Video: Planning an investigation to find rate of reaction

﻿Hello, everyone. I hope you're all doing well. Welcome back to chemistry. It's Dr.Dang here, and I'm taking you through the unit rate and extent of chemical change. In this lesson, we will be looking at how to plan an investigation, to find rates of reaction. This is a really important topic, that will prepare us for all the practicals that we will be looking at soon. So hope you're excited. Before we begin, please find yourself a quiet space where you can sit and work through this lesson without any distractions. Close down all the tabs on your device, so you can make the most of this lesson. when you're ready, let's begin. We are going to start by recapping how to measure rates of reaction. Then we're going to look at how to plan and write a method for a practical. Then we're going to look at how to display data, or present our results from the practical. And finally, we're going to look at graphs. But specifically, how to describe and explain trends. In terms of the equipment that you need for this lesson, you will need a writing pen, some paper or something to write on, and also a ruler. If you do not have any of these on you, please pause the lesson and go get them. Then resume when you are ready. Okay, so let's go over how to measure rates of reaction. To calculate the rate of reaction, there are two equations that we looked at previously. The first one is, mean rates of reaction equals quantity of reactant used, divided by the time taken. The second one is, mean rate of reaction equals quantity of product formed, divided by time taken. As you can see, what's in common is time. We need to make sure that when we're looking for rates of reaction, we are always recording the time that it takes for a reaction to complete. In terms of the equipment that we need to find rates of reaction, early on, we said, it's so important for us to record time. We can do this using a digital stop clock in the lab. And next, if you remember the equations, you can either find the quantity of reactants used, or the quantity of product formed. Now, if the quantity that we're measuring is the volume of gas released from the reaction, then we can collect and measure, the volume of gas using a gas syringe. However, if the quantity we're measuring is the change in mass, and remember from conservation of mass, mass of the reactants should always be the same as the mass of products. Unless one of the products is gas. So if the gas is released to the surrounding, we will notice the change in mass, or the mass will decrease. And we can measure the change in mass, using a top pan balance. Now let's take a look at how to write a good method for a practical. In this lesson, we are going to look at the reaction between magnesium, and hydrochloric acid, which make magnesium chloride salt, and hydrogen gas. In the diagram on the screen, you can see all the apparatus that we need and how it sets up, in order for us to find the rates of reaction. From the word equation, hydrogen gas is going to be released as one of the products. Are we going to find the rates of reaction by collecting the hydrogen gas and measuring the volume of gas produced from the reaction over time, using the gas syringe? Okay, so let's take a look at how to plan and write a method for this practical. On the screen, you can see an example of a method that has been poorly written. What I would like you to do is pause the lesson here and identify the different sections in this method, that needs to be improved. On your piece of paper, write down some notes and think about how we can improve this method. Then resume the video when you're ready. Okay, well done for your effort. Now let's take a look at the different parts of this method, where we can improve and how we can improve them. The first step says, "Pour 0.5 molar hydrochloric acid into the flask." So what is wrong with the sentence here? Firstly it says pour, but it doesn't specify what volume of hydrochloric acid to use. So we need to be careful here when we're writing a method, it needs to be specific. We need to mention what is the volume of the reactant we are going to use and how are we going to measure this? What else is wrong with this sentence? Now, if you remember from the diagram early on, the reactants were placed in a conical flask. So we do need to be careful with our language here and be specific with the equipment that we're using. In this case, it is a conical flask. And step two, it reads, "Add magnesium strips into the flask and close the flask." So what's wrong here? Well, it says, add magnesium strips, but it doesn't tell us how many strips to add and do we know the size of the magnesium strips? Because they may not be the same size or the same mass. Once again, we need to be specific and control this so that our results are reliable. What else is wrong with this sentence? So at the end of the sentence, it says close the flask. If we think about the diagram, where the equipment was set up early on, this sentence is too simplified. So we need to describe this better. Once again, we need to say conical flask, instead of just a flask. Step three, "Read the volume of gas in the syringe." What's wrong with the sentence? It says read, but it doesn't specify how many readings do we need to take, and how frequently should we take these readings. So we need to be specific with this. And finally, step four says, "Write down the results." Of course, it's important for us to be able to record and display our results appropriately. So I'm going to show you how I have improved this method. So for step one, measure 20 centimetre cubed of 0.5 molar, hydrochloric acid using a measuring cylinder. Then pour the acid into a conical flask. You can see I've improved the step by specifying the volume of hydrochloric acid used. So 0.5 molar is the concentration. So that has been given already. I also mentioned what I'm going to use to measure this 20 centimetre cubed of hydrochloric acid. I'm going to use a measuring cylinder. Then pouring the acid into a conical flask. Step two, "Place two 2 centimetre magnesium strips into the conical flask. Insert a rubber bung into the conical flask and attach it to a delivery tube that is connected to a gas syringe." You can see that I've specified, the number of magnesium strips that are being used, as well as the size of the magnesium strip, which is two centimetres. Next, I've described the setup of the equipment more specifically. So I mentioned having to use a rubber bung, to close the conical flask, then attach it to a delivery tube or the glass delivery tube, which is connected to a gas syringe. So that any hydrogen gas that's been produced, can be collected and measured in the gas Syringe. Step three, "Measure the volume of gas in the syringe, every 20 seconds for three minutes." As you can see, the sentence is much more specific and it allows the person who's reading this to find out how frequently they should be taking a reading, and for how long. And step four, "Record the results in a table." Once again, I specified, where I'm going to record the results. So this is what a good method looks like. It should include all the different measurements. It should also include the specific equipment that you need. And a good method will allow the person who's reading it, to be able to set up and carry out the practical with ease. So once you got your results or data from a practical, we need to think about how we can display it. So that is well understood, I will control my information from the data collected. One way of displaying data or recording results is by drawing a table. In the specific reaction, we have recorded time, as well as the volume of gas produced. Remember, when you're preparing a table, you need to include the units. And finally, we can then use the data from the table to plot a graph. This is a table that I've prepared to record the results for this investigation. In the left column of the table, I've recorded time, and I know I'm taking a reading every 20 seconds, for three minutes. So three minutes is the equivalent of 180 seconds, which is why I've displayed my table this way, and I've left room to record the volume of hydrogen gas produced in centimetre cubed. It's good practise to prepare a table before you start your practical. As this will allow us to record the data more effectively, and it also serves as a good reminder when we should be taking our next reading. And we would do this until our table is filled with the data from our results. Next, we can use the data from the table. to plot graph. Now in this case, we haven't done the practical. So we don't have any data on the volume of gas produced, but we can prepare a graph. In this graph on the x-axis, you can see time is in seconds and on the y-axis, volume of hydrogen in centimetre cubed. In the previous lesson, we looked at a couple of things that we must remember when plotting a graph. We need to be careful and choose a suitable scale. So as you can see on my x and y axes, I've chosen a suitable scale that will allow me to plot data points from the table. Next, we must ensure our axes are labelled, with the quantity measured as well as the units. And don't forget, when you are finally plotting the graph, you must use a sharp pencil and using crosses to plot the data points. And finally, you can join up the data points and draw the line of best fit. Okay, so let's take a look at how to describe and explain trends in graphs. This part is really important because it involves exam skills and you need to be able to differentiate between describe and explain. So when you see questions that ask you to describe or ask you to explain, you need to know clearly, what you need to include in your answers. So let's practise. When you answer a question that asks you to describe what's going on, you need to say what you see. But remember, you also need to use data to support your claim. So the data can either be from a graph from the question or from a diagram. You just need to look carefully. So let's take a look at this question. Describe what is happening to the rate of reaction: a, in the first 20 seconds, b, between 40 and 70 seconds and c, between 100 and 120 seconds. So there are three separate questions here. Let's take a look at how to answer the first one. Okay, let's describe what is happening to the rate of reaction in the first 20 seconds. In the previous lessons, when we looked at graphs where time is on the x-axis and quantity of either reactants used or the quantity of product formed on the y-axis. The gradient of the graph will tell us the rates of reaction. So we're going to use this knowledge to help us describe what is happening, in the first 20 seconds of this reaction. I'm going to highlight what we're looking at. So how can we describe the gradient and link it to the rate of reaction in the first 20 seconds? As you can see, the gradient of the graph is very steep between zero and 20 seconds, which is why I'm going to say, the rate of reaction is increasing very quickly, during this time. And I am also going to include data from reading of the graph, and this is what I'm going to include in the answer. The volume of gas produced increase from zero to 39 centimetres cubed in twenty seconds. Now let's take a look at question b. Describe what is happening to the rate of reaction between 40 and 70 seconds. Once again, we are going to use our x-axis to identify which section of the graph this is at, then we're going to look at the gradient. We don't have to work out the exact calculation. If you want to, of course you can do so because you know how to do that now. But looking at the gradient or the slope of this graph, I can tell that the rates of reaction is starting to slow down towards the middle. Because the gradient has become less steep. And I'm also going to include data in my answer. So this is how I'm going to answer this question. Rates of reaction started to slow down as the gradient of the graph became less steep. I am describing, I am stating what I see. And next I'm going to use data by saying, the volume of gas produced increased from 59 to 70 centimetre cubed in 30 seconds. And I've worked this out from reading the graph. And finally, let's answer this one. Describe what is happening to the rate of reaction between 100 and 120 seconds. Once again, identifying where that is on the graph. And I can see that the graph has started to flatten or plateau. This means, the volume of gas has not changed between a hundred and 120 seconds. The volume of gas produced is going to be zero. So what this tells me is that the reaction has stopped. So I'm going to describe what I'm seeing. The graph flattens. And of course, you can say the reaction has stopped. In terms of data, we can just say, no gas has been produced. All right, now let's take a look at the same questions, but changing the word describe to explain and look carefully at what the differences are. So when you're answering questions that ask you to explain, you need to use your own knowledge to say why something is happening. Now, the question here is, explain what is happening to rate of reaction: a, in the first 20 seconds. Question b, between 40 and 70 seconds and c, between 100 and 120 seconds. So you can see the question is similar to the one before, the only difference is the word describe that has been replaced with explain. So let's take a look at how to answer these questions. The first one. Explain what is happening to the rate of reaction in the first 20 seconds. And I am going to annotate this on the graph again, where this section is. And previously, where we were describing what's going on here, we mentioned that the gradient is very steep and therefore, the rate of reaction is increasing very quickly, but now we need to use our own knowledge to explain why is the reaction happening so quickly at the beginning. Pause the lesson and have a think why this is. So this is how I'm going to answer this question. Reaction took place quickly as more reacting magnesium is present in the conical flask with hydrochloric acid. Therefore, particles collide more frequently. As you can see, I've used my own knowledge of collision theory, to explain what is happening here between zero and 20 seconds. This is different from when we describe what is happening in the first 20 seconds. Next, explain what is happening to rate of reaction between 40 and 70 seconds. I'm going to annotate this on the graph where the section is. So when we were describing, we said that the gradient has become less steep, and therefore, the rate of reaction has started to slow down towards the middle. Now, when it comes to explaining, we need to use our own knowledge to say why this is happening. And this is how I'm going to answer this question. The reaction started to slow down as little magnesium is left in the conical flask, reacting with hydrochloric acid. Therefore, particles collide less frequently. Once again, I've used my own knowledge of collision theory to explain why this is happening. And finally, explain what is happening to rate of reaction between 100 and 120 seconds. Annotating that on my graph, where the section is. Earlier on, we said the graph plateaus or flattens, and this tells us that the reaction has stopped. And no gas has been produced because the volume of gas has remained the same from 100 to 120 seconds. So how do we explain this? Reaction has stopped because all the magnesium has reacted with hydrochloric acid. No hydrogen gas is released. And this is how I explained it, by saying the magnesium, which is the reactant, has openly reacted with hydrochloric acid. And therefore the reaction is complete. Okay, Thank you for listening. Now, I would like you to practise a couple of questions independently. So I'm going to show you a question on the next slide, and what I would like you to do is pause the lesson and attempt the question. Give it a good go, and write down the answers on your sheet of paper. Then you can check your answers afterwards. So here it is. Read the question carefully and give it a good go. Pause the lesson now to complete the task, then resume when you're finished. Well done. I hope you found that quite useful. Now let's check your answers. Here's the answer for describe and explain what's happening to the rate of reaction, in the first 10 seconds. Pause the lesson to assess your work, then resume when you're finished. Excellent, now let's take a look at the next question. Here are the answers for describe and explain what's happening to the rates of reaction between 15 and 25 seconds. Pause the lesson here to assess your work, then resume when you're finished. Brilliant. And finally, here are the answers for the last question. Describe and explain what is happening to rates of reaction, between 35 and 40 seconds. Pause the lesson here to assess your work, then resume when you're finished. To summarise what we looked at in this lesson. At the beginning, we recapped measuring rate of reaction. Then we looked at how to plan and write a good method for a practical. Then we looked at how to display data, whether that's in the form of a table or in the form of graph. And finally, we looked at graphs, but specifically how to answer, describe and explain questions. Well done for your excellent effort. We have now arrived at the end of the lesson. Don't forget there are downloadable resources for you to complete after you have watched this lesson. Please make sure you also complete the Exit Quiz, so I know how well you've been listening. I hope you found this lesson useful, and I look forward to seeing you in our next lesson, where we will look at more practicals. Thank you for joining me in this lesson and I will see you soon, bye.

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Completed: Planning an investigation to find rate of reaction

This lesson will prepare students for the required practicals in the following lessons. Students will look at how to write a method for a practical and learn how to collect and display data appropriately. At the end of the lesson, students will learn how to describe and explain trends in graphs.

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Planning an investigation to find rate of reaction

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