This quiz will test your knowledge on the previous lesson and previous lessons in the unit

What happens to bonds during a chemical reaction?

What is the activation energy?

Which arrow in the energy level diagram represents activation energy?

Which arrow in the energy level represents the overall energy change?

What type of reaction is shown in the energy level diagram?

Energy level diagrams

﻿Hello, I'm Mrs. Begum. And welcome to another energy changes lesson. This lesson we'll be focusing on calculating bond energies. This lesson is only for students doing the higher paper. For this lesson, you will need pen, paper and a calculator. Please make sure that you are working in a quiet environment. Make sure that you are distraction-free. All the tabs are closed on your computer, your mobile phone is on silent. This is so that you can get the most out of this lesson. As always, don't give up. Make sure you persevere and continue the lesson right to the end. Okay. So when you're ready, let's begin. So here's a plan for the lesson. First of all, calculate bond energy values and use them to predict whether the reaction is exothermic or endothermic. And then relate these bond energies to the correct part of an energy level diagram. Okay, so before we start looking at calculating bond energies, let's do a quick warmup. Pause the lesson here to answer these questions. Resume once you have finished. Okay, how did you do? Let's look at the answers. So in that molecule, there are three bonds. Well, so its NH, there's three NH bonds. What type of bonds are they? Well, they are covalent bonds because that's a simple molecule. The minimum amount of energy needed to start a reaction is called the activation energy. Which part of any reaction is endothermic? Well, it's the breaking bonds in the reactants part that's endothermic. And what type of reaction takes place when methane burns in air? Well, you can call it a combustion reaction because burning in oxygen, oxidation because oxygen is being added. Or you can call exothermic. During chemical reactions we looked at what happens to chemical bonds. And chemical bonds are broken and new ones are made. The bonds are broken in the reactants and new bonds are made in the products. Okay. So what we need to look at before we can calculate bond energies is looking at the bond energy values. So the amount of energy required to break a bond is the same as the energy that's released when the same bond is made. Bond energy do differ depending on which atoms are bonded. Can we look at some bond energy values in a minute. Bond energy values are always given in kilojoules per mole. That is the unit for bond energy values. So these are some examples of some average bond energy values in kilojoules per mole. Really important that you understand that you don't ever have to memorise any of these numbers. In an exam question, if you're asked to calculate bond energy values you will be given these values. So what does this mean? Well, if you look at C-H, so what that means is if a C-H bond is broken, you need to supply it with 413 kilojoules per mole of energy. Subsequently when a C-H bond is made, the same amount of energy is released. So 413 kilojoules of energy is released. Let's look at another example, let's look at oxygen-oxygen double bond. Oxygen-oxygen double bond needs 498 kilojoules per mole of energy to break. It also releases 498 kilojoules per mole of energy when the oxygen-oxygen double bond is made. Okay. So time for a quick quiz. Pause the lesson here to complete this task. Resume once you have finished. Okay, so how did you do? Let's look at the answers. So question number one, what units are used for bond energies? Is kilojoules per mole. Number two, what do bond energies represent? Well, they represent the amount of energy needed to break a bond, but also the amount of energy released when a bond is made. Number three, if 413 kilojoules per mole of energy is needed to break a C-H bond, that's a carbon hydrogen bond, how much energy is released when a bond is made? Well, it's the same, 413 kilojoules as the energy taken in and released is the same. Okay. So let's start looking at applying these to bond energy calculations. So the reaction on the screen shows your reaction between hydrogen and fluorine to produce hydrogen fluoride. So it's got a word equation and a symbol equation. H2 plus F2 to produce 2HF. So when you're looking at bond energy calculations, it's always a good idea to look at the bond energy for the reactants, the energy going in, the endothermic part and also looking at the, separately, the products. So the energy that's released, the exothermic part separately, and it's easy to determine whether the reaction is exothermic or endothermic. So if we take the hydrogen and the fluorine reacting together first, so those bonds are broken. So to break one H-H, hydrogen-hydrogen bond, it needs 432 kilojoules per mole of energy. And to break one fluorine-fluorine, F-F bond , requires 159 kilojoules per mole of energy. So altogether the energy that's needed to break those bonds is 591 kilojoules because you just add those two values up. So that's how much energy is needed to break the bond. Now, the energy that's released when those bonds are made. Now, if you have a look, there's a two in front of the HF, which means there are two moles of HF that's produced. So one HF bond releases 567 kilojoules per mole of energy. So therefore two of those would release 1,134 kilojoules of energy. So the total in is 591 kilojoules. And the total energy released is 1,134 kilojoules. So the overall energy change is 591. That's the energy that's in, take away the energy out, that's energy that's released, is 1,134. So the overall energy change is minus 543 kilojoules. So this means that the reaction is exothermic because it's minus by 543 kilojoules per mole. Now it's always a good idea so when you are doing the overall energy change to do the energy in take away the energy released, it's that way that you get the either positive or the negative sign correct. Every time you'll get it right. So if it's a negative number, is exothermic. Okay. So to calculate, for successful bond energies calculations first one is to make sure that you calculate energy required for bond breaking. So first of all, calculate the energy needed to break the bonds of the reactants. Second, is calculate the energy released during bond making. So in the products, how much energy is released when those bonds are made? And then overall energy change is always energy required, that's number one, take away the energy released, that's number two. And if you do it this way, you will always, always get successful bond energy calculation. It will always be correct. Just a quick recap. Making and breaking bonds. So if a reaction is exothermic overall, then more energy has been released during bond making than has been required for bond breaking. And if a reaction is endothermic overall, then more energy has be required or taken in during bond breaking than has been released during bond making. Okay. So time for you to have a go. Pause the lesson here to complete this task. Resume once you have finished. Okay, so how did you do? Let's look at the answers. So if you look at hydrogen and chlorine so one H-H bond, one Cl-Cl bond. So the total energy required for bond breaking is 674 kilojoules. And if you look on the other side, the product side, two HCl that means there are two HCl bonds. So each HCl bond releases 431 kilojoules of energy. So the total energy released is 862 kilojoules. So the overall energy change is 674 take away 862, which is minus 188. So the reaction is exothermic by 188 kilojoules per mole. Okay. So that was the answer to question one. Here are the answers to the rest of the questions. Can you pause the lesson here, please, to check your answers. Okay. So, so far the examples that we've looked at have all had one bond in the molecules. What if there are more than one bond? So if you look at the molecule of CH4, that has four bonds in the molecule and each C-H bond requires 413 kilojoules per mole to break. So how much energy would be required to break all the bonds in a mole of methane? Well, you simply do four times 413 and the answer is 1,652 kilojoules per mole of energy would be needed to break the bond, all the bonds in one mole of methane. So how much energy would be released if one mole of methane was made? It would be exactly the same. It'll be 1,652 kilojoules per mole. So let's look at another example. So reaction between hydrogen and oxygen to produce water. So it's easier for things that have got more than one bond to draw the structures out. So there's two H-H, one oxygen-oxygen double bond. And remember you have to work with a balanced, simple equation. And there's two H2O and they've been drawn out. So it's easy to see how many bonds there are and what the bonds are. So if you look at the, take the bond energy values. So H-H is 432 kilojoules per mole. O-O double bond requires 498 kilojoules per mole. So to break the bonds, there's two H-H bonds. So it's two times 432, plus there's only one oxygen double bond, oxygen bond to break. So it's 498. So the total energy required to break those bonds is 1,362 kilojoules per mole. What about how much energy is released when those, when water is made? So the bond energies, the only bond energy values you need is O-H, O-H-O which releases or requires 463 kilojoules per mole. And if you're making four of those, so the four times 463, which is 1,852. So therefore the overall energy will be the energy in for the endothermic, the bond breaking part, which is 1,362 take away the energy that's released, which is 1,852. The overall energy is minus 492 kilojoules. So because it's a minus, we know that the reaction is exothermic by 492 kilojoules. That's how much energy is released into the surroundings. Okay. So here is one for you to try. Remember to look at the number of bonds in each molecule. Calculate all the energy in bond breaking and the energy released in bond making then work out the difference. Pause the lesson here to have a go at this one and then restart when you're ready to check your answer. Here is the answer. Pause so you can check your working out and your answer. Well done if you got this one right. Remember, a minus sign shows the reaction is exothermic. One more for practise, pause the lesson here to have a go at it. Be careful with your language when doing question two. Restart the lesson, when you're ready to check. Here is the answer to question one. Please pause to check your answer. And here is the answer to question two. Be really careful with your language around whether the energy is needed or whether the energy is released. Sometimes after you've worked out the overall energy values and you know what the reactants and products are, you may be asked to use this information and represent it in a energy level diagram. So, first of all, you always put the reactants, because this is an exothermic reaction, you put the reactants higher up on the energy level. And then the products low down because it's an exothermic reaction, so products have less energy. You label where the overall energy changes making sure the arrow is pointing downwards. And you may even need to write down what the energy value is, is minus 543 kilojoules per mole. And also identify where the activation energy is. These are the essential parts that you need to be able to label in an exam if you're asked to do so. So to summarise, this lesson, we looked at how to calculate bond energies, and then looking at the overall bond energy values and predicting whether the reaction is exothermic or endothermic. And then looking at bond energies and relating them to the correct parts of energy level diagrams. So that's the end of another energy changes lesson. Please complete the lesson and do the plenary quiz. Well done, for your fantastic efforts. And I look forward to seeing you all again in the next lesson.

This quiz will assess you on your knowledge from this lesson and previous lessons

What is the minimum amount of energy needed to start a reaction called?

What is the unit for bond energy values?

If the overall energy change is negative, what type of reaction is it?

During a reaction, 2462 kJ/mol of energy is needed to break bonds and 1363 kJ/mol of energy is released when bonds are made. What type of reaction is it?

During a reaction 988 kJ/mol of energy is needed to break bonds and 1892 kJ/mol of energy is released when bond are made. What type of reaction is it?

Calculating bond energies - Higher

Calculate bond energy values and use them to predict whether a reaction will be exothermic or endothermic

Relate bond energies to the correct part of energy level diagrams

Explain why bond energy calculations have a margin of error

This lesson, students will calculate bond energy values and use them to predict whether a reaction will be exothermic or endothermic. Students will relate bond energies to the correct part of energy level diagrams.


Combined Science

Y10 Combined Science (Higher)

Key Stage 4

Year 10

Calculating bond energies

Energy changes (HT)

UNIT

Exothermic and endothermic reactions

Required Practical: Temperature change (Part 1)

Required Practical: Temperature change (Part 2)

Writing a method to test a hypothesis

Review combined

Case study

What is the minimum amount of energy needed to start a reaction?

What type of reaction does this energy level diagram represent?

A reaction is overall endothermic because...

To break a C-C bond it requires 348 kJ/mol of energy, how much energy is released when a C-C bond is made?

A student is investigating the effect of changing the concentration of acid on the temperature change when reacting an acid with an alkali. Identify the independent variable.

What happens to the temperature of the surroundings when an exothermic reaction occurs?

Why is it important to use a polystyrene cup in an experiment where the temperature change is measured?

Why is it important to do at least 3 repeats?

Select 4 examples of exothermic reactions:

What measurement could we take to determine if a reaction was endothermic or exothermic?

Hypothesis: A student investigated the temperature change when zinc reacts with different concentrations of copper sulfate. What is the independent variable?

Hypothesis: A student investigated the temperature change when zinc reacts with different concentrations of copper sulfate? What is the dependent variable?

Hypothesis: A student investigated the temperature change when zinc reacts with different concentrations of copper sulfate. What are the control variables? Choose 2 answers.

Lesson overview:Calculating bond energies

Energy changes (HT):