Video: Rate of reaction required practical (Part 2)

﻿Hello everyone. I hope you're all doing great. It's Dr. Deng again and welcome back to chemistry, where we will continue on this journey, looking at the unit rate and extent of chemical change. In this lesson, we are going to look at the second required practical in this unit where we will investigate the effects of changing concentration on the rate of reaction. This required practical is slightly different from the one we looked at previously, as we will use a method that involves turbidity, or in other words, reactions where the solutions go cloudy. So before we begin, as always, please find yourself a quiet space where you can sit and work through this lesson without any distractions, switch off your mobile phones, or turn off all the notifications, and close down all the tabs on your device so you can make the most of this lesson. When you are ready, let's begin. We are going to start the lesson by going over the different signs of a chemical reaction which we looked at in previous lessons. This leads very nicely into the main part of this lesson, where we will look at the required practical, investigating the effects of changing concentration on the rate of reaction. We will also look at how to develop a hypothesis for this particular investigation. We're going to determine the independent, dependent, and control variables and planning for the required practical. Finally, we will look at how to display data appropriately and draw conclusions from our results. In terms of equipment that you need for this lesson, you will need a writing pen, paper, or something to write on, pencil, rubber, and a ruler. If you do not have any of these with you, pause the lesson now and go and get your equipment. Then resume when you are ready. Let's recap the different signs of a chemical reaction. So how do we know if a chemical reaction is taking place and how quickly is it occurring? On the screen, you'll see different signs of a chemical reaction taking place. If you're feeling confident, you can pause the lesson now to work out what can be observed or the quantities being measured in each of these diagrams which tell us that a chemical reaction is occurring. Otherwise, please continue listening as we go through each one of these. Okay, as shown in the first diagram on the top left corner, some reactions will allow us to observe bubbling or effervescence, which tell us gas has been produced. In the previous required practical, we looked at how we can collect and measure the volume of gas produced from a reaction using either an inverted measuring cylinder or gas syringe. In the second diagram on the bottom left corner, we can see a top pan balance. This is a method we can use to determine the mass of gas produced from a reaction that is lost to the surrounding. The diagram in the middle shows the method we will be focusing on in this lesson. So we will look at this further in the next slide. And the last diagram on the right hand side of the screen shows a chemical reaction where temperature change can be observed as the thermometer is involved, which is used to measure temperature. In the required practical we will be looking at in this lesson, we will be using a method that involves the solution going cloudy as the reaction takes place. As we're not able to measure the quantity of the reactants used or the quantity of the products formed, what we can do instead is place the reactants in a conical flask over a cross that is drawn on a piece of paper, then record the time it takes for the cross to no longer be seen as a precipitate starts to form. However, there are a couple of disadvantages using this method. Can you think of any? Firstly, it will only work if the reaction involves a visual change. This usually happens if one of the products formed is an insoluble solid. Secondly, the results are subjective as it does depend on the judgement by the person carrying out the practical. Before we move on, let's do a quick check to make sure you understand what was explained. Pause the lesson now to answer this question and resume once you are finished. Excellent. Now let's take a look at the answer. If the reaction involves the reacting solution going cloudy, we can draw a cross on a piece of paper before placing the conical flask containing the reactants onto the piece of paper, then record the time it takes for the cross to disappear as a precipitate is formed. Well done if you've got this right. Now let's take a look at the required practical. You will need to learn this very well as you can be assessed on this in your exams. In this investigation, sodium thiosulphate reacts with hydrochloric acid to form sodium chloride, sulphur, water, and sulphur dioxide. Pause the lesson now and see if you can write out a word equation for this reaction. If you're feeling brave, go ahead and write out the symbol equation with the state symbols as well, then resume once you have done this. Off you go. Well done. Now let's take a look if you got that right. When we are writing a word or symbol equation, you need to make sure that you're using plus signs and arrows, not equals. You also need to make sure that all the reactants are written on the left hand side of the arrow and the products are displayed on the right hand side of the arrow. This equation is particularly long, so you need to be extra careful, making sure that the products do not occupy the line after on the side of the reactants, if you have attempted the symbol equation, you can check if all the symbols are written correctly and that the equation is balanced. Please also check that the state symbols are written correctly. A-Q stands for aqueous solution. L for liquid. G for gas. And S for solid. So my next question is explain why the solution goes cloudy as the reaction occurs. Pause the lesson and have a think, then resume once you have an answer. Okay, so hopefully you were able to work this out based on the state symbols we looked at from the symbol equation. So the reason why the solution goes cloudy is because sulphur is formed as one of the products and sulphur is an insoluble solid. Now let's come up with a hypothesis for this practical, where we will investigate the effect of changing concentration of hydrochloric acid on the rate of reaction with sodium thiosulphate. Remember, a hypothesis is a prediction made with limited evidence to begin with, but we can use our knowledge of collision theory to help us come up with a sensible hypothesis. Pause the lesson now and write down on your piece of paper what you think the hypothesis for this particular investigation is going to be. Well done. Now let's see if we have written down a similar hypothesis. So our hypothesis for this investigation is going to be "Increasing the concentration "of hydrochloric acid increases the rate of reaction "when reacting with sodium thiosulphate." Before we begin our required practical, let's take a look at the variables that are changed, measured and controlled in this investigation. Remember that independent variable refers to the variable that has changed in this practical. The dependent variable is the variable that is being measured, and finally, the control variables are variables that need to be kept the same throughout the practical. This is so that our results are valid and reliable. Pause the lesson now, and on your sheet of paper, write down what you think these different variables are, then resume once you're finished. Great work. Now let's go over these different variables and see if you've got them right. For the independent variable, that will be the concentration of hydrochloric acid and the unit for this is molar. This is the variable that we will be changing in this practical, as we will be looking at how different concentrations of hydrochloric acid affect the rate of reaction. The dependent variable is the time taken for cross to disappear. This is measured in seconds, which we will use a digital stop clock to record. For the control variables, I've come up with four of them. These include: the concentration of sodium thiosulphate, the volume of hydrochloric acid, the volume of sodium thiosulphate, and temperature. We must keep these different variables the same throughout the entire practical. Okay, now let's take a look at how to carry out the practical. The diagram on the screen illustrates how this required practical can be conducted. You can pause the lesson here to take a better look at the setup but we are going to watch a video for this required practical that is carried out in a science lab in school. In this required practical, we're going to look at the effect of increasing concentration of hydrochloric acid on rate of reaction with sodium thiosulphate. Sulphur is one of the products that's going to form, which is an insoluble solid that will make the solution go cloudy which we can observe. The first thing we need to prepare is a cross drawn on a piece of paper. This will allow us to observe the turbidity, or as the solution gets cloudy, it will become opaque and we can observe this through a conical flask that's placed directly on top of the cross. The cross will slowly disappear as the solution becomes opaque. Next we're going to prepare our sodium thiosulphate solution. For this, we are going to use a measuring cylinder to measure out 10 centimetre cubed of sodium thiosulphate. Now when we are measuring our sodium thiosulphate using a measuring cylinder, we need to make sure that we are reading the volume at eye level. Then we're going to pour this into the conical flask. Okay, so now we need to prepare our hydrochloric acid. We're going to start with the lowest concentration first, which is 0.25 molar hydrochloric acid. Then we will repeat the procedure using the other concentrations of hydrochloric acid and looking at the effect it has on the time it takes for the cross to disappear. The next piece of equipment that we need in this practical, or in any other rate of reaction practical, is a stopwatch to record time. So we're going to get this ready first before we measure out our hydrochloric acid. So we need 10 centimetre cubed of this. We're going to measure it out using a different measuring cylinder. We're not going to use the same one that we use to measure out the sodium thiosulphate earlier on as we don't want the reaction to start yet. Now we're going to get our stop clock or stopwatch prepared, then we're going to pour the hydrochloric acid into the flask and press go on the stopwatch. I'm going to pick up the conical flask and swall it a couple of times and then leave it. We can observe the solution as it goes cloudy from above or we can see from the camera. We're just going to wait and record the time it takes until the cross can no longer be seen. Okay, we can start to see a precipitate forming. However, we can still see the cross, so we're not going to press stop on the stopwatch yet. I'm going to use my own judgement from the camera over here. You can still see the cross. You can still see it. Some of you may still be able to see it. And stop. Okay, I'm going to record the time it has taken for the cross to disappear as I can no longer see the cross. Okay, so that took around 78 seconds. Now this reaction produces some gas that is not very pleasant, so I wouldn't get too close when we're observing from above because we don't want to be breathing in the fumes. Now we're going to empty out the contents of that conical flask, making sure we dispose of the contents safely, then we're going to use a new conical flask that is the same shape and the same size for our next reaction. This is important because it could affect the way we perceive how quickly the cross disappears. We're also going to use the same cross. We're not going to use another one that is drawn in pencil, for example, as this could also affect how quickly we see the cross disappear. Now we are going to prepare sodium thiosulphate solution for our next reaction. We're going to use a measuring cylinder that is not contaminated to measure out 10 centimetre cubed of sodium thiosulphate and then we're going to pour this into the conical flask. This time the concentration of hydrochloric acid that we're using is 0.5 molar. Once again, we're going to use a clean measuring cylinder that is not contaminated to measure out 10 centimetre cubed of hydrochloric acid. Now we need to get our stop watch ready and we're going to pour the hydrochloric acid into the conical flask and press go on our stopwatch, swall the conical flask a couple of times and leaving it. Now we are going to wait for the precipitate to form and observing carefully from above. Okay, we can see the solution is starting to get cloudy, but we can still see the cross so we're not going to press stop yet on the stopwatch. Can still see it. We can still see the cross. And stop. Okay, so that took around 66 seconds. I'm going to record this down in my table. Now, sometimes your results may not always be what you predict them to be. So it is good practise to repeat each one three times to find a mean. This is so that we can discount any anomalous results. So we're going to repeat this practical again and ideally we're going to use five different concentrations of hydrochloric acid. This is so that we can collect enough data to plot a graph later on, but for now we're going to skip a couple of the concentrations in between and we're going to jump straight to using a concentration of two molar hydrochloric acid. Now we need to prepare our sodium thiosulphate solution, so let's measure out 10 centimetre cubed of that using a clean measuring cylinder that is not contaminated and then pouring the sodium thiosulphate into the conical flask that is placed on top the cross. Now let's measure the hydrochloric acid. We're going to use 10 centimetre cubed of this two molar hydrochloric acid. Before we pour it in, making sure we get our stopwatch ready. And pour and press go. Swall it a couple of times and then leaving it. Making sure we can see the cross clearly from above. We can see that the precipitate has started to form. We can still see the cross, so we're not going to press stop yet on our stopwatch. And stop. So this time it has taken 43 seconds, which is much faster than the reactions earlier on where we used the other two concentrations of hydrochloric acid. So what we're going to do now is repeat this three times to find a mean, then repeat the procedure with the other concentrations of hydrochloric acid. Then we're going to collect our data and record it down in our table to find a mean. So what we're going to do now is head back to the slides to give this a go. Okay, so I have collected all this data from the reaction between sodium thiosulphate and five different concentrations of hydrochloric acid, HCl. I've repeated each one three times. So now we need to calculate the mean for each concentration of HCl. To calculate mean, we need to total up the numbers and then divide it by how many numbers there are. In this case it's three. This is so that we can discount any anomalous results. Pause the lesson now to do this and then resume when you have calculated the mean for each concentration. Excellent. Now let's check your answers. You can pause the lesson again to check your answers. Okay, so now let's plot a graph using the data that we've collected. I've transferred the table from the previous slide, but simplifying it by only including the concentration of hydrochloric acid and the mean time taken for the cross to disappear. So let's take a look at how we can plot this graph together. On the X axis, we can see concentration of hydrochloric acid in molar and the Y axis is for the mean time taken for cross to disappear in seconds. So when the concentration of HCl is 0.5 molar, the mean time taken for cross to disappear is 66 seconds. Now I've identified where this is on my graph paper and I'm going to draw a cross where this data point is. I'm going to continue doing this for when the concentration of HCl is one molar and the mean time taken for the cross to disappear is 35 seconds. Next, when concentration of HCl is 1.5 molar, the mean time taken is 19 seconds. When the concentration of HCl is two molar, the mean time taken is 11 seconds. And finally, when the concentration of HCl is 2.5 molar, the mean time taken for the cross to disappear is seven seconds. Now I'm going to draw a line of best fit. In this case, a curve of best fit, by joining up the data points. From the graph, it is very clear that as the concentration of hydrochloric acid increases from 0.5 molar to 2.5 molar, the mean time taken for the cross to disappear decreases. So this tells us that the reaction is happening quicker. Alternatively, we can calculate rate for each concentration. To do this, we can use the equation rate = one / time, in seconds. The values we end up with will be very small, so we can multiply each by 1000. I've done this, as shown in the third column in the table on the screen. This makes it easier to plot on a graph. So now, we can plot a graph of this, where concentration of hydrochloric acid is on the X axis and rate is on the Y axis. We can draw the line of best fit and look at the relationship between concentration and rate. So there you go. As the concentration of hydrochloric acid increases, the rate of reaction increases as well. As a conclusion, "As the concentration "of hydrochloric acid increases, "the mean time taken for cross to disappear decreases, "the faster the rate of reaction with sodium thiosulphate." So how can we explain this using our knowledge of collision theory? Pause the lesson now and on your sheet of paper write down why you think this is, making sure you include all the necessary, important key terms that we've looked at previously. Give this a go. Excellent. Now let's take a look at why this is. "The higher the concentration of reacting particles, "the more particles per unit volume. "Therefore particles collide more frequently," and the "rate of reaction increases." I've underlined the key terms that are very important that you must include in your explanation, so make sure you pay attention to these. Well done if you got this right and you have included all the important key terms in your explanation. Now let's summarise what we have looked at in this lesson. At the beginning, we recalled the different signs of a chemical reaction and we looked at how we can measure the rate of reaction using different quantities. Next, we looked at how to determine rate from the disappearing cross experiment, which is the required practical. And finally, during the required practical, we looked at how to write a hypothesis for this particular investigation, then we talked through the method and identified the variables that need to be changed, measured, and controlled, and lastly, how to display data appropriately and how to draw conclusions from our graph. We have now arrived at the end of this lesson. There are downloadable resources and a worksheet for you to complete. 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 the effects of changing surface area of solid reactants on the rate of reaction. Thank you for joining me in this lesson and I will see you soon. Bye.

Worksheet: Rate of reaction required practical (Part 2)

Exit Quiz: Rate of reaction required practical (Part 2)

Completed: Rate of reaction required practical (Part 2)

In this lesson, students will complete the required practical: Investigate the effect of concentration on rate of reactions by a method involving a change in colour or turbidity.
After the required practical, students will explain observations and draw conclusions using particle theory.

Combined Science

Rate of reaction required practical (Part 2)

Video

Worksheet

Final Quiz

Lesson Complete