Video: Rate of reaction

﻿Hello everyone, I am Dr. Deng, and we will be embarking on a journey together, looking at the rate and extent of chemical change. One of the hobbies I picked up lately is baking. However, one thing I do not have the patience for when it comes to baking is the wait time. I'm always so eager for the cake to come off the oven so I can start digging in. So this got me thinking about rates of reaction, which is what we're going to look at in this lesson. Now, this is a really interesting topic because it allows us to think about all the chemical reactions that happen around us. So before we start, make sure you find yourself a quiet room so that you are free from any distractions, switch off your mobile phones, close all the tabs you have open so that you can get the most out of this lesson. Now, when you're ready, let's begin. We are going to start by recapping what is a chemical reaction and look at some examples of chemical reactions that you may or may not have come across before. Next, we will look at the signs of chemical reaction, so we know whether a chemical reaction is taking place. Finally, we will focus on rate of reaction and look at the different methods we can use to collect data in order to measure different variables and calculate the rate of reaction. It's so important you understand why you're learning this topic. On the screen, you will see different jobs that require the knowledge of rate and extent of chemical change. Pause the lesson, and maybe have a think about these different careers and how they involve the knowledge of chemical reactions. In terms of the equipment that you need for this lesson, you'll need a pen, paper, or something to write on, and a calculator. If you do not have any of these with you, pause the lesson now and go grab them, then resume the video once you have your equipment. When we talk about chemical reactions, students often forget that chemical reactions do not only happen in a science lab in school. They happen all around us. Let's take a look at some examples of chemical reactions that happen in our everyday lives. The first example is rusting. You may have seen this on the chain of an old bicycle, and this is because a chemical reaction has taken place between the metal and oxygen or its vapour in the air. Another common example of a chemical reaction is when we're cooking. Raw materials or ingredients they react to form a new product, or in this case, very delicious food. Another important chemical reaction that happens around us is photosynthesis, where plants convert carbon dioxide and water into glucose and oxygen. And finally, the colourful fireworks that you typically see on new years day that is a result of chemical reactions as well. Now let's take a look at some examples of chemical reactions that happen in a science lab in school, which we will primarily be looking at in this chemistry lesson. You may have come across the first example previously, which is a neutralisation reaction when acid and base react together to make salt and water. Now you can see the solution is cream because a universal indicator has been added to it to show that the solution is indeed neutral. The second example of a chemical reaction, as shown in the picture in the middle, is when a metal is added to acid. Again, this is how salts are made, and you can see bubbling or fizzing in the boiling tube. And that is because a gas is being produced. Finally, another chemical reaction that you may have come across in a science lab previously is thermal decomposition. This is when heat is used to break down a substance to form two or more new products. So how do we define a chemical reaction in words? Let's take a look. In a chemical reaction, atoms in the reactants rearrange themselves and then join back together in a different way to form new substances, products. In the diagram on the screen, you can see four examples of chemical reactions where the atoms in the reactants, which are the atom shown before the arrow, on the left, these rearrange themselves to form new products, which are shown after the arrow, on the right-hand side. And this is when we say a chemical reaction has taken place. So how do we know whether a reaction has taken place and how fast is it happening? There are a couple of signs that you should be aware of, pause the lesson and see if you can figure out what we can measure, observe in each one of these diagrams, which tells us that a chemical reaction is happening. Give it a go. Okay, let's take a look at these. Now, the first diagram on the left, you may observe bubbling or effervescence sometimes. And this means a gas is being released. It will be very difficult to count the number of bubbles, though. So what we can do instead is collect and record the volume of gas produced. Now you may have noticed I use the word volume, and this is because we should never ever use the word amount, which is what students typically say, amount of gas. That is not correct. Never say amount. We don't use amount in chemistry for measurements. Okay, we always talk about volume or mass or concentration. So remember that. So the next diagram here shows us a top pan balance. This is not a scale that we use in the kitchen or in the bathroom. We can use this to record a change in mass where the mass of reactants has either increased or decreased. Now, if the mass of the reactant has decreased, it would be because gas is produced from the reaction and released to the surrounding. If the mass increases, a gas has, in fact, reacted with the reactants. Now, the diagram in the middle here shows a conical flask placed over a cross that's drawn on a piece of paper. Now, this allows us to observe turbidity in a solution, which means that the solution goes clouding. We can record the time it takes for the cross to no longer be seen. And finally, another sign of a chemical reaction is when a temperature change is observed. This happens when the reaction is either endothermic or exothermic, which means energy is either taken in from the surrounding or given out to the surroundings. It's important for you to know which signs you should be looking out for when you're carrying out a chemical reaction. So what is the rate of reaction? Let's take a look. The rate of the chemical reaction is a measure of how quickly a reactant is used up, or a product is formed. Now, when we say how quickly, we need to bear in mind that we should always make reference to time. So it's very important that we record the time taken for either the reactants to be used up or the time taken for the products to be formed. The rate of reaction can be found by measuring the quantity of reactant used or the quantity of product forms over time, which is what we just spoke about. Rate of reaction can be measured in grammes per second. If the quantity measured is mass over time, or we can also measure rate of reaction in centimetre cubed per second if the quantity measured is volume over time. Before we carry on, let's do a quick quiz to test how much you know. So when the questions pop up on the screen, I will read the questions out, and I will get you to attempt the questions by pausing the lesson. It's important that you give each question a go. First question, which is the correct definition for a chemical reaction? Please pause the lesson and resume when you have an answer. Well done, B is correct. Atoms rearrange to form new products. This is the correct definition for a chemical reaction. Next question, which of the following is NOT a chemical reaction? Please pause the lesson and resume when you have an answer. Fantastic, the answer is indeed C, ice melting. Now it's important to bear in mind when ice melts, it turns into water. Ice and water are made up of the same compounds, H2O. So therefore, no atoms have been rearranged, and no new products are formed. This is simply a change in state. Next question, which of the following is NOT a sign that a reaction has taken place? Pause the lesson and resume when you have an answer. Brilliant, the answer is D, evaporation. Once again, no atoms are being rearranged. No new products are being formed. Next question, which of the following is a possible unit for a rate of reaction? Please pause a lesson and resume when you have an answer. Well done, the answer is A, grammes per second. Again for rate of reaction, we are always making reference to time. Let's take a look at the last question. How could we measure the rate of reaction if the reaction produces a gas? Pause the lesson here and resume when you have an answer. Brilliant, the answer is D. Time how long the reaction takes and record the volume of gas produced. Well done. Let's take a look at how we can collect data in order to measure rate of reaction. Remember, when we measure rate, we must make reference to time. Now in a science lab, we use a digital stop clock to record time. Next, we measure either the quantity of reactant lost or the quantity of products formed. Now, this depends on what you are measuring. If gases are involved, it's best to use a gas syringe. This allows us to collect the gas and measure the volume of the gas produced. If you are recording a change in mass, which is likely because gas is either reacting with the reactants, or gases released to the surroundings causing the change in mass, we need to use a top pan balance. If you are measuring temperature change, you need to use a thermometer to record it. And finally, if a reaction involves a colour change or turbidity, you need to use the disappearing cross method and observe the change carefully. All right, let's take a look at how we can calculate rates of reaction. The first equation we can use is mean rate of reaction equals quantity of reactant used divided by the time taken. And the second equation we can use is the quantity of product formed, divided by the time taken. Now, when we find the quantity of either the reactants or product, we are referring to either volume or mass. We're not going to use temperature change or turbidity to calculate rate of reaction because neither of those tell us anything about the quantity of products or reactants. Let's take a look at a couple of examples where we can apply these equations to calculate rate of reaction. First example, a student investigated the rate of reaction between calcium carbonate, also known as marble chips, and hydrochloric acid. The student had collected 30 centimetre cubed of gas produced after 15 seconds. Calculate the mean rate of reaction from 0 to 15 seconds. In order to identify the right equation, I need to identify the variables and values given. Now, in this case, a student collected a volume of gas, which is 30 centimetre cubed over a period of time, which is 15 seconds. So I know in this case, I can use this following equation, which is, mean rate of reaction equals quantity of product formed, divided by time taken. Then I will substitute these values in to the equation and then solve it using my calculator. And the answer is 2 centimetre cubed per second. The unit for rate, In this case, is centimetre cubed per second, as we mentioned previously, that quantity measured is volume of gas. Let's try another example. Now, if you're feeling confident and would like to attempt this independently, pause lesson now and do so. Otherwise, I'm going to explain how to work out this question. A student investigated the rate of reaction between calcium carbonate, also known as marble chips, and hydrochloric acid. The student had recorded initial reactant mass of 27 grammes. The mass of the product recorded was 24.9 grammes after 30 seconds, calculate the mean rate of reaction from 0 to 30 seconds. Once again, I'm going to identify the variables and the values given. So, in this case, I've been given the initial reactant mass, which is 27 grammes. And I've also been given the mass of the product, which is 24.9 grammes. I've also been given the time, which is 30 seconds. I'm going to identify the right equation to use, which is this one. Mean rate of reaction equals quantity of reactant used over time taken. Now, the quantity of reactant used this case, is going to be the change in mass. Now, don't forget, mass doesn't get lost only if a gas is produced and released to the surrounding, which explains why the mass has decreased. So I'm going to find the change in mass by substituting these numbers into the equation, making sure the quantity of reactant used is 27 - 24.9 to find the difference. And taking that divided by the time taken for the reaction to take place. I'm going to solve using my calculator, and the answer is 0.07 grammes per second. Once again, being very careful with my units here because masses involved, I'm using grammes per second. Now I would like you to attempt this question independently. So read the question carefully and write down how you would work this out. Pause the lesson now. Well done, now let's check if you've got the right answer. Pause the lesson here to check your answer. Now, we are going to look at how to calculate rate of reaction from a graph. Now, when we're calculating rate from a graph, we need to make sure that time is on the X-axis, then we're going to look for the gradient, which would tell us the rate of reaction. To calculate the gradient of a line, you'll need to find the change in the Y-value divided by the change in the X-value. Now the triangle symbol that you see here simply means change. Let's try and apply this to a couple of examples together. Now, the first graph we're looking at here, you will see time on the X-axis and volume of gas produced on the Y-axis. You can see that there's a straight line through the origin, which tells us that the relationship between time and volume of gas is indeed directly proportional. Now, when you're using a line graph to calculate the gradient, a good tip would be to draw the biggest line possible using your ruler. However, I'm going to show you how it's done on my screen instead as if I were using a ruler to draw these straight lines. So to find the change a Y-value, I need to look at the line and refer to the Y-axis. I'm going to draw a vertical line from 0 to 100, which is where the line starts and stops. And the change of Y will be the difference, which is 100 - 0. Then, I'm going to draw a horizontal line from 0 to 48, which is where the line starts and stops. And this is how I find my change in X-value, 48 - 0. Now, I'm going to substitute these numbers into the equation to find rates of reaction, then use my calculator to solve. Now my calculator shows that the answer is 2.083333. And I'm going to use those dots to represent that this is recurring. However, I'm going to present my answer in three significant figures. So that will be 2.08-centimeter cubes per second. Now you need to be careful with the unit, and the unit comes from the volume of gas produced as you can see from the Y-axis over time, which is seconds as seen on the X-axis. Let's take a look at this next graph, which slightly different. And actually, this one is more representative of a chemical reaction, but similarly, we can find the rate of reaction by finding the gradient. Here the students would need to learn how to draw tangent lines to find the gradient at different points on a graph. But this will be discussed in the next lesson. For now, I'm going to show you, once again, how to find the mean rate of reaction using a gradient. Now, in this case, we've been asked to find the mean rate of reaction in the first 15 seconds. I'm going to start by finding the change in X because I know the time is 15 seconds. So I'm going to draw a horizontal line from 0 to 15 seconds. And the change in X is 15 - 0. From there, I'm going to draw another line vertically from that very same point on the graph down to when the volume of gas is zero centimetre cubed, as this is the start of the reaction. We can see from the graph that the change in Y is 45 - 0. I'm going to substitute these Y and X values into my equation and solve for the rate of reaction. And the answer is 3 centimetre cubed per second. Once again, looking carefully at what is being measured on the Y and X-axis to determine the correct unit. Now, I would like you to give this next graph question a go independently. So you need to find the mean rate of reaction in the first 20 seconds from this graph. Pause the lesson now to try. Fantastic, now let's take a look at whether you worked it out correctly. Once again, you can pause the lesson here to check your answer. Fantastic work. Now I would like you to try some more questions independently so you can check your own understanding from what we have covered so far. Please pause the lesson and work out the answers on a piece of paper, then resume the video once you have completed this task. Pause the lesson now to answer these questions. Great job. Now, I'm going to show you the answers. So pause the lesson again to assess your work. Now, just going over what we have covered in this lesson. At the beginning, we looked at what are chemical reactions. We have seen some examples of chemical reactions that take place around us, as well as in the science lab. Next, we looked at some signs of chemical reactions. So we know when a chemical reaction is taking place. Then we looked at how to collect data in order to measure rates of reaction. And finally, we looked at how to calculate rate of reaction, as well as how to calculate rate of reaction from graphs. We have arrived at the end of this lesson. There are downloadable resources and a worksheet available for you to complete after you've watched this video. And don't forget, make sure you complete the exit quiz, so I know how well you've been listening. Well done for your excellent effort. Thank you for joining me, and I will see you soon.

Worksheet: Rate of reaction

Exit Quiz: Rate of reaction

Completed: Rate of reaction

In this lesson, students will go over what chemical reactions are and the signs of a chemical reaction occuring. Students will learn and apply the equation used to calculate rate of reaction. At the end of the lesson, students will learn how to determine mean rate of reaction from graphs.

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Rate of reaction

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