Lesson video

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

This lesson forms part of the Energy Changes in Reactions unit and is called bond making and bond breaking.

So let's start the lesson.

During today's lesson, we're going to describe how bond breaking and bond making use or release energy.

Here are the key words for today's lesson, endothermic, exothermic and activation energy.

Pause the video here, read through the definitions, and write down any notes that you feel you need to.

Today's lesson is divided into two parts, breaking and making bonds, and overall energy changes.

We're going to start 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. If we look at this diagram, we can see that the nucleus is positively charged.

A nuclei is the plural for the word nucleus.

The shared pair of electrons are negatively charged, so they're 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.

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 going to 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 recognised 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 a 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 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 bond is formed.

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

We are now going to have a go at task A.

So one A, 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 recognised 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 mol 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 mol, '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 got that correct.

Moving on to part two of the lesson, the overall energy change.

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 got hydrogen and bromine reactants.

The bonds are broken in those molecules, so we 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 and 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 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 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 recognised 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 are now going to 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 mol, that is endothermic because the number is positive.

The minus 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 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 then 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 then is released forming the bonds in the products.

So well done if you got that correct.

Here is 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 reactant.

Energy is released when bonds in the products are formed.

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

So thank you very much for joining me for today's lesson.