Lesson video

Hello, my name is Mrs. Collins and I'm going to be taking you through today's lesson.

This forms part of the unit Energy Changes in Chemical Reactions and is called "Breaking and Making Bonds Using Bond Energies." So let's get started on the lesson.

During this lesson you will describe how bond breaking and bond making use or release energy, and use bond energies to calculate whether reaction is exothermic or endothermic.

Here are the key words for today's lesson.

Endothermic, exothermic, activation energy and bond energy.

Pause the video here, read through those descriptions, and make any notes you feel you need to.

Today's lesson will be broken down into three separate parts, breaking and making bonds, overall energy change, and bond energy calculations.

So let's get started with part one, breaking and making bonds.

Now remember, non-metal atoms join together covalently.

They're covalently bonded together in compounds.

And the atoms share electrons in covalent bonds.

A good example is hydrogen, H2.

A covalent bond, remember, is the strong electrostatic attraction between a pair of electrons and the nuclei of the bonded atoms. We look at this diagram, we can see that the nucleus is positively charged.

And nuclei is the plural for the word nucleus.

The shared pair of electrons are negatively charged, so their electrostatically attracted to the nucleus.

Here you can see they've been paired together and they're shared between the two atoms. So this is a hydrogen molecule showing a pair of shared electrons.

Covalent bonds need energy to be broken and to release those atoms needed in the chemical reaction.

So here's a molecule of hydrogen.

You've got two hydrogen atoms covalently bonded together.

They're sharing the electrons there in the centre.

They absorb energy from the surroundings, and this breaks that covalent bond to form two hydrogen atoms. So notice the difference between the hydrogen molecule, which is sharing electrons in that covalent bond, and hydrogen atoms. We can show this in this equation here.

So we've got the two hydrogen atoms covalently bonded together, and then the two hydrogen atoms not bonded together.

And we use this line or this stick to show the covalent bond between the two hydrogen atoms. We're now gonna do a question based on the learning so far.

So look through those examples, select the correct examples showing bonds breaking.

Pause the video here and I'll see you when you're finished.

So hopefully you recognise that the correct answers are A and B.

So, well done if you got that correct.

So the minimum amount of energy needed for particles to react is called the activation energy, remember.

And this is the energy needed to break the bonds in those reactants, to enable them to react together to form the products.

So here we've got an example.

We've got hydrogen and chlorine.

So we've got hydrogen molecule, two hydrogen atoms bonded together, and a chlorine molecule with two chlorine atoms bonded together.

And here they're breaking down to form two hydrogen atoms and two chlorine atoms. And this is an intermediate step during the reaction.

Breaking bonds is an endothermic process because energy is absorbed from the surroundings.

The amount of energy that's needed depends on the bonds that are being broken in the reaction.

So if we go back to hydrogen and chlorine again, so we've got the hydrogen molecule and the chlorine molecule being broken down into hydrogen and chlorine atoms, and we can show that in this way.

So we've got the covalent bonds that are being broken between those hydrogen atoms in the molecule and the chlorine atoms in the molecule to form the hydrogen atoms and the chlorine atoms. The atoms released when the bonds are broken in a chemical reaction then rearrange, remember, to form the products.

New bonds are formed between those atoms, and we can show that again taking place here.

So we've got those two hydrogen atoms and the two chlorine atoms then bonding together to form the two hydrogen chloride molecules.

We can also show the covalent bonds between those atoms in the hydrogen chloride molecules.

So you can see there's a covalent bond between each hydrogen atom and chlorine atom forming a hydrogen chloride molecule.

Making bonds from the rearranged atoms is exothermic because energy is released to the environment.

So bond breaking is endothermic and bond making is exothermic.

Now we have a true or false question.

So what I'd like you to do is to read through that question, pause the video here, and I'll see you when you're finished.

Welcome back.

So, bond breaking is an exothermic process.

Is that true or is it false? It's false.

Bond breaking is actually an endothermic process because the bonds absorb energy from the surroundings.

Well done if you've got that correct.

So the amount of energy absorbed to break a particular covalent bond between two non-metal atoms is the same as the amount of energy released when the same bond is formed in a reaction.

So we can look at an example to explain that in a bit more detail.

So the amount of energy absorbed to break a carbon-hydrogen covalent bond equals the amount of energy released when that same bonds is formed.

The units of energy used in this case are kilojoules per mole.

We are now gonna have a go at task A.

So 1A, using the information in the table provided, circle the atoms which do not have the correct number of covalent bonds, and explain why.

So if you have a look in the table, it shows you that a carbon atom can form four bonds, a hydrogen atom can form one bond, and an oxygen atom can form two bonds.

Then move on to B, making bonds is what type of process and why.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So, let's go through those answers.

So hopefully you recognise that the hydrogen here has got two bonds, and it should only have one.

The carbon here has five bonds, and it should only have four.

The oxygen has one bond, and it should have two.

And the carbon has three bonds, and it should have four.

Making bonds is what type of process and why? It's an exothermic process as energy is released to the surroundings.

So if you've got that correct, well done.

Moving on to question two.

This time, complete the table below by counting the number of the different types of bonds in the following molecule.

So look at the structure of the molecule, how many carbon-hydrogen bonds are there? How many carbon-carbon? How many carbon-oxygen? And how many carbon-oxygen double bonds? So you can see there are two sticks there showing it's a double bond.

Now, it may be helpful to draw that molecule out on a piece of paper and tick off each bond as you include them.

And then part B, if a carbon-hydrogen bond takes 412 kilojoules per mole of energy to break, how much energy will be released making a carbon hydrogen bond.

So pause the video here, answer those questions, and I'll see you when you're finished.

Welcome back.

So let's go through those answers, then.

In terms of the table, there are eight carbon-hydrogen bonds, two carbon-carbon bonds, two carbon-oxygen bonds, and one carbon-oxygen double bond.

Part B is 412 kilojoules per mole 'cause remember, it takes the same amount of energy to break a bond as is released when that bond is made.

So well done if you've got that correct.

Now we're moving on to the second part of the lesson, overall energy changes.

So during a chemical reaction, bonds are broken in the reactants, and during this process energy is absorbed.

The atoms released are then rearranged and new bonds are formed making the products.

And during this part of the process, energy is released, and we've got an example there below.

So we've got hydrogen and bromine reactants.

The bonds are broken in those molecules, so we've got free atoms which can then rearrange, and then bonds are formed between the hydrogen and the bromine forming hydrogen bromide as the product.

Bond breaking is endothermic because energy is absorbed into the reaction, and bond forming is exothermic because energy is released.

So the breaking and making of bonds can also be shown in a reaction profile.

Now remember, the reaction profile is a diagram which shows the energy changes during endothermic and exothermic reactions.

So here we have the energy on the Y-axis.

We've got the reactants at the bottom here, and then the products are higher up.

So that shows you the reactants contain less energy than the products.

We've got the energy loop here showing you what's happening in terms of energy in the reaction.

And this is an endothermic reaction, and we know that because the reactants contain less energy than the products.

So the products have absorbed energy from the surroundings.

And then we've got the second diagram, again, energy on the Y-axis.

We've got the reactants higher up this time than the products, so there's more energy in the reactants than in the products, and that must mean energy has been released into the environment or into the surroundings.

We've got the loop here, and this time an exothermic reaction.

And we know it's an exothermic reaction because the reactants contain more energy than the products, so energy must have been released into the surroundings.

So here's a question based on that learning.

Select the exothermic reaction profile where R equals the reactants and P equals the products.

So pause the video here and I'll see you when you're finished.

So hopefully you've recognised that the answer to the question is B.

And it's B because the reactants contain more energy than the products, and this is because energy has been released into the surroundings during the reaction.

So, well done if you've got that correct.

So the reaction profile for hydrogen reacting with bromine looks like this.

So we've got the energy up on the Y-axis, we've got the reactants here, we've got hydrogen and bromine molecules, and we've got the loop.

And at the top of the loop we've got atoms being released.

So on the way up, those molecules are being broken.

And at the end we've got the products, the two hydrogen bromide molecules.

Now we can see that's an exothermic process because the products have got less energy in them than the reactants.

This is the activation energy.

Remember that's the energy needed to break the bonds in the molecules for the reactants.

And during this process energy is absorbed to break those bonds.

That's an endothermic process.

Energy is released when the bonds are made.

And the overall energy change is the difference between the energy in the reactants and the energy in the products.

And in this case, it's a negative energy change because energy is released to the environment.

So this is an exothermic reaction as the energy released making the bonds is greater than the energy absorbed to break the bonds.

And if you look at the two green arrows on that diagram, you can see that.

so the energy released when the bonds are made is greater than the energy absorbed to break the bonds.

The overall energy change for a reaction is the difference between the energy absorbed to break the bonds and the energy released to making the bonds.

So we can say that's the energy absorbed to break the bonds minus the energy released making the bonds.

So for an exothermic reaction, the energy absorbed to break the bonds is less than the energy released making the bonds.

This means the overall energy change is negative.

Here is a question based on the learning so far.

So, the overall energy change for an exothermic reaction is? Pause the video here and answer the question.

Welcome back.

So, hopefully you've recognised that the answer to that question is negative.

And the reason is the energy being released making the bonds in the products is greater than the energy required to break the bonds in the reactants.

So the reaction profile for the decomposition of water looks like this.

So we've got the energy again on the Y-axis.

we've got the reactants, which is the water molecules.

We've got the loop, and at the end of the loop we've got four hydrogen atoms and two oxygen atoms being released.

So that's where the covalent bonds are broken.

And here are the products.

So we've got two hydrogen molecules and one oxygen molecule.

Here we can see is the activation energy.

So this is the energy required to break the covalent bonds in the water.

That's the energy absorbed to break those bonds.

And then here is the energy released when the bonds are made.

So look at those two arrows and think about how they're different to the previous example we did.

And here is the overall energy change, and we can see that that's positive.

So this is an endothermic reaction because the energy absorbed to break the bonds is greater than the energy released making the bonds.

And we can see that in the green arrows.

So the energy absorbed to break the bond, the arrow there, is much larger than the arrow for the energy released when the bonds are made.

So the overall energy change for a reaction, remember, is the difference between the energy absorbed to break the bonds and the energy released making those bonds.

And in the case of an endothermic reaction, the energy absorbed to break the bonds is greater, this time, than the energy released making the bonds.

This means the overall energy change is positive for an endothermic reaction.

So now we've got a true or false question.

So the overall energy change in an endothermic reaction is positive.

Is that true or false? And then justify your answer using the statements below.

Pause the video here and answer the question.

Welcome back.

So hopefully you recognise that that statement is true and that is because the energy required to break bonds is greater than the energy released making the bonds.

So, well done if you've got that answer correct.

We're now gonna have a go at task B, and it says, "Look at the values for the overall energy change for some reactions.

Complete the table and decide whether they are exothermic or endothermic." So we're interested in whether those numbers are positive or negative.

So for each one decide is it exothermic or endothermic.

So, pause the video here and answer the question.

So let's go through the answers.

So the top one, 50,500 kilojoules per mole.

That is endothermic because the number is positive.

The -105 is exothermic because the number is negative.

And if we go down each of those, the next one is exothermic because it's a negative number, endothermic because it's a positive number, and exothermic because it's a negative number.

So remember the differences there between the two, the endothermic and the exothermic.

So, well done if you've got that correct.

Here's question two.

So, explain the overall energy changes for the following reactions.

Consider whether they are positive or negative and why that is the case.

So for A, we've got combustion of methane, and for B we've got thermal decomposition.

So take your time with this one, really think it through, and I'll see you when you're finished.

Welcome back.

So, let's go through the answers.

So the combustion of methane is an exothermic reaction.

And the overall energy change would be negative.

And this is because more energy would be released forming the products than was absorbed breaking the bonds in the reactants.

And then we've got B, thermal decomposition of copper carbonate is an endothermic reaction.

So the overall energy change would be positive as more energy would be absorbed breaking the bonds in the reactants than is released forming the bonds in the products.

So, well done if you got that correct.

So let's on to part three of today's lesson, bond energy calculations.

Now you may find some of this work challenging, but I'm gonna help support you through it.

So, the energy required to break a covalent bond, or is released when making a covalent bond, is known as the bond energy.

And each covalent bond between two non-metal atoms has a known bond energy.

So for example, hydrogen, H2, the value for this bond energy is 436 kilojoules per mole.

So if we wanted to break that hydrogen-hydrogen bond, that would mean that 436 kilojoules per mole of energy would be absorbed.

And if we were to make that hydrogen-hydrogen covalent bond, then 436 kilojoules per mole of energy will be released.

The size of the bond energy value reflects the strength of the covalent bond.

So for example, a bromine-bromine single bond has got a bond energy of 193 kilojoules per mole, and that is lower than the hydrogen-hydrogen covalent bond of 436 kilojoules per mole.

This means it needs less energy to break a bromine-bromine bond, and it releases less energy when it's formed.

The overall energy change can be calculated using the following equation, and it's important to note that that symbol at the start means "sum of." So the overall energy changes equals the sum of the bond energies in the reactants minus the sum of the bond energies in the products.

It may be worth pausing the video here and just writing down that equation.

So, here's a question based on the learning so far.

If the amount of energy needed to break an oxygen-oxygen double bond is 498 kilojoules per mole, how much energy would be released making this bond? So pause the video here and answer the question.

So hopefully you've recognised the answer to that question is 498 kilojoules per mole.

And this is because the bond energy value needed to break the bond is the same as the amount of energy released when making the bond, even if it's a double or a triple bond.

So you don't need to multiply it by two because it's a double bond, you just have to take the same value.

So, well done if you've got that correct.

We are now going to go through a few examples together.

So we've got hydrogen reacting with bromine here to form hydrogen bromide.

And I want to draw your attention to the table of data first of all.

So that gives you all the bond energies for the different bonds.

Remember, that's the amount of energy that is absorbed to break those bonds, but it's also the amount of energy that's released when those bonds are formed.

So step one is to write down the balance symbol equation for the reaction, and then we can display each of the individual bonds that need to be broken or made.

So the bonds that need to be broken are the hydrogen-hydrogen single bond and the bromine-bromine single bond.

And then the bonds that are going to be made are two hydrogen-bromine bonds.

So we look in the table, identify the bond energies for each of those individual bonds, and then we can display them underneath.

Then what we need to do is sum, or add together, the values on both sides of the equation.

So we've got the value for the total bond energies for breaking those bonds is 629 kilojoules per mole.

And then the sum for making the new bonds is 732 kilojoules per mole.

And then we can put that data into that equation that you wrote down earlier to find the overall energy change.

So it's the sum of the energy is required to break the bonds minus the sum of the energy released when those bonds are made.

We put the data into the equation, carry out the calculation, and notice that we end up with a negative number.

So we've got -103 kilojoules per mole.

And because that's a negative value, we know that it's an exothermic reaction, so that we know during that reaction energy is released into the surroundings.

So let's go through a second example.

This time we've got methane plus oxygen forming carbon dioxide plus water, and we've got a new table of bond energies there.

So step one, write down the balanced symbol equation.

And then underneath we do the displayed formula for each of those different molecules so that we can see what bonds are present.

So we can see, if you look in the methane, you've got four carbon-hydrogen bonds there.

In the oxygen molecules you've got two oxygen-oxygen double bonds.

In the carbon dioxide you've got two carbon-oxygen double bonds.

And then in the water you've got four oxygen-hydrogen bonds.

So it's really important, you can see, to put those displayed formula out so that you can really see how many bonds you've got.

So again, underneath we will write down what bond energies we have on both sides of the equation again.

So you can see that there.

I'll just give you a moment to absorb that information.

Then we sum both sides.

And then we can work out the overall energy change using the equation.

And that gives us a value of -816 kilojoules per mole.

And again, it's exothermic because it's a negative value.

So you might want to pause the video here and just look carefully through that calculation.

Okay, we're going to do another example, and this time I'm going to walk you through it, and then you are going to do your own version of the calculation afterwards.

So what is the overall energy change when decomposing water? We've been given all the different bond energy values there for the different bond types.

You've been given the equation, and you've been given the displayed formula as well.

So step one is to look and count up how many of each different type of bond we've got.

So in terms of the oxygen-hydrogen bond, we've got four bonds there.

And then on the other side we've got two hydrogen-hydrogen bonds and one oxygen-oxygen double bond.

So we then look at the data and we'd include the data.

So we know we've got four lots of 463, we've got two lots of 436, and then we've got 498.

So we add those values together to give us the values on both sides of the equation, write down the equation for putting the data in, input the data into the equation, and that gives us a value of 482 kilojoules per mole.

And because that's a positive number, we know it's an endothermic reaction.

So you are going to go through this example yourself.

So what is the overall energy change when hydrogen reacts with iodine? You've got the bond energies listed there, the balance symbol equation, and you've got the displayed formula to show you what bonds are present.

So pause the video here and answer the question using my example as a model.

Welcome back.

So your answer should be displayed like this, similar to mine.

So we're going to write down the bond energies, first of all, for both sides of the equation.

We're gonna add together and do the calculation so we know what the value is for each side of the equation.

We're going to write down the equation.

Input the data.

Carry out the calculation.

And we can see the value is -6 kilojoules per mole.

And we know it's exothermic because it's a negative value.

So, well done if you've got that answer correct.

Now we're going to go on to task C.

So we're going to correct Andeep's calculation as the answer should be -123 kilojoules per mole for this equation.

And Andeep's got that wrong.

He's ended up with -466 kilojoules per mole.

So what I need you to do is look through that calculation, maybe carry out the calculation by yourself on paper, and then you'll be able to see where Andeep has gone wrong.

So pause the video here and then work through the answers.

Welcome back.

So let's go through the answers, then.

So the first thing that's incorrect is here at the start.

So Andeep's written down that there are two lots of the chlorine-chlorine single bond, but in actual fact there's only one.

So let's just correct that part of the answer there.

The next thing is that he's recognised that there are three carbon-carbon hydrogen bonds, but he's actually forgotten the carbon-chlorine bond here.

So we need to add that.

And then the next bit he's got wrong, as we just do the calculation there, the next bit he's got wrong is the overall energy equation.

So he's got those the wrong way round.

So it's break minus make.

So if we switch around the values here, we end up with the correct answer of -123 kilojoules per mole.

So well done if you got the answer to that correct.

Moving on to the next question, calculate the overall energy change for the combustion of ethene.

So this time you're going to go through the calculation yourself.

They've given you the displayed formula there to help you.

You might want to write that out on a piece of paper and just tick off each bond as you've included it.

So pause the video here and answer the question.

Welcome back.

So let's work through that answer together.

So first of all, we've got one carbon-carbon double bond.

We've got four carbon-hydrogen bonds, and we've got three oxygen-oxygen double bonds.

We've got four carbon-oxygen double bonds, and four oxygen-hydrogen bonds.

So let's pop that data in, first of all.

So your data should look like that.

If we then sum up both sides of the equation before we move on, place those numbers into the equation here, carry out the calculation, and we end up with a value of -1,314 kilojoules per mole, which makes it exothermic because it's a negative value.

So, well done If you got that answer correct.

You might want to pause the video here just to check through your answer.

If you made a mistake, see if you can identify where that is.

So here's a summary of today's lesson.

In all chemical reactions, bonds in the reactants need to be broken before the reaction can happen.

Energy must be supplied to break the bonds in the reactants.

Energy is released when bonds in the products are formed.

The energy needed to break bonds and released making bonds can be calculated using bond energies in kilojoules per mole.

And the difference between the energy needed to break bonds and the energy released making the bonds is the overall energy change.

So, well done for today's lesson.

Some of that was a little challenging, but I'm sure you've worked through it well.

Thank you for joining me for today's lesson.