete graph of the results is shown below. So, you can see here that they're all starting with the same amount. This is mass of reaction mixture and flask.

[00:02:58]It starts off all of it the same and then you can see that there's different gradients experienced by A, B, and C.

[00:03:05]The gradient of the graph is different.

[00:03:07]The steepness of the graph is different.

[00:03:09]The steepness of the graph or the gradient would be representing reaction rate. It would be change in mass because here it's mass of reaction over change in time. And I know we just defined reaction rate as change in concentration of reactants or products per unit time, but it can also be change in mass or change in moles or change in volume per unit time. So, dividing by time. So, let's see what they want from us. Give a reason why the mass of the reaction mixture and flask decreases.

[00:03:40]Very valid question. Why would the mass decrease? We have to go back to our balanced chemical equation to see why.

[00:03:46]So, we're combining magnesium carbonate, which is a solid, with excess hydrochloric acid, which is aqueous and you can see here they react together in the solution over here in this mixture, and this looks like an open conical flask. The things that are made or produced is pure liquid, magnesium chloride, which is also aqueous, so that would be in the flask.

[00:04:04]So, then why would the mass of the flask decrease? It's because of this over here. Carbon dioxide is a gas, and as we know, the gas would escape through the mouth of this flask. And with the gas escaping, you will see the mass on the scale decreasing. So, that would be your reason over there.

[00:04:21]Now, 5.4 years where my stoichiometry, my calculations come in. It says, "For curve B, which would be this curve over here.

[00:04:30]There's B. We've got a time value associated with B over there, and we've got values over here associated. Okay, so for B, calculate the average rate at which carbon dioxide gas is produced for the first 120 seconds in cubic decimeters per second." So, that is what they want rate in. So, if they want rate, remember, this is the unit for rate.

[00:04:53]If they want the rate in cubic decimeters per second, it means that we need to calculate rate by using change in volume over change in time, because dm cubed is volume. So, it would be dm cubed at the top volume, seconds at the bottom, and when seconds goes upstairs, that's why it becomes dm cubed per second. That's where that unit for rate comes in. So, we are going to have to work with volume of carbon dioxide in my final steps of the calculation. They also give me molar gas volume, and you should know that when you see molar gas volume, that is VM, and that's from the following formula.

[00:05:33]Remember, at standard temperature and pressure, then my molar volume is 22,4, but they're giving me a different molar volume over here.

[00:05:46]Which is 0,63 g. If you get that difference in mass, that difference in mass is the mass of the carbon dioxide gas that escaped. So, now what I can do is I convert that to moles by using this formula, which you should be familiar with.

[00:06:02]My mass is 0.63.

[00:06:04]My molar mass for carbon dioxide, remember CO2 from your periodic table, carbon is 12 and then this is 16 * 2.

[00:06:12]And that gives me 44.

[00:06:15]And the number of moles, remember this is moles of carbon dioxide that I get here is 0.01431818 and so on moles. I'm not going to round off because I'm not the end at the end of my sum. So, that is the moles of carbon dioxide gas that is produced that escapes from the flask. Remember, I need to calculate the rate in cubic decimeters per second and I need to use this molar volume over here. I need to calculate the volume of carbon dioxide gas that was produced.

[00:06:45]So, that is when you use your molar gas formula, this one, which may only be used with gases, very very important.

[00:06:52]That number of moles goes in there.

[00:06:54]So, I'm just going to write it off as if I've rounded it off, but I'm not going to actually round it off. And then my molar volume they gave me in the question is 24.5.

[00:07:05]Obviously, I'm going to multiply, take 24.5 that way, multiply the number of moles by 24.5 and what I end up getting is for volume is 0.350795 and so on. Now, this will give me an answer in cubic decimeters because the molar volume, look at the unit there, cubic decimeters per mole. So, this is going to be in cubic decimeters. Then to work out rate, we are going to say, okay, rate is the change in volume over the change in time. Now, because we're working out how much volume was produced, we know that initially we produced zero volume of carbon dioxide because initially nothing was produced just when we started the reaction. So at time zero, there was still all of the reactants in the flask, no carbon dioxide has been produced. So I guess you could say V initial is zero and V final, the final amount of volume produced after 120 seconds is that. So remember, change in volume is always your final volume. So that's minus your initial volume over your final time would be 120 seconds, your initial time would be zero. 120 minus zero. So you are dividing by 120. I still haven't rounded off and what I get according to my calculator is 0.002923 and so on. Now you can round off. So what I know a lot of members also do is they will write this in scientific notation. So 2. 92. If you had to move the decimals, 1 2 three places. 2.92 * 10 to the -3 and your units they gave it to me is cubic decimeters per second because I'm using volume and time in seconds. Cubic decimeters per second, like that. If you want to keep it like this in decimals, that's fine. So it'll be 0.00. Obviously the rules in chemistry and physics is round off to at least two decimal places. So you're not going to round off to 0.00. That's just silly. So if you want to keep it like maybe round it off to 0.0029 or something like that. That's also fine. So where would you get your marks for this? You would get marks for subtracting the masses, formula, substitution, then another substitution over here. So that's four marks already.

[00:09:28]Then your rate calculation over here and your final answer.

[00:09:33]Our next question, which curve represents experiment one? Choose from A, B, or C. So, let's just go back and double-check what experiment one was.

[00:09:43]So, experiment one had the lowest concentration that was used. Remember we said experiment three had the highest concentration. So, experiment one had the lowest concentration, so you would expect lowest concentration, lowest or smallest rate of the reaction.

[00:10:00]And so, you would expect a gradient that is less steep, not as steep. And I know that that might be a weird thing to determine. Yeah, what is got the steepest gradient? But imagine you are a little person, and you want to slide down these slides. You've got three slides in front of you, A, B, and C.

[00:10:17]Which one would be the scariest to slide down? Which one is the steepest? I hope you say C. You can see how steep C is, which means that C would have the steepest gradient, the biggest gradient, the highest rate of reaction. And A would be the least scary slide. Do you see that? The gradient is not as steep.

[00:10:37]Okay? So, our answer here would be A.

[00:10:40]So, you would get one mark for saying A, one out of five. And then it says, "Use the collision theory to explain the answer." So, we already said that A has the less steep gradient, or the gradient is less steep. So, it has the lowest rate of the reaction. So, it basically has the smallest amount of gas produced in 120 seconds. You can also think of it like that.

[00:11:05]And you might be saying, "But ma'am, A's got the biggest number here." Yes, it's got the biggest number because its flask still has the most reactants inside of it. Not as much gas has been produced, so not as much gas has escaped. So, the scale is still weighing quite a lot. I hope that makes sense. Okay, so you're going to speak about the gradient. You have to connect to the gradient. And then explaining in terms of the collision theory, you would say it's because it's got the lowest concentration, which means it has less particles per unit volume or per cubic decimeter. Therefore, the lowest amount of effective collisions per unit time.

[00:11:43]And that corresponds to the lowest rate of reaction.

[00:11:46]And there's the memo, that is exactly how you would write it for your five marks. So, there's mark two, mark three, and mark four, and mark five. Okay, our last question says, "How will the final mass of CO2 produced in experiment two compared to that of experiment three?"

[00:12:03]Notice how they say final mass. So, basically, once the limiting reagent has run out, the limiting reagent, remember we said right at the beginning, is magnesium carbonate. So, once that is run out, once the experiment has come to an end for both reaction two and three, how will the final mass of product compare? So, if you look at experiment two and three, this is very important.

[00:12:25]If you compare experiment two and three, yes, experiment three has double the concentration. So, it will take place in half the amount of time. It'll have double the rate of the reaction. So, reaction three will have a higher rate of reaction, but the final amount of carbon dioxide produced, the final yield, the amount of product produced for two and three will be the same. The reason why is because if you read the question very carefully, they say that they're using magnesium carbonate, excess hydrochloric acid. They don't say that they use a different amount of the limiting reagent. If they change the amount of limiting reagent in experiment two versus three, then the amount of product will change. Because remember, it's the limiting reagent that determines the amount of product produced. In this case, the product is CO2. So, if they're not changing the amounts of magnesium carbonate, the limiting reagent, then the amount of the product will be the same. It does mean that in experiment three, the product will be made faster, so we'll get to that final amount of carbon dioxide faster. It'll be made quicker. The rate of the reaction is higher, but you will have the same amount of product. So, like when I explain to my classes, I say, "If you bake a cake and you keep the ingredients the same, the same amount of your limiting reagent, same amount of flour, same amount of eggs, and you start increasing the temperature, you're going to bake the cake faster, but you're not going to suddenly make more cake." I hope that makes sense. So, let me show you how you would answer this.

[00:13:58]You would say the same. "Same amount of magnesium carbonate, which is the limiting reagent, is used in each experiment." I hope you enjoyed that past paper practice. I can't wait to see you in another video very soon. Bye, everyone.