Video: Collision theory

﻿Hello, everyone. I hope you're all doing great. I'm dr. Deng. And welcome back to chemistry. I am going to take you through this next lesson in the unit rates and extent of chemical change. In this lesson, we are going to look at collision theory, why it's important and what role does it play in chemical reactions? So make sure you find yourself a quiet space where you can work through this lesson free from any distractions, switch off your mobile phones, close all the tabs on your device. So you can get the most out of this lesson. When you're ready, let's begin. We are going to start by recapping what is rate. We are going to go over what measurements we need to take in order to find rates of reaction. Next, we're going to look at collision theory. We're going to let his definition and why it's so important in chemical reactions. Next, we're going to look at activation energy. And what role does it play in starting a chemical reaction? And finally, we're going to go over the five factors that affect rates of reaction. In terms of equipment you need for this lesson, you will need a writing pen and some paper or anything to write on. If you do not have any of these on you, please pause the lesson and go get them, then resume once you're ready. Okay. So let's go to vote. What is the rate and how can we measure it? The rates of a chemical reaction is a measure of how quickly a reactant is used up or how quickly a product is formed. The rates of reaction can be found by measuring the quantity of reactant used or the quantity of product forms over time. As we mentioned in previous lessons, it's so important that we make reference to time when we are talking about rate, because it is a measure of how quickly either the reactant is being used up or how quickly the product is being formed. Rates of reaction can be measured in grammes per second. If the quantity measured is mass over time. The other unit that we can use for rate is centimetre cubed per second. And this is if the quantity that we measure is volume over time. Now let's take a look at collision theory. You're going to learn about its definition and why it's so important in chemical reactions. On the screen, you will see a very simple diagram representing a reaction between atom A and atom B. Now, I have a question for you in order for atoms A and B to react together, what do you think must happen? I'm going to give you two hints in the form of diagrams. There are some very important key terms that you need to mention, and hopefully you'll be able to identify them from the hints in the diagram. So pause the lesson now and have a think. And resume when you have an answer. Okay. So in order for atoms and B to react, what must happen? Let's take a look. Collision theory states that, for chemical reactions that occur, the reacting particles must collide with enough energy. And you can see a similar diagram on the screen right now, representing reactions between these different atoms and eventually forming new products. The greater the frequency of successful collisions between reacting particles, the greater the rate of reaction. So without two different factors, we need to talk about in order for reaction to take place, the reacting particles must collide. Not only that they must collide with enough energy and this ties in very nicely to what we're going to look at next, which is activation energy. So what is activation energy? And why is it so important in order for chemical reactions to start? The activation energy is the minimum amount of energy needed for a collision to be successful is different for different reactions. So remember collision theory states that, in order for successful collisions, reacting particles must collide with enough energy. And the minimum amount of energy needed for the successful collision is known as the activation energy. For successful collisions, particles must collide with energy equivalent or more to the activation energy. So what do you think will happen if particles collide with insufficient energy or energy that is lower than the activation energy? Pause the lesson to have a think then resume when you think you've gotten answer. Okay. So if reacting particles do not collide with sufficient energy or with energy that is lower than the activation energy, the reaction is not going to take place. These particles will simply bounce off each other. So it's very important for you to bear mind in order for reactions happen, the activation energy level must be met. I want you to take a look at the diagram that's on the screen. This is a reaction profile, which you may have come across previously in the topic, Energy Changes. Don't worry if you haven't. I will explain a little more. Our reaction profile shows how the energy of the reactants and products changes during a reaction includes the activation energy, which is the minimum energy needed for reaction to start. And as you can see, the activation energy is shown as a hump in the line. It starts at the energy of the reactants and it ends, or it goes up to the peak of that curve or the peak of that hump. Okay. Now let's do a quick quiz just to test if you've understood what we've talked about so far, these are multiple choice questions. So I'm going to read the questions out and then I'm good to get you to pause the lesson so you can work out the answer. Are you ready? Let's go. The following is a unit for rates. Pause lesson here to try then resume when you have an answer. Good. The answer is C, centimetres cube per second. As we mentioned previously, freight is a measure of how quickly reactants are being used up or how quickly products are being formed. So in this case, the unit centimetre cube per second is used because the quantity measured is volume over time. Collision theory States that, for reactions to take place, reacting particles must? Pause the lesson to try this question, then resume when you have an answer. Excellent. The answer is D, collide with enough energy. So two things you must remember here, reacting particles must collide, but they also must collide with enough energy in order for reaction to take place. What is the minimum amount of energy required for successful collision? Pause a lesson to answer this question, then resume when you have an answer. Fantastic. The answer is C, activation energy which is the minimum amount of energy needed in order for reaction to take place or in order for successful collisions. The rates of reaction depends on two factors. They are? Pause the lesson to answer this question, then resume when you have an answer. Okay. I hope we got that answer right, let's check. And the answer is A, how frequently particles collide and the energy with which they collide. In which of the following reaction profile diagrams is activation energy labelled correctly? Pause lesson to try then resume when you have an answer. Okay, let's see if you got that right. And the answer is B. So you can see where the arrow is used to show the activation energy, the starts at the energy of the reactants. And it goes all the way up to the peak of the hump. Fantastic. Now let's take a look at five different factors that affect the rates reaction. Then in this lesson, we are only going to have a general overview for each one of these factors. Then we will look at them individually, even further in the coming lessons, the factors which affect the rates of chemical reactions include, the concentration of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature, and finally the presence of catalyst. Now let's take a closer look at the first four factors individually and how they affect the rate of reaction. How does concentration of reactants affect the rates? Remember when we are linking concentration of reactants and rates of reaction, we need to bear in mind, that we are only referring to reactants in a solution. The higher the concentration of reactants in a solution, the more number of reacting particles per unit volume. So the volume stays the same. You just have more number of reacting particles. So what does this mean in terms of rates of reaction? This means that the particles will collide more frequently and the rates of reaction will increase. So therefore the higher the concentration of reactants in solution, the faster the rates of reaction, because the particles will collide more frequently. Now it's important to bear in mind that the mean energy is going to remain the same. So we're not going to talk about energy in this scenario. Next, let's take a look at pressure and how it affects the rate of reaction. When we talk about pressure, remember that we are referring to reacting gases. So the States is gas. The higher the pressure, the closer together the reacting particles are. And I want you to take a look at the diagram. That's on the screen, there are two containers there, in the first container on the left, you can see that the volume of gas is much greater than the volume of gas in the second container. And there is more space between those gas particles as they are bouncing off each other and bouncing off the walls of that container. In the second container on the right hand side, you will see that the pressure has increased as the volume of the gas has decreased. And now the particles are closer together. So what does this mean in terms of rates or reaction? Therefore, the particles will collide more frequently when the pressure increases and the rates of reaction will increase as well. Next, let's take a look at surface area of solid reactance and how it affects rates of reaction. On the screen, you will see two diagrams of potatoes being cooked or being boiled in water, in the first diagram on the left, the potato is being cooked whole. And in the second diagram on the right, you can see that the potatoes have been sliced into smaller pieces. So I want you to support the lesson and have a think which potatoes will cook faster. Okay. So what do you think which potatoes will cook faster? And I'm sure some of you have come across this scenario before, and I actually love making mashed potatoes. So I did this the other day, and I believe that we can all agree that the potatoes that have been sliced, they will cook faster in the boiling water. So let's take a look at the reason why. For a given mass of solid, powders or smaller lumps have larger surface area to volume ratios than larger lumps. So when we looked at those two pots of potatoes early on, in the first diagram, the potatoes were whole and in the second diagram, or in the second part, the potatoes were sliced into smaller pieces or in this case, smaller lumps. And they will cook faster because they have a larger surface area to volume ratio, even though the size of a potato. Now, the slice of potato are much smaller than the whole potato. And this is because the area of exposure or an area that's being exposed is much greater now. So what does that mean in terms of a reaction, particles will collide more frequently and therefore increasing the rates of reaction. So remember smaller pieces or small lumps, especially powder. That's very fine. They have a larger surface area to volume ratio. And this means that the area of exposed surface is much greater than those larger lumps and larger pieces. And so the particles will be able to collide more frequently and this increases the rates of reaction. Now let's take a look at how temperature affects the rates of reaction. So what happens when we increase temperature, the higher, the temperature, the particles gain more kinetic energy. And you may have learned about kinetic energy previously in physics. If not, don't worry about it. Kinetic energy is the energy that is stored in moving objects, or in this case, the particles that gave more kinetic energy are able to move more quickly. And therefore the particles will collide more frequently, and the rates, our reaction is going to be greater. As you can see on the graph in the screen right now, the reaction that took place at higher temperature has a much steeper slope or much steeper gradient. This tells us that the rates of reaction is actually greater than the reaction that is taking place at 25 degrees Celsius. Okay. Let's test how much you remember. Here's a question that I would like you to answer. What are the five factors that affect the rates of chemical reaction? Pause lesson here to answer this question, then resume once you are finished. Okay. So how many factors did you remember? Let's take a look if you've got all of them. The first one is, the concentration of reactants in solution. The pressure of reacting guesses. The third one is, the surface area of solid reactants the temperature, and finally the presence of catalyst. Well done if you've got all five factors, correct. Okay. So hope you have been listening well. Now what I would like you to do is practise a couple of questions independently. So when you see the questions pop up on your screen, I would like you to pause lesson, then attempt answering each one of them on your piece of paper, then resume, so we can find out the answers to get them. Okay. Here are the questions. Please pause the lesson now to try then resume to assess your work. Fantastic work. Now let's take a look at the answers and whether you got them right. They are the answers for the first three questions. Pause the lesson here to mark your work. Yeah, the answers for questions four to seven, pause the lesson again to check your work. Okay. Now let's summarise what we've talked about in this lesson. First, we recapped rates of reaction and we talked about how to measure it. Then we looked at collision theory that States that in order for a reaction to take place, particles must collide with enough energy. Then we looked at activation energy, which is the minimum amount of energy for successful collisions. And finally, we talked about the five different factors that affect the rate of reaction. Do you remember them? Yes. They are concentration of reactants in solution, pressure of reacting gases, surface area of solid reactants, temperature and the presence of catalyst, which we will look at even further in the next couple of lessons. We have now arrived at the end of the lesson. Don't forget that are downloadable resources for you to attempt. And please make sure you attempt the exit quiz. So I know how well you've been listening. Thank you for joining me today. And I hope you found this lesson very useful. I look forward to seeing you in our next lesson. Bye.

Worksheet: Collision theory

Exit Quiz: Collision theory

Completed: Collision theory

In this lesson, students will look at the 5 factors that affect rate of reaction. Students will also learn what collision theory is and what is activation energy. At the end of the lesson, students are expected to be able to explain how reactions occur using collision theory.

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Collision theory

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